Emission Spectrometry Marvin Margoshes and Bourdon F. Scribner, National Bureau of Standards, Washington, D. C.
T
the ninth in a series, covers optical emission spectrometry, flame photometry, and atomic absorption spectrometry for the years 1964 and 1965. In the previous review (280), it was estimated that the 565 references cited represented about one quarter of the publications which appeared in the years 1962 and 1963 within the subject range of these reviews. It appears that a similarly comprehensive coverage for the period of the present review would require approsimately 800 references! The authors have made a study of the rate of growth of the literature of emission spectrometry, including flame photometry and atomic absorption spectrometry, based on the citations in our files for thc years 1920 through 1960. From 1920 until 1941, the literature in these fields doubled each 4l/4 years. Following a drop in the rate of publication during World War TI, the number of papers published doubled every years. An extrapolation of this growth curve yields a predicted publication rate of 1500 articles in 1964 and 1700 in 1965. Xot only is the literature on emission spectrometry and closely related subjects growing a t a rapid rate, but it is also widely distributed. A survey of the literature on flame photometry for the year 1961 indicated that there were 155 publications which appeared in 106 journals and nonperiodicals. It is likely that the literature of all of emission spectrometry has a similarly diffuse distribution. It is clear that it is not possible to mention more than a small percentage of the publications on emission spectrometry in this review. In addition, not all this literature is known to us at the time of sriting, since we are aware only of that portion that has been cited in Chemical Abstracts or was located by a search of 20 journals which most frequently publish articles on emission spectrometry. From these we have selected a relatively small number that appear to be of interest to a substantial number of readers. HIS REVIEW,
BOOKS AND REVIEWS
A translation has been published (44) of a 1960 Russian practical text on spectral analysis, which will be of interest to Western readers mainly for its description of Soviet instruments and methods. Plsko (246) has written a practical textbook on optical methods of analysis, and a book by Zaidel et aZ.
(349) on the spectrometric analysis of atomic materials has been translated into English. Navrodineanu and Boiteux (199) have brought out a large, handsome volume on flame spectrometry. This book contains little information on specific methods of analysis, and is otherwise quite different from the several works previously published on flame photometry, but analytical spectrometrists will find it to be of value because of its excellent treatment of the theory of flame excitation and of methods for the excitation and measurement of flame spectra. Kuba et al. (175) have prepared a coincidence table for spectral analysis, covering the wavelength range from 2000 to 10,000 A. For each of more than 80 elements, there are listed an average of 12 characteristic lines as well as lines of other elements which are close in wavelength. Kerekes (157) has published an atlas of 12 charts and a set of tables listing analysis and interfering lines for scandium, yttrium, lanthanum, and the stable rare earth elements. Intonti and Cecchetti (139, 140) have prepared a similar atlas and tables for the rare earth elements. An atlas of the spectra of aluminum, carbon, copper, iron, germanium, mercury, silicon, and the hydrogen molecule in the wavelength range from 1100 to 2250 A. has been published by Junkes, Salpeter, and RIilazzo (147). Committee E-2 of the American Society for Testing and Materials has issued new editions of two valuable reference works for emission spectrometrists. These are the fourth edition of “Methods for Emission Spectrochemical Analysis” (6) and a new edition of the “Report on Available Standard Samples, Reference Samples, and High-Purity Materials for Spectrochemical Analysis” ( 5 ) . Bochkova and Shreider (28) have written a book on spectral analysis of gas mixtures. A 1948 publication by Mitchell on the analysis of agricultural materials has been reissued (213) with the addition of new material describing recent developments. A handbook on soil analysis (27) includes descriptions of some flame and emission spectrometric methods. Volumes 3 (92) and 4 (60) of “Developments in Applied Spectroscopy,” the proceedings of the MidAmerica Symposium, have been published; each of these volumes includes several articles on emission spectrometry, some of which have been published elsewhere.
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Volumes 5 and 6 of Part I of the “Treatise on .halytical Chemistry” contain some sections of direct interest to the readers of this review. These include chapters by 1Ieehan on spectrometric apparatus and measurements (203) and the emission and absorption of light by atoms and molecules (LOW), as well as a chapter on emission spectrometry by Scribner and Margoshes ( % S I ) and one on flame photometry by Vallee and Thiers (324). A book on plasma spectrometry by Griem (113) will be helpful to analytical spectrometrists who wish to improve their knowledge of the relations between spectral ernhion and source parameters, such as temperature and particle density, and of the methods of measuring these parameters. More and more such nieawrements are being made on the light sources used in analytical emission spectrometry (see below). A useful supplement to Griem’s book is the set of tables by Drawin and Felenbok (72) which includes partition functions and values of the Saha-Eggert equation for 38 elements as well as other data and equations needed for the spectrometric study of plasmas. Nedler (228) has reviewed advances in emission spectral analysis, and a number of more specialized reviews have also appeared. Eckhard (SO) has discussed the development of photoelectric methods, and Romand (264) described spectrometry in the 10- to 2000-A. region, including analytical applications. The determination of trace elements by emission spectrometry was the subject of a review by Peter (241). Trucco (321) considered applications of emission spectrometry in the nuclear field. Nakajima and Takahashi (224) discussed isotopic analysis by emission spectrometry. Foster (93) and Penkin (239) have summarized the methods for the measurement of atomic oscillator strengths. There have been several reviews of the rapidly growing field of atomic absorption spectrometry, including articles by Bermejo Martinez (22), David (59), Lockyer (1867, Parellada Bellod (238), Poluektov and Zelyukova (249), RubeBka and Velickh (269), Schleser (276),Tardon (SI%’), and Thilliez (313). Herrmann (128) prepared a review which touches on atomic absorption spectrometry as well as several aspects of flame emission spectrometry. Other recent reviews on flame photometry are by Dean (61), Gilbert (106), Hankiewicz (120), and Prugger (250). VOL. 38, NO. 5 , APRIL 1966
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SPECTRAL DESCRIPTIONS A N D CLASSIFICATIONS
Considerable progress has been made in the seemingly endless task of measuring and classifying the spectral lines of the elements in their various states of ionization. In the past several years there has arisen increased recognition of the need for such data and new methods have been devised to speed the work of making the measurements and analyses. These methods include automatic devices for reading spectral plates and computer techniques for evaluating the data. New measurements in the far ultraviolet and in the infrared regions have also been of great value. Penkin and Shabanova (2.40) have measured absorption spectra of aluminum, gallium, indium, and thallium in the 2000- to 2300-A. region, and with these new data have reclassified 47 lines of aluminum, 61 of gallium, 14 of indium, and 25 of thallium, and have given new values for the first ionization potentials of aluminum, barium, calcium, gallium, indium, strontium, and thallium. Gutmacher, Hulet, and Lougheed (116) have measured the spark spectrum of berkelium between 2500 and 4500 A. and have listed the wavelengths and relative intensities of 20 lines of this element. Johansson (145) has given new measurements of the wavelengths of about 340 lines of C I between 3400 and 9700 A. A remeasurement and partial analysis of the second spectrum of germanium was made by Shenstone (287). Diago (67)has revised the analysis of the second spectrum of nickel on the basis of new measurements in the ultraviolet, visible, and infrared regions. Bockasten et al. (29) have made new measurements of the spectra of multiply-ionized nitrogen and oxygen between 200 and 8000 A., with a highcurrent pinch discharge as the source; they reported 38 lines for 0 111, 31 for 0 V 17 for X V, and 3 for N VI. Shenstone (286) made new measurements on 1110 lines of Pd I11 between 680 and 3000 A. and classified 1028 of these. Richards and Ridgeley (263) in a preliminary classification of the first spectrum of plutonium have assigned about 1200 lines to transitions between 284 energy levels. Radziemski and Andrew (263) have reinvestigated the first spectrum of silicon, observing 179 new lines and classifying nearly all of them; these observations have permitted the identification of 80 additional energy levels for this element. The spark spectrum of tellurium was investigated between 340 and 9040 A. by Crooker and Joshi (579, who have assigned each line to spectra from Te I to Te VI. Brill (37) has written a thesis on the arc spectrum of tin, and Laun (181) has made a study of the second spectrum of tungsten, assigning 298 R
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2173 lines to transitions between 194 energy levels. The effort that has been made in the past several years to classify the spectra of the rare earths and related elements has resulted in considerably improved knowledge of their energy levels, particularly for the ionized elements and the lower energy levels. This effort has been advanced by more accurate wavelength measurements, by methods of identifying lines corresponding to transitions involving the lower energy levels, and by improved theoretical techniques for predicting the approximate positions of the energy levels. Hook and Thekaekara (155) have used nearly automatic methods to measure the positions of 977 lines of yttrium with an accuracy of 0.005 A. and have classified 162 of these lines on the basis of known energy levels. Sugar and Kaufman (306) have measured 45 new lines of the third spectrum of lanthanum in the vacuum ultraviolet and have established 13 new terms for this ion. New observation of the third spectrum of cerium, including 1700 new lines between 750 and 11,000 A,, were made by Sugar @OS), who established 126 new energy levels and revised the values for some previously known levels. Calculations were made of the positions of some energy levels of doubly ionized praseodymium by Spector (296) and Trees (319), who found reasonably good agreement between their calculations and experimental observations. Sugar (304, 305) has measured the spectrum of Pr IV between 800 and 3100 A. and has established a number of energy levels and made numerous line assignments. Smith and Wybourne (295) have made a partial assignment of energy levels of the europium atom. Marquet and Davis (196) measured about 8000 lines of the first and second spectra of erbium and have assigned the seven lowest levels of the atom and the eight lowest levels of the singly ionized atom. Preliminary reports on energy level assignments for the erbium atom were given by ,Marquet and Behring (195) and by Spector (297). Bryant (42) investigated the spectrum of ytterbium between 600 and 11,000 A. and assigned 41 energy levels for Yb+2 and 22 for Ybf3. Several measurements have been made of isotopic shifts of spectral 'lines. In most cases, the data were obtained to provide information on the atomic nucleus, but they nevertheless are of interest to analytical spectrometrists. Haynes and Ross (113) measured the shifts of seven lines for six isotopes of neutral erbium, and Rao and Gluck (259) measured isotope shifts for approximately 50 lines in the first spectrum of neodymium. Isotope shifts for six lines and seven isotopes of ruthenium were measured by King (160); and
Striganov and Kulazhenkova (302) reported on isotope shifts for eight lines and five isotopes of samarium. Stacey ($98) determined the shift of the Sn I1 line a t 3283 A. for ten isotopes of this element, and Rao (258) described the results of measurements of isotope shifts for uranium, but did not find any lines which were more favorable for isotopic analysis than those presently being used. Measurements of the shifts of three ultraviolet lines of zinc for two isotopes were reported by Les and Les (184). hlore precise measurements of the wavelengths of spectral lines are invaluable in classifying complex spectra, and these are made possible by better wavelength standards. Improved mavelength standards are also of particular importance in the vacuum ultraviolet. Peterson (2.42) has measured the wavelengths of some lines of argon, krypton, neon, and xenon between 700 and 1300 A. with an accuracy of 0.003 A. Iglesias (137) has calculated the wavelengths of numerous lines of Mn I1 between 1100 and 2000 A. with an accuracy of 0.001 to 0.002 A. from the known energy levels of this ion. Measurements of the vacuum wavelengths of 95 lines of KrS6between 3400 and 6900 A. were made by Phelps (245) with a reported accuracy of about A. Interferometric measurements for lines of thorium, with a similar accuracy, were reported by Giacchetti et al. (103), Littlefield and Wood (186), and Meggers and Stanley (204). Giuliani and Thekaekara (107) have made interferometric measurements on 218 lines of titanium between 3340 and 4190 A. with an accuracy of better than 0.002 A. SPECTROMETRIC INSTRUMENTATION
Kozlov (171) has given a novel treatment of the problem of describing the resolving power of a spectrograph or monochromator. Rather than considering the resolving power in terms of the shape and separation of hypothetical monochromatic lines, he has given a statistical treatment of the probability of distinguishing between two lines. An advantage of this treatment is that the effect of noise can be specifically included. Although spectrometers are ordinarily thermostated to prevent thermal line shifts, shifts caused by pressure changes have usually been neglected. Carlsson (50) has made a study of the effects of temperature and pressure on line positions in spectrometers and has developed a compensation method which counteracts the effect of pressure changes by means of controlled temperature changes. A method of providing an intensity scale on a photographic plate was described by Gerharz (101, 102). A coarse isometric transmission grating,
with the bars equal in width to the clear areas, is mounted inside a stigmatic spectrograph in such a way as to disperse a spectrum a t right angles to the main dispersion of the instrument. Because of the particular spacing of the grating, all of the even orders are missing, so each spectral line appears in the zero, first, third, etc., orders, with intensities which may be calculated from the known grating relationships. This arrangement gives a wide intensity scale on the photographic plate; for example, the intensity in the fifth order will be 1.6 per cent of the intensity in the zero order. It should also be possible to distinguish between the different orders appearing on the plate when, for example, an 13bert spectrograph is used a t large angles of the grating. Gabriel, Swain, and Waller (97) have designed an instrument for the photographic or photoelectric recording of spectra between 5 and 950 A., with the grating mounted a t grazing incidence. Thorn (314) described an inexpensive Littrow spectrograph which can be built from readily available parts and also the design and construction of a comparator - microphotometer. Vierle (326) designed two modifications of the Ebert mount. In one arrangement, up to 50 cm. of spectra can be photographed with low aberration with a focal length of only 1.5 meters. The other arrangement is an f / l O monochromator in which the slit heights can be as great as 100 mm. The design and performance of a prototype Soviet vacuum spectrometer for the determination of carbon, phosphorus, sulfur, and other elements in ferrous alloys were described by Klimova et al. (162); the instrument appears to be similar t o those made in the United States and Western Europe. Belyaev, Ivantsov, and Kikitina (21) compared the Russian DFS-10 spectrometer with an American instrument, and reported that the two spectrometers gave comparable results. Ikonnikova, Podmoshenskaya, and Frolova (138) investigated the sensitivity of detection of impurities in copper and lead with the DFS-10 spectrometer. They found that several elements could be detected a t concentrations as low as 1 to 10 p.p.m., with the sensitivity limited by the intensity of the background. Giavino (104)has given a description of a photoelectric attachment for a plane grating spectrograph. Two slits and photocells can be moved to selected positions along the focal plane as needed for nonroutine analyses or for research. Kessel and Jecht (159) have developed a novel method of photoelectric spectrometry. A sample and standard are sparked alternately in a rotating electrode holder, and a synchronous switch directs the signals alternately to
a pair of capacitors. At the end of the exposure period, the difference between the two stored charges is read out. The apparatus has been applied to the determination of aluminum and iron in glass and to the study of the intensity of a line as a function of element concentration in the sample. However, no marked advantage of this system as compared to the usual methods of photoelectric spectrometry was demonstrated. Weekley and Norris (335) have described a modification to a commercial spectrometer to permit automatic correction for the continuous background. The background is detected by one photocell placed on the focal curve, and signals proportional to the background a t this position are electronically subtracted from the line-plus-background signals which are stored on each of the element capacitors. As a result of the background correction, the analytical curves are made to be linear a t low element concentrations, and a relatively simple analog computer was then used to convert the instrument readings to element concentrations which were printed out as four-digit numbers. Kennedy (156) gave the results of experience with a similar background correction system in the photoelectric analysis of stainless steels. He found that, for these samples, the intensity of the background was related to the chromium content, so that the background correction minimized the matrix effect. Another approach to background correction was developed by Svishchev (309) ; a narrow spectral range is scanned rapidly and repeatedly by means of a rotating biprism, and the output of the photocell is amplified by an a. c. amplifier. In a tebt, the system excluded 99% of the background intensity. Several special-purpose photoelectric spectrometers have been described, three of these for the determination of gases in metals. llatsumoto, Fassel, and Kniseley (198) constructed an instrument for the determination of oxygen in steels. A small Ebert monochromator was employed to isolate the 7772-A. line of oxygen and an interference filter selected a group of argon lines as the internal standard. A precision of +5.6 % (relative standard deviation) was reported for the determination of oxygen in steel, but the authors were not successful in applying the instrument to the determination of nitrogen. Webb and Webb (334) described an instrument built around a small grating monochromator fitted with two photocells to measure the intensity of an oxygen multiplet and background. The samples were melted in a hollow-cathode discharge which also excited the spectrum of the oxygen. Output was
recorded digitally. Analyses could be made a t a rate of 2.5 minutes per sample with a sensitivity of 1 pg. of oxygen and a relative standard devi& tion of 6 to 15%, depending in part on sample homogeneity. Baum and Eckhard (14) developed a spectrometer using interference or glass filters to isolate the spectra of nitrogen and carbon monoxide following hot extraction of the gases from metals. The higher light intensities transmitted by filters compared to monochromators made it possible to employ less expensive electronics in this device. Hagenah and Laqua (118, 180) developed an instrument for the continuous determination of the iron and manganese content of dust from a Thomas converter during the production of steel. Dust is drawn through a tube into a discharge vessel, the spectrum is excited with a spark, and the radiation is recorded photoelectrically. Rozsa et al. (266) have described a spectrometer which automatically monitors the beryllium content of air each 75 seconds. Air containing beryllium particles is drawn through a filter paper which is then moved into a spark discharge. The intensity of a beryllium line is measured photoelectrically, and if the content of this toxic element in the air reaches an unsafe level, an alarm is automatically actuated. Three novel microphotometers have been described, each intended for a different purpose. An instrument built by Steinhaus, Engleman, and Briscoe ($00)provides data to a digital computer to locate line positions. The transmittance values are recorded a t equal intervals along the plate and these data are fed to the computer which is programmed to locate the centers of the lines. Overlapping lines can be resolved when their shapes are known. It has been shown that line positions can be located to 0.1 micron in this way. A microphotometer for semiquantitative analysis was designed by de Villiers and van Wamelen (328) to permit approximate density measurements by matching the intensity of light transmitted by a line with a light spot of variable intensity. An instrument described by Met (208) was designed for the analysis of spectra on Polaroid positive film; a novel feature of this microphotometer is the use of a gas laser as the light source. Sterghiu-Frimescu and Sandulescu (301) have reviewed various types of detectors which may be used in photoelectric spectrometry, with data on their properties and techniques for their optimum application. Schneider (277) has discussed the use of image converters in time-resolved spectrometry. These devices combine light amplification with a capability for rapid on-off switching. Kendall (155) employed a magnetically VOL. 38, NO. 5, APRIL 1966
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driven mechanical shutter for time resolution; this device permits the time during which the slit is open to be controlled between 50 and 1000 psec. EXCITATION
Properties of Arcs and Sparks. One of the most interesting developments in the period covered by this review has been an increased emphasis on basic studies of the properties of arc discharges. These studies have emphasized measurements of arc temperatures and atom concentrations, in the hope that a more detailed knowledge of these parameters and their dependence on such factors as arc current, sample composition, and composition of the surrounding atmosphere will make possible a more rational choice of operating conditions for an analysis. Czernichowski (58) has reviewed spectral methods for temperature measurement including relative line intensities, line contours, line reversal, line width, and the analysis of the continuum. The first method is the one most commonly used, generally by adding a thermometric element to the plasma being studied. Hefferlin and Rouse (124) have commented on one of the hazards of this procedure, namely the possibility that the element added may change the plasma temperature. Another hazard is the possibility that the line intensities may be reduced by self-absorption, changing the apparent temperature of the plasma. Ovechkin (237) described a method for correcting for such self-absorption. Any temperature measurement assumes thermal equilibrium, an assumption which is often taken for granted even though exact thermal equilibrium is theoretically impossible in a radiating plasma. Gurevich and Podmoshenskii (115) have tested the approach to thermal equilibrium for arcs in air and argon a t atmospheric pressure and have found in each case that the difference between the excitation temperature indicated by the line intensities and the true gas temperature is small enough to introduce no serious error in measurements. Trich6 and TrichC ($20)have derived a formula relating the temperature of an arc plasma to the concentrations of electrons and atoms. Volk-Levanovich (331) made measurements in an a. c. arc of the intensities of two groups of iron lines, and from these data he obtained temperatures of 5380 270' K. and 5100 rt 300' K. Desai, Vaidya, and Bidaye (66) measured the vibrational temperature indicated by the BaO bands in an arc, obtaining a result of about 2 5 0 0 O K., which seems to be much too low to be representative of true arc temperatures, indicating either a serious departure from thermal equilibrium for this molecule or else the existence of the
*
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molecule only in the coolest regions of the arc. Bokova and Sukhanova (31) have made measurements of the temperature distribution over the cross-section of an arc in nitrogen containing 4'3, of argon. Several studies have been made of the distribution and lifetimes of atoms in arc plasmas. Perhaps the most extensive of these studies was by de Galan (98), who established the element distributions from the line intensities taking into consideration the distribution of temperature and electron density in the arc. He found that the radial distribution of the atoms is nearly the same for all elements and that there is an appreciable radial decline of atom density only near the lower electrode (anode). However, the axial distribution and the lifetimes of the atoms were observed to be different for different elements and to be dependent largely on the degree of ionization of the element; therefore de Galan fitted his data t o an empirical equation in terms of the degree of ionization of the element, which is in turn a function of arc temperature and the properties of the atom. From the results of this study, de Galan and Bounians (99) made calculations of the arc temperature which would give the highest line intensity for a given element concentration in the sample for elements of different ionization potentials and lines of different excitation potentials. Rubeaka (267) has made measurements of the distribution of excited atoms in the free-burning arc in air and in the arc in air and argon in a Stallmood jet. The results obtained with the Stallwood jet in air were similar to those obtained in the freeburning arc, but there was observed a much greater radial separation of the excited atoms by element in argon. One of the difficulties in making spectral measurements of arc parameters is to obtain reliable values for the oscillator strengths or transition probabilities. Only relative values are needed to estimate temperatures from relative line intensities, but absolute values are required to obtain data on element concentrations and the absolute values are less reliable than the relative values. Rice and Ragone (262) have described a method for measuring both oscillator strengths and atom concentrations a t the same time, based on absorption measurements made a t lorn atom concentrations where the vapor is optically thin and a t high atom concentrations where the vapor is optically thick. Measurements of the lifetimes of lithium, sodium, thallium, and mercury atoms in flames and a. e. and d. c. arcs were made by llalykh and Serd (190), who found that the residence times of these elements were two to three times greater in the a. c . arc than in the d. c. arc. Rukosueva (270) made measure-
ments on element concentrations and the degree of inhomogeneity in arcs based on reabsorption of light reflected back through the arc from a mirror. Naumenkov (227) has measured the atom concentrations and the heterogeneity in a low-voltage pulsed discharge from the contours of the spectral lines, and Burakov and Naumenkov (43) have applied the same method to study the composition of the vapors in an a. c. arc. Krempl (173) has reported the results of a theoretical study of the effects of third elements on line intensities from arc discharges. One cause of interelement effects is a shifting of ionization equilibria, caused by changes in electron density in the discharge plasma. Krempl calculated that such influences should be marked a t flame and arc temperatures, but that they will be small at the much higher spark temperatures. Grechikhin and Tyunina (111) made an experimental study of the effect of gas pressure on the properties of an arc in argon. In the pressure range from 1 to 30 atmospheres, increasing the pressure increased the temperature and ion concentration in the arc, decreased the diameter of the arc column, and made the plasma more homogeneous. A study of voltage fluctuations in the d. c. arc was reported by Mellichamp (206), who reported that the pattern of voltage us. time depends on the operating conditions, such as arc current, electrode shape, and size of the gap, as well as on the properties of the sample, but under standard operating conditions the pattern forms a recognizable characteristic of the sample compound. Fewer publications on time-resolved spectra of sparks appeared during the past two years than had been the case before. However, Walters and Malmstadt (332) have written an extremely detailed report based on a survey of the literature and the results of their own investigations. Their paper includes too much material t o be covered here. It summarizes what is known about the processes occurring in spark discharges and an attempt is made to gather all of this material into one cohesive model. Sickel described the results of two studies on processes occurring in the electrodes of d. c. arcs. In one study (233) he used samples containing Fe69 and followed the movement of the iron by autoradiography and counting techniques. In the same report he describes the results of studies made by adding boron as B&; the boron became concentrated at the anode spot. In another study (232) Sickel added various boron compounds to the graphite anodes and employed x-ray diffraction to identify the compounds formed when the arc was burned in different atmospheres. He interpreted his results in terms of the kinetics of reactions and temperature
distributions within the electrodes. Doerffel and Geyer ('71) studied vaporization of samples from the arc electrode, correlating observations with calculations based on thermodynamic approximations. h4ilyus (220) studied the effect of compounds in the sample on atom concentrations in the arc when a powdered sample is blown into the discharge. He concluded that the influence of the composition of the sample on line intensities may be minimized only by establishing conditions which favor complete evaporation of the particles. Yanusik (348) considered the possibility of identifying chemical compounds in the sample by emission spectrometry. In one experiment, samples consisting of either ferric oxide or ferric sulfate mixed with calcium carbonate and silica were arced on a copper diqk electrode. He found that the intensity ratio of a Si/Cu line pair was an indication of the type of iron compound in the sample. In mobt cases, the spectrometrist is less concerned with determining the form of compounds in the sample than with obtaining reliable data on .elemental composition regardless of the nature of the sample. Buffers are often used when samples of diverse types must be analyzed to reduce the effect of the matrix on line intensities. Maritz and Strasheim (193, 194) have made a study of buffer efficiencies based on two criteria: the amount of buffer which must be added to reduce the matrix effect to a minimum and the effect of the buffer on arc temperature. Similar results were given by both criteria. In general, buffer efficiency was found to depend on both the anion and the cation in the buffer. Contrary t o what many have thought, Naritz and Strasheim found that the ionization potential of the cation in the buffer is not the primary factor in determining buffer effectiveness. Doerffel and Geyer (71) stated that the best buffer efficiency is found when the buffer does not affect the arc voltage. Excitation of Solutions. Several investigations were made of the application of plasma jets and highfrequency discharges as analytical sources. Frish and Startsev (95) studied some properties of a plasma jet, including the temperature, and found that the discharge may be considered as two overlapping sources, a high-current arc a t the cathode with a temperature near 5000" K. and a compressible plasma beam with a temperature of 11,000 to 14,300" K. Serin and Ashton (282) reported on the effects of the tangential gas flow, the arc current, water-miscible organic solvents, and acids on emission intensities from a plasma jet; they found that optimizing the operating conditions could improve the sensitivity of detection considerably. Sirois (290) has also
studied the effect of operating parameters, including flow rate of the tangential gas, arc gap, arc current, sample aspiration rate, and position of view in the arc gap, on line intensities in the plasma jet. While the operating parameters found to be optimum in these two studies may not be best for all analytical problems, the optimization techniques reported in these papers should be useful in other laboratories. In a companion paper, Sirois (291) made a study of interelement effects in the plasma jet. While he did find interelement effects to be present, he was able to establish conditions which permitted the determination of manganese in copper, nickel, iron, zinc, and aluminum alloys with a single analytical curve. A test of the precision of analysis with a plasma jet was made by Lerner (283). A relative standard deviation of 0.77' was found for the photoelectric determination of 300 p.p.m. of manganese in acetone, and a slightly poorer precision when water was used as the solvent. These data are based on 10-second integrations with no internal standard. Ishida (141) compared a plasma jet and a flame source for the determination of the alkaline earth elements and found the former to give better sensitivity but poorer precision than the latter. A number of workers have investigated high-frequency plasma discharges for spectrometric applications. ;ilthough many sources of this type had been described in previous years, they had not been satisfactory for many reasons, including difficulty of use and severe matrix effects. Some of the newer high-frequency torches may overcome these difficulties. Greenfield, Jones, and Berry (112) compared a highfrequency torch with a plasma jet. The latter gave slightly better precision in the determination of calcium, but the torch had the advantages of operating without electrodes and of having a low background emission. In addition, this source was found to be relatively free of matrix effects. A somewhat similar torch was described by Wendt and Fassel (336), who stressed the differences between various designs. A torch designed by West and Hume (338) gave considerably better detection limits than did those described in References (119) and (336),but it must be kept in mind that low limits of detection on singleelement solutions are not always the most important criterion of the quality of a source, since matrix effects can be more or less serious in different discharges. Another design of a high-frequency source was given by Tappe and van Calker (4.9, S l l ) , who found serious matrix effects when the sample contained elements of low ionization potential. Undoubtedly, much more research will be needed before an optimum design is
achieved. Several of these torches use ultrasonic atomizers, and Dunken, Pforr, and Mikkeleit (75) have stressed the advantages of this method of dispersing compared to pneumatic atomizers. The flame-like discharges have not replaced other methods for the analysis of solutions. S z a k h (310) and Russmann and Brooks (274) have reviewed the comparative advantages and disadvantages of several excitation sources for the analysis of solutions. Eardly and Clarke (79) investigated several factors affecting the precision of analysis with the rotating disk electrode, including rate of rotation, type of graphite in the electrode, solution depth, and prespark time. Fukushima and Kuroha (96) modified the copper spark method by precoating the ends of the electrodes with gelatin to obtain a more even distribution of the sample on the surface; they found that the intensities of the lines of the rare earth elements were increased two to four times in this way. Kocsis and Erdey (167), Mitkov and Baklanovskaya (214), and Zhuravlev and Nemtseva (351) investigated spark-in-spray excitation of solutions, with the last study concentrated on the effect of elements in the sample on the discharge characteristics. Other Excitation Studies. There has been a somewhat increased interest in analysis with the hollow cathode source. Coetzer and Iiessler (54) described the design of a hollow cathode source and methods for the analysis of solutions and nonconducting solids. Falk (86) employed a hollow cathode tube for the determination of chlorine, fluorine, and arsenic in glass with a sensitivity of 1 part in IO4 for a 2-mg. sample, and Gladushchak and Shreider (108) determined helium in air with a pulsed discharge in a hollow aluminum cathode. Korostyleva (169) studied the effect of gas pressure and composition on the spectrum of plutonium in a hollow cathode discharge; either factor affected the relative intensities of arc and spark lines, but did not distinguish spark lines from arc lines of high excitation energy. Leichnam and Capitini (182) compared hollow cathode lamps to electrodeless discharges for the isotopic analysis of uranium and found the latter to be preferable. The advantages of controlled atmosphere excitation in some circumstances are well known, but application of controlled atmospheres has been limited in the past in part because of difficulties in the use of chambers. Wang and Cave (333) have described a chamber which is easier to use than most earlier types. Several systems for controlling the atmosphere around the arc by a flow of gas around the electrode, thus eliminating the difficulties encountered with closed chambers, have been described by VOL. 38, NO. 5, APRIL 1966
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Helz (126), Margoshes and Scribner (192), Abdellatif, Vilnat, and DebrasGuedon ( 2 ) , and Vilnat and Vignet (330). In the last two references, the use of this system for the determination of the halogens in a helium atmosphere is described. Boyd and Goldblatt (35) and Dilworth (70) described similar systems for spark excitation, stressing reduction of matrix effect in point-toplane spark excitation in atmospheres of argon or nitrogen (see below). Babadag (10) observed an improvement in the sensitivity of detection of arsenic, phosphorus, and selenium in germanium oxide by arc excitation in an argon atmosphere. Bogershausen and van der Walt (30) developed methods for the analysis of gases, solutions, and powders in a high-current arc in argon between a water-cooled copper anode and a tungsten cathode. The discharge mas found to have a temperature between lG,OOOo and 2G,iOO" K., considerably higher than normal arc temperatures. Moore, et al. (216) employed an arc in inert atmospheres a t reduced pressure to excite and distinguish between second and third spectra of several elements. Brooks and Boswell (41) have made a comparison of excitation in an arc with the sample in the anode or the cathode. They found that, for a graphite matrix, cathode excitation gave better precision of analysis for the more volatile elements and higher line intensities for most elements. Uman (323) investigated the spectra emitted near the electrodes outside of the arc gap, again with the sample in the anode or the cathode. Plotting the line/background ratio as a function of distance from the center of the gap, two maxima were found near the cathode with either polarity, one in the cathode layer and another about 1 mm. outside the gap. Rusanov and Vorob'ev (273) described an apparatus for uniform introduction of powders into an arc in a stream of gas. llooney, Schoder, and Garbini (215) made a comparison of line and background intensities in an arc when the sample was contained in electrodes made of ordinary graphite or of pyrolitic graphite with the c axis of the graphite aligned parallel or perpendicular to the electrode axis. The anisotropic properties of pyrolytic graphite may be advantageous when it is necessary t o control the temperature distribution in the electrode. Namitokov (226) examined the pits formed in metallic and composition electrodes by an impulse discharge, finding considerable evidence of thermal changes near the pits. Minnhagen (211) has compared the properties of high-frequency discharges and sliding sparks for the excitation of atomic and ionic spectra. He concluded that high-frequency discharges are preferable for samples in the form of
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gases and vapors, while sliding sparks should be used for solid samples or for the excitation of spectra of gases above the third stage of ionization. Alexander, Feldman, and Fraenkel (4) compared the spectra between 75 and 400 A. generated by a sliding spark and a triggered spark, and concluded that the ionization state of an element producing a particular line could be identified by comparing its relative intensity in the two sources. Few laboratories will have a Van de Graaff accelerator available for spectrometry, but Bashkin (12) has studied the applicability of such a device t o excite spectra which would be difficult to obtain from more conventional sources. STANDARDS A N D CALIBRATION
Koch, Christ, and Weber (166) investigated the production of aluminum alloy standards for emission and x-ray fluorescence analysis in molds cooled by air, water, and a water spray, and found that the last type gave the most homogeneous standards. Milos and Studencka (209) described methods for the large scale production of aluminum standards for spectrometry, while Klimecki (161) advocated the production of standards of high alloy steels by continuous casting. For the determination of oxygen in metals by excitation of the evolved gas in an argon atmosphere, Fassel and Goetzinger (88)reported that congruent analytical curves were obtained from analyzed metal specimens, from the decomposition of metal oxides in the arc, or simply by synthesizing mixtures of carbon monoxide and argon. Yamamura (347) pointed out the danger of assuming that high-purity materials used to synthesize standards are stoichiometric. Kalinkina et al. (151) described the properties of some emulsions developed for recording vacuum ultraviolet and soft x-ray radiation, including some which have little response to visible light. Some work has been published on the derivation of theoretical plate calibration curves based on particular models for the photographic process. KBntor (152) derived such an equation in which the emulsion is regarded as optically thick and the Beer-Lambert law is assumed to apply. The equation was found to apply t o three emulsions tested. Torok (315 ) also compared a theoretical equation with experimental data. Such studies are certainly preferable to the empirical derivation of transformation equations which may be useful only in one laboratory working with a particular emulsion and equipment. Trandafir (518) has developed a special graph paper, based on a variation of the Seidel function, for the direct plotting of analytical curves from plate transmittance values including a background correc-
tion when necessary. This approach may save time, but can introduce errors under some circumstances. Kaiser (148) has given a general treatment of the shape of analytical curves based on the light-conductance of the spectrograph or spectrometer and empirical knowledge of the radiance function of the light source, and has related the results of this study to limits of detection by photoelectric and photographic photometry. Kerekes and Ag (158) have derived an equation for the shape of an analytical curve, taking into account the effect of self-absorption a t high element concentrations, and have found that the equation gives a good fit to experimental data. Torok (316) has reviewed the evaluation of spectrometric data, including intensity measurements, background correction, and corrections for interelement effects and for changes in the concentration of the internal standard element. He also described an electronic function generator for linearization of analytical curves to assist in automatic evaluation of result s. There has been a marked increase in the use of electronic digital computers for the evaluation of spectrometric data recorded both photographically and photoelectrically. In some cases, computers are used to replace the usual graphical methods of calculation, but in other cases the computers perform calculations which would be difficult t o do otherwise. For example, dslund and Cronhjort (9) employed a digital computer t o calculate element concentrations from the output readings of an emission spectrometer. The computer was programmed to make corrections for overlapping lines and for other matrix effects, based on data from standards, with a resultant improvement in the accuracy of analysis of stainless and low alloy steels. Tunnicliff and Weaver (5%) employed a computer to evaluate plate readings in a common matrix semiquantitative method. The program converts plate transmittance values to element concentrations and checks for possible interferences and applies corrections by a matrix technique. Dickens et al. (68) described a computerized data evaluation and transmission system for control of furnace charging and melting procedures in the production of steel. Not all computer evaluation methods have relied on large computers. Torok and Bajaki (317 ) described a small computer designed for conversion of microphotometer readings to element concentrations, including background correction. They reported that evaluation of the data for an average of four elements in each of 200 exposures on 10 plates required only 2l/4 hours. Computer methods to replace graphical calculation of data from photographic plates have been described by
Franke, Post, and Schmotz ( 9 4 , Joyce (146), Shaw (284), and by Schalge, Thurnau, and Miller (275). With the increased availability and reduced cost of computers, particularly the new timesharing systems which permit almost instant access to a computer, automatic data reduction techniques can be expected to become commonplace. K’ot only is the evaluation speeded up [Franke, Post, and Schmotz (94) report that data are processed 200 times faster with the computer than by the usual procedures], but there should also be fewer human errors due to fatigue when processing large amounts of data. Regardless of how photographic data are processed, the results can be no more precise than the microphotometer readings. Slavin (292) has given a theoretical treatment of the error of microphotometer readings as a function of the transmittance value. His calculations agreed with experiments and showed that the minimum random error in the derived intensity is about 1% a t a transmittance of 30%, even though the reproducibility of the photometric readings may be better by a factor of three to five. Burmistrov, Nalimov, and Nedler (45) have considered errors in the measurement of weak spectral lines, stresying the importance of fluctuations in the source and in the conditions of recording the spectrum. Plsko (247) studied errors in quantitative analysis when extrapolation is required. Nepekoichitskii and YankovskiK (231) demonstrated the possibility of quantitative analyses based on the measurement of the intensity us. time curve in the arc. Linear curves were obtained for zinc a t trace levels and a t concentrations of a few per cent by plotting the intensity at the maximum against the concentration. In the selection of internal standards, Nakimovskaya (225) pointed out that for optimum results the analyte and internal standard lines should preferably have equal probabilities of self-absorption. Chupeev (52) investigated errors in visual estimation of intensities from photographic plates, and concluded that the precision of such estimates is dependent on the contrast of the emulsion. Lontsikh and AIeshalkin (187) reported on the results of a study which appear to justify the usual assumption that semiquantitative analytical data follow a normal distribution of errors. Meshalkin (207) studied the errors in semiquantitative analysis further, and derived a rational scale for representation of such results. ANALYSIS, GENERAL
The reduction of matrix effects through point-to-plane spark excitation in atmospheres of argon or nitrogen has been mentioned previously. Arrak (8)
has shown a considerable reduction of matrix effect in the analysis of ferrous alloys in this way. Dilworth (70) has investigated this method for the analysis of ferrous and nickel ba.e alloys, where the problem of spectrometer calibration is made extremely difficult because of the large number of different alloy compositions in use. He found that, with the proper excitation conditions, line intensities are proportional to element concentrations with little or no evidence of interelement effects, and that the background intensity is relatively unaffected by sample composition. I n addition, the line intensities rise to a constant value in a very short time, so that little presparking is needed before the exposure is started. Boyd and Goldblatt ($5) have reviewed some applications of spark excitation in controlled atmospheres. A few practical applications of the plasma jet have been described. Vigler and Failoni (327) used this source for the determination of 1 to 10 p.p.m. of boron in gasoline with a relative standard The only deviation of about 8%. sample preparation was t o mix the gasoline with ethanol containing nickel as an internal standard. Collins and Pearson (55) determined beryllium in oil field waters to concentrations below 1 part in 109 with plasma jet excitation after concentration by extraction. Iluntz (220) applied the plasma jet for the determination of titanium and zirconium in molybdenum. Several methods were described for preparing metallic specimens for analysis. Clarke (63) discussed recommended practices for preparing samples of cast and pig iron, and described a machine by which molten metal could be solidified in a nitrogen atmosphere as a very thin ribbon. Holler (132) investigated the preparation of metallic samples by melting in a small arc furnace in an argon atmosphere and casting in a watercooled copper mold. He described methods to minimize losses through evaporation, and also pointed out the advantages which can be gained by making additions to the sample in the furnace. For example, aluminum, zirconium, or titanium can be added to deoxidize the sample, and other alloying techniques can be employed to minimize interelement effects. By diluting pig iron samples with pure iron, he was able to analyze them as steels. Eckhard (81, 82) gave the results of a study of the conversion of chip samples of steel by pressing into disks suitable for pointto-plane excitation. The precision of determination of manganese and silicon improved with increasing pressure, and for disks formed a t a pressure of 80 tons per square inch the standard deviation of the analysis was poorer only by a factor of two than for the analysis of cast samples. He felt that it would be
better to improve the precision of analyses by averaging the results of several sparkings than to attempt to press samples a t even higher pressures. Baskov and Palladin (13) reported on the application of contact-spark sampling for the analysis of heat-resistant steels. They found that coating the surface with cadmium chloride by evaporating a solution on the metal surface before sampling decreased the time required for sampling, increased the line intensities, and resulted in improved analytical precision. Interest has continued in improving techniques for the determination of the nonmetallic elements. Shaevich et al. (283) employed a low-voltage pulsed discharge for the determination of oxygen and hydrogen in copper and of hydrogen in nickel. One difficulty found in this last determination was strong adsorption of water vapor by nickel surfaces. Heating the sample to a high enough temperature to desorb the water caused a loss of hydrogen from the sample, and it was necessary t o grind off the surface of the nickel just before the analysis to obtain accurate results. Berneron and Romand (24) reviewed improvements in methods for the determination of carbon, phosphorus, and sulfur in steel and considered the possible extension of the analyses to include nitrogen and oxygen. Korovin (170) determined fluorine and chlorine in beryllium to concentrations as low as a part per million by measuring the intensities of BeF and I3eC1 bands excited in a hollow cathode discharge. Dunken, Mikkeleit, and Haucke (77) analyzed mixtures of light and heavy water by exciting the samples in a quartz capillary discharge and observing the OH and OD bandheads. h system for continuous monitoring of hydrogen in nitrogen was developed by Dickinson and Wheeler (69), with excitation in an electrodeless discharge. For the determination of carbon in sulfur by spark excitation, Guidry, Ilatson, and W e wiorowski (114) formed electrodes by pressing a mixture of the sulfur with a metal powder which provided electrical conductivity and also served as an internal standard. Iesue and Cecchetti (136) described a spectrographic method for the determination of graphitic carbon in rocks. Carbonate carbon was removed by dissolving the sample in acid, and the dry residue from this solution was analyzed either in an arc between copper electrodes or by exciting in a spark electrodes made by pressing the sample with copper powder. In the area of isotopic analysis, Rerthelot and Lauer (25) studied the isotope shift of the uranium atom line at 5027.4 -4.;they made determinations of U235 and U2s with this line and also studied the possibility of the determination of the isotopes of mass 233, 234, VOL. 38, NO. 5, APRIL 1966
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and 236. Starik and Aleksandruk (299) determined Sr87 in rocks for the measurement of absolute ages, a determination normally done by mass spectrometry. They found good agreement between ages measured by the emission and mass spectral methods, and also reported that the emission analysis required only 40 to 50 minutes and that the sensitivity was adequate for the analysis of samples containing 10 pg. of strontium. Smith (294) employed excitation in an electrodeless discharge to determine Hg20r. Two very unusual methods Lvere described for the spectral analysis of powdered samples. Raikhbaum and Stakheev (254) fed powdered minerals into a flame slowly enough so that the spectra resulting from the vaporization of individual grains could be recorded photoelectrically. They reported an improvement in sensitivity by three orders of magnitude compared to common spectrographic methods for those elements that are heterogenously distributed. Schroll and Hauk (279) investigated the area under the time-intensity curves obtained with arc excitation of powdered substances, and found that the area is a function of both element concentration and of particle size, so that for a sample of known composition the particle size, and thus the surface area, can be determined spectrometrically. MICROANALYSIS AND LASER APPLICATIONS
Local area analysis of conductive samples by sparking between the sample and a fine wire or needle as the counterelectrode is a well known technique. Krishtal, Davydov, and Korvachev (174) found that this technique could be used for the local determination of carbon in steel, and Korolev and Faivilevich (168) employed this method, as well as a solution technique, for the analysis of nocmetallic inclusions as small as 50 microns in diameter in steel. Laser vaporization appears to offer many advantages for the analysis of small inclusions and other microsamples, compared to the local area spark. One major advantage is that laser vaporization applies equally well to nonconducting or conducting samples. In some cases, the vapor produced by the laser has been further excited by an electrical discharge, while at other times spectra have been recorded directly from the laser-heated vapor. Karyakin, Akhmanova, and Kaigorodov (153) investigated the application of lasers in spectrographic analysis with different methods for focusing the laser beam and with and without cross-excitation of the vapor produced by the laser. They employed a neodymium glass laser with a pulse energy of 20 joules, which vaporized about 4 mg. of material when the beam was focussed with a simple lens.
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Smaller samples, about 100 microns in diameter and 10 microns deep, were vaporized when the laser beam was focussed through a microscope objective. The spectra emitted by the vapor, without cross-excitation, had broad, diffuse lines, often self-reversed. Cross-excitation with a spark or a x . or d.c. arc produced more intense spectra with sharper lines. These observations are in agreement with results which have been reported a t meetings but which have not yet been published. Runge, Minck, and Bryan (271, 272) reported on studies of the determination of nickel and chromium in stainless steels using a laser to vaporize the sample and observing the spectra without cross-excitation. The pits formed in the samples were about 0.4 mm. in diameter and 0.05 mm. deep, and satisfactory line intensities were obtained by superimposing the spectra from four laser flashes. The analytical precisions were comparable to those obtained with arc excitation, but the sensitivities were relatively poor. DebrasGuedon and Liodec (63, 64) have described the application of a similar system to analysis, with emphasis on ceramic materials. Preliminary results were given for the application of laser vaporization with spark cross-excitation by Maxwell (200) for minerals, and by Sherman et al. (288) for teeth and bone. Rosan (265) employed a laser with spark cross-excitation for the analysis of small volumes (100 pl.) of solutions which were deposited on a fine-grained photographic film. Extremely good sensitivity of detection was reported; as little as 5 X 10-13 gram of magnesium could be detected. I t has not been our policy in these reviews to include unpublished information presented a t meetings, but a group of papers presented a t the XI1 Colloquium Spectroscopicum Internationale a t Exeter, England in July 1965 give an exceptionally interesting range of methods of applying lasers in analytical emission spectrometry. Vilnat, Liodec, and Debas-Guedon (329) used a powerful ruby laser, with an output energy of 100 joules, t o vaporize samples and observed the spectra without cross-excitatation. They were able to establish analytical curves for 0.02 to 2% chromium and 0.05 to 1% nickel in steel, and reported a standard deviation of 4y0for a chromium content of 0.92%. The limit of detection was 0.002% chromium and 0.005% nickel. They also discussed the application of this method of excitation to other samples, including highpurity materials. Felske, Hagenah, and Laqua (90) described the analysis of larger surface areas with a system that automatically moved the sample to expose fresh areas t o successive pulses from a laser with a repetition rate of one flash per second. This system may give
the high precision obtained with pointto-plane spark excitation, which is essentially derived from the averaging of a large number of events, but will be applicable equally well to nonconductive samples. Rasberry, Scribner, and blargoshes (260) summarized their experience with a laser probe, with spark crossexcitation, in the qualitative analysis of microsamples including inclusions in nonconducting samples, surface films, and individual grains of metal powders. They also described some problems which have been encountered in the application of this method of excitation and means for avoiding these difficulties, as well as the results of some experiments in quantitative analysis. Hagenah, Laqua, and Mossotti (119) presented a discussion of the atomic absorption spectrum of the vapor produced by a focussed laser beam, with a xenon flash lamp as the emitting source. They reported on possible analytical applications and on the use of this method to study the distribution of atoms in the vapor cloud. In addition to these papers, Karyakin, dchmanova, and Kaigorodov summarized their published work (153), which is mentioned above. TRACE ANALYSIS
While emission spectrometry is inherently a sensitive method of analysis, the increasing requirements for purity of materials and the development of interest in knowledge of the trace constituent composition of samples of many kinds has resulted in efforts to extend the limits of detection and analysis. In addition to the development of techniques for improving sensitivity, several investigators have studied the factors which presently limit the determination of trace elements. A comparison of spectrometric and other methods for the analysis of very pure materials has been made by Ehrlich and Rexer (85). Kaiser (149) has summarized the considerations entering into a statistical definition of the limit of detection as the concentration of an element which will give a signal three times the noise level, and has defended the choice of the factor three on practical grounds. To describe adequately the negative outcome of an analysis, he has introduced the concept of a “limit of guarantee of purity” which will be somewhat greater than the limit of detection. Zaidel, Jialyshev, and Shreider (350) described how the sensitivity of detection is related to errors in the intensity measurements. Preliminary enrichment of the sample is one of the most frequently used methods for improving the sensitivity of detection, since it permits the concentration of impurities from a relatively large sample into a small enough volume to be placed in a spectrographic elec-
trode. Table I lists several enrichment procedures, using chemical and physical methods, which have been employed prior to spectrographic analysis. Brooks (39, 40) has studied the use of a n automatic counter-current solvent extraction apparatus for concentration of trace constituents prior to spectrographic analysis. He obtained enrichment factors as large as 400,000 for several elements in silicate rocks. Abakumov and Konovalov (2) described an apparatus employing zone melting for preliminary concentration, and with it they were able t o determine silver, copper, and thallium in bismuth at conyo. Dvorak centrations of 10+ to (78) reviewed the application of fractional distillation to preliminary concentration of impurities, and O’Connell (635) described a n apparatus for this purpose which can be used with controlled atmospheres a t either positive or negative pressures. Not all methods for improving the sensitivity of spectrographic analysis have relied on a preliminary concentration. Pevtsov and Krad’shchik (243, 244) reported on the analysis of pure acids, volatile liquids, silica, and other materials by mixing the sample (or the residue from its evaporation) with carbon and forming briquets which were then excited in a hollow cathode discharge. They reported that this method of excitation affords much better sensitivity than can be obtained with the d. c. arc. Novoselov and Aidarov (234)
Table 1.
also investigated the sensitivity of analysis of solutions in a hollow cathode discharge, and reported detection limits of gram/ml. for aluminum and gram/ml. for bismuth and 3 X copper, cadmium, lead, and silver. In the determination of molybdenum, tungsten, and tin in rocks, Ivanova (143) improved the sensitivity of detection in the d. c. arc by adding poly(fluoroethylene), cupric chloride, or silver chloride to the samples as halogenating agents to enhance the volatility of these elements. Haftka (117) described a technique for the analysis of zone-refined metals which gave detection limits of 0.01 to 10 p.p.m. with samples weighing only a few milligrams. One of the peculiarities of trace analysis is that the results may not be normally distributed, since contamination introduces a n asymmetry in one direction. Belyaev (20) has stressed the advantages of studying the distribution of results of multiplicate analysis, rather than taking a simple mean as the most probable result, since the distribution may be skewed and the high results are due to contamination. FLAME SPECTROMETRY
The period covered by this review has seen several noteworthy advances in atomic emission and absorption flame spectrometry. I n addition, atomic fluorescence has emerged as a third analytical method employing the flame to convert samples to atomic form.
Methods of Preliminary Concentration for Spectrometric Analysis
Elements Matrix detd. Arsenic, phosphorus, anti- Ca, K, Li, Na mony Bismuth Ag, All Cd, Cu, Mg, Mn, Ni, Pb Boron Cerium dioxide Bi, Cu, Fe, Pb, Sn, Zn Cobalt, nickel Germanium As, Bi, Gal In, Sb Ag, All Bi, Cd, Cr, Indium Cu, &In, Nil Pb, Zn Molybdenum Ta, Ti, W, Zr Plutonium 24 elements Plutonium Ca, Gal K, Na Selenium As, P Tungsten oxide Uranium Rare earths 19 elements Vanadium Yttrium Zirconium Aluminum-alumina mixt. Silicon halides Germanium tetrachloride Iron, steel Rare earth oxides Rocks, water Fluorite Ores, oxides, alloys Water Water Silicates
Ca Rare earths B B
Ag, Al, As, Bi, Pb Rare earths 11 elements Rare earths Cd 70 elements Hf, Ti, Th, Zr
cs
Method of sepn. Reference Vaporization of matrix Pptn. of BiON03 Evapn. of Bz03 Ion exchange Solvent extn. Evapn. of GeC14 Solvent extn. Evapn. Solvent extn. Ion exchange Evapn. of SeBrl Chem. ppn. Ion exchange Evapn. of VC14 or vocl3 Solvent extn. Vaporization of ZrClr Chromatography Evapn. Evapn. Solvent extn. Ion exchange Solvent extn. Chem. pptn. Vaporization of Cd Ion exchange Extn., ion exchange Chromatography
Winefordner and Vickers (344, 345) have derived equations for calculating limits of detection in atomic absorption and emission flame photometry. The derivation takes into account the processes in the flame which contribute to the signal and to the noise, and also changes in the signal/noise ratio introduced by the monochromator and detector. I n these calculations, it was assumed that amplifier noise mould be negligible, but the effect of this factor was considered in a paper by Winefordner and Veillon (342). A comparison of the results of the calculation with experimentally determined values was satisfactory in the case of sodium, but for cadmium the calculated limit of detection was lower than the experimental value by a factor of twenty for both emission and absorption spectrometry. It was thought by Winefordner and Vickers that the difference in the case of cadmium was due to extensive compound formation in the flame which was not considered in the calculations for this element because there had been no previous indication of such compounds. The value of such calculations will probably not be in calculating detection limits, which can usually be measured more easily. The calculations will be of value in optimizing instrument design, since the effect of any instrumental modification can be estimated without actually making the change. They should also be of value in indicating areas where more experimental work is needed, as in the case of cadmium where a search for evidence of compound formation in flames is indicated. I n addition to limits of detection, spectrometrists are also interested in knowing the concentration range which will give a linear analytical curve. I n flame photometry, an analytical curve plotted on logarithmic coordinates will have a unit slope at low concentrations, but at high concentrations when selfabsorption becomes significant the slope of the curve will be 0.5. Winefordner, Vickers, and Remington (346) have developed equations for calculating the concentration at which the slope of the curve will change. One difficulty in applying theoretical calculations is the lack of accurate experimental data on some important factors, such as dissociation energies of molecules and spectral line widths, Hollander, Kalff, and Alkemade (134,150)reported some new measurements of the dissociation energies of the alkaline earth oxide molecules, and compared their results with previous measurements. For example, for CaO, they obtained a dissociation energy of 3.96 e.v., while earlier measurements by flame photometry and mass spectrometry gave values ranging from 3.2 t o 4.6 e.v. More experimental data are also needed on line widths, particularly VOL 38, NO. 5, APRIL 1966
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for atomic absorption and fluorescence spectrometry, as these are affected by flame conditions. Behmenburg, Kohn, and Nailaender (17 ) described an acetylene-oxygen burner designed to permit such measurements to be made in flames diluted with perturbing gases such as nitrogen, helium, or argod. Behmenburg (15) reported on results obtained for measurements on the 5890 A. line of sodium, and Behmenburg and Kohn (16) gave the results of similar measurements on the resonance lines of sodium and strontium. Any theoretical study of flame emission or absorption spectrometry must take into account the fraction of an element sprayed into a flame which is actually converted into atoms. Typically, large losses occur from incomplete vaporization, from ionization, and from compound formation. Hollander (133) has written a thesis on ionization of the alkali elements and dissociation of the alkaline earth oxide molecules in flames of acetylene and carbon monoxide with air. He also investigated theoretically the effect of self-absorption on intensity measurements. Pueschel, Simon, and Herrmann (252) reported on the loss of sodium atoms sprayed into a turbulent oxygen-hydrogen flame, and they concluded that less of the atomic element is lost by ionization than by incomplete vaporization of the spray particles. Flame temperature is another important variable, affecting the rate of particle evaporation as well as dissociation and ionization equilibria and the excitation of the resulting atom?. Janin and Bouvier (144) have made an experimental and theoretical study of the temperature of the cyanogen-oxygen flame as a function of gas and solution flows. One of the difficulties in the use of this flame as a source has been the severe decrease in flame temperature on the introduction of aqueous solutions, making it necessary to limit the sample flow to small values to obtain maximum intensities for a given element concentration. Janin and Bouvier found that acetic acid as a solvent produces a much smaller drop in flame temperature than does water a t equal flow rates. Some improvements have been made in the design of atomizers and burners for flame spectrometry. Dunken et al. (76), compared pneumatic, electrostatic, and ultrasonic atomizers and concluded that the last type of atomizer gave the best sensitivity of detection. Shipitsyn, Kiryushkin, and Ermolaev (289) described a novel system for introducing powdered samples into a flame. They used a flat burner, below which was a chamber with a metal bottom which was struck by a pulsating hammer. This fed the powder into the burner through a small hole in the top of the chamber. The system was successfully applied to the determination of potassium in soils. 306 R
ANALYTICAL CHEMISTRY
D’Silva, Kniseley, and Fassel (73) described an improved attachment to a commercial atomizer-burner for use with fuel rich flames, and reported significant improvements in the limits of detection for cerium, hafnium, tantalum, thorium, uranium, and zirconium with this flame compared to results obtained with other burners. Britske (38) reported on a study of the spectra produced by laminar and turbulent flames of hydrogen and acetylene with air and oxygen. He concluded that hydrogen with oxygen is the best gas mixture for flame spectrometry and that turbulent flames are superior to laminar flames because of their lower background emission. He also investigated the design of atomizers, stating a preference for atomizers operating a t high pressures, above about 4 atmospheres, because these were found to be less sensitive to changes in gas pressure. Mackison (188) described the modification of a commercial burner to permit the sample to be atomized in nitrogen, and May, Dinnin, and Rosenbaum (201) gave the design of a simple oxygen sheath which they are using in the flame determination of sodium and potassium. Herrmann, Lang, and Ruediger (129, 130) reported on results with periodic addition of samples t o the flame, thus modulating signals for emission and absorption measurements. In this way, the system can be made insensitive to radiation emitted or absorbed by the flame gases, indicating only the signals from the constituents of the sample solutions. Lang (178) investigated the recording of an electrically differentiated signal in scanning flame photometry. He found that flame background, zero drift of the amplifier, and the effect of atomizer crusting were eliminated as disturbing influences, that there was possible a better separation of closelying lines, and that the signal/noise ratio could be improved by partial integration of the differentiated signal. Among analytical applications, Ramirez-hluiioz (255) reviewed the analysis of metallurgical materials, with particular emphasis on preliminary separation by solvent extraction. D’Silva et al. ( 7 4 , studied the analytical application of the flame spectra of the rare earth elements and scandium excited in a fuelrich oxygen-acetylene flame. Kuper (176) has written a thesis on the flame spectrometry of lead, and Adkins’ (3) thesis was on the flame spectrometric study of cobalt with emphasis on the reaction zone of a fuel-rich oxygenacetylene flame. Debras-Guedon (62) studied the effects of alcohols and of 8hydroxyquinoline on the emission spectrum of aluminum in an oxygen-acetylene flame. She reported that the effect of the alcohols is thermal in origin, while the chelating agent acts through the formation of a complex.
Crider (66) developed an instrument for the determination of sulfur compounds in air by flame photometry. The source was a shielded air-hydrogen flame. With less than 1 p.p.m. of SOz in the air, a blue glow could be seen outside the combustion zone of the flame, and with a simple filter photometer as little as 0.1 p.p.m. of SO, could be detected. Fenimore and Jones (91) investigated the effect of flame conditions on the green bands attributed to HPO which may have analytical applications for the determination of phosphorus. Eshelman, Dyer, and Armentor (85) made a general study of the extraction and flame spectrometric determination of palladium and rhodium. Brandenberger and Bader (36) investigated the extraction of gallium, indium, and thallium into ethyl ether and their determination by flame photometry; the method was applied to determine the distribution of thallium in the body of a poisoning victim. Lacy (177) described the design and performance of an automatic instrument for the analysis of soil extracts for potassium and calcium by flame photometry and magnesium by atomic absorption spectrometry. The instrument is capable of performing 30 to 40 determinations per hour. There have been several instrumental developments of interest in atomic absorption spectrometry. Sullivan and Walsh (308) described a simple atomic absorption spectrometer in which the line to be measured is selected from the total radiation by resonance scattering by the vapor in a hollow cathode lamp. A double-beam atomic absorption spectrometer was constructed by Hinson and Kitching (131) who reported that the improvement in stability as a result of double-beam operation contributed to a significant reduction of detection limits for magnesium and calcium. An atomic absorption spectrometer designed for the analysis of water was the subject of a thesis by Herrin (127). Boling (32) described an output unit for atomic absorption spectrometers which integrates the signal for a selected period of time and supplies an output which is linearly proportional to concentration. The advantage provided by integration is increased precision of reading, which in turn makes it possible to improve the sensitivity of detection because smaller changes in the signal can be observed than is possible by the usual means. Absorption of a few hundredths of a per cent of the light in the flame could be detected with a 10-second integration. Eickhoff and Sykes (84) and Murie and Bourke (221) have described adjustable burner mounts for atomic absorption spectrometers; in the latter reference there is also described a simple device for testing alignment of the burner with the optical axis of the spectrometer. Winefordner and Veillon
(341) used a light pipe to couple the burner to the monochromator of an atomic absorption spectrometer and reported a large reduction in flame emission signal reaching the detector. Lang (179) reported that the advantages of a double-beam system could be obtained on a single-beam spectrometer by periodically deflecting the flame out of the light path. Atomic absorption spectrometry in reducing flames was reviewed by Bermejo Martinez (23). R a m and Hambly (256) studied the distributions of temperatures and of atoms in rich and lean air-acetylene flames in a slotted burner. Some studies were also reported on flameless methods of generating atomic vapors. Mislan (212)was able to determine cadmium in water a t concentrations as low as 0.01 p.p.m. by vaporizing the sample in a heated quartz tube, and Goleb and Yokoyama (110) were successful in determining Li6 and Li7 in solutions of lithium hydroxide which were vaporized in a hollow cathode discharge. There have been several significant developments in light sources for atomic absorption spectrometry. Sullivan and Walsh ($0’7) have developed a new type of lamp in which the vapor produced in the hollow cathode is further excited by an auxiliary discharge a(’ross the open end of the cathode. The resonance lines were found to be markedly enhanced relative to the other lines of the metal or the lines of the filler gas, with no indication that these lines are broader or more self-absorbed than in the usual hollow cathode lamp. Other experiments with light sources have been intended to permit the determination of several elements without changing lamps. Massmann (197) reported more successful operation of lamps made with different metals in the form of rings in the cathode than with cathodes made of alloys. Butler and Strasheim (4’7) employed a similar multi-element cathode or else lamps in tandem in conjunction with a spectrometer fitted with exit slits and photomultipliers for the determination of up to four elements simultaneously; they reported that an instrument for the determination of six elements is under construction. This instrument also permitted use of an internal standard, with a resultant improvement in analytical precision. Other workers have experimented with sources of continuous radiation in place of line sources. Ginzburg and Satarina (106) compared results obtained with a hydrogen lamp to those with hollow cathode and electrodeless discharges. They found that the sensitivity of detection with the hydrogen lamp was 30 to 100 times poorer than with a hollow cathode source. Belchev, Beleva, and Dancheva (19) also investigated the use of a hydrogen lamp as a light source, with photographic
registration of the spectrum. Another study of atomic absorption spectrometry with a hydrogen source was made by Ivanov and Kozyreva ( f 4 2 ) , who reported much poorer sensitivities than can be obtained with hollow cathode lamps but who nevertheless felt that the technique is promising. The hydrogen lamp is probably not the best continuum source for this purpose, since sensitivities of detection are a function of the band width of the spectrometer and with a low intensity source such as the hydrogen lamp it is necessary to work with relatively wide slits on the spectrometer. Sheklein and Popov (285) reported on the application of a pulsed xenon lamp as a continuum source for the investigation of absorption spectra of atoms and molecules in flames. Even with a sharp line source, such as the hollow cathode lamp, the line shapes have an effect on the results of atomic absorption analysis. Both Prugger (251) and RubeBka and Svoboda (268) have discussed the relation between emission and absorption line profiles and the shapes of analytical curves. The fact that the emission line widths are generally nearly as large as the widths of the absorption lines may account for most of the departures from Beer’s law a t high absorbance values, but RubeSka ar,d Svoboda pointed out that it will be necessary to have more precise data on line widths to resolve this question. Xossotti and Fassel (219) have made a study of atomic absorption spectra of the rare earth elements in fuel-rich oxygen-acetylene flames, and have commented on analytical applications and also on the value of these measurements in deducing the energy level structure of these elements. Operating conditions for determination of selenium were given by Rann and Hambly (267), for mercury by Poluektov, Vitkun, and Zelyukova (2.49,for aluminum by Amos and Thomas (Y), for silver by Belcher, Dagnali and West (18),and for chromium by Feldman and Purdy (89). Harrison (122) investigated the sensitivity of detection of cobalt with more than 40 lines of this element and the linearity of the analytical curves with many of these lines. This information may be of some use to other workers, but most of it is predictable and some of the results are not directly transferable to other equipment. Among particular analytical applications, Burrows, Heerdt, and Willis (46) developed a method for the determination of chromium, copper, iron, lead, and silver in used lubricating oil, and Bordoneli, Biancifiori, and Besazza (33) determined copper, iron, lead, manganese, and nickel in an organic moderator from a nuclear reactor. Atomic fluorescence spectrometry is new as an analytical method, although
it had previously been applied as a method for studying processes in flames. All of the published reports on analytical applications have come from one laboratory. Winefordner and Vickers (343) discussed the theory and techniques of atomic fluorescence spectrometry, and compared this method with atomic absorption and emission methods. Other reports (191, 339, 340) from this laboratory considered optimizing the operating operating parameters and reported limits of detection. The limits of detection are often much lower than can be obtained by flame photometry or by atomic absorption spectrometry. Some values which have been reported (191) are 2 X p.p.m. for cadmium, p.p.m. for zinc, 0.04 p.p.m. for thallium, 0.1 p.p.m. for mercury, and 10 p.p.m. for indium and gallium. The limit of detection is primarily a function of the intensity of the source, and in the future it may be possible to improve on these limits of detection and to extend the method to additional elements. LITERATURE CITED
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