Review of Fundamental Developments in Analysis
Absorption Flame Photometry Paul T. Gilbert, J r .
Beckman Instruments, Inc., Fullerton, Calif.
A
flame photometi y now boasts a literature of perhaps 4000 publications, including a dozen books. The rate of publication remains unabated, even mith omission of papers that meiely mention flame photometry as the analytical method employed (these are now so numerous that i t is pointless to keep track of them). illthough a century ago spectrochemistry used only the flame, this source later succumbed to the arc and spark. Thanks to Lundeggrdh, spectrochemical analysis with the flame has been struggling back up. I n versatility the flame now begins to rival the arc and spark, and nearly all of the less familiar elements can now be determined by flame photometry. This review covers publications and papers that have come to light since the 1960 review on emission spectroscopy by Scribiier (9SL4), with a few earlier ones of particulars ignificance. Flame photometry has generally been regarded as a branch of emission spectrochemistry. But atomic absorption spectroscopy has now reared its head. This very vigorously burgeoning analytical method cannot avoid being classified n i t h conventional flame photometry, though it is not emission spectroscopy. M7e need some new terms The present writer, vAth several others, favors dividing analytical flame photometry into emission flame photometry and absorption flame photometry. This review corers abqorption flame photometry only. NALYTICAL
TERMINOLOGY
To argue the terminology, n classification is in order: Vapor-Phase Absorption Spectroscopy in t h e Optical Range.
1. Atomic spectroscopy A . K i t h continuous emitter 1. Hot (emitting) absorbing vapor: The Fraunhofer lines and stellar spectroscopy (Le., much of astrophysics) ; King furnace; line-reversal temperature measurement; underwater 210 R
ANALYTICAL CHEMISTRY
spark (Meggers, 1926) : lunar spectroscope (Ostergren, 1960). 2. Cool vapor: Interstellar absorption; various physical studies; mercury vapor (Kaye, 1961). B. K i t h discrete (line) emitter 1. Hot vapor a. Produced by flames: Atomic absorption spectroscopy in the narrow sense (Walsh) ; studies of absorption and self-absorption in flames (Gouy, 1876-80; Zahn, 1902, 1913; Locher, 1928; Child, 1930; Sobolev, 1950; Alkemade, et al. 1954). b. Produced by other sources : Self-absorption in arcs, etc.; sputtering (Gatehouse and X a l s h ) ; spark-in-spray (Robinson) ; L'vov furnace (see below). 2 . Cool vapor: Detection of mercury vapor. 11. Molecular spectroscopy -4.With continuous emitter 1. Hot vapor: Stellar spectroscopy (YO, TiO, etc.) ; King furnace; molecules in flames (e.g., SnO, BiC1, 013); absorption-emission pyrometry (Ihrlbaum). 2 . Cool vapor: Ultraviolet, visible, and infrared molecular spectroscopy, theoretical and analytical; atmospheric absorption (terrestrial, planetary). B. With discrete emitter (employing same molecule): -4 few studies on flames (Kondrstjen-, 1936).
First, it is clear t h a t Walsh's atomic absorption spectrascopy (proposed independently but slightly later by Alkemade and Milatz) in its present embodiments is only one subdivision
(Section I, B , 1) of atomic absorption spectroscopy in general. Second, there is no reason (except low oscillator strengths) why molecular absorption spectroscopy of flames (11, B ) should not be counted with Walsh's method. Third, there is nothing (except the high resolution needed) to prevent our using continuous emitters for absorption flame photometry (I. -4; 11, A). Fourth, there are many ancient techniques that must be called atomic absorption spectroscopy (I, -4,1, 2; I, B, 1, 2). Indeed atomic absorption spectroscopy has been going on since before the days of Bunsen and Kirchhoff. Thus, the term absorption flame photometry neatly covers 95% of the work on analytical atomic absorption spectroscopy, leaving room for molecules and for continuous emitters, should they prove useful; but it excludes the ho1lon.cathode sputtering vapor source and other nonflame techniques. These lntter are, however, so interesting and as yet so few that they will be included in this review, although they are really not flame photometry. GENERAL
In the follom-ing survey of recent advances in absorption flame photometry, the earlier references are not heeded; they may be found in any of the reviews, and indeed they constitute only a fen- per cent of the total bibliography, so rapid has been its expansion. Likeivise, some of the papers concerned only with the design of hollow-cathode tubes, even though intended for absorption flame photometry, are omitted. I shall not t r y to duplicate the reviens already in e'iiqtence. some of which (9, 104) are especially well balanced and authoritative; instead, a condensed but fairly thorough guide to the recent literature of various topics in absorption flame photometry will be provided. Reviews. The early paper on applications by Rusqell, Shelton, and R a l s h (94) discussed many aspects of the subject. The method has been patented by Kalsh (106, 109); additional patents are listed in (104). Menzies (76) delves into some of the earliest work on atomic absorption
spectroscopy. Walsh (108) reviewed the subject in 1959, discussing interferences in particular. I n 1960, David (26) gave a thorough review and Gilbert (45) presented a compact survey; there was a brief account in ($0), and Robinson (86)prepared a less technical account of the subject, with photographs of instruments; there was a review by Pungor (86), with emphasis on magnesium, and one by Malmstadt (68), with a discussion of his null-point instrument. There were surveys in 1081 by Robinbon (91), Leithe (59), and Poluektov (81); Walsh’s review (104) is among the best, as is that of Allan (9), not yet published. The latter discusses interferences and methods of avoiding them, the newer flames, organic solvents, the factors governing srnsitivity, and the unsolved problems, the chief of nhich is the matter of producing atomic vapors of all the elements. Other surveys of recent developments, presented or to be presented a t meetings, are those of Wells ( I l l ) , Manning (YO), and Walsh (105). Some reviews of a more specialized character, discussing particular applications or experimental nork (to be mentioned again a t the proper junctures) are those of Shelton and Walsh (99), Menzies (71, 73-75), Lockyer (GS), and Walsh (107). It is interesting that mhile the bulk of the work has come from Australia, S e w Zealand, England, and, more lately, the United Statw, there are I ecent contributions from Germany, Hungary (82, 83), Russia (67, 121-123), Italy (17), and South Africa ( I O I ) , not to mention the early work by Alkemade and hIilatz in Holland.
INSTRUMENTATION
Xbsorption flame Photometry is so young that many workers are assembling their own apparatus, often constructing some of the components. h typical setup comprisrs a hollow-cathode tube as emitter, a long, narrow air-acetylene flame fed by an external atomizer with a spray chamber (which may be the barrel of the burner), a prism monochromator, a photomultiplier tube, and a galvanometer. When flame emission n ould be troublesome, the light from the emittrr is modulated either by a chopper or by running the tube on alternating current. Dual-beam systems are sometimes used t o compensate emitter drift or instability. Sinw most of the papers describe the instrumentation, only certain selected topics will be reviewed here. However, n e might mention as an example of a good general-purpose absorption flame photometer the one described by Box and Walsh (15). They show the circuitry for the lamp power supply and
the photometer amplifier. The emitter was modulated, and the instrument could Le converted from absorption to emission flame photometry by placing a chopper between the flame and the monochromator. A modified E E L flame photometer burner handled air-coal gas or air-acetylene. The Beckman D E monochromator was equipped with 1P28 and 1P22 photomultipliers. Precision was 0.25%. Instrumental problems have been surveyed by Sawyer (97, 98), who used a Perkin-Elmer Model 13 monochromator, and by Herrmann and Lang (48), mho studied in detail the factors affecting sensitivity. With their best arrangement, sensitivities for copper, sodium, potassium, and magnesium, though very good, were 1 to 2 orders of magnitude short of the calculated theoretical sensitivities. Among other things, they tried a continuum source, but the dispersion of the prism monochromator was too low to enable the continuum to compete with line emitters. A continuum source (tungsten or hydrogen lamp) is sometimes used (8) with a good spectrograph in a preliminary photographic survey of the absorption spectrum of an element introduced into the flame, for the purpose of selecting the best lines. Emitters. A general discussion of the construction and characteristics of hollow-cathode lamps for absorption flame photometry was given by Jones and Walsh (56). Large lamps have longer life, since life is limited by cleanup of the filler gas. They built lamps with some rather unusual cathodes, including titanium, zirconium, hafnium, and iridium. Robinson (87) reviewed the technology of hollow-cathode lamps; he evidently used lamps with cathodes of tantalum and tungsten. David (5’7) described a molybdenum hollow-cathode lamp. Burger and Gillies (18) described the hollow-cathode lamps now made by Westinghouse, discussing problems of gas fill, voltage and current, and the battle among spectral output, electrical characteristics, and life. Russell and Walsh (96) found that pure resonance radiation is emitted at right angles to the axis of a hollow cathode; normally the lamps put out a full and rather complex spectrum, from which the desired line has to be isolated by a fairly good monochromator. However, the monochromator can be left out if the lamp rndiation is dominated by the desired line, as may be the case XTith sodium (14). llalmstndt and Chambers (19, 68,69) used only filters to isolate the sodium and potassium lines. Lockyer (63) discussed electrodeless discharge tubes for absorption flame photometry, as well as hollowcathode and ordinary discharge lamps. Entrance Optics. Various lens and diaphragm systems have been de-
scribed for passing the light from the emitter through the best part of the flame. Inhomogeneity of absorbaixe in the part of the flame viewed causes premature curvature of the working curve ( 7 1 ) as well as loss of sensitivity. Dawson and Ellis ( S I ) , for example, diaphragmed the beam. David (27) diaphragmed the lamp and restricted the slit height. Various mirror systems have been used for passing the beam through the flame more than once. In their carly work, Russell, Shclton, and n’alsh (94) used as many as 12 passes, whivh mhanced the sensitivity 6-fold. Ilcrrmann and Lang (48) tried passing the beam longitudinally upward through the flame, but gained an advantage only with sodium. They tried multiple horizontal passes, along or across an elongated flame, but found that emission rose faster than absorption ~ + i t hadditional passes; they compromiwd on three passes, using plane or convex mirrors, and also semitransparent mirrors to get superimposed beams. The Jarrell-Ash designers ( 5 5 ) got best sensitivity with five passes (n-ith concave mirrors) through three atomizerburner flames. llillikan (77), in a theoretical study of sooty flames, enhanced the absorption that he wanted to measure by passing the beam noncentrally through the flame eight times with eight mirrors arranged in a circle around the flame-four plane mirrors close to the &me and four concave mirrors farther back. Atomizers. Herrmann (47) made a detailed study of atomizer characteristics (effect of dimensions on suction, gas flow. etc.) for emission and absorption flame photometry. Some critical comparisons of different kinds of atomizers, as used Rith spray chambers and separate burners, have been published. Allan (‘7) presented data for four atomizers used with various organic solvents. Herrmann and Lang (48, 49) also compared atomizers, finding their home-made high-pressure atomizers superior to five other atomizers from different commercial flame photometers. They also examined the effect of spray chamber dimensions on sensitivity, response time, and deposition on the n-alls. They got better efficiency n-ith tangential injection of swondary air into a vortex chamber into nhich the sample n a s sprayed axially, and also with a heated spray chamber, but in the latter troublesome deposits formed. HOTT ever, Malmstadt and Chambers (69) used a heated spray chamber successfully. Burners. 84ost of the work in absorption flame photometry has been done with elongated burners burning premixed air-gas or air-acetylene. A good example is the one described by VOL. 34, NO. 5, APRIL 1962
21 1 R
Clinton (21). It is water-cooled, made of aluminum, with a gas port 12 cm. long and 0.7 mm. wide, rotatable to permit varying the thickness of flame through which the beam passes and hence the sensitivity and range of an analysis. Of more interest, perhaps, is the use of atomizer-burners in absorption flame photometry, in which the sample is sprayed directly into the flame to achieve efficient utilization of the analyte. Baker (11) used an atomizerburner with premixed oxygen-propane. Killis (117 ) tried the Beckman atomizerburner but preferred the more conventional burner of Box and Walsh (15). Robinson used the Beckman burner with oxyhydrogen (87, 88) and, suitably modified, with oxycyanogen (87, 93). He also used i t (nith oxyhydrogen) for determining lead in gasoline (90). He tried an adapter above the burner to spread the flame out and provide more thickness, but i t was not very successful although the sensitivity for platinum was good (93). Herrmann and Lang (48) got good sensitivity with an atomizer-burner, especially with a highpressure one of their own design, but they could do still better with the vortex chamber described above and a n elongated premixing burner. Tabeling and Devaney (103) and Devaney and Brech (33) described the use of the Beckman atomizer-burner, three of which in a row are used in the atomic-absorption attachment of the Jarrell-Ash spectrophotometer (64,55). Robinson described a new kind of atomizer-burner with four concentric tubes (93), in which a high pressure (150 p s i . ) of oxygen supplied through the central tube sprays the sample pumped (with a syringe) up through the second (annular) tube, to be burned with added oxygen coming through the third tube and hydrogen or acetylene through the fourth. The central tube is completely retracted within the sample orifice. For absorption flame photometry the outer oxygen and fuel flow are kept very low; for emission the device operates like a n ordinary atomizer-burner, with the central oxygen turned off and the outer oxygen doing the atomizing and providing suction. Flames. First we should mention the interesting results obtained with a fuel-rich air-acetylene flame. This flame, by shifting t h e dissociation equilibrium of a metal oxide in favor of t h e free metal, enhances atomic absorptions t h a t may be weak or undetectable in the normal (stoichiometric) flame. Allan ( 1 ) first reported this for magnesium. Molybdenum is a particularly good example of this phenomenon, as observed by David (27, 98). Gatehouse and Willis (40) got the best sensitivity for both molybdenum and tin in a very rich air-acety21 2 R
0
ANALYTICAL CHEMISTRY
lene flame. Allan (8) observed the same effect for chromium and ruthenium, as well as molybdenum and tin. The use of flames other than air-gas, air-propane, and air-acetylene in absorption flame photometry is reviewed next. The Jarrell-Ash bulletin (64) states that the air-hydrogen flame (from an atomizer-burner) is best for many elements-as might be eypected from its rather lo^ cmissive power. Honever, oxyhydrogen is also used in this instrument, and a ith the multiplepass optics i t can yield exceptional Fensitivity. I l l a n (,9) rmien s oxyhydropcn, oxyacetylene, and oxycyanogen, pointing out that thcy offer less sensitivity, a t least for those elements that are adequately vaporized in atomic form in the cooler flames. Herrniann arid Lang (48) reach similar conclusions. Warren’s flamp spectrophotometer (110) employed a special atomizer-burner of his own design (a modified Keichselbauni-Varney) burning oxyhydrogen; his sensitivity for zinc was not exceptional. The same can be said for sodium in oxyhydrogn (88). Robinson’s successful use of oxyhydrogen for gasoline analysis (90) and for other elements (87, 93) (which showed only fair detection limits) were mentioned above. His special atomizer-burner (93) can burn both ovyhydrogen and oxyacetylene. but data were presented only for nickel, which showed a detection limit (for 1% absorption) of 0.07 p.p.m. in organic solvents; this is rather good. Willis (117) used oxyhydrogen and oxyacetylene for calcium, but found more sensitivity and less anion interference in the conventional air-acetylene flame. Gidley (41) mentioned the use of oxyacetylene for certain elements with oxides hard to dissociate. Allan (10) reported the use of a nitrous oxide-acetylene flame; this has nearly the temperature of oxyacetylene but a lower burning velocity, hich makes burner design easier. Oxycyanogen has been used for absorption flame photometry only by Robinson (87, 89. 93). It is fairly good for the usual elements, but not as good as other flames; and of the elements n i t h stable oxides, it gave a feeble response only for vanadium. Holyever, it must be admitted that oxycyanogen has shov, n oddly disappointing behavior in emission as n-ell, and it certainly deserves further study. Photometry. Various double-beam and/or modulated-beam systems are employed in absorption flame photometers. The principal re\ iens should be consulted for the fundanientals of these systems. Shelton and K a k h (99) used a double-bram system, one beam passing through and one around the flame, the one beam being chopped a t tnice the frequency of the other and 90’ out of phase; the signals are rectified and the absorbance of the flame is measured
with a ratio recorder. Baker (11, 12) also used a double chopped-beam system, as did Herrmann and Lang (&). The latter coupled this with a n auxiliary lamp with photocell and phase shift to provide a reference signal, the two signals passing through a phase-sensitive rectifier to a galvanometer. Leen and Atwood (58) described the PerkinElmer instrument, IThich employs a double chopped beam and two photomultipliers (but only one monochromator), with a counter or recorder showing the per cent absorption directly. The recorder can be used for integrating. The double-beam feature apparently reduces the effect of lamp drift about 40-fold. Saunderson (96) described the new Research and Control atomic-absorption instrument, having a doublebeam, optical-null photometer. Alenzies (71) described a single-beam system in lyhich the intensities of two lines from the hollow-cathode tube are compared, one of ahich is absorbed by the flame and one not. This offers ratio-reading without beam-splitting, but the intensity ratio of the two lines depends on the lamp condition. LZalmstadt and Chambers (19, 68, 69) dpscribed a null-point absorption flame photometer in which the sample and standard are sprayed alternately while the standard concentration is adjusted by adding analyte from a buret until the two readings match. This method, although slower than most, enjoys the greater precision and dynamic range inherently available with differential absorption photometry. Despite the contribution of volumetric error, the standard error of a determination amounted to 0.1 to 0.5%, rather better than usual, while the dynamic range, 4 decades, was greater than that reported by any one else for absorption flame photometry. Commercial Instruments. I n addition t o the absorption flame photometer patented by JJ7alsh for the C.S.I.R.O., and manufactured, I believe, by Techtron Appliances in South Melbourne (I h a r e no direct inforination on it), the following instruments are now available. Hilger and Watts, Ltd., make the H909 atomic-absorption attachment for the Uvispek spectrophotometer, based on Ralsh’s design ( I , 50, 51, 71, 86). It is a single-beam instrument with an elongated burner for air-con1 gas. Optica, Inc. (also Optica United Kingdom, Ltd., and Optica Milano, the original company) has the ATA atomicabsorption attarhment for the CF-4 grating spectrophotometer, using airacetylene (17, 31, 7 9 ) . I t , too, is a single-beam instrument. Optica also sells the AT-6 atomic-absorption spectrophotometer (7$), a rompact, doublebeam, d.c. instrument with conventional (water-cooled) air-acetylene burner and
quartz monochromator. It has a 6lamp turret for quick change from one element to another, and it is adaptable to digital readout. The Perkin-Elmer Corp. offers the Model 214 atomicabsorption spectrophotometer (58, 80), with the chopped double-beam system nientioncd in the preceding section. It incorporates a Model 98G grating monochromator. The atomizer is built into the base of the elongated airacetylene burner. The Jarrell-,ish Co. produces :in atomic-abporption attachment for thc 82000 scanning grating spectrometor 133, 64, 65, 103). I t is a d.c., singlc-beam instrument, using three Beckman buiners in a ron, nith mirrors to proiide as many as 5 passes of the beam through the flames; airhydrogen and oxyhydrogen are recommendcti. Lqst, at the 1962 Pittsburgh Conference, Iteseaich and Control Instruments, Iiic., prwented a new atomic-absorption spectrophotometer (85,96). I t has an array of hollowcathode lamps, each placed opposite an entrancc :.lit on the Rowland circle of a concave grating monochromator, so that the selected line of each element emcrges through the single exit slit. Thus, the element can be changed without focusing or warmup. The instrument has a double-beam, opticalnull photometer. Hollow-cathode lamps are now manufactured by a t least half a dozen concerns, and are available for a wide assortment of elements. Discharge lamps for the alkali metals and a few other volatile elements such as mercury and cadmium are also available from several companies.
ANALYTICAL CHARACTERISTICS
Wavelengths. The strongest resonance line in absorption is not necessarily the strongest in emission. The best absorption line often lies in the deep ultraviolet (2000 to 3000 A , ) , and for several elements it lies in the vacuum ultraviolet, where it is not accessible to present-day absorption flame photomet>ers. Anyway, the opacity of flames a t these shorter wavelengths limits the number of elements that can be determined by atomic absorption in the flame. Selenium and mercury are two elements for m-hich bett er sensitil ity could be mpected in flame absorption by a slight extension of the navelength range to their stronger resonance lines a t 1961 and 1850 A., respec1 ivelj . Lilies of .hart wavelength can always be uied in a d.c. photometer, since flame ernkion below 2500 A. is usually riegligible compared with the lamp intensity. Allan (3, 6, 6, 8) has done some of the most interesting work with For these short-wavelength lines.
example, he has shown detection limits of 0.5 p.p.m. for antimony a t 2068 A. and tellurium at 2143 A., and 5 p.p.m. for selenium a t 2040 A. Of course, the resonance line of zinc a t 2139 A. has been used since Walsh’s first work, b u t zinc has no longer-wavelength lines of consequence to mislead one. Gatehouse and W-illis (40)have also presented some interesting absorption data for elements a t short wavelengths-e.g., antimony a t 2311 A. One point that has been commonly overlooked is the fact that weaker absorption lines are useful for determinations a t higher concentration of analyte without the need for dilution, aiid they can thus greatly extend the otherwise limited dynamic range of absorption flame photometry. Working Curves. Nearly everyone has had to examine the working curve of his analyte. I t s shape is not as simply predictable as might be hoped. It depends on various lamp characteristics, which may change with timc, on the nature and homogeneity of the flame, the entrance optics, the spectral resolution, the method of photometry, and the solvent and concomitant solutes. Meneies (?‘I), for instance, showed quantitatively the effect of flame inhomogeneity on n orking-curve curvature. Some typical working curves are published by Perkin-Elmer (SO). Absorption flame photometry has the same limitations as conventional absorption spectrophotometry in dynamic range of concentration. When absorbance is measured directly, it is impractical to work beyond 3 or a t most 31/2 decades above the detection limit, and normally curvature begins to be bad a t 2 to 21/2 decades. However, the differential method of Malmstadt and Chambers (69), described above, has gone to 4 decades, and there are no serious obstacles to its going farther. Precision. This depends chiefly upon the stability of the emitter and the atomizer. More attention seems to have been paid to the former, b u t atomizer technology is far from being well understood. In emission flame photometry with atomizer-burners, stability of the atomizer is not of first importance, ou-ing to the type of self-compensation discussed by Gilbert ( 4 4 , but this is not true for absorption. See the earlier section on atomizers for references on their performance. Some excellent precision has been obtained in absorption flame photometry, but more commonly the reproducibility is no better than 1%. This is partly due to the mathematical limitation inherent in direct absorbance measurement, which can be overcome by differential absorption photometry. The differential method of Rlalmstadt and Chambers (69) should in principle
be able to get well below 0.170, provided that the atomizer and emitter can be made to function well enough. Box and Walsh (16), for example, found that fluctuation of the hollow-cathode or discharge tube limited their accuracy to I/&. Leen and Atwood (58), by ubing two beams and a recorder to obtain a timeaverage, shorn-ed a noise level as low as 0.170, but the practical reproducibility was still no better than seveial tenths of 1% of the unabsorbed Pignal. Tabeling and Devaney (103) and Ilevaney arid Biech (33) examined the relative noise contributions from the hollow cathode, the flame, and the photometer. In work nith closely dehned beams of small aperture and iiai row bandn idths, the aniount of light reaching the photomultiplier may be $0 small that shoteffect noise imposes the limitation. Sensitivity. The earlier work by Walsh and his colleagues examined the question of attainable sensitivity in absorption flame photometry. More recently, Herrmann and Lang (48) compared the theoretically with the practically attainable detection limits. Clearly, there is plenty of room for further instrumental improvement. Table I lists the best detection limits yet reported for all the elements detectable by absorption or emission flame photometry. 130th organic solvents and commercially available grating spectrophotometers are counted fair means for improving detection limits. Detection limit is defined as the eoncentration of element yielding an absorption of 1%) of the passing beam or an emission of 1% above the background continuum. Because the ratio of analyte emission to background, for lines and sharp bands, depends on spectral bandwidth, the definition permits taking the narrowest bandwidth for which shot-effcct noise in the photometer does not exceed the flame flicker. These definitions for absorption and for emission are about equally realistic, and, when it comes t o an euperimental demonstration of practical deteetability, they are found to be on the con-ervative side. For absorption, n e h a w to contend chiefly with emitter noise and atomizer noise; for emission, nith flame noise and photometer noise. By and large, these are about equal in the best instruments for the two kinds of flame photometry under the ddined detectionlimit conditions. In the table, the detection limits are expressed in p’%, Le., the negative logarithm of the concentration in per cent. Thus, 10 p.p.m. = 0.001% = 3 p%. Most of the data are given only to the nearest p% (Le., the nearest order of magnitude), but some are quoted t o a half p%. To date, of courbe, all useful flame absorptions have been atomic lines. VOL. 34,
NO. 5, APRIL 1962
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yo
Table 1. Detection Limits in for Absorption and Emission Flame Photometry
Element -4luminum -4ntimony Arsenic Barium Beryllium Bismuth Boron Bromine Cadmium Calcium Carbon Cerium Cesium Chlorine Chromium Cobalt Copper 1 )J.sprosium
Erbium Europium Fluorine Cradolinium C;allium Germanium Gold Hafnium Holmium Indium Iodine Iridium Iron Lanthanum Lead Lithium Lutetium Magnesium Manganese Mercury Molvbdenum r\'eodymium Nickel Niobium Xtrogen Osmium Palladium Phosphorus Platinum Potassium Praseodymium Radium Rhenium Rhodium Rubidium Ruthenium Samarium Scandium Selenium Silicon Silver Sodium Strontium Sulfur Tantalum Tellurium Terbium Thallium Thorium Thulium Tin Titanium Tungsten Uranium Vanadium Ytterbium Yttrium Zinc Zirconium
214 R
sorpAbtion n 4 ? 4 1.5 4 n n 5.5 5 n n 5 n 5 5 6 n n ? n n 4 n 5 n n 5
n n
5.5 n 4.5 5.5 n 6.5 5.5 3 4.5 n 6 n n n 5 n 4 6 n 4? n 4.5 5 4.5 n n 3 n 6 6 5 n n
4 n 5.5 n n 3.5
n
L
Emission B W C
5 4 4 6 5 4 ( 2 ) ;3*
5 4 5 3
5 2
i.5 i
6 7 3
3*
4
4
7 2
6 5.5 6 4 4 5 3 6 3.5 3
5 5 5 5 5 5 2 4 5
31 3?
4
5 5
7 3 21 6 3 3 6 5 9 3.5 5 5 . 5 ; 6* 5 7 3.5 4 5 3.5 5 6 5.5 4 ? 3 2* 3? 6 4 4 3; 4* 9 3 4 5 5 3 5 8 6 4 3 4 6 9
3 6 9 7
2 r
I
3 3* 3 3 6
3 5 5
4 5 4" 4*
5 5 3 4 5
n n 1 . 5 * 3; 5* ? 6 n 3.5 6 4 n 3.5*
3?
4?
31 4 4
5 5
6
ANALYTICAL CHEMISTRY
4 3 3
But flame emission makes free use of lines, sharp bands, diffuse bands, and even continua for analytical purposes. These four categories of emission, in order of diminishing desirability, are listed as L , B, TI', and C in the table. When, for a given element, a line offers the best sensitivity in flame emission, the bands and continua of that element arc ignored unless thry are strong, since they do not conti ibute to the comparison of emission \I ith absorption. Under absorption, an n signifies that the element has not been or is unlikely to be detected in the usual flames. Since oxycyanogen is not ranked among the usual flames, data for this flame are marked with an asterisk. It is clear that many of the n's under absorption and a t least a few of the blank spaces under emission lines 1% ill be removed n-ith further research, and it is equally clear that all of the data are likely to shorn improvement with time. It will be seen that this tabulation differs widely a t many points from that of Robinson (86),even though I have used some of his data. Solvents. a l t h o u g h the early TT ork was done with aqueous solutions, there is increasing interest in nonaqueous solutions, which can be used both for separating and/or concentrating the solute by extraction, as in t h e methods popularized by Dean for emission flame photometry, a n d for enhancement of sensitivity. Malmstadt and Chambers (68) added 37, isopropyl alcohol to their solutions t o improve performance. Nenzies (76) revierred work by colleagues, which was subsequently published (66), and quoted enhancements by 507, isopropyl alcohol. I n examining the role of surface tension in enhancement, he pointed out that neither octyl alcohol nor other surface-active agents enhanced absorption, and concluded that reduction of surface tension is not a major cause. However, he failed to distinguish static and dynamic surface tension. Under some conditions surface-active agents (which affect chiefly the static surface tension) will enhance emission, but as a rule solutes such as alcohol that lower the dynamic surface tension too will have a much more profound effect. Robinson (89, 99) has discussed the enhancenicnt of flame absorption (and emission) by organic solvents. He prcsentcd data io1 nickel a t 3414 A. in oxycyanogen. and somewhat later (87) he tabukited detection limits for many elements in oxyhydrogen and oxycyanogen n ith aqueous and organic sokents. These results. hon ever, shon- such anomalous inconsistencies (v hich are not explained by Robinson) that one would certainly like to see more of the experimental data. Willis (116) extracted lead from urine
with ammonium pyrrolidine dithiocarbamate into methyl n-amyl ketone, thereby concentrating i t as much as 100-fold, and analyzed the extract by absorption flame photometry. The method could be made sensitive to a few parts per billion of lead in urine. Lockyer, Scott, and Slade (66) reported the enhancement of magnesium and other elements by alcohol>. A l a n ( 2 , 6, 7 ) has contributed perhaps the most basic nork on this subject. He extracted copper JTith ammonium pyrrolidine dithiocarbamate into ethyl acetate (6) and zinc with the same reagent into hexone ( 2 ) . I n a detailed study of solvent enhancement with various atomizers for iron, manganese, copper, zinc, and magnesium (79, he concluded that the enhancement of absorption is due chiefly to the increased efficiency of introduction of analyte vapor into the flame and only slightly to thermal effects. He was able t o verify this conclusion quantitatively. His review (9) covers these matters concisely. Interferences. The subject of interferences in absorption flame photometry has been critically reviewed by lJyalsli (104, l o g ) , Allan (9),and David (2.9). Earlier reviews have been given by David (30) and Baker ( 1 1 ) . One may classify interferences broadly as spectroscopic (instrumental), chemical (element-specific) , and physical (nonspecific). The first category includes interference from inadequate resolution. This is often a limiting factor in emission flame photometry, where the emission of the analyte cannot be fully resolved from continuum, bands or lines emitted by a n interferent. This difficulty is vastly smaller in absorption flame photometry, n-here the effective bandwidth is essentially the width of the selected line emitted by the hollomcathode tube. Indeed, isotopic analysis is possible by absorption (see below). Emission flame photometry will have to adopt iriterferometric monochromators before it can match absorption in this respect. AIeanwhile, absorption will continue to enjoy far greater specificity and freedom f i o m spectral interference. JTc must keep in mind, though, that flame emission (though often negligible) is a form of interference in absorption flanie photometrv, and it is never wholly eliminated by modulation, since the emission hns a noise component at the frequency of modulation. Another difficulty is the presence of continuum and neighboring (unabsorbed) lines from the emitting source, lyhich may pass through the monochromator along with the analytical line; their effect is to diminish sensitivity. Chemical interference may occur in the vapor phase (ionization, excitation,
dissociation) or in a solid or liquid aerosol phase in the flame. Evcitation interference, consisting in a shift of population of the several energy levels of an atom, and common in emission, is negligible in absorption, since the ground state, which does the absorbing, is predominant. Ionization and dissociation affect emission and absorption equally. Some of the lesser mutual enhancements observed among alkali and alkaline-earth elements ( e g . , 78) may be due t o repression of ionization. Dissociation or formation of (nonabsorbing) molecules depends largely on the flame matrix and temperature. A good example is the marked enhancement of molybdenum absorption by e w e s of acetylene (W7), which presumably encourages dissociation of a stable oxide species. Stability of OYides is the chief factor preventing the detection of all the elements having accessible resonance lines, and attempts are being made to circumvent this limitation by use of hotter or richer flames (see the earlier section on flames). The second kind of chemical interference, sometimes called condensedphase interference (45), has been widely studied in recent years both in emission and in absorption, which it affects equally David (24, 2 5 ) , Killis (113, I 1 7 , 1IS),and Leithe and Hofer (60,6 1 ) studied the depression of calcium and magncsium absorption by phosphate, silicate, sulfate, and aluminum-these being perhaps the commonest examples of thic kind of interference. The severe depression of niolybdrnum by magnesium. calcium, strontium, manganese, mid iron observed by David (27) may be similarly due to formation of involatile double oxides of molybdenum and the interferent. hIenzies (71, 75 studied the depression of niagnesiuiii by aluminum, which takes a very peculiar form. Baker and Carton (12) made a special study of the effect of phosphate on calcium and strontium. There are various ways of circumventing condensed-phase interference. Hotter flmies-e.g., (60)--and/or finer sprays usuallv help, but not enough, and there has been almost no study of interference in the hottest flamescertainly none in absorption. The straightforn-srd way is to remove the interferent (or the anslyte) bodily, but flame photometrists are usually a t pains t o avoid this recourse unless it earl be done elegantly, ;is by extraction or ion exchange. The other advantages of extraction (see the section on solvents above) often justify such a separation. Another method is to buffer the absorption with a sufficient quantity of either the interferent in question or some other interferent, to make the absorption insensitive to variations of interferent concentration in the original sample.
This is easier to do in absorption than in emission flame photometry since we have no spectral interference from the buffer. Perhaps the most popular method a t present is the use of releasing agents (competing ions) or protective chelation (complexing agents). Here again absorption has the advantage, for emission is often troubled with the spectrum of the releasing agent. A releasing agent ties up the interferent, freeing the analyte from its interference. David (84) favored the use of a large concentration of magnesium and sulfuric acid to free calcium from interference by phosphate, etc., and Leithe and Hofer used calcium and sulfuric acid (60) or calcium alone (61)to release magnesium from the interference of aluminum, etc. I n another paper (85)David recommended strontium (1500 p.p.ni.) for releasing calcium and magnesium from interferences in soil samples; he tried lanthanum also. I n his study of molybdenum (27) he found that aluminum and phosphate released the molybdenum from interference by calcium, iron, etc.; this is the inverse of the usual situation, in which a more electropositive element suffers interference from a less positive one, and is freed by a metal of its own kind. Willis (117, 118) used strontium to release calcium and magnesium from phosphate. I n the analysis of urine (113), he found lanthanum more efficacious for releasing calcium from phosphate. Newbrun (78) also used strontium and lanthanum for this purpose. Menzies (71, 75) found calcium and strontium useful in releasing magnesium from aluminum. Killiams (112) used lanthanum to release calcium from interferenccs in soil analysis. The commonest protective chelating agent lias been EDTA. Bv complexing the analyte, it prevents it from precipitating as an involatile compound with the interferent during the evaporation of the spray. Killis uscd EDTA to protect magnesium (11 $, 11s) and calcium (113, 117). A general method of coinpcnsating interferences, useful also in the presence of condensed-phase interferences (though caution must be exercised here), is self-standardization [standard addition, addition method, etc. ( / t 5 ) ] . It was used for magnesium by Willis (115) and for strontium and molybdenum by David (28). David also discussed it a t Pittsburgh in 1962 (29). Physical interference, the third category, is due to the effect of solute or solvent characteristics on the efficiency or rate of transport of analyte into the flame, and (to a minor extent in absorption flame photometry) the consequent change of flame temperature (and sometimes change of flame size, luminos-
ity, and composition). The enhancement by organic solvents (see the earlier section) belongs here, but it is ordinarily used deliberately and only rarely occurs as a n unintentional interference. Orgallic solutes-e.g., protein-can also raise the efficiency of transport. See Allan’s discussion ( 7 ) . acids, by altering the physical properties of the solution, can cause a slight transport interference (60). An extreme case was noted by David (87), in which a high concentration of salt actually clouded the flame with unvaporized particles, reducing its transparency. Another effect that belongs here is the loss of transparency of the flame a t shorter wavelengths that occurs when certain organic solvents are used ( 7 ) . -1 similar effect was thought to occur in the presence of halogen acids ( $ 2 ) : the short-wavelength zinc and copper lines from the hollow-cathode lamp ere all weakened, although the sample was free from these metals. I t was subsequently realized ( 2 , 43) that this was due to attack of the brass burner by the acids. The usual method of coping with physical interference is to match the matrix composition of the standards to that of the sample-a method which is very widely used to balance chemical interferences also. ELEMENTS DETERMINED
For condensed reviews see (9, $5, 104). Lithium was listed with a detection limit of 0.03 p.p.m. by Gatehouse and Willis (40). The separate isotopes of lithium were determined by absorption flame photometry by Zaldel and KorennoI (123) (see below). Sodium was studied by Russell et al. (94) as well as by earlier n-orkers in absorption flame photometry. Box and Kalsh (14) determined sodium without a filter, using the entire radiation from the sodium lamp. The detection limit has been given as 0.03 p.p.m. (SO), 0.008 p.p.m. (48),and 0.02 ?.p.m. (69). General analytical studies are reported by Brownell (16),Pungor and Konkoly-Thege (83,and Robinson (88). llalmstadt and Chambers (69) studied sodium with their null-point instrument. Sodium has been determined in soils (25, SO), nonferrous metals, including zirconium, copper and aluminum alloys (36, S I ) , ferrous metals ( $ I ) , blood serum (31, 49, 119), and other biological fluids (31). Potassium. See ( 9 4 ) ; a m-orking curve and other data are given in (80). Detection limits of 0.03 (40) ond 0.02 p.p.m. (48) are mentioned. Malmstadt and Chambers (69) studied potassium with their null-point instrument. Potassium has been determined in soils (15, SO), blood serum ( S I , 49, 119), other VOL. 34, NO.
5, APRIL 1962
215 R
biological materials ( S I ) , and sodium iodide (65). Rubidium and cesium were studied by Russell et at. (94) and Gatehouse and Willis (do), who listed detection limits of 0.1 and 0.15 p.p.m. for the two metals, respectively. Lockyer et al. (66) observed an enhancement factor of 1.3 for cesium in 50% isopropyl alcoliol. Beryllium was not detected by Gatehouse and Willis (40) in an air-acetylene flame. Allan (8) reported a detection limit of 300 p.p.m. a t 2349 A. in airacetylene. Magnesium is the metal most studied by absorption flame photometry, and with good reason, since it needs to be determined in many kinds of material and absorption offerr better sensitivity than even the most elaborate emission flame photometer can provide. For general studies see (1, 21, 71, 82, 94). For a working curve and other data see (80). Detection limits are reported as 0.01 (8, 40) and 0.002 p.p.m. (48) in air-acetylene and 0.01 p.p.m. (54, 55) in a hydrogen flame. Enhancement by organic solvents has been reported (7, 66). hfagnesiuin has been determined in agricultural materials (1, 4). especially soils (1, 4, 25, 30, 60, 65), plants (1, 4, 23, 26, 30, 60), and soil waters (1,J), in blood serum (1, 4,31,32, 114, 118), urine (31, 32, 113), feces (23, Sd), milk (1, 4 ) and food (32), in nonferrous alloys (36, 41), especially aluminum alloys (41, 61), in steel ( 4 1 ) , ores and slags (60, 7 5 ) , in particular limestone, gypsum, and cement (60). Calcium is less popular with absorption flame photonietrists, owing to the longer wavelength (4227 A,) of its resonance line and the fact t h a t flame emission offers better sensitivity. Still, the absence of spectral interference makes absorption attractive. Detection limits of 0.1 (8),0.08 (io), and 0.03 to 0.1 p.p.m. (58) are reported in air-acetylene. I n the last study, 0.95 p.p.m. could barely be distinguished from 1.00 p.p.m. ,4working curve, etc., is given in (80). Enhancement by 50y0 isopropyl alcohol is reported as 8-fold (75) and %fold (66). Baker and Garton (12) studied phosphate interference, as did nearly everyone else who carried out analyses for calcium. Calcium has been determined in soils (25, 30, 112), plants (24, 26, 30) biological materials ( S I ) , blood serum (115, 117), urine (113), saliva (78),and culture solutions (28). Strontium, at its resonance line 4607 A., shows a detection limit of 0.05 p.p.m. (28) or 0.15 p.p.m. (40). The interference of phosphate has been studied (12, 28). Strontium has been determined in soils and plants (28),in which self-standardization was employed to overcome the severe interferences. Barium was reported to show a detection limit of 8 p.p.m. at its resonance 2 16 R
ANALYTICAL CHEMISTRY
line 5536 A. (do), this poor value doubtless being due to the occurrence of barium largely as oxide or hydroxide in the air-acetylene flame. Titanium could not be detected in air-acetylene (40, 7 6 ) . Zirconium has not been tried. Hafnium could not be detected ($4). Vanadium could not be detected in air-acetylene (40, 76) or oxyhydrogen (54, 87, 93). Robinson (93) stated that he had had no success with vanadium in oxycyanogen, but his paper (87) s h o w a detection limit of 300 p,p.m. in that flame a t 3183 A. Kiobium could not be detected in nor tantalum in airair-acetylene (40)) acetylene (7’5) or oxycyanogen (87). Chromium n-as studied by Russell et al. ($4). The best line is 3579 A,, offering a detection limit of 0.15 p.p.m. (40) or 0.05 p.p.m. (8) in air-acetylene, the latter value in a rich flame, which is more favorable. Air-gas is inadequate (40). Robinson (87’) reported detection limits in aqueous and organic solvents in oxyhydrogen a t 3579 and 3605 A. and the violet triplet. The best value was 0.2 p.p.m. Molybdenum has aroused appreciable interest since David (28) first reported that it could he determined by absorption in a rich air-acetylene flame, with a sensitivity of 0.5 p.p.m. I n a thorough study (27) he examined interferences and means of removing them, the effect of flame composition, and the shape of the working curve. The best line is 3133 A. Molybdenum was determined in superphosphate and stainless steel. See also Allan (8) and Gatehouse and Willis (40), who reported equal sensitivity and showed the effect of the acetylene ’air ratio. Robinson (93) said that he could not reproduce David’s results, and he shows no detectability in oxyhydrogen (87). Tungsten was not detected in airacetylene (40, 75) or in oxyhydrogen or oxycyanogen (87). Manganese was found to absorb most strongly a t 2795 A. by Allan ( 5 ) . Clinton (21) tested manganese ~ i t his h burner. A detection limit of 0.05 p.p.m. is listed (8, 40). I n a study of organic solvents, Allan ( 7 ) concluded that manganese is not present entirely as atomic vapor in the air-acetylene flame. Manganese has been determined in soils and plants (1, 4, 6 ) and in bronze (52, 7 1 ) . Iron shows the best sensitivity a t 2483 A. (5), although earlier work (23, 94) made use of the longer-wavelength ultraviolet lines. Clinton (21) and Sawyer (98) have studied iron. Detection limits are reported as 0.1 p.p.m. (8, 40) in air-acetylene and 0.03 p.p.m. in hydrogen (64, 65, 103). Robinson (87, 93) presented data a t 3720 A. in oxyhydrogen. Enhancements by isopropyl alcohol as high as 10-fold
were reported (66, 7 5 ) . dllan (7) also studied organic solvents and reached the same conclusion for iron as for manganese. Iron has been determined in soils and plants (1, $, 6). Cobalt absorbs most strongly a t 2407 A. (3); this line is 50 times as intense in absorption as 3527 A, the strongest emission line. The detection limit is 0.2 p.p.m. (8, $0, in air-acetylene. Robinson (87) gave some d a t n for oxyhydrogen and oxycyanogen a t the longer wavelengths for aqueniis and organic solvents. Clinton (21) studied cobalt, and Allan (10) determined i t in agricultural materials, espeeidly fertilizers. Iiickel has its best line a t 2320 A., rrhich is 15 times as strong as 341.5 A. (3). I n earlier n-orli (94))the longerwavelength lines were used. Detection limits are listed as 0.13 p p.m. ( 8 ) or 0.2 p.p.m. (40) in air-acetylene, 0.01 p.p.m. in hydrogen (54, iOd,, and 0.1 p.p.m. (at 3414 A.) in oxycyanogen (87); less favorable data are given for oxyhydrogen (87’). Working curves are given in (40, 80). The effect of organic solvents has been studied in air-acetylene (66, 7 5 ) , oxycyanogen (87. 83,92,93), and oxyhydrogen (87, 931. Robinson’s forced-feed burner, described earlier, showed identical enhancements (4-fold over m t e r ) for many organic solvents a t the 3414-A. line. Clinton (21) studied nickel n-ith his air-acetylene burner. Killis (120) discussed its determination in biological materials. Nickel has been determined in agricultural materials, particularly fertilizers (IO),in urine 165), and in bronze (52, 7 1 ) . Ruthenium shon-s a detection limit of 0.25 p.p.m. a t 3499 or 3782 A. in a rich air-acetylene flame (8). Rhodium has a detection limit of 1 p.p.ni. (8) or 0.3 p.p.m. (40) a t 3435 -4. Lockyer and Hames (64, 65, 7 1 ) obtained a working curve, observing a detection limit (1% absorptinn) of 2 p.p.m. but a standard error of only 0.4 p.p.m. S o interferences were caused by sih-er, gold, platinum, palladium, lead, or iron. Palladium has a detection limit of 0.3 p.p.m. a t 2448 A. or 2476 A. (8) or 0.8 p.p.m. (40). Lockyer and Hames (64, 65, 71) observed a detection limit of 0.7 p.p.m. and a standard error of 0.12 p.p.m. As with rhodium, no interferences were noticed. Osmium has not been tried. Iridiuin could not be detected (75). Platinum a t its best line, 2659 A., offers a detection limit of 5 p.p.m. (io), or 0.7 p.p.m. (8),or 10 p.p.m. with a standard error of 2 p.p.m. (64). Again, there were no interferences. Robinson (93) claimed a sensitivity 10 times that of Lockyer and Hames (64). Robinson (87) shows data for oxyhydrogen, but the entries for 2852 A. probably belong to magnesium.
Copper is best determined a t 3248
A. (941, :it which the detection limit is 0.1 p.p.m in air-acetylene (8, 40, 48) or 0.01 p p.m. in hydrogen (54, 55). Working (wrleb are given in ( S I , 52, 80). Herrmanri nnd Lang ( $ 8 ) studied the distribution of absorption with height in a premixing and an atomizerburner fldiiie. Dayid (25) also studied copper. Absorption a t shorter wavelengths has been reported (8, @). 1 1 e effwt of orgmic solvents has been noted in :iir-acet?lene ( 7 , arid in oxyhydrogen and oxycyanogen (&?). Copper has been determined in brass or bronze (52, 71, D ) ,iron ore (loa),longrade copper ore (101), and soils and d11:~n (6) extracted the complexed copper into ethyl acetate. Silver, at 3281 A. (06) shows a detection iiniit of 0.1 p.p.m. (8) or 0.05 p p.m. ($0). Lockjer and Hames (64, 65, 71) found a liinit of 0.2 p.p.m. (1% absorption) and observed a standard error of 0.04 p.p.m. Isopropyl alcohol n as reported t o double the absorption (66, 7 5 ) . Robinson (87) listed data in oxyhydrogen and oxycyanogen for aqueous and organic solvents. Rawling, Greaves, and Amos (84) determined silver in lead-silver-zinc ores and concentrator products, especially lead sulfide concentrates. Gold was studied a t 2428 A. by Russell et al. (&), who observed a sensitivity of 2 p.p.m. in air-gas. Detection limits of 0.3 p.p.m. (40) and 0.6 p.p.m. (8) are reported. Lockyer and Hames (64, 65, 7'1) observed a standard error of 0.15 p.p.m. They showed a working curve and noted no interference from iron, lead, platinum, palladium, silver, or rhodium: but the iron m u d be fully oxidized. Zinc, like inngiiesium, has been very popular IT itli absorption flxnie photometrists, because I t is an important element :ind e:isil> determined by absorption, n hereas no practical emission flame method has been found. Basic studies are reported in air-gas (21, 94), air-acetyiene (21. 52, 98), and oxyhydrogen (110). The detection limit is 0.03 p p ni. ( 2 , 8, $0) in water and as lon as 0.005 p p m. in hexone ( 2 ) . There are other studies of organic solvents (?, 66, 87) and oxyhydrogen and oxycyanoqen flames (87) Zinc appears to exist entiiely as zitornic vapor in the flame ( 7 ) . It has been determined in soils (2, d), plants (2,4,23,SO),fertilizers ( 2 , as), serum and whole blood (120), urine (40, nonferrous alloys (36), particulai 1y copper alloys (41-43, 61, 7f), aluminum alloys (41, " I ) , and zirconium ($1, 6 5 ) , in ferrous alloys (41) and iron ore (102). Zinc seems nearly immune to interferences. Allan (2) extracted zinc as a complex into hexone for very sensitive determinations. Cadmium enjoys a detection limit of 0.1 p.p.m. in air-gas (94) or 0.025
p.p.m. in air-acetylene (8, 60). Sawyer (98) mentioned it and Robinson (87) listed data for aqueous and organic solvents in oxyhydrogen and oxycyanogen. Cadmium can be determined directly in urine (40, 120). Mercury is run a t its less sensitive intercombination line 2537 A., since the resonance line 1850 A. is out of reach of most instruments. However, %ye (57) has worked a t 1850 A. with a continuum source (see section on cool vapor below). The detection limit in flames is 5 p.p.m. (8) or 10 p p.m. (40). Robinson (87) shows poorer limits in oxyhydrogen. Willis (1$0) discussed its determination in biological materials. Aluminum is undetectable in air-gas or air-acetylene (40,75,94) or oxyhydrogen or oxyacetylene (87, 93). Sawyer mentioned aluminum (98). Gallium and indium have been studied by Gatehouse and Willis (40)and Allan ( 8 ) . The best gallium line is 2874 A., with detection limit 3 or 1.5 p.p.m., and the best indium line is 3040 A. with limit 0.2 p.p.m. (8). Thallium was studied a t 2769 A. by Russell et al. (96) and Gatehouse and Killis (4O), who found detection limits of 8 and 0.8 p.p.m., respectively. Allan (8) found the 3776-A. line more sensitive, offering a limit of 0.03 p.p.m. in air-acetylene. Robinson (87)listed similar data for oxyhydrogen. Silicon could not be detected (94). Germanium has not been studied. Tin is best determined a t 2863 A. (sa),a t which the detection limit is 5 p.p.m. (8, 40), a rich flame being favorable (8). Robinson (87) could not detect it in oxyhydrogen or oxycyanogen. The stability of SnO makes trouble. Lead has received more attention. At 2833 A. the detection limit is 0.3 p.p.m. (8) or 0.5 p.p.m. (40). Robinson (87) gave data for oxyhydrogen. Lead has been determined in urine by extraction into a ketone, with a mean error of 0.02 p.p.m. (216, 120), in gasoline with an oxyhydrogen atomizerburner (no interferences occurred) ( g o ) , in nonferrous metals (36, @), cspecially copper alloys (55, 52, '71, Y d ) , in steel (35, 41, 102), and iron ore (102). In the metallurgical applications interferences were unimportant. Arsenic has not been studied. ilntimony and bismuth have been examined by Gatehouse and Willis (40) and Allan (8). The latter finds 2068 A. and 2176 A. (Sb) and 3068 A. (Bi) to be the best lines, with detection limits of 0.5 p.p.m. for each. The former show working curves for bismuth and find that air-coal gas gives the best sensitivity, viz., 2 p.p.m. For antimony they give 1.5 p.p.m. a t 2311 A. Willis (120) mentioned the determination of bismuth in biological materials. Gilbert (46) noted absorption by bis-
muth of the light from an effectively hotter OH band a t 3068 A. in the oxyhydrogen flame. Selenium and telluriuni have been studied by ,411an (S), who finds the best lines (above 2000 -4.)to be 2040 and 2143 A., respectively, with detection limits of 5 and 0.5 p.p. m. This is encouraging. The analytical chemistry of these and other neglected elements in the above list could benefit greatly from absorption flame photometry. The metals not mentioned above (ignoring those that are too rare or radioactive) include scandium, yttrium, the lanthanides, the actinides (primarily thorium and uranium) and rhenium. Among these, a t least europium and ytterbium will show absorption in the conventional flames, and many of the others should succumb to the hotter flames. Boron and phosphorus have resonance lines in the accessible region, but their oxides will dissociate only in the hottest flames.
MATERIALS ANALYZED
Agricultural Materials. Davey (22) and Allan (10). Soils and soil extracts have received a good deal of attention. David (16, 30) determined sodium, potassium, calcium, and magnesium. The interferences of aluminum, phosphate, silicate, and sulfate were suppressed by strontium. Allan determined magnesium (1, 4 ) and iron and manganese (1, 4, 5 ) . He found no interferences with the latter metals. Lockyer and Hames (65) and Leithe and Hofer (60) determined magnesium. Williams (112) determined magnesium and calcium; he used lanthanum to release the calciuin from the serious interferences of aluminum, phosphate, silicate, and sulfate. David (28) determined strontium in ammonium chloride soil extracts, using self-standardization to overcome the severe interferences that were present. -4Ilan (6) determined copper in EDTA soil extracts; when its concentration was too low, he complexed the copper with ammonium pyrrolidine dithiocarbamate and extracted it into ethyl acetate. Accuracy was about 1 p.p.m. of copper in soil. Allan (2) also determined zinc in the same way, preferring hexone as solvent. Fertilizers. David (27, 28) determined molybdenum in superphosphate containing 0.0570 110. He used a rich acetylene flame and avoided interferences by adding aluminum. The sample was extracted with boiling nitric-hydrochloric acid. Allan (10) determined cobalt and nickel in fertilizers. He also determined copper a t 0.2 to 0.5%, using the weaker line a t 2187 A. to avoid undue dilution (6), VOL. 34, NO. 5, APRIL 1962
217R
and zinc by the method mentioned above ( 2 ) . Plants are usually ashed before analysis. Leithe and Hofer (60) determined magnesium. Allan (1, 4) determined magnesium, manganese, and zinc. David (25, 30) determined magnesium and calcium, correcting interferences, as above, with lanthanum. David (93) determined magnesium and zinc, b u t could not do copper or iron, Later (24) he determined calcium, removing the interference of phosphate, aluminum, and silicate by adding magnesium and sulfuric acid, and compensating the interference of sodium and potassium by adding these metals to the standards. David also determined strontium (28), employing self-standardization. Allan (5) determined iron and manganese, encountering no interferences. He also determined copper (6) and zinc (2) by the methods mentioned above. lfiscellaneous agricultural materials. h l h n (1, 6) determined magnesium in drainage and lysimeter waters. David determined zinc in sheep dung (23) and calcium in microbiological culture solutions ($8)Medical Applications. TVillis (120) has discussed t h e determination of lead, mercury, bismuth, nickel, cadmium, and zinc in biological materials, b u t no details are available. Blood serum mas analyzed for sodium and potassium by Willis (119) and by Herrmann and Lang (49). The latter obtained satisfactory results a t &fold dilution, using standards matched t o the serum. Dawson and Ellis or Heaton (31, 52) determined magnesium and apparently also sodium, potassium, and calcium Rith the Optica equipment. Like Willis, they merely diluted and acidified the samples to 0.1N in hydrochloric acid. Interferences were unimportant. Accuracy was within 2% for magnesium. They analyzed erythrocytes also. Allan (1, 4) and Willis (114, 118) determined magnesium in serum. Willis diluted the serum (0.05 ml.) with 1% disodium EDTA or a strontium solution to eliminate interferences. Recovery experiments yielded 100 to 102%. He also determined calcium (126, 117), using the same diluent to avoid phosphate interference, and he determined zinc (120) in serum and whole blood, merely diluting with water. Urine was analyzed for calcium and magnesium by Willis (1I S ) , using various methods of preparation t o avoid the phosphate interference with calcium. H e favored direct dilution with a lY0 lanthanum solution; mean recovery error was 0.15 p.p.m. of calcium. Magnesium he determined by dilution with water, observing a recovery error of 0.01 p.p.m. (on the diluted sample). Dawson and Ellis or Heaton (31, 32) analyzed urine as well as serum for 21 8 W
ANALYTICAL CHEMISTRY
magnesium. Tabeling and Devaney (55, 103) determined nickel in urine down to about 0.05 p.p.m., using the Jarrell-Ash equipment with Beckman burners. Willis (40, 120) found that cadmium and zinc could be determined directly in urine without separation. He also determined lead, extracting i t with ammonium pyrrolidine dithiocarbamate into methyl n-amyl ketone a t pH 1.5 to 4.5, to concentrate the lead (116). The mean error was 0.02 p.p.m. of lead in the urine. Other biological materials. Kewbrun (78) determined calcium in saliva by WilliT’s method (117). Recoveries were 99y0 using EDTA without deproteinization or strontium with deproteinization. Allan ( 1 4 ) determined magnesium in milk. Danson and Heaton (32) analyzed ashed food and feces for magnesium. Industrial Products. Ferrous metals w r e discussed by Elwell (35) and Gidley (41, 44). Together (35) they reported the determination of lead in steel a t 0.1 to 0.2%. Interferences from many other elements were found to be unimportant. Stuinpf and Gonsior (109) also determined lead a t 0.1 to 0.4% in steel without concentrating the analyte. David (27, 28) determined molybdenum at 0.3% in stainless steel, using a fuel-rich acetylene flame. The sample was dissolved in nitric-hydrochloric acid, diluted and mixed with aluminum (2000 p.p.m.) to remove interferences, in addition to m-hich he used self-stmdardization. Nonferrous metals. Gidley and Jones (42, 43) described the determination of zinc in various copper- and aluminumbased alloys and in zirconium and its alloys. The method is useful over a wide range of zinc concentrations. Only silicon interferes, and i t should be removed. Lockycr and Hames (65)reviewed the analysis of zirconium for zinc. Menzies (52] 71, 72) reported the determination of copper, zinc, manganese, nickel, and lead in bronze n i t h the Hilger instrument. iigreement r i t h chemical analysis was extremely good. He also determined zinc in archeological materials and reviened Gidley and Jones’s m-ork, described above. Elwell and Gidley (36, 41) discussed the determination of sodium, magnesium, zinc, and lead in nonferrous metals (copper and aluminum alloys), and they published (36) a method for lead in copper-based alloys n i t h a re1a t‘ive standard devintion of 2% (see above for steel). Sonmetallic materials. Lockyer and Hames (65) mentioned the analysis of soda ash and the determination of potassium in sodium iodide. Leithe and Hofer (60) determined magnesium in limestone, gypsum, cement, and its raw materials, and slag, as well as soil and plant ash. They resorted to
no separations (except of silicon), and removed the interference of aluminum or phosphate, when i t was noticeable, by adding calcium and/or sulfuric acid. This interference was smaller in acetylene than in city gas. Rawling et al. (84) determined silver in lead-silverzinc ores and concentrator products, especially in lead sulfide containing 0.05% siher. The lead did not interfere; the relative standard deviation was 1%. Strasheim, Strehlow, and Butler (101) determined copper in ores (0.27, copper), evpelling silica. Interferences v-ere negligible. Stunipf and Gonsior (102) determined lead (0.1 to 0.4%), zinc (0.01 to 0.5%) and copper (0.03 to 0.2y0)in roasted iron ore, v-ithout separations. Robinson (90) determined lead in gasoline diluted 10-fold n-ith iso-octane, using a Beckman burner and Perkin-Elmer spectrophotometer. The mean error was about
1.570. Isotope Analysis. R a l s h suggested the possibility of determining the individual isotopes of a n element by absorption flame photometry, uqing a n emitter containing a single isotope. Zaidel in Russia (121) has worked out the theory of the method, which necessarily suffers from a certain amount of spectral interference owing to the closeness of the lines of the different isotopes and their width. Zaldel and KorennoI (122, la5) reported a successful application of this method to the isotopic analysis of lithium. For the analysis of Li6concentrates. a Li7 lamp n-as used. Its light passed through an acetylene flame into which the sample was sprayed. The total lithium was determined by emission spectrophotometry while the Li7 content was found h;y absorption; a chopper sent the transmitted light from the lamp and the emitted light from the flame alternately to the two monochromators. With corrections for interference, the relative standard deviation of the Lie concentration was 0.6%. ATOMIC ABSORPTION SPECTROSCOPY WITHOUT FLAMES
Hollow-Cathode Sputtering. Five or six methods of spectrochemical analysis have come to light which are not absorption flame photometry but which certainly are atomic absorption spectroscopy in the sense envisioned by Kalsh. I n addition there is an oral paper by Sikorski and Copeland (loo), entitled “Application of Atomic Absorption Spectroscopy to Solids.” Considering the date, 1957, one wonders what methods mere used; the abstract of the talk gives no real clue. Sputtering as a means of producing atomic vapor for absorption spectrophotometry was proposed by Gatehouse and Walsh (37-39, 107). The solid sample, with a hole through it, is made
the cathode of a hollow-cathode lamp and run on low current to keep it cool. This lamp is placed between another hollow-cathode lamp containing the analyte and a spectrophotometer. The emitter lamp is run on alternating current to avoid interference from emission by the absorber lamp. The absorber lzmp develops a reproducible vapor pressure of the sample including the analyte. A pressure of 1 mm. of neon or argon is used in the absorber. By this method silver mas determined in copper in the range 0.005 to 0.05%, using the pure resonance radiation from the emitter lamp in the manner of Russell and \j7alsh (95). The working curve of the silver line 3383 A. was straight and the relative standard deviation was 207, a t 0.005% silver and 7% a t 0.05%. Gatehouse, Sullivan, and Kalsh (37) mentioned aluminum, strontium, and titanium. They succeeded in carrying out analyses in a few cases hut said that the method is still unsatisfactory for aluminum. Robinson (93) also used the hollon--cathode sputtering method for silver, copper, and nickel. Discharge Tube. Poluektov (81) in his review of atomic absorption spectroscopy mentioned a method described by Bochkova and Shreider ( I S ) . They were able to determine tlie noble g w s in a mixture by placing tlie sample in a discharge tube and ohserving the absorption spectrum. I have no further details, but since the resonance lines lie between 584 A. (helium) and 1470 A. (xenon), it seems likely that absorption \vas measured a t longer-wavelength lines of excited states popul:itcd by the agency of the discharge. Spark-in-Spray. This source of atomic vapor, now commonly used for emission analysis, was tried by Robinson (93) in absorption. He placed the sparked spray between the hollow-cathode emitter tube and a Quantometer, and tested the equipment on aluminum. Although emission n-as very strong, he observed a n absorption of 50% a t 10 p.p.m. of aluminum. He recommended further effort on this technique. Cool Vapor. T h e familiar mercury vapor detector is, of course, a n atomicabsorption spectrophotometer. Mercury is unique in being t h e only element having appreciable pressure of atomic vapor a t room temperature together with a n absorption line outside the vacuum ultraviolet. Kaye (57) hn? recently presented a n absorption spectrogram of cool mercury vapor obtained with a Beckman far-ultravicolet DK spectrophotometer with hydroqen lamp, slion-ing the 1850-A. line to be much more intense than the 2537-A. line. Two recent applications deserve mention. Liiidstrom (62) determined nano-
gram quantities of mercury by spraying the dissolved sample through a Beckman atomizer-burner and passing the combustion products through a condenser and filter into a Kruger mercury vapor meter. Sensitivity was 0.001 p.p.m. in solution, and precision was 1%. H e applied the method to urine, disinfectants, paper and pulp products, and seed-treating materials. A question comes to mind: since the mercury is observed in the floning streim, how is it that regular absorption flame photometry is only 10-4 as sensitive? Possibly the cooling of the vapor, the removal of the water, and the greater optical path account for this large discrepancy. More recently, Jacobs, Goldnater, and Gilbert (53) described a n equally sensitive method for mercury in blood. The sample is digested with sulfuric acid, oxidized with permanganate, and extracted with dithizone in chloroform. The extract is heated in a furnace and the evolved mercury vapor determined by absorption. Furnace. T h e time-honored King furnace has never been suited to quantitative spectrochemical analysis. L’vov (67), however, has ingeniously adapted it to this purpose. The weighed solid sample (below 0.1 mg.) is carried on the tip of a carbon electrode which fits into a hole in the middle of an electrically heated (2200°C.) graphite tube lined with tantalum, suitably 10 em. long and 3 mni in bore, situated in a n argon-filled chamber with quartz windows. As the electrode enters the hole, i t is momentarily heated by an arc to another electrode outside the furnace tube, t o vaporize the saniple abruptly. Chopped light from a hollowcathode lamp passes through the furnace and is monitored by a recording spectrophotometer. The absorption by the analyte a t the selected wavclength is seen as a sudden drop in transmitted light a t the moment of vaporization followed by a fairly rapid decay as the vapor diffuses out of the furnace. The peak absorption is taken as the analytical value. This method has the tantalizing advantage that owing to the lorn partial pressures of 0 and OH, stable oxides are almost completely dissociated and all the elements appear as atomic vapor. L’vov gives n orking curves for potassium. cesium, strontium, barium, aluminum, indium, thallium, titanium, lead, chromium, and manganese. H e detected no interferences a t all, even a t interferent ’analyte ratios of lo4. The mean deviation of the armlytical readings n as 2%. h determination takes 3 minutes. He lists detection limits for the aboye elements, and s h o w t h a t the sensitivity of the method is 0.1 to 10 p.p,m. of analyte in the solid sample, depending on the element. Another scheme, described by Miller (?6),was conceived by Ostergren in
1060 dt Beckman Instruments. This is a portable atomic-absorption spectroscope to be landed on the moon, where it will analyze the surface material and telemeter the results back to earth. The sample, confined in a chamber under a moderately lorn pressure of flowing inert gas, is vaporized completely and quickly in the focus of a high-aperture solar furnace. A secondary collimated beam of sunlight passes through the escaping vapor and is compared with a reference beam in a high-resolution scanning spec‘trophotometer. The Fraunhofer lines must be compensated, of course. The resulting absorption spectrogram provides a quantitative analysis of the sample. The theoretical details were worked out fully by Ostergren and Gilbert, and the instrument is nom being developed under contract n i t h the Jet Propulsion Laboratory of the California Institute of Technology. LITERATURE CITED
(1) Allan, J. E., Analyst 83, 466-71
(1958). (2$-IbG., 86,530-4 (1961). (3) Allan, J. E., Nature 187, 1110 (1960). (4) Allan, J. E., Second Australian Ppectroscopy Conference, Melbourne, June 1959; See Ref. (46). (5) Allan, J. E., Spectrochim Acta 15, 800-6 ( I 959). 6) Ibtd.. 17,459-66 (1961). 7) Ibzd., 17, 467-73 (1961). 5) Ibzd., in press. 9) Allan. J. E.. Third Australian Soectroscopy Confefence, Sydney, August 1961; Spectmchzm. Acta, to be published. 10) Allan, J. E., Ibid.; Y a t u r e 192, 929 (1961). 11) Baker, C. A., Soc. Anal. Chern., Poole, England, May 1960; Analyst 85, 461 (1960). 12) Raker, C. A.> Garton, F. JT7. J., U. I., Symposium on Spectroscopy, pp. 13-156, Am. SOC. Testing Materials, Spec. Tech. Publ. hro. 269 (1960). (46) Ham, Y . S., Xature 184, 1195-7 11959) : Proc. ROT/.Sustralian Chem. Inst. 1959, 293-8;’ Bz~stralian J . Sci.
22, 64 (1959). (47) Herrmann, R., Optik 18, 422-30 (1961). (48) Herrmann, R., Lang, W.,Colloquium SuectroscoDicuni Internationale IX. L;ons. June 1961. (49)“ H e k a n n , R., Lang, W , 2. ges. ezptl. M e d . 134, 268-79 (1961). (50) Hilger and Watts, L t d , London, Bull. CII 407, 5 pp. (1959). (51) Hilger and Watte, Ltd., London, Hzlger J 5, S o . 1, 12-14 (Sug. 1958). (52) Hilger and Watts, Ltd , London, Research Rept. BR 4 10 pp (1957). (531 Jacobs, RI B , &oldwater, I,. J., Gilbert, H., Am l n d . Hya Assoc J . 22, 276-9 (1961). (54) Jarrell-Ash Co., Nea tonville, Mass., Bull. 82020, 3 pp. (1961). (55) Jarrell-Ash Co., Newtonville, Mass., Jarrell-Ash A-ewsletter, No. 11, 1-2 (Aug. 1961). (56) Jones, W. G., Walsh, A., Spectrochim. Acta 16, 249-54 (1960). (57) &ye, W. I., A p p l . Spectroscopy 15, 130-44 (1961). (58) Leen, M. K., Atwood, J. G., Pitts-
220 R
ANALYTICAL CHEMISTRY
(1961). (79) Optica, Inc., Washington, D. C., Bull. 6-2000, 4 pp. (1961). (80) Perkin-Elmer Corp., Norwalk, Conn., Atomic Absorption Data Sheets: Potassium; Nickel; Copper; Calcium; Magnesium; 1 p. each (1961). (81) Polvektov, S. S.,Zavodskaya Lab. 27, 830-6 (1961). (82) Pungor, E., Magyar Kkm. Lapja 15, 133-5 ( 1960). (83) Pungor, E., Konkoly-Thege, I., Acta Chirn. Acad. Sca. Hung. 28, 133-9 ( 1961). (84) Rawling, B. S., Greaves, M. C., Amos, 11.D., Suture 188, 137-8 (1960). 185) \ - - , Research and Control Instruments. Inc., Woburn, Mass. (86) Robinson, J. W., ANAL.CHEM.32, YO. 8, 17A4-29A(July 1960). (87) Ibid., 33, 1067-71 (1961). (88) Robinson, J. W., Ana!. Chim. Acta 23, 45841 (1960). (89) Zbid., pp. 479-87. (90) Zbid., 24, 451-5 (1961). (91) Robinson, J. W., 18th Intern. Congr. Pure and -4ppl. Chem., Montreal, August 1961. (92) Robinson, J. W.,Soc. Appl. Spectroscopy, 11th -4nnnal Symposium on ~
Spectroscopy, Chicago, June 1960. (93) Robinson, J. W., SOC.Appl. Spectroscopy, 12th Annual Symposium on Spectroscopy, Chicago, May 1961. (94) Russell, B,. J., Shelton, J. P., Walsh, A,, Spectrochzm. Acta 8,317-28 (1957). (95) Russell, B. J., Walsh, A., Spectrochim. Acta 15,883-5 ( 1959). (96) Saunderson, J. L., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1962. (97) Sawyer, R. R., 7th Ottawa Symposium on Applied Spectroscopy, October 1960 (98) Sawyer, R. R Pittsburgh Conference on Analyt?cal Chemistry and Applied Spectroscopy, March 1960. (98a) Scribner, B. F., Anal Chein. 32,229 R (1960). (99) Shelton, J. P., Walsh, 4.,Proc. 16th Intern. Congr. Pure and Applied Chem., Vol. 11, pp. 403-9, Lisbon, September
1956 (published 1958).
(100) Sikorski, M. E., Copeland, P. L., Spectrochim. Acta 9, 361 (1957). (101) Strasheim, -4.> Strehlow, F. W. E., Butler, L. R. P., J . S . African Chem. Znst. 13, 73-81 (1960). (102). Stumpf, K. E., Gonsior, T.: Collo-
quium Spectroscopicum Internationale IX, Lyons, June 1961. (103) Tabeling, R. W., Devaney, J., SOC.appl. Spectroscopy, 12th Annual Symposium on Spectroscopy, Chicago, May 1961. (104) Walsh, A., “Advances in Spectroscopy,” Vol. 2, pp. 1-22, W.H. Thompson, ed.; Interscience, S e v York, 1961. (105) Walsh, A . , Colloquium Spectroscopicum Internationale X, Washington, D. C., June 1962. (106) Walsh, il.,German Patent, huslegeschrift 1,026,555 (1958). (107) Walsh, A., Pittsburgh Conference on Analytical Cheinistry and Applied Spectroscopy, March 1960. (108) Walsh, A., Second Australian Spectroscopy Conference, Xelbourne, June 1959; see Ref. (46). (109) Walsh, A., U. S. Patent 2,847,899 (1958). (110) Warren, R. I,., Colloquium Spectroscopicum Internstionale VIII, Lncerne, 1959, pp. 213-15; published by Sauerlander, Aarau, Sx-itzerland. (111) R‘ells, R., 7th Ottan-a Symposium on Applied Spectroscopy, October 1960. (112) Williams, C. H., -4nal. Cham. Actu 22, 163-71 (1960). (113) Willis, J. R., ANAL.CIIEX.33, 556-9 (1961). (114) Willis, J. €3.. Y a t u r e 184, 186-7 ‘ (1959). ’ (115) Ibid.. 186. 249-50 11960). 191,’381-2(1961). (116) Zbid.: 191,’381-2(1961).’ (117) Killis, Willis, J . B., Spectrochim. Acta. 16, 2.59-72 1 960). 259-72 _.. .~ ((1960). (118) Zbid., pp. 273-8. (119) Ibid., pp. 551-8. (120) Willis, J. B , Third Australian Spectroscopy Conference, Sydney, August 1961; Lq7ature192, 929 (1961). (121) Zaidel, A. N., Optika i Spektroskopiya 4, 701-2 (19.58). (122) ZaIdel, -4.N., Uspekhi Fis. S a u k 68, 123-4 (1959). (123) Zardel, A. K., KorennoI, E. P., Optika i Spebtroskopiya 10, 570-6 (1961). \ - - - - ,
