Decay of Atom Populations Following Graphite Rod Atomization in Atomic Absorption Spectrometry R. D. Reeves,’ B. M. Patel,* C. J. Molnar, and J. D. Winefordnera Department of’ Chemistry, Uniwrsity of Florida, Gainesuille, Fla. 32601 The variation in atom population of Ag, Cr, Cu, Fe, Mn, Ni, Pb, Sn, and Z n with height above a graphite rod atomizer in both argon and argon:hydrogen atrnospheres is studied. The rate of decrease of atom population is considerably less in the presence of the hydrogen diffusion flame as compared to the argon atmosphere. Also, particularly with the argon atmosphere, improved sensitivity and reduced detection limits occur if a limited field viewing technique is used, whereas with the argonlhydrogen atmosphere, the improvement in sensitivity and detection limits is much less when using a limited field viewing method. A special problem involving loss of silver from aqueous silver nitrate solutions held within the stainless-steel needle of a 1-pI syringe is also discussed.
THEPAST FEW YEARS have seen the development of a number of graphite-rod atomizers for use in atomic absorption and atomic fluorescence spectrometry. The devices of West and Williams ( I ) , Amos er ul. ( 2 ) , and Dipierro and Tessari (3) used a graphite rod in a n enclosed chamber which was purged with argon or nitrogen. Light enteIed and left the chamber through appropriately-oriented quartz windows. Amos ( 4 ) reported a modification of the apparatus with the enclosure removed, and the replacement of the inert gas by a hydrogen flow of 0.5-1.0 1. min-’. Amos et ul. ( 2 ) found a reduction of interferences in the determination of lead when they used a n open system with a n argon-hydrogen o r nitrogen-hydrogen gas flow, rather than an inert gas alone. Such a mixture leads t o the ignition of a diffusion flame when the rod temperature is high enough. Much recent woIk has been carried out with systems open to the atmosphere. An inert gas flow alone was used in the apparatus of Alder and West (9,and in the “minifurnace” carbon rod atomizer of Matougek and Stevens (6), and Brodie and Matougek (7). Glenn et ul. (8) used a central flow of argon, surrounded by a hydrogen diffusion-flame sheath, and Molnar et ul. (9) have described a n apparatus in which a simple hydrogen diffusion flame is employed. The inert atmosphere serves the dual purposes of protecting the graphite rod against oxidation, and restricting the loss of atoms by recombination (e.g., with atmospheric oxygen). On leave, Department of Chemistry and Biochemistry, Massey University, Palmerston North, New Zealand. On IAEA Fellowship from the Atomic Energy Commission, India. Radiochemistry Division, Bhabha Atomic Research Center, Trombay, Bombay 85, India. Author to whom reprint requests should be sent. (1) T. S. West and X. K. Williams, Aiial. Chin?.Acta. 45, 27 (1969). (2) M. D. Amos, P. A. Bennett, K. G. Brodie, P. W. Y . Lung, and J. P. MatouSek. ANAL.CHEM.,43,211 (1971). (3) S. Dipierro and G. Tessari, Talanta, 18, 707 (1971). (4) M. D. Amos. Amer. Lab., p 33 (August 1970). (5) J. F. Alder and T. S. West, Atinl. Clzim. Acta, 51, 365 (1970). (6) J. P. MatouSek and B. J. Stevens, C h .Chem., 17, 363 (1971). (7) K . G. Brodie and J. P. MatouSek, ANAL.CHEW,43,1557 (1971). (8) M. T. Glenn, J. Savory, L. P. Hart. T. H. Glenn, and J. D. Winefordner, Anal. C/iim. Acta, 57, 263 (1971). (9) C. J. Molnar. R. D. Reeves. J. D. Winefordner, M. T. Glenn, J. R. Ahlstrom, and J. Savory, Appl. Spectrosc., in press 246
In graphite tube furnaces and in the “minifurnace,” where atoms spend a longer time in a high-temperature environment, the use of an inert gas alone is generally satisfactory. With a n open graphite-rod system, the atoms pass more quickly into a cooler environment, with a rapid loss by recombination, in addition to the usual process of diffusion and convection in the gas stream. An approach to examining the behavior of the metal atom population above the graphite rod has been demonstrated by Anderson et ul. (IO). The light beam was collimated and focused above the rod to give a small “field of view” measuring 4 mm wide and only 0.5 mm high. By using such a restricted height, a reasonably accurate picture can usually be obtained of the decrease in atom population with incredsing height above the graphite rod. In their work Anderson et ul. used hollow cathode lamps as light sources, and an argon flow of 1.7 1. min-l around the graphite rod. The populations of lead, copper, and nickel atoms decreased approximately in an exponential manner with height above the rod. Aluminum was detected only with the beam grazing the rod, the atoms being lost before traveling 1 mm vertically; concentrations of nickel, lead, and copper were reduced to just-detectable levels about 5-7 mm above the rod. In the present work, the system described previously (9) has been used for a similar kind of study. It was of particular interest to compare, for several elements, the decrease of the atom populations in the argon/hydrogen,’entrained-air flame with that in a n argon gas stream alone. In cases where there is a considerable (or complete) loss of atoms within about 5 mm of the heated graphite surface, the use of a limited field of view should improve the analytical sensitivity considerably. However, the collimation and stopping of the primary radiation under these conditions necessitates the use of higher photomultiplier gains, leading to poorer limits of detection and undesirably larger standard deviations for replicate measurements. In this work, this problem has been partially overcome by using temperature-stabilized electrodeless discharge lamps (EDL’s) (11) as light sources. These are much more intense than hollow cathodes, and, even with collimation, lead to baseline noise levels only a few times greater than those of the corresponding hollow cathode lamps operating under optimum conditions. EXPERIMENTAL Apparatus. LIGHTSOURCES. The electrodeless discharge lamps (EDL’s) used as spectral light sources were both singleelement and multiple-element EDL‘s ( 1 2 ) . The single-element EDL’s were: Cr (as CrC13) and Zn (as metal). and the multiple-element EDL’s were : Ag-Cu-Mn-Pb-Sn and Ag(10) R. G. Anderson, H. N. Johnson, and T. S. West, Aizal. Chim. Acta, 57,281 (1971). (11) R. F. Browner. B. M . Patel, T. H. Glenn, M. E. Rietta, and ‘ J. D. Winefordner, Spectrosc. Lett., 5, 311 (1972). (12) B. M. Patel. R. F. Browner, and J. D. Winefordner, ANAL.
CHEM.,44, 2272 (1972).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
Cu-Fe-Mn-Ni-Pb-Sn. Single-element EDL’s were operated at temperatures 340”C, respectively. Multiple-element EDL’s were operated in the “simultaneous mode” (12) a t a temperature of 410 “C. The temperature-control unit and the microwave power source used were identical to those described previously (11). An “A” type antenna (Model 2254-5002G1, The Raytheon Co.) was used for microwave coupling with the EDL’s. OPTICALAND ELECTRICAL SYSTEM. A schematic diagram of the optical setup is shown in Figure 1. The aperture used was a diaphragm with a rectangular slit and was placed between the antenna and the mechanical chopper (PAR Model 125, Princeton Applied Research Corp., Princeton, N.J.). Optical-grade biconvex quartz lenses (2-in. diameter, 2.5-in. focal length; ESCO Products, Oak Ridge Road, Oak Ridge, N.J.) was used throughout for focusing. An image measuring 3.7 mm wide and 0.7 mm high was produced by the lens above the graphite rod. This image was then focused on the monochromator entrance-slit using the second lens L2. The atomic absorption measurements were carried out with a 0.35 m, 1-6.8 Czerny-Turner monochromator (Model EU700/E, Health Co., Benton Harbor, Mich.) and using a RCA 1P28A photomultiplier, lock-in amplifier tuned to 665 Hz (PAR Model 220). pre-amplifier with variable gain (PAR Model 21 1) and photometric pre-amplifier (PAR Model 221). Source modulation was performed with the mechanical chopper and the data were recorded o n a potentiometric recorder (Sargent, Model SR). Reagents and Procedure. For each element (Ag, Cr, Cu, Fe, Mn, Ni, Pb, Sn, Zn), the sample used was 0.5 pl of an aqueous solution, sufficiently concentrated to give a peak absorption of 30-60z when the light beam grazed the rod. Suitable concentrations were generally in the 0.5-5.0 pg ml-l range, depending on the element. The atomizing procedure was similar to that described previously (9). The micrometer screw-thread, o n which the rod assembly was mounted, was used to vary the height between the rod surface and the beam. Control to within 1 0 . 0 2 mm was possible. Duplicate o r triplicate runs were made at heights between 0 mm (lower edge of the beam grazing the rod surface) and 20 mm. Absorption measurements were made for each element (i) with a n argon flow of 8.4 1. min-l, and (ii) with a mixture of argon (7.2 1. min-1) and hydrogen (1.2 1. min-l), as used in previous work with this atomizer (9). All other conditions were identical for the purpose of this comparison. In the case of aqueous silver solutions, poor reproducibility was obtained. A study of this problem, carried out with a n uncollimated beam from a hollow cathode lamp, using a Perkin-Elmer 303 Atomic Absorption Spectrophotometer, showed that it arose from the partial loss of silver on the inside of the stainless-steel syringe needle (Hamilton 7101 NCH). This is discussed in more detail below. Consequently, solution of silver 2-ethylhexanoate in a synthetic oil was used for the atom-population studies o n silver in this work. RESUTS AND DISCUSSION
Loss of Silver in a Stainless-Steel Syringe Needle. The variations in the atomic absorption signals obtained from a 0.5-111 sample of a 0.5-wg ml-l aqueous silver nitrate solution were found t o be related to the length of time for which the solution occupied the needle of the syringe. (In the type of syringe used here, the solution is not drawn up into the glass barrel.) Under normal sampling conditions, this residence time would be 10-20 seconds. It was found, however, that if the solution remained in the syringe needles for two minutes, the atomic absorption peak height was reduced to about half of its former value. The results of duplicate and triplicate runs with various residence times between 10 and 340 seconds are shown in Figure 2. Short and long times were alternated, to eliminate the possibility that the effect
M
Figure 1. Schematic diagram of the optical setup Legend: A, microwave antenna; E, temperature-stabilized EDL assembly; S, aperture; C, chopper; L1, L2, lenses; R, graphite rod; M, monochromatorentrance slit
.05
.04 absorbance
\\
Aqueous Ag solution \
\
-03
0 . 5 ~ 1x
0,Sppm
4
.02
.01
0
1
2 Residence
time
3 in
4
5
ryringt/minutes
Figure 2. Loss of silver in the stainless-steel syringe needle with varying residence time
was caused by some other systematic change in the solution o r the atomizer. The syringe was rinsed twice with dilute nitric acid and twice with distilled water between each sample. It is apparent from the figure that, after the solution has been in the needle for about 5 minutes, about 80% of the original 0.25 ng of silver has been lost by a n electrochemical or adsorption process o n the wall of the needle. The difference between 10-second and 20-second residence times is also large enough to lead to measureable errors. A stability problem with solutions of the silver--APDC complex in methyl isobutyl ketone has been noted by Bratzel et al. (13). Where such problems occur, the use of a syringe with a needle of platinum (or with other suitable inert coating) is necessary. In the present work, there was no difficulty in using the stainless-steel needle with solutions of silver 2ethylhexanoate in a synthetic oil. Atom-Population Studies. Factors contributing t o the decrease in the measured peak absorbance as the graphite-rod assembly is lowered include (i) the loss of atoms by chemical reaction; (ii) the transport of atoms outside the light path by diffusion; (iii) the decrease of the solid angle subtended by the atoms at the sample cavity. Even in the absence of condensation, oxide-formation, and other possible chemical losses, a gradual decrease from factors (ii) and (iii) would be expected. Graphs of absorbance plotted against the distance between the rod su.rface and the light beam are shown in Figure 3. (13) M. P. Bratzel, C. L. Chakrabarti, R. E. Sturgeon, M. W . McIntyre, and H. Agemian, ANAL.CHEM., 44, 372 (1972).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
247
oecu
w
0
0
5
10
15
Fe
Sn
Pb
I
:
He igh t / rn m
0
0
Haiyirt/mm
5
10
15
Height/rnrn
Figure 3. Decay of the atom population with height above the graphite-rod atomizer for nine elements, as measured by atomic absorption spectrometry (Ag, 338.3 nm; Cr, 357.9 nm; Cu, 324.7 nm; Fe, 248.3 nm; Mn, 279.5 nm; Ni, 352.5 nm; Pb, 283.3 nm; Sn, 286.3 nm; and Zn, 213.9 nm) -0-
Ar (lower curves)
-0-
Ar/H2 (upper curves)
For each element, with a n argon flow alone, the decrease of atom population with increasing distance shows an approximately exponential behavior similar to that found by Anderson et al. (10) for copper, lead, and nickel. The slightly more rapid atom loss found here for these elements may be a dilution effect resulting from the larger argon flow rate used in this work. The most striking feature of these graphs is the way in which the presence of the hydrogen diffusion flame helps to prevent this rapid attenuation of the atom populations. In all cases, there was measurable atomic absorption as much as 15 mm above the graphite-rod surface. In the presence of hydrogen, it is apparent that the already slower decrease of the atom populations between about 0 and 3 mm is further arrested, in most cases, between about 3 and 10 mm above the rod. The evidence of previous studies on the diffusionflame temperature with this system (9) suggests that this is a region characterized by the presence of an excess of unburned 248
hydrogen. The hydrogen/argon flame serves to remove much of the entrained oxygen in its outer zone, and the flow-rates used here provide unbu.rned hydrogen in the region of importance above the atomizing swface. The tendency for the atom-population curves to show a plateau in this region may be an indication that the hydrogen is not only protecting the metal-atom cloud from oxide formation, but also may be assisting in metal-atom production from molecules formed previously, by reactions such as MO
+ H, * M + H?O
(1)
MO
+ H * M + OH
(2)
or In the case of lead and silver, the two elements atomized at the lowest temperatures, the improvement with hydrogen is marked, but there is no perceptible plateau between 3 and 10 mm. The volatilization of tin occurs readily, but the
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
efficiency of atomization is low in the absence of hydrogen. In this respect, the behavior of tin here parallels that in the various atomic absorption flames, the improved atomization of tin in hydrogen-containing flames being well established. Chromium requires high temperatures for atomization, and in the absence of hydrogen, the atom concentration is low, even close to the surface of the rod. (Values for tin and chromium absorption at heights of less than “0 mm” were obtained by allowing the lower part of the beam to strike the rod; the absorption signal then represented an average over only about the first 0.3 mm above the rod.) The following conclusions may be drawn as a result of this work. Improved sensitivity can certainly be achieved by the use of a limited-field vewing technique, as Anderson et al. (IO) have shown. The problem of reduced intensity of the primary radiation can be overcome, in part, by using temperaturestabilized EDL’s as light sources, with a n improvement in detection limits. The concurrent operation of the hydrogen/argon/entrainedair flame often makes only a small difference in atom populations within 1 mm of the rod surface, large improvements being found here only for Cr and Sn. However, because the _
_
_
_
_
_
_
_
~
~
flame helps the atom populations to be maintained at a relatively high level for at least 10 mm above the rod, good sensitivity can be obtained by using standard hollow cathode lamps with beams of 3-8 mm diameter focused above the rod. Thus the presence of the flame makes it unnecessary, in general, to use either limited-field techniques or EDL’s for regular analytical work with this type of atomizer. The relative position of the beam and the atomizing surface is not as critical with the flame as with argon alone. With the diffusion flame and a 4 mm-diameter beam from a hollow cathode lamp, there is often only a small loss of sensitivity when the rod is moved from a grazing incidence to a position 2 mm lower. The use of the lower position of the rod allows one to avoid any possibility of interference from continuum emission from the rod, which may be observed at high atomizing temperatures close to the surface of the rod.
RECEIVED for review July 17, 1972. Accepted October 13, 1972. One of the authors (B.M.P.) would like to thank the International Atomic Energy Agency for the award of a fellowship. This work was supported by AF-AFOSR-701880 H .
~
Determination of Vanadium in Titanium Dioxide by Ultramicro Atomic Absorption Spectrometry on a Carbon Filament Atom Reservoir K. W. Jackson and T. S. West Department of Clzernistry, Imperial College of Science and Technology, London S W7 2A Y , England
Leonard Balchin Organics and Pigments Dicision, Laporte Industries Ltd., Grimsbj, Lincoln, England A modified Alder-West carbon filament atom reservoir with a rapid temperature rise time of up to 2500 O C in 0.18 sec yields a detection limit of 3.3 x 10-lo g for vanadium at 318.4 nm using 1-pI samples. The effect of up to 10,000-fold excesses of 22 metals i s studied and a simple and rapid method is evolved for the determination of vanadium ( > 1 5 ppm) in titanium dioxide. The dioxide is fused with Na2C03and the extract is submitted directly for analysis on the carbon filament. The water is dried off ca. 80 OC as the sample is added, the sodium carbonate is driven off at an intermediate temperature, and the vanadium is measured by flash atomization ca. 2500 OC. The procedure is calibrated by standard addition. The results obtained by AA compare with those obtained by an independent coulometric procedure.
VANADIUM, BEING A METAL of comparatively low volatility, and forming a stable refractory oxide, is one of the more difficult elements to determine by atomic absorption spectrometry, and for this reason few methods involving the use of nonflame absorption cells have been reported to date. Manning and Fernandez ( I ) used an electrically heated graphite tube furnace to determine 26 elements in pure aqueous solution and obtained a sensitivity abs) of
(lz
(1) D. C. Manning and F. Fetnandez, At. Absorption Newslett., 9, 65 (1970).
3.2 x 10-IO gram for vanadium at 318.4 nm using 20-p1 aliquots. This method was later applied (2) to the determination of amounts of vanadium in excess of 20 pg/ml in mineral oils. No sample preparation other than dilution with xylene was necessary and good agreement with spectrographic results was reported. A nonflame cell in the form of a tantalum boat (3) accommodating 50-100 p1 of solution gave a sensitivity (1 abs) of 6 x 10-10 gram for vanadium at the same wavelength. The carbon filament atom reservoir (CFAR) designed by West and Williams ( 4 ) was modified by Matousek ( 5 ) who used a carbon rod of larger diameter with a hole drilled transversely to give a miniature tube furnace (“Mini-Massmann”). A sensitivity of 9.2 X gram (1 % abs) for smaller aliquots (0.5 p1) of vandium solution was claimed with this cell, but when applied to the determination of vanadium in petroleum products ( 6 ) , some difficulties were reported. These were due to incomplete vaporization of the sample from the rod and spectral interference from particulate carbon produced as a result of the high temperatures required for atomization of vanadium.
x
(2) (3) (4) (5) (6)
S. H. Omang, Aiial. Cliim. Acra, 56, 470 (1971). H. M. Donega and T. E. Burgess, ANAL.CHFM., 42, 1521 (1970). T. S. West and X. K. Williams, Anal. Cliim. Acta. 45, 27 (1969). J. P. Matousrk, Amer. Lab., 3, 45 (1971). K. G. Brodie and J. P. Matousek, ANAL.CHEM., 4,3,1557 (1971).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
249
