CONCLUSIONS
organic and inorganic forms of mercury. Standards prepared from mercuric acetate, mercuric chloride, methyl mercury chloride, and methyl mercury iodide were pyrolyzed and areas obtained for mercury measurement were equivalent. Acid gases which would give positive readings with the ultraviolet detector were effectively scrubbed from the gas stream, while mercury vapor passed through. One-ml quantities of SO2 and NOz gas introduced into the open end of the combustion tube gave no reading on the detector. Area measurements obtained for pyrolyzed mercuric acetate standards were equivalent to area measurements obtained by the stannous chloride reduction method. It became evident early in the studies that recorder peak heights were not adequate for mercury quantitation. The cause was traced to the nonreproducible rate of mercury release from various specimens in the combustion zone of the furnace. Measurement of peak area with a digital integrator overcame this problem and, in effect, further decreased the overall analysis time.
A rapid and convenient analysis scheme for the determination of total mercury in fish has been presented. While not tried, its possible application for mercury analysis in other materials is obvious. Measurement precision appears to be adequate for present requirements and further increases in sensitivity are inherent to the system if required. ACKNOWLEDGMENT
The authors thank R. D. Householder for the data provided in the chemical procedures for total mercury, w. A. Nichols for the methyl mercury analytical data, and R. C. Rittner for assistance in establishing the micro-elemental conditions. RECEIVED for review September 3, 1971. Accepted October 29,1971.
Homogenization of Nonconducting Samples for Spark Source Mass Spectrometric Analysis G. H. Morrison and A. M. Rothenberg Department of Chemistry, Cornell University, Ithaca, N . Y. 14850
A method has been developed for the homogenization of powdered geological samplesfor use in spark source mass spectrometry. Uniform blending with high purity graphite has been achieved and precision of analysis of +lo% attained. In the application of the procedure to material from the meteorite Allende, 28 elements were determined with an average precision of *7%.
MANYINVESTIGATORS have evaluated the precision and accuracy of determining average elemental concentrations by spark source mass spectrometry employing photoplate detection. However, all of these studies have involved the use of fairly homogeneous metal samples containing a limited number of well-characterized impurities (1-5). Halliday et al. ( I ) have determined the reproducibility of analysis of a homogeneous aluminum sample to be 10-20x. Ahearn (2) reported standard deviations of 5-12x for CA2 copper standard and 10-20x for the CA4 sample. Vossen (3) obtained approximately 10 relative standard deviation for the aluminum standard 1791 but only 13-20z for steel sample SS53. Franzen et al. ( 4 , 5 ) made detailed studies of the precision of photoplate evaluation and presented a computer program for its improvement. However, many materials currently being studied by spark source mass spectrometry are nonconducting substances of a complex nature where sample heterogeneity is a primary source of error (6-12). In addition, these samples must be
x
(1) J. S. Halliday, P. Swift, and W. A. Wolstenholme, Aduan. Mass Spectrom. 3, 143 (1966). (2) A. J. Ahearn in “Trace Characterization, Chemical and Physical,” Nut. Bur. Stand. (US.)Monogr. 100, 1967. (3) P. Vossen, ANAL.CHEM.,40, 632 (1968). (4) J. Franzen and K. D. Schuy, 2.Anal. Chem., 225, 295 (1967). (5) J. Franzen and K. D. Schuy, Z . Natdrforsch., 219, 1479 (1966).
powdered and uniformly blended with graphite (or another suitable conducting matrix) in order to sustain the R. F. spark. Analytical precision generally achieved with these samples is A20-30x on the average (6, 12, 13). Attempts have been made to improve precision by using devices such as a rotating electrode (14) or an ion beam chopper (3). According to Nicholls et al. (9), the homogeneity of geological samples can be improved by means of successive fusions and grindings, but this method introduces the possibilities of loss of sample o r contamination, and those elements present in the flux material cannot be determined. Much work has been accomplished in this laboratory on the spark source mass spectrometric analysis of geological, meteoritic, and lunar samples blended with high purity graphite (15-17), where the homogeneity of standards and (6) D. W. Oblas, D. J. Bracco, and D. Y. Yee, in “Symposium on
Trace Characterization-Chemical and Physical,” National Bureau of Standards, Washington, D.C., 1966, p 486. (7) R. K. Skogerboe and G. H. Morrison, ibid., p 589. (8) R. K. Skogerboe, A. T. Kashuba, and G. H. Morrison, ANAL. CHEM., 40, 1096(1968). (9) G. D. Nicholls, A. L. Graham, Elizabeth Williams, and Margaret Wood, ibid., 39, 584 (1967). (10) Richard E. Honig, Aduan. Mass Spectrom., 3, 101 (1966). (11) A. Cornu, ibid., 4, 401 (1968). (12) E. B. Owens, ibid., 3, 197 (1966). (13) John Roboz, “Introduction to Mass Spectrometry,” Interscience, New York, N.Y., 1968. (14) F. Aulinger, 2.Anal. Chem., 221,70 (1966). (15) G. H. Morrison, J. T. Gerard, A. T. Kashuba, E. V. Gangadharam, A. M. Rothenberg, N. M. Potter, G. B. Miller, Geochim. Cosmochim. Acta, Suppl. I , Vol. 2, p 383 (1970). (16) G. H. Morrison, J. T. Gerard, N. M. Potter, E. V. Gangadharam, A. M. Rothenberg, and R. A. Burdo, Geochim. Cosmochim. Acta, Suppl. 11, Vol. 2, p 1169, (1971). (17) G. H. Morrison and A. T. Kashuba, ANAL.CHEM.,41, 1842 ( 1969).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
515
samples were of vital importance. The United States Geological Survey standard diabase W-1 was used as a comparative standard for obtaining elemental sensitivity factors, As originally prepared, W-1 was ground to pass 80 mesh (18). The heterogeneous nature of such a powder has been evaluated, and investigators have found the sampling error to be quite high for trace elements, especially those present in minor accessory minerals or at very low concentrations (19, 20). Since the amount of material consumed in the mass spectrographic analysis is limited, the powder must be made more uniform: A. D. Wilson (19) has calculated that a 10mg sample would have to be ground fine enough to pass 325 mesh in order to minimize sampling errors. Therefore, a procedure has been developed in our laboratory for W-1 which significantly improves the uniformity of the electrode material without any chemical or physical pretreatment, except the grinding of W-1 in an agate electromagnetic micromill and the blending with a high purity graphite conducting matrix. The average precision of a single determination (calculated from the elemental constants of a graded series of exposures for each element on a single plate) is =k12 for the 28 elements used in the study, this value reflecting the accumulation of errors from sample heterogeneity, interference effects, and measurement deviations, including photoplate variability. When those elements for which interference and measurement problems are minimized are evaluated, the precision is calculated to be % l o % relative standard deviation. This homogenization technique has been applied to the analysis of meteoritic and lunar materials (15-17). In the analysis of the meteorite Allende, the average reproducibility for 28 elements is +7z.
z
EXPERIMENTAL
Sample Preparation. In order to further homogenize the W-1 powder, a Geoscience Instruments Corp. Micro-Pulverit 0 was used. Pulverization is achieved through the transmission of a vertical oscillation of an agate mortar mill into a vibratory motion of the agate ball. Finer particles are tossed to the upper portions of the mortar, while larger grains are selectively triturated. As the range of grain size decreases, the motion of the ball is changed to a tumbling motion which achieves the desired pulverization and homogenization through a friction effect. Cleaning of the mortar and ball can be simply achieved by the pulverization of high purity fused quartz in the micromill, followed by rinsing with distilled water to remove the quartz dust. To evaluate contamination of samples from grinding in this agate mortar, additional pieces of the quartz were ground in the cleaned micromill and then analyzed by neutron activation analysis and high resolution gamma spectrometry along with samples of the unground material. Replicate analyses revealed no contamination at the 0.01-ppm level. Taking W-1 as originally received, five grams of the powder were further crushed in the Geoscience micromill for four hours. After rolling of the sample in a glass vial to prevent segregation due to particle size, one gram of the sample was then sieved and about 80% of the material passed 400 mesh. The remaining 4 grams of the crushed but unsieved sample were separated into four equal parts, each of which was mixed with an equal amount of high purity graphite (Spex Industries lot 600-HP). The aliquots were subsequently blended in the micromill for periods of 2, 3, 5 , and 6 hours, respectively. Past six hours grinding, it had been observed that electrodes formed from the powder were ex(18) M. Fleischer, Geochim. Cosmochim. Acfa, 33, 65 (1969). (19) A. D. Wilson, Analyst, 89, 18 (1964). (20) A. W. Kleeman, J. Geol. Soc. Aust., 14, 43 (1967). 516
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
tremely brittle and difficult to mount in the source, presumably due to the excessive crushing of the graphite thereby destroying its binding property. A second sample of 5 grams of the original W-1 was successively sieved and crushed until the entire sample was made to pass 400 mesh. The sieved material was mixed with the graphite and blended as before. A final 1-gram portion of the original rock was simply blended with the graphite for 1 hour in a Spex Industries Mixer/Mill, a procedure often followed in sample preparation. Each sample was pressed into a suitable electrode according to the method previously described (17). Mass Spectrograph. A Nuclide Analysis Associates GRAF-2, double-focusing spark source mass spectrograph with recently installed solid state electronic components was used in this study (21). Optimization of experimental parameters as previously described (17) was performed. Exposed photoplates were developed for 3 minutes in a modified Ilford ID-13 solution. Internal Standards. In this work no internal standard was added to the sample since the limited size of most samples analyzed in this laboratory precluded the precise addition of the internal standard. A given element present in the sample could function well as an internal standard for those elements in the same local mass range, but the relative standard deviation rose sharply and accuracy decreased when the mass range was extended. Therefore, four elements present in the W-1 were selected as internal standards for each of the four mass ranges studied. The isotopes used were 9Be, 5Cr, *9Y,and 139La,where the concentrations of their elements were 0.63, 120, 25, and 12 ppmw, respectively. This was done to minimize both the effects of varying line widths arising from instrumental factors and the nonuniformity of the Ilford 4 2 emulsion lengthwise across the plate (22). Since the elemental concentrations were unknown in the analysis of lunar and meteoritic materials, internal standards could not be employed without independent analysis; hence both the reference standard W-1 and the sample were analyzed on the same photographic plate as suggested by Franzen and Schuy ( 4 ) . Data Reduction and Interpretation. A Jarrell-Ash console microdensitometer was used to measure the following parameters for each spectral line of interest: the peak height transmittance (Tlb) including the superimposed background level, the background transmittance (Tb) near that line, and the saturation transmittance (T,) of the species represented by that line. The value of T, was determined by interpolation of nearby T, values for similar species when no saturated line of a given species was available. Calibration curves for several regions of the photoplate were constructed according to a linearized form of the Hull equation (23): where Tiis the transmittance of the line corrected for background, E is the beam monitor exposure in nanocoulombs, A is the isotopic abundance, and K is a constant proportional to the concentration of the element in the ion beam and to the plate sensitivity. The left-hand side of the equation is plotted cs. log E for a graded series of exposures of the same isotope to yield a calibration curve of slope R. Since the values of T Idepend on the value of R, that is, Ti
where LB (Tb -
=
[l
=
T8)]1'B,
+ T, (LB - B)"]/[l + (LB - B)"]
(2)
[(l - Tih)/(Tib - T 8 ) ] 1 ' R and B = [(l - Tb)/ then it is convenient to assign an initial value
(21) G. H. Morrison, B. N. Colby, and J. R. Roth, ANAL.CHEM., in press. (22) A. J. Ahearn, Sixteenth Annual Conference on Mass Spectrometry and Allied Topics, Pittsburgh, Pa., 1968, p 271. (23) F. Degreve and D. Champetier de Ribes, Int. J . Mass Specfrom. Ion Phys., 4, 125 (1970).
of R in order to obtain the values of TLrequired for the initial plot of Equation 1. The method of least squares fitting of straight lines is used t o compute an interim value for the slope R of the initial plot. A computer-controlled cycling procedure employs successive interim R values to redetermine TZ values and refit the plot of Equation 1 until two successive R values differ by less than 0.5% relative. The last calculated value of R is considered to be representative of the “true” calibration curve for a given region of the photoplate near the calibrated isotope, provided that the region has similar background levels throughout and that the absolute plate sensitivity remains relatively constant over that region. The value of the constant ( K ) for a single analyte ( x ) line in the calibrated region is then determined by solving Equation l for K:
Kz
=
Il/(E~Az)IKI
- TI)/(TI- TJ1”l’”
(3)
The mean value 17, and its relative standard deviation (RSD) are then computed from the individual values of K , for a graded series of exposures. A similar mean value, and its RSD are obtained for one or more isotopes of an element defined as t_he internal standard (8) in the calibrated region, The ratio K J K , is proportional to the concentration ratio of the analyte and-standard elements in the ion beam. The RSD of K , or K , is a measure of the deviations contributed by several agents including variations in vertical plate sensitivity, uncertainty in the calibration technique itself, inadvertent or uncontrollable changes in electrode parameters (gap, position, geometry), statistical sparking fluctuations, errors in the beam monitor measurement, densiometric error, and inhomogeneity in the sparked sample. The cumulative effect of all deviations except inhomogeneity is to impose a limit on the degree of precision obtainable for even_ “homogeneous” sample types. The use of the RSD of K, or K , as an indicator of inhomogeneity within an aliquot is significant only insofar as it exceeds the total deviations caused by all other limiting-errors or when the average RSD of several or more E. or K. values for a sample blended in one manner is statistically different from that of the same sample run under the same conditions and limiting deviations but blended in a different manner. Since the average limiting RSD depends on a complex relationship of contributing errors, then it may vary according to the instrumentation and techniques used by the investigator. - However, the average limiting RSD of 5-7x for K , or K, values determined in this laboratory is consistent with the 5 % intraplate sensitivity variations quoted by Honig (IO), the 3.7% intraplate and 5.SZ interplate sensitivity variations given by Halliday et al. (24), and the approximate 5 precision errors caused by combinations of plate sensitivity and replicate exposure variations as stated by Cornu (21) and Owens and Giardino (25). The intraplate limiting RSD the ratio R./E. will include the deviations of both K. and K. and can be estimated to be 7-10% by taking the square root of the sum of the squares of the individual limiting RSD’s of 5 - 7 x each. The interplate RSD of the R,/K, ratios for different aliquots of the same blended sample is a measure of the degree of uniformity between aliquots (4), and also includes variations in grain response, statistical sparking fluctuations, and experimental errors introduced by inconsistencies in electrode positioning which can alter relative sensitivity of analyte and standard as suggested by Wadlin and Harrison (26).
a,
L Figure 1. Micrograph of graphite particles (lot 600-HP)
cf
(24) J. S. Halliday, P. Swift, and W. A. Wolstenholme, Aduun. Mass Spectrom., 3, 131 (1966). (25) E. B. Owens and N. A. Giardino, ANAL.C ~ M35, . 1172
(1963). (26) W. H. Wadlin and W. W. Harrison, ifiid.,42,1399 (1970).
Figure 2. Micrograph of rock-graphite mix from Mixer/Mill RESULTS AND DISCUSSION
The maximum degree of homogeneity of a powdered sample is closely related to the particle size of the material, the type of mineral in which a given element is present in the rock, its concentration, and the total amount of sample studied. Under microscopic examination, the effects of the further ANALYTICAL CHEMISTRY, VOL. 44,NO. 3. MARCH 1972
517
Table I. Effect of Homogenization on Precision in Determination of the Elemental Constant K , Mixer/Mill Element blending, concn RSDZ, Micromill blending, ppmw 1 hr 2 hr 3 hr 5 hr Isotope 0.63 22 19 10 6.0 OBe 11.6 28 17 20 10 'OB 'Ti "V
'cr 'acr 6'CO
8PRb SQY
oozr 9aNb lraBa lJ9La 140ce
Figure 3. Micrograph of - 4 00 mesh W-1
* J
Figure 4. Micrograph of homogenized rock-graphite mix crushing of the sample and the blending with graphite are evident. Figures 1 and 2 show the original graphite powder and the graphite/rock mixture blended in the MixeriMill, respectively. Little reduction of the graphite particle size was achieved and, as expected since the opening of an 80mesh sieve is 0.2 mm, grains of W-1 as large as 0.16 mm were 518
0.64% 240 120 50 22 25 100 10 180 12 23
8.8 22 30 _. ZI 21 26 23 28 20 22 28 35
8.4 19 14 15 6.7 27 27 27 24 15 37 34
9.5 6.8 IO 13 17 7.1 14 7.4 6.7 12 11 19 12 19 18 20 10 17 8.8 10 15 13 15 12
the
RSDZ 6 hr 8.2 1.2 9.7 11 9.0 6.9 7.5 12 7.3 21 8.2 11 10 I1
frequently observed, in contrast to the much smaller particle size of the rock powder passed through 400 mesh (Figure 3). The results of blending the fine rock powder with graphite for six hours in the micromill are seen in Figure 4. The significance of decreased grain dimensions has been determined by Kleeman (ZO), who calculated that the maximum error is reduced by one third when the number of grains in a given sample is effectively increased ninefold in the grinding of the rock powder to a finer particle size. Such an increase in the number of grains can be attained by reducing the particle size from 80 t o 400 mesh; comparable improvement in precision is expected. In the evaluation of the reproducibility of the mass spectrometric analysis as related to the amount of grinding of a sample, 14 isotopic lines representing 13 elements with minimum suspected interference were selected. Since there appeared to be no analytical differencebetween the rock powder crushed for four hours, 80% of which passed 400 mesh, and the sample crushed and sieved until all material passed 400 mesh, only the latter values are reported. The precision of determination of the average elemental constant 8, for each of the 14 isotopes is listed in Table I. [Several plates were run for each mix to ensure that variations in precision were the result of changes in uniformity of the sample rather than fluctuations in the measurement step.] With the exceptions of Ti which seems to be fairly homogeneously distributed as would be expected for a concentration of 0.63% and of Zr which appears heterogeneously distributed in all samples (see later for discussion), all other elements show a general increase in the uniformity of distribution in a given sample as indicated by the improvement in the precision of determination of 8%.As the amount of grinding is increased, the average precision for the 13 elements steadily improves to approximately *lo% for the five and six hour mixes (Figure 5). Although this type of information regarding the precision of individual analyses is not normally reported, it is an important measure of the accuracy of the constants determined on each plate. When precision is poor, the final concentration value reported for that plate must he considered unreliable, and numerous replicate analyses, which are timeconsuming and often impractical due to limited sample size, must be performed in order to obtain better accuracy. The replication of analysis is a common method of reducing the standard deviation due to sample heterogeneity and thus improving the accuracy of analysis. Improving the
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
3 0 9 Table 11. Plate-to-Plate Reproducibility of RZ/i?* I
I
H t
1
IX”
Mixer/Mill Micromill blending blending, 10 10 Element/internal replicates, replicates, RSD % Calculated Fa standard RSD % 10B/9Be 30 9.8 3.21 *‘Ti15aCr 28 13 3.23 slV/Wr 26 12 2.50 baCr/baCr 6.3 5.3 2.07 W O / ~ ~ C ~ 32 4.7 6.19 ssRb/sgY 18 19 1.33 goZr/SgY 15 15 1.12 24 10 3.54 g3Nb/8gY 1asBa/l 39La 17 13 4.09 10.6 4oCe/1 38La 21 6.1 a The F statistic is calculated by dividing the larger variance by the smaller. In all cases except Rb and Zr, the micromill sample had the smaller variance. This value is compared with the tabular F 0.95(9,9) of 3.18.
33 0
I
2
3
4
5
6
Time of Grinding, hours
Figure 5. Effect of homogenization on precision of determination of K,
homogeneity of a sample should result in increased precision, thus minimizing the number of replicate analyses required to obtain accurate results. For the samples homogenized according to the method developed in this laboratory, the effects of the increased uniformity of electrode material are evident in the reproducibility of measurement from replication. The RSD of the values of &/R, calculated using ten plates each of the uniform blend a n a the nonhomogeneous mix are reported in Table 11. The reproducibility of Cr as determined by the isotope ratio 52Cr/53Cr, which is independent of sample heterogeneity and variations in sparking, is about =t5 % for both samples, approaching the limiting precision expected in mass spectrographic analysis as discussed earlier. The concentration ratios for most of the elements listed show a marked improvement in precision with the homogenization of the W-1 ; the reproducibility of the 13 elements is about f10 for the ten plates. Variance estimates included in the Table indicate that 60% of the elements are significantly more uniformly distributed in the sample mixed in the micromill. The average precision achieved for most of the 28 elements normally determined in rock samples is 5-12Z RSD. The fact that the reproducibility of the ten plates is not improved over the precision of a single determination indicates that other factors are contributing to the variation; high RSD values for several elements can also be traced to these factors. In particular, the elements Zr and R b (Table 11) show no improvements in precision as a result of the homogenization procedure. The possible presence of Zr as the accessory mineral zircon in W-1 could account for its unusual degree of heterogeneity. Wilson (19) calculated the effect of the mode of occurrence on sample error; for lithium present at 400 ppm of lithium oxide, the standard deviation is 1.3 ppm when found as a trace element in the major mineral pyroxene and 20 ppm as a major constituent of spodumene. Rubidium is subject to deviation as a result of sparking conditions; the analytical line at mass 85 is often extremely sharp and dense, suggesting thermal ionization rather than the normal mechanism. Measurement errors also affect the precision obtained. The variability of several elements present at low concentrations (W, Ge, and Mo) can be attributed to problems arising from their determination near their actual detection
Table 111. Analyses of Allende Meteorite Av concn of 4 analyses, ppmw Re]. stand. Element SSMS NAA dev, Z 1.3 ... 6.8 Li 14 ... 0.03 Be ... 7.8 1.o B 3.3 55 ... F 11 89 83 V 2.0 610 ... co ... 3.8 11 Ge 11 ... 1.4 Rb 3.3 ... 27 Sr 6.1 Y ... 3.1 5.0 11 11 Zr 2.8 ... 0.74 Nb 19 ... cs 0.1 ... 3.0 12 Ba 0.6 1.7 0.56 La 6.6 1 1.2 Ce 8.0 ... 0.2 Pr 1.7 ... 0.93 Nd 3.0 0.35 0.36 Sm 11 0.1 0.1 Eu 1.7 ... 0.42 Gd 2.3 0.1 0.08 Tb 5.9 ... 0.4 DY 8.0 0.1 0.1 Ho 9.1 ... Er 0.3 10 ... 0.3 Yb 3.5 0.2 Hf 0.2 27 1 ... Pb
limits in a complex system. Other sources of error include variations in plate sensitivity, inadvertent or uncontrollable changes in electrode parameters, statistical sparking fluctuations, fluctuations of the beam monitors, especially at low charge collections, and densiometric measurement. One of the primary causes of error which is often neglected or underestimated is the presence of minor unresolved interferences on some of the analytical lines in the spectrum of a complex material. The precision and accuracy reflect the contribution of these species; in several instances, such as the determination of As, Ag, Cd, and Br in W-1, analysis is not possible. To demonstrate the effectiveness of the homogenization technique in the analysis of a complex sample, both the W-1 used as a reference standard and samples of the recent meteANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
@
519
oritic fall known as Allende were similarly treated. Belonging to the class of Type I11 carbonaceous chondrites, 6.8 kg of Allende has been recovered and has been prepared by the Smithsonian Institute as a possible standard meteorite powder. Its detailed analysis was therefore most appropriate. Since the powder has not yet been well characterized and suitable internal standards could not be selected, the W-1 and Allende were both analyzed on the same photoplate (4). Analysis of four different samples of the meteorite were performed and results of the mass spectrographic and the neutron activation analysis (included for comparison) are reported in Table 111. The average RSD for the 28 elements determined is 7 %, comparing favorably with the precision of analysis of simpler metallic samples. The higher deviations for Be, Cs, and Pb can be attributed to analytical difficulties associated with their determination at low concentrations. Although comparative data are limited, agreement with the results available from the neutron activation analysis of
the same samples is quite good. A more extensive evaluation of the reported results of the analysis of Apollo 12 soil sample 12070 using the homogenization method has been completed (27). The accuracy of the mass spectrometric determination of 33 elements in 12070, for which comparable precision was attained is 9%. ACKNOWLEDGMENT
The authors thank J. T. Gerard for assistance in developing the crushing procedures.
RECEIVED for review September 3,1971. Accepted November 3, 1971. Financial support was provided by the National Science Foundation under Grant No. GP-6471X and the Advanced Research Projects Agency (DAHC 15-67-C-0214) through the Cornell Materials Science Center. (27) G. H. Morrison, ANAL.CHEM.,43 (7), 22A (1971).
Determination of Technetium by Atomic Absorption Spectrophotometry Willard A. Hareland,' Earl R. Ebersole, and
T. P. Ramachandran
Argonne National Laboratory, Idaho Division, Idaho Falls, Idaho 83401 The atomic absorption characteristics of technetium have been investigated with a laboratory-constructed technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 pg/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. The absorption of spectral radiation was studied as a function of wavelength, fuel-to-oxidant ratio, hollow-cathode lamp current, burner height, and spectral bandwidth. Studies with 32 cations at concentrations of 50 and 500 pg/ml indicated that only calcium, strontium, and barium caused severe technetium absorption suppression with 60 pg/ml of technetium in 2N hydrochloric acid solutions. The cationic interferences are eliminated by adding aluminum to the test solutions. The applicability of atomic absorption spectrophotometry to the determination of technetium in uranium and a uranium alloy was demonstrated.
ATOMIC ABSORPTION SPECTROPHOTOMETRY is a rapid, accurate, and extremely versatile analytical method for performing routine analysis and has been applied successfully to the determination of nearly 70 elements. However, no published information exists pertaining to the determination of technetium by atomic absorption spectrophotometry or to the atomic absorption characteristics of this element. Technetium does not exist as a primordial element in terrestial minerals but is produced in high yield in neutron-irradiated uranium and plutonium. However, traces of technetium have been detected in uranium ores as a result of spontaneous fission (I). Several analytical methods are available for the determination of trace quantities of technetium. Neutron acti-
vation is the most sensitive. For the reaction 99T~(n,y)100T~, the neutron capture cross-section is 30 barns, which permits the determination of 2 x lo-" gram of g9Tc (2). Boyd (3) described a polarographic method capable of determining 5 X lo-* gram. Technetium is determined spectrophotometrically as a thiocyanate (4, 3,or 1,5-diphenylcarbohydrazide(6, 7) complex with a sensitivity of 1 X lo-' gram. Using the 4031 A spectral line, as little as 1 X 10-7 gram can be determined by emission spectrometry (8). However, direct analysis of highly radioactive nuclear fuel by conventional analytical techniques is not practical because of interference from other fission products. Thus, separation of technetium is generally required. A direct atomic absorption method would be a valuable asset to the nuclear power industry since it would provide a rapid and accurate method for determining burnup. In addition, nuclear fuel processing waste could be analyzed conveniently to aid in establishing the economic factors in the recovery of this valuable element. This paper describes the design and construction of a technetium hollow-cathode lamp which was used as a spectral line source for the atomic absorption determination of technetium. The absorption of technetium was studied as a function of wavelength, fuel-to-oxidant ratio, hollowcathode lamp current, burner height, and spectral bandwidth. The optimum parameters were established by varying one factor at a time while keeping the others constant. Studies
Present address, Department of Biochemistry, College of Biological Sciences, University of Minnesota, St. Paul, Minn. 55101.
(2) G. Boyd and Q. Larson, J . Phys. Chem., 60, 707 (1956). (3) G. Boyd, J . Chem. Educ., 36, 3 (1959). (4) C. Crouthamel, ANAL.CHEM.,29,1756 (1957). (5) 0. Howard and C. Weber, ibid., 34, 530 (1962). (6) F. Miller and H. Zittel, ibid., 35, 299 (1963). (7) R. Villarreal, M.S. Thesis, University of Idaho, MOSCOW,
(1) B. Kenna and P. Kuroda, J. Znorg. Nucl. Chem., 26,493 (1964).
(8) W. Meggers, Spectrochim. Acfa, 4, 317 (1951).
Idaho, 1967.
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