Determination of Trace Amounts of Sulfur by Atomic Absorption and Emission Spectrometry H. E. Taylor, J. H. Gibson,l and R. K. Skogerboe Department of Chemistry, Colorado State Uniuersity, Fort Collins, Colo. 80521 The excitation of a sensitive atomic sulfur wavelength with a low-power, microwave-induced discharge is reported and the atomic absorption, emission, and fluorescence characteristics of this previously unused line are presented. Instrumental parameters have been optimized for sulfur absorption and emission in argon, helium, and air atmospheres; analytical curves developed; detection limits defined; spectral interference effects investigated; and example analyses have been carried out. In general, fractional microgram quantities of sulfur can be determined by either absorption or emission. The utilization of a microwave-induced plasma as an atomic absorption cell is discussed.
AN EXAMINATION of the literature indicates that the spectrochemical methods for the determination of sulfur have been essentially limited to X-ray fluorescence ( I ) , vacuum ultraviolet spectroscopy (2), gas chromatography with emission spectrometric detectors (3), colorimetric procedures ( 4 , 5 ) , and flame spectrophotometry (6, 7). The lack of spectrochemical methods based on the observation of atomic absorption or emission in the metallurgical region of the spectrum apparently originates from the fact that there are few sensitive sulfur lines in this region and from the tendency of sulfur t o form molecular species with carbon and oxygen in most atomization-excitation media. Consequently, the unusually high sensitivity reported by Bache and Lisk (3), who excited the visible region ion lines in a low-pressure, microwaveinduced helium plasma, suggested that this might serve as a means for solving a variety of sulfur analysis problems. In an attempt t o avoid the general inconvenience accompanying operation of a plasma a t reduced pressure, a n investigation of an atmospheric pressure discharge which might encompass the desirable sensitivity features of the low pressure system was initiated. Although diverse combinations of excitation conditions were examined, the ion spectra reported (3) were not observed. Long-term photographically integrated spectra of the low background argon discharge covering the 21008200 A region were subsequently used as a means for determining the possibility of exciting atomic sulfur. A single, intense, unidentified line, the intensity of which was proportional t o the sulfur concentration, was observed in this region. A similar investigation of an electrodeless discharge lamp containing sulfur produced the same result. The wavelength of this line was measured as accurately as possible with Present address, American Chemical Society, 1155 Sixteenth Street, N.W., Washington, D. C. 20036 (1) L. S. Birks, E. J. Brooks, and H. Friedman, ANAL.CHEM., 25,
692 (1953). (2) G. Milazzo and G. Cecchetti, Appl. Spectrosc., 23, 197 (1969). (3) C. A. Bache and D. J. Lisk, ANAL.CHEM.,39, 786 (1967). (4) D. F. Boltz “Colorimetric Determination of Non-Metals,” Interscience Publishers, New York, 1958, p 261. ( 5 ) C. M. Johnson and N. Nishita, ANAL.CHEM., 24, 736 (1952). (6) W. L. Crider, ibid.,37, 1770 (1965). (7) R. M. Dagnell, K. C. Thompson, and T. S. West, A m l y s t , 92, 506 (1967).
the facilities available (2169 h 1 A) and a search of the appropriate spectral compilations was carried out. A sulfur line is listed at 2168.9 A only by Wiese, Smith, and Miles (8) citing Lawrence ( 9 ) as the reference. The wavelength given, however, is calculated from energy level differences (8) and is classified as a 3P4-1D2intercombination transition. It might be concluded, therefore, that the present report constitutes the first actual observation and analytical application of this particular wavelength at least for low sulfur concentrations. The spectroanalytical utility of this wavelength has been evaluated for atomic absorption, atomic emission, and atomic fluorescence with negative results for the latter. The results reported herein apparently represent the first direct atomic absorption measurements for sulfur and the first usage of a microwave discharge as a n atomizing medium for atomic absorption. Instrumental parameters have been optimized for absorption and emission in argon, helium, and air atmospheres; analytical curves were developed; interference effects have been investigated; and example analytical applications have been used as a means for practical evaluation. Detection limits for gaseous sulfur compounds, sulfur containing pesticides, and sulfur impurities in air have been defined. In general, fractional microgram quantities of sulfur can be determined by either technique. The sensitivity realized, coupled with the relative simplicity of the instrumentation required, suggests that both atomic absorption and emission may find widespread application in the solution of problems involving the determination of trace amounts of sulfur. EXPERIMENTAL
Instrumentation. The basic spectrochemical facilities used in this investigation have been previously described (IO) with the exception that a lock-in amplifier (Ithaco, Model 353B) tuned to a frequency of 20 Hertz replaced the dc amplifier. A variety of sample introduction, atomization, and excitation techniques were utilized and are listed in the following appropriate sections. ATOMICEMISSION.Emission studies were carried out at both atmospheric and reduced pressure, thereby necessitating the use of different sample introduction systems. The atmospheric pressure system which is operable only with argon contained in a small diameter (1-mm i.d.) discharge tube has been described (10). Two sample delivery systems were used : the thermal filament assembly described by Runnels and Gibson ( I O ) for direct vaporization into the plasma, and a gas chromatographic system which separated and vaporized compounds prior to the discharge. Acetone solutions of sulfur compounds of known purity were deposited on the filament, the solvent was evaporated, and the sample vaporized into the argon stream by resistance heating. The (8) W. L. Wiese, M. W. Smith, and B. M. Miles “Atomic Transition Probabilities,” Volume 11, NSRDS-NBS22, 1969. (9) G. M. Lawrence, Astrophys. J., 148, 261 (1967). (10) J. H. Runnels and J. H. Gibson, ANAL. CHEM.,39, 1398 (1967).
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0.01
O
’
O
8
0.06
0.0 W
W
z
z
0 In
0 In
8
/
I
/e
I
0
0
I a
I a
:
m
i
0.04
0.0
0.02
5
10
15 20 ABSORPTION CELL PRESSURE, TORR
0
Power
=
40
50 60 70 80 MICROWAVE POWER (WATTS)
90
I
Figure 2. Effect of microwave power on sulfur absorption 0 Argon at 12 Torr A Helium at 5 Torr
Argon
A Helium 0 Air
80 watts
gas chromatograph (Carle Instruments, Inc., Model 8000) was attached to the inlet of the discharge tube; compounds such as hydrogen sulfide and sulfur dioxide were separated on a Porapak Q column (11); and eluted directly into the plasma. A gas sampling valve (Carle, Model 8030) immersed in liquid nitrogen was used as a trap for analyses requiring the collection of evolved sulfur compounds with subsequent introduction into the chromatograph cia warming the trap and opening the valve. Reduced pressure operation utilizes essentially the arrangement described above with the addition of a vacuum gauge (Hastings-Raydist, Model VT-4) and a mechanical vacuum pump upstream and downstream from the discharge, respectively. A quartz cell, 10 cm by 1-cm i.d. with 6-mm i.d. sidearms for entry and exit of the support gas and quartz windows cemented t o the ends was used as the discharge tube. Gaseous sulfur compounds were injected directly through a silicone rubber septem located 10 cm upstream from the cell. The pressure gauge is located 5 cm upstream from the injection port to reduce contamination of the gauge filament. The light sample was taken from the central portion of the cell without observation of significant selfreversal effects. ATOMIC ABSORPTION.Absorption measurements were made with the reduced pressure system described above. As previously indicated, the atmospheric pressure discharge can be reliably sustained only in a tube of capillary dimension. A low pressure plasma can, however, be readily sustained in a cell of larger diameter which is necessary for sufficient passage of the incident light beam. Chopper modulated radiation from a microwave powered sulfur electrodeless discharge lamp (Westinghouse, Model WX31844) was focused (11) H. M. McNair and E. J. Bonelli, “Basic Gas Chromatography,” Varian Aerograph, Walnut Creek, Calif., 1969, p 64. 0
I 30
1
25
Figure 1. Absorbance of sulfur as a function of cell pressure
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Y
a
0.0
0 Air at 3 Torr Sulfur concentrations are constant for each gas but differ between gases
through the cell and, thence, on the slit of the monochromator with a two-lens system. Gaseous samples were injected through the septum, as above. ATOMICFLUORESCENCE. Again, the same cell was used. Modulated radiation from the discharge lamp was focused on the sample cell from a position above the optical path. Reagents. Chemicals used were of reagent grade or known purity. Argon and helium were both welding grade. RESULTS AND DISCUSSION
For convenience, the analytical characteristics for the three spectrochemical techniques studied are presented separately. Atomic Absorption. OPTIMUM ANALYTICALCONDITIONS. The number of experimental conditions affecting the degree of absorption was limited by two factors. First, the end-on absorption cell was completely filled with radiation from the light source which precludes the necessity for determining the region of maximum absorption. Second, the pumping rate of the vacuum pump was much greater than the flow rate of the gases studied. Consequently, the internal cell pressure rather than the gas flow rate served effectively as a measure of the optimum condition. Thus, the only additional parameter requiring optimization was the microwave power input, These variables were optimized using response surface experimental methods (12, 13) using both hydrogen sulfide and sulfur dioxide in the support gases. The optimum conditions determined were the same for each impurity gas in the respective support gases but differed between (12) R. K. Skogerboe, “Developments in Applied Spectroscopy,” W. Baer and E. L. Grove, Ed., Plenum Press, New York, 1969. (13) W. G. Cochran and G. M. Cox, “Experimental Designs,”
Wiley and Sons, New York, 1960, Chapter 8A.
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0.
0.01
0.01 w V
z
2 (r
2m a
0.0
a 0.0
IO
20
MICROGRAMS SULFUR
Figure 3. Absorption curves for the direct injection of sulfur as hydrogen sulfide Key is same as for Figure 1
/ IO MICROGRAMS SULFUR
support gases as might be anticipated. The effects of each parameter on the absorption signal measured at the optimum settings of the other parameters are presented for illustrative purposes in Figures 1 and 2. Figure 1 indicates the effect of cell pressure on the absorption signal when a constant amount of hydrogen sulfide was injected. The absorption signal observed is transient in time, lasting a few seconds. The absorption at the maximum of the transient peak serves well, however, as a reproducible measure of concentration. The differences in the relatively sharp maxima occurring for the three support gases are due primarily t o differences in gas density. In any case, regulation of the pressure t o within + l o % of the optimum is required-a condition that is easily satisfied with the system used. It must be emphasized, however, that the pressures given are not the actual internal cell pressures because of the location of the vacuum gauge and the gradient within the system-ie., location of the gauge i n closer proximity t o the cell would indicate a lower pressure. Equivalent optimal cell pressures were observed when sulfur dioxide was used in place of hydrogen sulfide. The effect of the microwave power is indicated for the three gases in Figure 2. Although the input power has a relatively small effect on the absorption in air, the choice of a n operating level of 80 watts is readily justified on the basis of the signal enhancement in helium and argon. Again, the effects were the same for both hydrogen sulfide and sulfur dioxide. ANALYTICAL Smsmvrry. Analytical curves are given in Figures 3 and 4 for the determination of sulfur in hydrogen sulfide and sulfur dioxide, respectively. A comparison of the two figures suggests a marked difference in the degree of dissociation of the molecular species to atomic sulfur which will be discussed below. It should also be noted that the sensitivity discrepancy for the air discharge suggested by
20
Figure 4. Absorption curves for direct injection of sulfur as sulfur dioxide Key is same as for Figure 1
Table 1. Detection Limits of Sulfur Analyses Operating Detection limit, pg pressure, AbsorpSupport gas Torr Sample Emission tion HzS 0.2 ... Argon 630a SO? 0.2 ... Argon 630 HS 0.8 0.6 Argon 12 SO? 1.7* 0.3 Argon 12 HS 0.02 0.3 Helium 3 SO? 0.4b 0.7 Helium 3 H2S 1.2 0.7 Air 5 Air SO2 2.6b 0.4 5 0.02 ... 630 Parathion Argon Ethion 0.02 ... 630 Argon Guthion 0.02 ... 630 Argon a Local atmospheric pressure. b Detection limit limited by background interference due to recombination of carbon and oxygen in plasma. The residual carbon impurity level in helium and argon was approximately 10 and 20 ppm, respectively.
comparison of Figure 2 with Figures 3 and 4 is due t o the use of unequal concentrations between the support gases in obtaining the data for Figure 2. Detection limits, defined as described by Skogerboe et al. (14) are given in Table I. The measurement precision and (14) R. K. Skogerboe, A. T. Heybey, and G. H. Morrison, ANAL. CHEM., 38, 1821 (1966).
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’“1
H,S
0
so*
Table 11. Optimum Excitation Conditions for Sulfur Determinations in Argon at Atmospheric Pressure Microwave power 15 watts Flow rate 300 ml/min Region of discharge 2 cm= a From leading edge of discharge.
the curve slopes were determined within a factor of 2-3 of the detection limit by five successive measurements. The t statistic for the 9 5 z confidence level and four degrees of freedom was used in place of the frequently used confidence constant of the Normal Distribution (15) because the latter is applicable only for large numbers of measurements. Consequently, this approach produces a comparatively conservative estimate of the detection limit. Atomic Emission. EXCITATION CONDITIONS. The reduced pressure conditions maximizing emission intensities were identical with those determined for atomic absorption. Optimum conditions for excitation in argon at atmospheric pressure are given in Table 11. These coincide with the conditions reported for the determination of molecular impurities in argon (16). ANALYTICAL SENSITIVITY. Figure 5 presents the curve determined for atomic emission in argon at atmospheric pres-
.
0
MICROGRAMS SULFUR
Figure 5. Emission curve for direct injection of sulfur compounds at atmospheric pressure
I.
2
’
,
4
6
’
,
IO
I2
MICROGRAMS SULFUR
Figure 6. Reduced pressure emission curves for the direct injection of hydrogen sulfide Key is same as for Figure 1. Concentration scale for helium is multiplied by 10
sure. The data indicate that all points lie on the curve within experimental error and imply that the degree of decomposition for hydrogen sulfide and sulfur dioxide are equivalent. Moreover, the fact that SO and SOs emission bands are not observed suggests that decomposition is essentially complete for SOZ. Atomic emission curves for sulfur as hydrogen sulfide (Figure 6) and sulfur dioxide (Figure 7) at reduced pressure are also presented. Note that the slope of the curves for excitation in helium are greater than the corresponding curves utilizing argon or air (the concentration scale for He has been multiplied by lo). Comparison of these curves leads to the conclusion drawn for the atomic absorption results-Le., the degree of compound dissociation and excitation depends on the identity of the sulfur compound and the support gas. SPECTRAL INTERFERENCE. Spectral scans of the region containing the atomic sulfur line are presented in Figure 8. The band degraded to the red occurring at 2173 is attributed t o the neutral CO molecule (17), but does not overlap the sulfur wavelength. The interfering band on the low wavelength side (2164 A) is due to excitation of CO+ (17). These two molecular species originate from the combination of carbon- and oxygen-containing impurities in the support gases. Although the CO+ band interference can be eliminated by removal of the impurities from the support gases as evidenced by Scan D of the figure, the presence of carbon- and oxygen-containing entities in the sample negates the purification and will be
A
(15) H. Kaiser, “Limit of Detection,” Adam Hilger Ltd., London,
1968.
(16) H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, ANAL. CHEM., 42, 876 (1970). 1572
(17) R. W. B. Pearse and A. G. Gaydon “The Identification of Molecular Spectra,” John Wiley & Sons, New York, 1963, p 116.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
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4 6 b MICROGRAMS SULFUR
215.0
IO.
Figure 7. Reduced pressure emission curves for the injection of sulfur dioxide Key is same as for Figure 1. Concentration scale for helium is multiplied by 10
Table 111. Analyses of Cast Iron Samples ManuWt facturer's R el Sample number sulfur value5 Dev, stand dev 1 2 3 a
0.135 0.197 0.219
0.139 0.235 0.224
-2.9 -16.2 -2.2
4.1 3.8 2.9
Thorn Smith, Chemist.
manifested as a background interference in a manner to be illustrated below. In order to evaluate the apANALYTICAL APPLICATIONS. plications potential of the emission technique, the following example analyses were investigated. The sulfur content of cast iron samples was determined by dissolving the sample in hydrochloric acid, sweeping the evolved hydrogen sulfide into the liquid nitrogen trap described above, separating the contents of the trap by gas chromatography, and exciting the eluted sulfur compound in the argon discharge at atmospheric pressure. Calibration was accomplished by direct injection of measured amounts of hydrogen sulfide. The results of triplicate analyses of standard iron samples are presented in Table I11 with the deviation of the average from the certified value and the standard deviation of replicates. The results are quite precise and are accurate within the limits of precision with the exception of sample 2 for which the results were anomalously low. Failure to obtain complete dissolution of the sample as evidenced
216.0 217.0 218.0 219.0 WAVELENGTH (NANOMETERS)
Figure 8. Atomic sulfur and background spectra A. B. C. D.
Purified argon Purified argon plus 1 volume per cent air Purified argon plus sulfur Sulfur electrodeless discharge lamp
by an insoluble residue remaining in the reaction vessel is the most plausible explanation for this anomaly. The analytical throughput for this procedure is determined by the time required for dissolution. On the basis of the detection limits for the atmospheric pressure discharge, sulfur can be determined at levels as low as 0.2 ppm by weight when a l-gram sample is utilized. Moreover, a solvent extraction method followed by vaporization from the thermal filament into the plasma might also be used for determining trace amounts of sulfur. Hydrogen sulfide and sulfur dioxide may be chromatographically separated under the conditions specified by McNair and Bonelli (11). The compounds are also adequately separated t o permit individual determination with the discharge as the detector. An analytical curve obtained by injection of 10-ml air samples doped with measured amounts of the two compounds is presented in Figure 9. Considering that all points lie on the same curve and that SO and SO2bands are not observed, it seems reasonably certain that both compounds are totally dissociated to atomic sulfur by the discharge as previously noted. Injection of large volumes of air tends to broaden the elution peaks and the transient emission signal with a corresponding loss in absolute sensitivity relative to injection of microliter amounts of the pure compounds. Examination of the magnitude of the relative detection capability as a function of the size of air sample used indicates a continuously increasing relationship. Ten milliliters is, however, the maximum volume that can be physically injected into the
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0
/ IO0
200 300 MICROGRAMS CARBON
400
508
Figure 10. Background emission curve for sulfur containing pesticides
J
100
200
300
400
500
A Parathion 0 Ethion
600
Guthion
PARTS PER M I L L I O N SULFUR
Figure 9. Analytical curve for determination of sulfur in air 0
0
HzS
so2
chromatograph used. Because most air pollution studies are concerned with determining sulfur at levels below 10 micrograms per cubic meter ( l a ) , the sensitivity observed for this particular procedure would be inadequate without preconcentration through use of a chemical scrubber, for example. Sulfur-containing pesticides were determined cia emission measurements in the atmospheric pressure plasma using the thermal filament vaporization technique. The compounds are largely decomposed to atomic and diatomic fragments (10, 16) including carbon monoxide which serves as a source of background interference for the sulfur line. Figure 10 shows the background intensity measured at 2162 A expressed in terms of the amount of carbon introduced since it is the element present in greatest excess. Although the height of the transient emission peak serves as an effective measure of intensity, the area of the signal is presented in this instance to demonstrate that it, also, is linear with concentration. Evaluation of the data in Figure 10 indicates that an equivalent amount of carbon from each of the three pesticides contributes equally within experimental error to the background intensity. The background corrected sulfur analytical curve is presented in Figure 11. Correction was made by subtracting the predicted carbon contribution (using the Figure 10 relationship) from the measured intensity of the line. The results verify that pesticides may be determined by extraction and direct vaporization from the filament in the absence of other sulfur compounds. (18) U. S. Department of Health, Education, and Welfare, Public Health Service, “Air Quality Data from the National Air Sam-
pling Networks and Contributing State and Local Networks 1964-65,” Cincinnati, Ohio, 1966. 1574
A statistical evaluation of the data confirms that all points lie on a single curve at the 95 confidence level and suggests that the degree of fragmentation to atomic sulfur and carbon is the same for all three compounds. The inability to observe the highly probable CS band spectra even at high pesticide concentrations further implies that dissociation to atomic sulfur is complete. If this conclusion is valid and applicable to the many sulfur-containing pesticides, it may be possible to utilize a single, universal analytical curve for determining all such compounds. A knowledge of the identity of each particular compound would be necessary, however, to permit making the appropriate compensation for the carbon contribution. The more practical alternative would be to determine a curve for each compound previously separated by gas chromatography. In this case the background, plus line intensity, would be due to the pesticide molecule as a whole and the detection capability would be improved by an amount characteristic of the particular pesticide. Utilization of this approach with a reduced pressure helium discharge would probably permit determinations at the concentration levels reported by Bache and Lisk (3). Atomic Fluorescence. Although a wide variety of experimental conditions were examined for the reduced pressure discharge, atomic fluorescence was not observed. Considering the origin of this line, this negative result would be anticipated. Dissociation-Excitation Mechanism. Examination of the data presented herein indicates some dissociation-excitation differences that can be generalized. First, the atmospheric pressure studies imply that H2S and SOzare dissociated to &he same degree by the plasma while this is not the case for the reduced pressure studies. Second, the position of the emission curves for helium relative to the other reduced pressure data (Figures 6 and 7) indicates approximately one order of magnitude improvement in sensitivity for a helium-supported discharge. Finally, the relative slopes of the absorption curves in Figures 3 and 4 suggest that, even though it has a
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
Table IV.
Comparison of Absolute Detection Limits Detection limits, Analysis technique ma Colorimetry ( 5 ) 5-50 10 X-Ray fluorescence ( I ) 0.1 Vacuum UI spectrometry (2) Gas chromatography, thermal conductivity detector ( 2 2 ) 10 Gas chromatography, microwave discharge 0.01 detector (3) Atomic emission, reduced pressure plasma in heliumb 0.02 Atomic emission, atmospheric pressure plasma in argonb 0.2 Atomic absorption, reduced pressure plasma in heliumb 0.7 Defined as the weight of sulfur that must be present to permit measurement. b Present study.
50
100 150 NANOGRAMS SULFUR
200
Figure 11. Background corrected emission curve for pesticides Key is same as for Figure 10
higher dissociation potential (Is), SOz is dissociated to a higher degree than H2S. The deduction of an explanation for these differences is complicated by the fact that the dissociation and excitation may originate from thermal, electronic collision (2O), collision with other atoms in a metastable state (IO), or chemiluminescent processes. Differences in plasma residence times for gases of varying density and quenching effects may also contribute to the complexity. Estimation of the thermal temperatures of all plasma combinations studied with a thermocouple produced temperatures in the 400-600 O C range suggesting that the thermal mechanism cannot be primary. Electronic temperatures of 4850, 4060, and 3350 OK were determined by the method outlined by Adcock and Plumtree (21) for atmospheric pressure argon, reduced pressure argon, and reduced pressure helium discharges, respectively. These results are consistent with the atmospheric us. reduced pressure differences, but cannot explain the increased emission sensitivity in helium or the dissociation discrepancy mentioned above. Although the curves in Figures 3 and 6 are reproducible,
(19) T. L. Cottrell, “The Strength of Chemical Bonds,” Butterworth, London, 1958, Table 11.5.1. (20) E. W. McDaniel, “Collision Phenomena in Ionized Gases,” John Wiley & Sons, New York, 1964. (21) B. D. Adcock and W. E. G. Plumtree, J . Quant. Spectrosc. Radiat. Transfer, 4, 29 (1964).
there is evidence of a change in the slope (see argon curves for example) which might be indicative of a quenching effect between atomic hydrogen and sulfur. Failure to observe this effect with sulfur dioxide supports the possibility of depopulation of the atomic state through quenching effects. Finally, the relative magnitudes of the energies of the metastable states for helium and argon, 21.3 us. 11.6 eV, imply that helium should be the most efficient support gas for the atomic collision mechanism and this is supported by the emission data. It must be emphasized, however, that the system is of sufficient complexity to preclude convenient proof of these possibilities. Comparison with Other Methods. Table IV summarizes the best detection capabilities of the techniques discussed herein with those that have been realized by other methods. To facilitate comparison, the limits are expressed in terms of the weight of sulfur that must be present in the measurement system to permit measurement. Thus, if a limit for a particular method was cited in relative concentration units, this was converted to absolute units through multiplication by the sample size utilized in the method. Because detection limits are somewhat nebulous quantities, it would be most reasonable to suggest that the values cited for two methods should differ by at least a factor of five to ten to be regarded as significant. On this basis, the only technique which compares favorably with the techniques utilizing the microwave induced discharge is vacuum ultraviolet spectrometry. The relative simplicity and low cost of the microwave systems would decisively favor the choice of this technique for those samples in a form amenable to introduction into the discharge. The fact that the low power discharge becomes unstable when the sample delivery rate exceeds one milligram per second does, however, impose a restriction on the relative detection capability of this approach. It can be concluded, however, that the utilization of the plasma technique to induce atomic emission or absorption at this wavelength will be applicable to a variety of sulfur analysis problems.
RECEIVED for review May 25, 1970. Accepted August 10, 1970. Research supported by NSF Grant GP-7986. (22) E. L. Obermiller and G. 0. Charlier, ANAL.CHEM.,39, 396 (1967).
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