tive decomposition. As can be seen from the table, neither DCA nor interferences in the method were detected. Although most unlikely, the possibility of an interference that would have the same retention time on PEG and stabilize the DCA still exists. This would make the analysis less than quantitative. A simple test for this would be to add a known partial pressure of DCA to the sample and to repeat the
analysis. If the newly-determined concentration agreed with the calculated value, it could be concluded that the original analysis was quantitative.
RECEIVE[)
for review September 27, 1971. Accepted January
10,1972.
Emission Spectrometric Determ ination of Trace Amounts of Mercury F. E. Lichte and R. K. Skogerboe Department of Chemistry, Colorado State University, Fort Collins. Colo. 80521
RECENTRESEARCH on the widespread contamination of the environment with mercury has generated a demand for highly sensitive, selective, and reliable means for determining this element in samples of diverse materials. Of the various analysis methods available, the cold cell atomic absorption method developed by Hatch and Ott ( I ) has received extensive attention because of its simplicity and sensitivity. Kalb ( 2 ) , for example, has adapted the technique for determining parts per billion concentrations in water and sediments while Uthe, Armstrong, and Stainton (3) have utilized it to analyze fish tissues. One can conclude that the method generally satisfies the analysis requirements for a majority of problems. There are cases, however, where the lack of adequate sample or the unusually low concentration of mercury precludes the use of this technique and greater sensitivity is consequently required. An interesting report by April and Hume ( 4 ) describes the use of a capacitively-coupled radiofrequency plasma torch in conjunction with a mercury reduction-vaporization cell for emission spectrometric determinations. Data presented suggests a limit of detection of 2 nanograms of mercury and a useful working range of 10 nanograms to 10 micrograms is cited ( 4 ) . Accepting the 10-ml sample size cited by the authors ( 4 ) , mercury concentrations as low as 1 ppb can be quantitatively determined in water with the system used. Thus, the method appears to be more sensitive than the atomic absorption technique, on an absolute basis, by approximately one order of magnitude (1-3). Previous reports from this laboratory have dealt with the utilization of a low power, microwave induced plasma as an excitation medium for spectrochemical analyses (5-7). In addition to the relative simplicity of this system, the high absolute sensitivity that can be obtained can be cited as a primary advantage. While this plasma resembles that used by Hume and associates ( 4 , 8 ) , the methods used to couple (1) W. R. Hatch and W. L. Ott, ANAL.CHEM.,40,2085 (1968). (2) G. W. Kalb, A t . Absorprioti Newsleft.,9, 84 (1970). (3) J. F. Uthe, F. A. J. Armstrong, and M. P. Stainton, Fisheries Research Board of Canada, Freshwater Institute, Winnipeg, Canada, 1970. (4) R. W. April and D. N. Hume, Science, 170,849 (1970). ( 5 ) J. H. Runnels and J. H. Gibson. ANAL.CHEM..39. 1398 11967). (6) H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, ibid., 42, 876 (1970). (7) Ibid., p 1589. (8) C. D. West and D. N. Hume, ibid., 36,412 (1964).
ANHYDRONE
+ARGON IN
-PLASMA ClNG SOLUTION U
Figure 1. Schematic diagram of mercury reduction chamber the power into the discharges are quite different as are the characteristics of the plasmas produced. The principal problem associated with the microwave plasma originates from the fact that the rate of sample introduction must be limited if a stable plasma is to be maintained (5-7). Consequently, the technique involving the reduction of mercury compounds to the metal followed by volatilization into the plasma (1-4) offers a nearly ideal means for eliminating the sample introduction problem. Experiments carried out using the reduction-vaporization approach indicate that microwave plasma excitation can be used to determine ultratrace concentrations of mercury in a variety of sample types. The accuracy and precision of the method, as inferred from measurements made at ppb to ppm concentration levels, is estimated to be generally better than 10%. EXPERIMENTAL
Apparatus. The instrumentation utilized is listed in Table I. Specific features of this system are discussed below where appropriate. A schematic of the closed reduction chamber is presented in Figure 1. Reagents. The reagents specified by Hatch and Ott ( I ) were utilized for reduction of the mercury. Acids used for preparation of the reducing solution, for dissolution of samples, and for preserving water samples, were redistilled from A.R. grades or were commercial products of a purity designated as higher than that of A.R. grade. Standards were prepared from triply distilled mercury. Procedure. Approximately 5 ml of freshly prepared reducing solution were placed in the reduction-vaporization chamber and argon was bubbled through for several minutes ANALYTICAL CHEMISTRY, VOL. 44, NO. 7 , JUNE 1972
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Table I. Experimental instrumentation^ Excitation system Microwave generator, Model Scintillonics, Inc. HV 15A, 100 watt Fort Collins, Colo. Microwave coupling cavity NBS Report (9) Evenson type (No. 5) Cavity mount Edmund Scientific Rack and Pinion No. 6028C Vaporization system See Figure 1 Reduction solution (1) Gas regulation Hoke 1315 G4B with Argon flow control vernier Flow measurement Brooks Rotameter, Model No. 1110-06K2GlZ Dispersing system Monochromator Jarrell-Ash No. 82000 scanning Ebert, 0.5 meter equipped with vibrating refractor plate (IO). Grating 1180 line/mm, blazed at 3000 "A Slits Entrance and exit fixed at 25 microns Readout system Fluke, No. 412B Phototube power supply Photomultiplier R-106 Amplifier Ithaco No. 353 Recorder Hewlett-Packard, No. 7101B with 17500 A preamp. a See Reference 5 for schematic diagram. Table 11. Comparison of Analyses by Microwave Emission and Cold Cell Atomic Absorption Mercury concentration, ppma Atomic Microwave Sample designation absorption emission Blood A 0.062 0.059 B 0.066 0.065 0.32 0.35 Leaves A B 1.10 1.20 Concentrated H2S04 0.06 0.057 Triply distilled water 0.0017 0.0017 a Biological sample concentration given on a dry sample weight basis. Others given in kg/ml.
to remove the residual mercury blank commonly present in the reagents used. Samples, acidified for preservation and varying in size from a few microliters to several milliliters, were injected directly into the chambers through a silicone rubber septum using the instrumental conditions specifie: below. The intensity of emission at the mercury 2536 A wavelength was recorded during the period of Hg evolution. Calibration using the height of the recorded peak was accomplished with standard solutions analyzed via the same procedure. Reagent blanks were determined for each specific reagent combination used. Sample and reagent blank analyses were based on the average of 2 to 4 measurements. Water samples were preserved via the procedures recommended by EPA (11) and analyzed directly. Freeze-dried (9) F. C. Fehsenfeld,K. M. Evenson, and H. P. Broida, NBS Rep?. 8705 (1964). (10) W. Snelleman, T. C. Rains, K. W. Yee, H. D. Cook, and 0. 42,394 (1970). Menis, ANAL.CHEM., (11) Environmental Protection Agency, Standard Methods of Water and Waste Analysis, May 1971. 1322
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
blood and plant leaf samples ranging in size from 0.1 to 0.5 grams were digested in nitric acid with hydrogen peroxide addition and diluted to 50 ml with 3M nitric acid for analysis. Hair samples of 5 to 20 mg were rinsed in acetone, methanol, and distilled water and dissolved in 1 ml of nitric acid and diluted to 25 ml for analysis. For most analyses, a sample solution volume of 0.1 to 0.5 ml was adequate for analytical purposes, but was increased to an appropriate size for unusually low concentration samples. Atomic absorption analyses were carried out on a Perkin-Elmer Model 303 spectrophotometer equipped with a commercial cold vapor system similar to that described by Hatch and Ott ( I ) . RESULTS AND DISCUSSION The design and geometry of the reduction chamber shown in Figure 1 was evolved through experimentation to provide a dead volume which permitted the delivery of mercury to the plasma in a short time-Le., 1 minute or less. The concentration of mercury in the plasma per unit time is thus increased while still maintaining an evolution period adequate for a precise measurement and a reasonable sample capacity. The total volume of the reduction cell is approximately 25 ml with a nominal dead volume of 20 ml and a total sample capacity of approximately 15 ml. Because small sample volumes of 0.1 to 0.5 ml were typically adequate for analysis and because the amount of mercury contained in such samples was in the nanogram range, 10 to 20 samples could be analyzed before it was necessary to replenish the reducing solution and/or remove the accumulation of depleted sample solution. The anhydrone, placed between the reduction chamber and the plasma to remove water from the argon stream, required replacement on a daily basis. The argon flow rate affects the rate of delivery of mercury to the plasma as well as the excitation characteristics of the plasma. Thus it was necessary to optimize this parameter in terms of the magnitude of the recorded emission intensity. At low flow rates a broad peak is observed which is difficult to quantitate except in terms of peak area. The mercury evolution rate increases with argon flow rate until the mercury concentration in the plasma per unit time and the plasma excitation conditions are jointly optimized as determined by the observation of maximum peak height and area. Further flow rate increases decrease the residence time of mercury in the plasma with a concomitant decrease in excitation efficiency as evidenced by a reduction in the peak height and width. For the system used herein, an argon flow rate of 600 ml/min controlled to i 5 % maximized the analytical signal. Investigation of the effect of the microwave power input using a dc readout system indicated that the line minus background intensity decreases slightly over the 30- to 90-watt range while the line-to-background ratio exhibits a rather sharp drop. The latter is due primarily to increased background emission from molecular species such as CO, nitrogen compounds, and hydrocarbons which are residual impurities in the argon (6) and may also be derived from the sample solutions. The noise level observed is essentially constant over the power range investigated but increases below 20-25 watts. Consequently, an operating level of 30 watts was selected as optimum. The background problem mentioned above does not preclude the use of a dc amplification-readout system but the use of the derivative spectrometry method (IO) permits the measurement of a net line intensity and conveniently eliminates problems which may originate from variation of background contributing species from sample to sample.
Measurements of emission intensity along the length of plasma which is nominally 8 cm in length indicate that the intensity was maximized with an optical arrangement which sampled the central region over a one-centimeter length. Analytical curves obtained under the conditions above were linear over three orders of magnitude as previously reported (5). The measurement precision, as estimated by five replicate determinations on standards, ranges from 5 % (relative standard deviation) down to 2 at nominal signal-to-noise ratios of 20 and 100, respectively. The detection limit, estimated as the amount required to produce a signal twice the standard deviation of the background, is 6 X lo-” gram. Quantitative measurements on 10-ml samples containing 0.01 ppb mercury (10-10 gram) have been made with a precision of =t10-12 %. Data indicative of the accuracy of the method are given in Table 11. Solutions of each sample were prepared as outlined above and analyzed by both the emission and the cold cell atomic absorption techniques. Sample volumes of 0.2 ml were used for the emission analysis while 5-10 ml was required for the absorption measurements. Duplicate results generally agreed within 10 for both techniques indicating that the comparative results are consistent within experimental error. It should be noted that the emission analyses were carried out on amounts of mercury ranging from 0.34 to 12 ng (distilled water and leaf B samples, respectively), while the absorption measurements covered the absolute range of 17 to 60 ng. This is indicative of the relative analysis capabilities of the two techniques. Moreover, because it is reasonable to anticipate that the accuracy and precision of the
I 1
analysis will be largely determined by the magnitude of the emission intensity, it might be inferred that the accuracy should be approximately i10 % or better at lower concentration levels than those in Table I1 when larger sample volumes are used, Emission spectrometric analyses of water samples collected from remote mountain lakes and streams lend credence to this supposition. Results obtained on analysis of 30 such samples ranged from 0.07 to 1.8 ng/ml. Duplicate emission measurements made on 10-ml sample volumes consistently agreed within *lo% or better. Analyses of small samples of human hair also produced duplicate results that agreed within that increment. On the basis of these results, it may be concluded that the absolute sensitivity of the microwave excitation technique is approximately two orders of magnitude better than those reported by April and Hume ( 4 ) and by the users of the atomic absorption method (1-3). The technique permits the analysis of a variety of sample types at unusually low concentrations and the analysis of small samples where sample size is limited. The analyses appear to be accurate to * l o % or better. By using the background correction system (IO), the interference problem from nitrogen oxides associated with the cold cell absorption method (12) does not present a problem. RECEIVED for review November 15,1971. Accepted February 16, 1972. Research supported by NSF Grant No. GP-21306. (12) W. J. Adrian, A t . Absorption Newslett., 10,96 (1971).
CORRESPONDENCE
I
Determination of Trace Lead in the Atmosphere by Furnace Atomic Absorption SIR: The use of nonflame methods for atomic absorption has become increasingly widespread in the last few years as more applications of nonflame techniques are being published every month in the literature. Among these the graphite tube atomic absorption furnace as designed by Woodriff et al. has demonstrated its sensitivity and precision for a number of elements (1-5). The very great sensitivity resulting from the application of this instrument suggests its usefulness for the determination of elements in particulates in air samples. There are, at present, no sampling methods which are readily applied to nonflame AA determinations. Most filtration methods that might be employed would require ( 1 ) R. Woodriff and G. Ramelow, Spectrocliirn. Acta, 24B, 665 ( 1968).
(2) R. Woodriff and R. Stone, Appl. Opt., 7, 1337 (1968). ( 3 ) R. Woodriff. R. W. Stone, and A. M. Held, Appl. Spectrosc., 22, 408 (1968). 141 R. Woodriff. B. R. Culver. and K. W. Olson, ibid... 24,. 250 _
I
(1970). ( 5 ) R. Woodriff and D. Skrader, ANAL.CHEM., 43, 1918 (1971).
either an ashing step or a dissolution of the filtrate (6, 7). For most other techniques this does not introduce substantial errors. Furnace techniques, on the other hand, are so sensitive that small volumes of air can be used and small amounts of the element of interest are determined. Under these circumstances, errors of greater relative magnitude are introduced with pretreatment. Since no pretreatment step is necessary using the following method, these errors are eliminated. EXPERIMENTAL
Materials and Equipment. Air samples are filtered through a graphite crucible of the type used for carrier distillation ASTM No. S-3. The dimensions are roughly 16 mm long (6) M. Katz, “Measurement of Air Pollutants,” World Health
Organization, Geneva, 1969. (7) W. Leithe, “The Analysis of Air Pollutants.” Ann ArborHumphrey Science Publishers, Ann Arbor, London, 1970. ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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