(1)E. L. Wynder and D. Hoffmann, "Tobacco and Tobacco Smoke", Academic Press, New York, N.Y., 1967. (2)W. B. Wartman, Jr., E. C. Cogbill, and E. S. Harlow, Anal. Chem., 31, 1705 (1959). (3)J. R . Stokely and M. R. Guerin, presented at the 27th Tobacco Chemists' Research Conference, Winston-Salem, N.C., October 1973,No. 31. (4)P. Nettesheim. M. R. Guerin, J. Kendrick, I. B. Rubin, J. R. Stokely, D. A.
(12) A. H. Laurene, L. A. Lyerly, and G. W. Young, Tobacco Sci., 8, 150 (1964). (13)G. Herzberg, "Infrared and Raman Spectra of Polyatomic Molecules", D. Van Nostrand Company, Inc., New York, N.Y., 1945. (14)K. N. Rao, C. J. Hurnphreys, and D. H. Rank, "Wavelength Standards in the Infrared", Academic Press, New York, N.Y., 1966. (15)W. J. Jones in "Infrared Spectroscopy and Molecular Structure", M. Davies Ed., Elsevier Publishing Company, New York, N.Y., 1963,pp 1 1 1-165. (16) "Tables of Wavenumbers for the Calibration of Infrared Spectrometers",
Creasia, W. L. Maddox, and J. E. Caton in "Proceedings of the Tobacco Smoke Inhalation Workshop on Experimental Methods in Smoking and Health Research", National Cancer Institute, Bethesda, Md., June 1974, DHEW Pub. No. (NIH) 75-906,p 17. (5)J. 0.Lephardt and G. Vilcins, Appl. Spectrosc., 29,221 (1975). (6)G. Vilcins and J. 0. Lephardt, Chem. lnd. (London), 22,974 (1975). (7) G. Vilcins and J. 0. Lephardt, in "Recent Advances in Tobacco Science", Philip Morris, inc., 1976,pp 123-146. (8) "Towards Less-Hazardous Cigarettes. Report No. I-The First Set of Experimental Cigarettes", Gio B. Gori, Ed., National Cancer institute, Bethesda, Md., 1976,DHEW Publication No. (NIH) 76-905. (9)M. R . Guerin, W. L. Maddox, and J. R . Stokely in "Proceedings of the Tobacco Smoke Inhalation Workshop on Experimental Methods in Smoking and Health Research", National Cancer Institute, Bethesda, Md., June 1974, DHEW Pub. No. (NIH) 75-906,p 31. (IO)P. R. Griffiths, "Chemical Infrared Fourier Transform Spectroscopy", John Wiley and Sons, New York, N.Y., 1975,p 89. (1 1) J. R. Newsome and C. H. Keith, Tobacco Sci., 9,65 (1965).
RECEIVEDfor review September 17, 1976. Accepted November 11,1976. This research was sponsored jointly by the Council for Tobacco Research-USA, Inc., and the U.S. Energy Research and Development Administration under contract with Union Carbide Corporation. The purchase of the FTS-20 Spectrometer a t the University of Tennessee was partially supported by the National Science Foundation Research Instrument Grant GP-41711.
LITERATURE CITED
I
International Union of Pure and Applied Chemistry, Butterworth and Co., London, 1961. (17)R. S.Rasmussen and R. R. Brittain, J. Chem. Phys., 15, 131 (1947). (18)N. R. Draper and H. Smith, "Applied Regression Analysis", John Wiley and Sons, New York, N.Y., 1966.
CORRESPONDENCE
Determination of Mercury by Atomic Absorption Spectrometry with Graphite Tube Atomization Sir: The high volatility of mercury-even a t room temperature-results in mercury being a very difficult element to determine by electrothermal atomization atomic absorption: even the drying of a mercury solution will produce a significant loss of mercury. It is this high volatility of mercury, and its ability to produce a monatomic vapor, which made possible Woodson's ( 1 ) flameless atomic absorption determination of mercury in 1939, a determination which predated the classical work by Walsh ( 2 ) on atomic absorption with flame atomization. The determination of mercury by atomic absorption with an air-acetylene flame is not very sensitive [sensitivity for 1%absorption = 7.5 pg/ml(3)], and although the toxicity of mercury has been recognized for many years, it is only within the past seven that methods have become available which are capable of determining mercury reliably at the low levels encountered in biological and environmental samples. These methods can be traced to the work of Brandenberger and Bader ( 4 , 5 ) and Hatch and Ott (6); variations of the reduction-aeration continuous recirculation technique for the cold vapor determination of mercury, introduced by Hatch and Ott, are now employed in many laboratories. These highly sensitive techniques have led to a large volume of data on mercury levels. While offering considerable sensitivity, the cold vapor method for mercury is essentially unique for this element, and consequently would not be easily applicable to a multielement atomic absorption determination utilizing a heated graphite tube as atom cell (7). Additionally, for the determination of trace metals in urine by ashing and atomizing directly in the 330
graphite tube, the high volatility of mercury will preclude any ashing of the urine matrix. The chemical stabilization of mercury in a graphite tube has been described by Ediger (8).He reported that the addition of ammonium sulfide to mercury in nitric acid solution allowed ashing temperatures of up to 300 "C to be employed; a detection limit of 0.2 ng was achieved. Issaq and Zielinski (9) studied the determination of mercury in a graphite tube atomizer, and reported a 50X signal enhancement by the addition of hydrogen peroxide to aqueous mercury solutions. They found that ashing temperatures up to a t least 200 OC could be used, and postulated the formation of a stable peroxy-mercury complex. A correspondence by Owens and Gladney (IO)concluded that the presence of hydrochloric acid is desirable as an additional stabilizer when hydrogen peroxide is used to stabilize mercury in a graphite furnace. This correspondence reports our study on the chemical stabilization of mercury in the heated graphite tube by the use of hydrochloric acid and hydrogen peroxide. EXPERIMENTAL Apparatus. The atomic absorption spectrophotometer used was a Perkin-Elmer Model 300s equipped with background correction; absorbance values were recorded with an Oxford Instruments series 3000 potentiometric recorder (response time for full scale deflection 0.25 s). For flame atomization an air-acetylene flame was supported on a 10-cm single slot titanium burner, but for the principal part of this work the Perkin-Elmer HGA 74 electrothermal furnace was
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
Table I. Absorbance from 50 ng Mercury in Solutions Containing Various Chemical Additions 5.0 ppm Hg solution
Absorbance from 50 ng Hg
Aqueous 4% "03 4% H202 4% "03 + 4% HzOz 4% HC1 4% HCI + 4% HzOz 4% HC1 t 4% HNOs + 4% Hz02
0.007 0.007 0.007 0.007
025-
o
20
5 0 n g Hg in 2 % H 2 0 2
9
50ng Hg
I
0.045 0.12 0.12 I
employed as atom cell. The spectral source was a Hilger and Watts mercury hollow cathode lamp. Reagents. A 1000-ppm mercury stock solution was prepared by dissolving 0.6768 g mercuric chloride (AnalaR, Hopkin and Williams Ltd.) in 500 ml distilled water. Working standards were prepared by serial dilution as required. Hydrochloric (34-37% w/w) and nitric (69-71% w/w) acids were of Aristar (BDH Chemicals Ltd.) or Ultrar (Hopkin and Williams Ltd.) quality, and the hydrogen peroxide employed was 20 volume (6% w/v), general purpose reagent grade (Hopkin and Williams Ltd.). Procedure. The mercury 253.65-nmline from the hollow cathode lamp operated a t 8 mA was monitored over a spectral band width of 0.7 nm. A 10+1 Eppendorf micropipet with disposable polypropylene tips was used to introduce the sample solution into the graphite furnace. The HGA-74 program employed consisted of: initial drying at 105 O C for 30 s, and ashing at 155 O C for a further 30 s; a 10-s atomization at 2000 "C under "gas-stop" conditions, and a final heating at 2700 O C for 5 s. Argon was used as purge gas.
RESULTS AND DISCUSSION Initial experiments failed to reproduce the results of Issaq and Zielinski (9). The addition of hydrogen peroxide (4%v/v 20 vol.) to a 5.0-ppm mercury solution gave no enhancement of the mercury signal, neither did the addition of nitric acid (4%v/v concd "Os), nor a combination of nitric acid and hydrogen peroxide (4% v/v 4% v/v). Comparison of our experimental parameters with those of Issaq and Zielinski (9) showed the only real difference to be the mercury stock solution used. Reasoning that their, commercially prepared, mercury solution might contain hydrochloric acid, test solutions were made up in hydrochloric acid. The results are given in Table I. The addition of hydrochloric acid gave some enhancement of the mercury signal, and a combination of hydrochloric acid and hydrogen peroxide gave approximately 20X signal improvement, but hydrogen peroxide on its own appeared t o have no effect. In order to clarify the situation, further experiments were undertaken. The effect of hydrochloric acid concentration was studied by analyzing 1 0 - ~aliquots 1 of 5.0-ppm mercury solutions in 0-18% (v/v) hydrochloric acid. The experiment was repeated after the addition of 2 ml of 20 volume hydrogen peroxide to the 100 ml of each solution, and the resulting graphs are given in Figure 1. The possibility that the hydrochloric acid might contain appreciable amounts of mercury was considered ( I 1 ), but mercury could not be detected in water blanks containing 0-12% hydrochloric acid. The effect of hydrogen peroxide concentration was studied by analyzing 10-yl aliquots of 5.0-ppm mercury solutions in 8% (v/v) hydrochloric acid, the hydrogen peroxide content of which was varied between 0 and 4% (v/v 20 vol. solution). The results are shown in Figure 2. The chloride ion concentration in solutions of: (a) 5.0 ppm Hg, (b) 5.0 ppm Hg containing 2% (v/v 20 vol.) hydrogen peroxide, (c) 5.0 ppm Hg containing 6% (v/v) nitric acid, and (d) 5.0 ppm Hg containing 2% hydrogen peroxide and 6%nitric acid, was varied between 0 and 0.6 M by the addition of 0-40 ml8Yo (w/v) ammonium chloride solution. Ten-wl aliquots of each solution were analyzed, and the resulting graphs are given in Figure 3.
+
0
,
, 2
,
, , , , , , , , , , 4 6 8 10 12 1L % Dy volume hydrochloric acid
,
, 16
,
,
18
Figure 1. Effect of hydrochloric acid concentration on mercury atomic absorption signal in the absence and presence of hydrogen peroxide 0.20j
1.0
20
3.0
L0
YO by volume H202
Flgure 2. Effect of hydrogen peroxide concentration on mercury atomic absorption signal
w
0
,
a
0
r-, 0.075 0.15
0.225 0.30 0.375 0.L5 0.525 0.60 CHLORIDE CONCENTRATION I M
Figure 3. Effect of chloride ion concentration on mercury atomic absorption signal Solutions of mercury containing 4,8, and 12% HC1, each with 2% Hz02, were ashed a t temperatures varying from 60 to 440 "C. For temperatures over 105 "C, a dry-ash program was used: drying a t 105 "C for 30 s, and ashing a t the second temperature for a further 30 s. Atomization a t 2000 "C gave plots of absorbance vs. ashing temperature (Figure 4). The temperatures over the range 0-500 "C were determined separately by the use of a thermocouple, and were found to be in broad agreement ( f 5 0 "C) with the data supplied by the manufacturer (12). Varying the atomization temperature for a two-stage ash of 106 OW30 s, 155 "C/30 s, gave the plot shown in Figure 5 , for the absorbance from 50 ng mercury. A calibration graph for mercury in a 10%hydrochloric acid and 2% v/v hydrogen peroxide solution was linear up to 120 ng Hg, and a sensitivity of 1.65 ng and detection limit of 3.8 ng were achieved. Solutions of: (a) 100 ppm mercury (aqueous), (b) 100 ppm mercury in 2% (v/v) hydrogen peroxide, (c) 100 ppm mercury in 8% (v/v) hydrochloric acid, and (d) 100 ppm mercury in 2% hydrogen peroxide and 8%hydrochloric acid, were examined by atomic absorption with atomization in an air-acetylene flame. Each solution gave the same absorbance value. ANALYTiCAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
337
Table 11. Effect of Varying pH with HzOz and "03 Hg Signal
0,28{
o'2Ll I
Sample 120 ng Hg, 6% HC1 120 ng Hg, 6% HCl, 2% HzOz 120 ng Hg, 6% HC1, HN03 120 ng Hg, 10%HC1 120 ng Hg, 10%HCl, 2% Hz02 120 ng Hg, 10%HC1, HNOs
0.20 W
pH
Y 0.162I: I
am 0.126
on
Absorbance
0.90
0.18
0.60
0.23
0.60 0.40
0.14
0.21 0.24 0.19
0.30 0.30
0.080,oq +,
60
Table 111. Mercury Determinations in Presence or Absence of HCl Gas at the Ashing Stage
I
110 155 210 280 355 TEMPERATURE I 'C Flgure 4. Effect of furnace ashing temperature on mercury atomic absorption signal in (a) 4 % , (b) 8 % , and (c) 12% hydrochloric acid
Absorbance HC1 gas No HC1 gas
Sample 10 p1 H20 10 p15.0 ppm Hg 10 pl5.0 ppm Hg, 2% H202 10 pl5.0 ppm Hg, 2% Hz02,8% HC1
i
0.01 0.15 0.15 0.26
0.005 0.005 0.005 0.14
Lu
5012 CT
50ng Hg in 8% HCI + 2%H202
Table IV. Mercury Determinations with Two Different Oxidizing Agents 69 110 138 16L 182 195 212 232 270 TEMPERATURE I 1 0 OC
Figure 5. Effect of furnace atomization temperature on mercury atomic absorption signal
Mechanism of Enhancement. It is difficult to envisage the formation of a peroxy-mercury complex, as suggested by Issaq and Zielinski ( 9 ) ,which would be stable in the strongly reducing atmosphere present in a hot graphite tube. I t was a t first thought that the hydrogen peroxide might, by reducing the Hg(I1) to Hg(I), be enhancing the measured absorbance, as has been observed in the atomic absorption determination of mercury with flame atomization (13,14). The fact that the observed enhancement occurred only in the graphite tube atomizer and not in the flame appeared to negate this argument. Additionally, when ascorbic acid was added to dilute solutions of HgC12, no signal enhancement was observed for graphite tube atomization. The facts which emerge from this study are that the mercury stabilization is effected by a combination of acid and chloride ion, i.e., hydrochloric acid or hydrogen chloride, and the presence of hydrogen peroxide causes a further enhancement, though hydrogen peroxide on its own will not effect the stabilization. The hydrogen peroxide enhancement was not a simple p H effect, as the data in Table I1 show. The amounts of hydrochloric acid and hydrogen peroxide required to effect the enhancement of the mercury signal are too large to postulate the formation of any kind of stoichiometric complex, and it seems likely that the enhancement/ stabilization is due t o the conditions present in the vapor phase in the furnace. The conditions are that HC1 gas is present in excess, and it is possible that mercury atoms will be held in the gas phase by the formation of HgC12. I t is interesting to note that the maxima of the ashing profiles (Figure 4) are near to the constant boiling temperature of hydrochloric acid (15).By passing HCl gas into the furnace, via the argon inlet, during the ashing stage, the results given in Table I11 were obtained. The data in Table I11 tend to support the gas phase theory vs. the formation of a complex in solution, but the role of the peroxide is not clear. If the peroxide is acting in its capacity 338
Sample
Absorbance
10 p15.0 ppm Hg 10 ~ 1 5 . ppm 0 Hg, 6% HCl 10 p15.0 ppm Hg, 6% HC1,2%H202 10 p15.0 ppm Hg, 6% HCl, 2%
0.005 0.06
ammonium persulfate 10 p15.0 ppm Hg, 6% HC1,2% ammonium persulfate AgNOB
+
0.16
0.05 0.05
of an oxidizing agent, then other oxidizing agents would be expected to effect the enhancement. The effect of ammonium persulfate was tested, in both the absence and presence of silver nitrate catalyst; the results are given in Table IV. The variation in the absorption maximum with change in the hydrochloric acid concentration, shown in Figure 4, is a possible cause of the discontinuities in the enhancements shown in Figure 1. CONCLUSION In the determination of mercury by atomic absorption spectrophotometry with graphite furnace atomization, it is possible to decrease the loss of mercury during the drying and ashing steps by the presence of hydrochloric acid. The formation of HgClz by the high concentration of HC1 gas in the hot graphite tube reduces the loss of Hg atoms, and results in a considerable enhancement of the Hg signal. Hydrogen peroxide effects a further enhancement, though this is neither a reducing, oxidizing, nor a p H effect of the hydrogen peroxide. I t is possible, however, that an HgC12-Hz02adduct, of higher thermal stability than HgC12, is formed. Hydrogen peroxide alone will not stabilize simple solutions of mercury in the hot graphite tube; we attribute the results of Issaq and Zielinski (9, 16) to failure to recognize the presence of hydrochloric acid in commercially prepared standard solutions. We should also like to point out that, if efficient background correction is employed, any peaks observed during drying or ashing steps must be due to mercury atomic absorption. ACKNOWLEDGMENT We thank T. S. West for his interest in this work, and for helpful suggestions.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
LITERATURE CITED
(15) "CRC Handbook of Chemistry & Physics", 534 ed., 1972-73, p 6-95. (16) H. J. issaq and W. L. Zieiinski, Anal. Chem., 48, 787 (1976).
(1) T. T. Woodson, Rev. Sci. Instrum., IO, 308 (1939). (2) A. Walsh, Spectrochim. Acta, 7, 108 (1955). (3) Perkin-Elmer, Norwalk, Conn., "Analytical Methods for Atomic Absorption Spectrophotometry", 1973. (4) H. Brandenberger and H. Bader, Helv. Chim. Acta, 50, 1409 (1967). (5) H.Brandenberger and H. Bader, At. Absorpt. Newsl., 7, 53 (1968). (6) W. R . Hatch and W. L. Ott, Anal. Chem., 40, 2085 (1968). (7) J. F. Alder, A. J. Samuel, and T. S.West, submitted for publication in Anal. Chim. Acta. (8) R. D. Ediger, At. Absorpt. Newsl., 14, 127 (1975). (9) H. J. issaq and W. L. Zielinski, Anal. Chem., 46, 1436 (1974). (10) J. W. Owens and E. S.Giadney, Anal. Chem., 48, 787 (1976). (11) E. 8. Sandell, "Colorimetric Determination of Metals", Interscience, New York, 1959, p 625. (12) Description and Operating Instructions, HGA 74, Bodenseewerk PerkinElmer & Co., GmbH, Uberlingen, 1975. (13) D. M. Hingle, G. F. Kirkbright, and T. S. West, Analyst(London), 92, 759 (1967). (14) R. F. Browner, R. M. Dagnail, and T. S.West, Talanta, 18, 75 (1969).
Exchange of Comments:
J. F. Alder* David A. Hickman' Department of Chemistry Imperial College of Science & Technology London, SW7 2AY England Department of Chemistry, Brookhaven N a t i o n a l Laboratory, Upton, N.Y. 11973.
RECEIVEDfor review August 6,1976. Accepted October 28, 1976. We are grateful to the Home Office Central Research Establishment, Aldermaston, for the provision of a research bursary (DAH); during the period of this work one of the authors (JFA) was the holder of an IC1 Fellowship.
Signal Flicker Noise and Noise Power Spectra
Sir: I have read the recent article by Talmi et al. ( I ) with great interest. Clearly this type of characterization of spectrometric sources is useful and necessary to make logical judgments about instrumental design and optimization of instrumental variables. However, I feel that some statements made about the data may be misleading and require clarification. On page 326, the authors state that "[The Poisson statistical] . . .behavior of PMTs is a direct result of their quantum efficiency being less than one. Thus in uv-visiblespectrometric studies, in which the P M T is operated in the dc mode (not saturated and not as a photon counter) the signal to noise ratio (S/N) is proportional to the square root of the signal regardless of whether the detector or the spectrometric source is the dominant noise source". The first sentence is not totally true because even if the quantum efficiency was one, the output would follow Poisson statistics if the input photon flux was Poisson. Because the guantum efficiency is less than one, the mean anodic photoelectron pulse rate is less than the photon rate incident on the photocathode. However, the Poisson nature of the incident flux is still retained, which has been proved mathematically by Fried ( 2 ) . Even if we consider a hypothetical noiseless photon flux (photons equally spaced with respect to time), the number of anodic pulses per unit time with a quantum efficiency less than one would follow a binominal distribution. The binomial distribution would approach the Poission distribution only if the quantum efficiency was very small. Of course, one must also consider the effect on current measurements of the statistical nature of secondary emission which follows a Polya statistics ( 3 ) . The basic idea expressed in the second quoted sentence in reference 1and used in other places in the paper is incorrect in certain situations. Noise from the spectrometric source (i.e., noise related to the magnitude of the analytical signal) can be divided into two types ( 4 ) :Signal shot noise and signal flicker noise. Signal shot noise is proportional to the square root of the signal and its noise spectrum is flat. Signal flicker noise is directly proportional to the signal. Its noise power spectrum may be white, exhibit l/f character, or exhibit higher magnitude at specific frequencies. Because of the difference in the dependence of the noise magnitude on signal level, signal shot noise will be dominant under low light level situations, whereas signal flicker noise will be dominant under high light
level situations. Since often signal flicker noise exhibits l/f character, it is likely to be more significant when measurements are made at frequencies of 1Hz or lower. It is felt that the importance of signal flicker noise (noise not proportional to the square root of the signal) has been underestimated in reference 1 since its significance in common spectrometric measurements has clearly been demonstrated in the literature (5-8). We have observed significant signal flicker noise in typical molecular absorption (91, molecular fluorescence, flame atomic absorption (IO), and flame atomic emission measurements. From a practical point of view, it is vital to realize that signal flicker noise exists. If measurements were always limited by signal shot noise so that the S/N was proportional to the square root of the signal, then the S/N could always be improved by changing instrumental variables to increase the magnitude of the signal. However, at higher light levels, the signal flicker limit is reached and the S/N is independent of the signal. The authors use their data to show that the signal shot noise limit applies in certain situations. In Figure 3, the slope of 0.5 in the log-log plot of noise vs. photocurrent indicates that these experiments are signal shot noise limited and Poisson statistics apply. However, other data in the paper show that signal flicker noise is present in addition to signal shot noise. Since for most measurements, the same PMT and bias voltage (Le., photomultiplier gain) were employed, the relative standard deviation (Le., the relative rms noise) at a given photoanodic current should be the same from experiment to experiment if only signal shot noise was important. Clearly from the data (e.g., Tables 111-V) this is not the case. In Figure 3, the noise from the dc arc is an order of magnitude larger than for other sources at the same photoanodic current or light level, and on page 329, the noise level for the argon microwave plasma was noted to be twice as high as for primary sources at signal levels of 5 X loT8A. The S/N is proportional to the square root of the signal in Figure 3 because measurements were made under low light level conditions where the S/N is not greater than one hundred. At higher levels, where signal flicker noise becomes more important, the slope in Figure 3 would decrease and approach a constant value, as has been experimentally verified (11). On pages 331-333 in reference 1,comments are made about the lack of signal flicker noise of l/f noise in many of the sources. The data presented do not necessarily indicate that
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
339
