tent with previous observations which indicated a variation of only 0.4% in relative silicon sensitivity for a change in pH from 1.1to 0.4. The success of the AAS method for glasses also applies to a variety of other materials including clays, refractories, feldspars, and limestone. This is illustrated in Table VI for eight certified NBS materials. Each of the materials was analyzed in duplicate or triplicate, with samplings taken a t time intervals of a week to a month. Replicate analyses for each material were obtained using standard solutions prepared from different stock batches and instrumental data collected on different days. Again $he mean error is better than 0.3% absolute. Table VI also allows the estimation of confidence limits for the AAS method through the employment of statistical analysis of paired samples according to Bennett and Franklin (19). By choosing pairs of results (taking the most deviant pair in the case of a triplicate), the estimate of precision is calculated to be about 0.37%absolute at the 95% confidence level and 0.54% absolute at the 99% confidence level for the range of 14 to 94% original silica content. The calculation of confidence limits assumes that the standard deviation of % Si02 is constant over the range of silica contents indicated in Table VI. The standard deviations quoted in this table lend support to the assumption. The average standard deviation for the five samples containing more than 40% Si02 is 0.14, which is comparable to an average standard deviation of 0.15 for the three samples containing less than 40% silica. The average of 0.14 agrees reasonably well with the estimates of standard deviation based on Equation 2 for samples of medium and high silica content. However, the average of 0.15 for samples of lower silica content is much higher than that predicted by Equation 2. Although the data are limited, they do suggest, a t least for the samples under consideration, that indeterminate errors tend to progressively discriminate against samples of decreasing silicon content. Errors associated with heterogeneous distribution of silica and with chemical in-
terference are, indeed, likely to increase as the silica level decreases. CONCLUSIONS The AAS method for silica is precise, accurate, and handles a wide variety of materials. Moreover, the method represents a 60 to 70% time saving over gravimetric analysis, and its basic simplicity extends the range of sample types that can be routinely analyzed without the special effort often required in gravimetric analysis. ACKNOWLEDGMENT The authors thank G. A. Machajewski for his comment,s and suggestions, Y.-S. Su for his information on ASTM glasses, and B. A. Swinehart and the Mathematical and Statistical Analysis Department of Corning Glass Works for their statistical help. LITERATURE CITED J. H. Medlin, N. H. Suhr, and J. B. Bodkin, At. Absorpt. Newsl., 8, 25 (1969). J. W. Yule and 0. A. Swanson, At. Absorpt. Newsl,, 8, 30 (1969). J. C. VanLoon and C. M. Parlssis, Analyst (London).94, 1057 (1969). P. L. Boar and L. K. Ingram, Analyst (London),95, 124 (1970). S. H. Omang, Anal. Chim. Acta, 48, 225 (1969). K. Govindarajuand N. L'homel, At. Absorpt. Newsl., 11, 115 (1972). R. J. Guest and D. R. MacPherson, Anal. Chim. Acta, 71, 233 (1974). F. J. Langmyhr and P. E. Paus, Anal. Chim. Acta, 43, 397 (1968). B. Bernas, Anal. Chem., 40, 1682 (1968). H. W. Knudson, C. Juday. and V. W.Meloche, lnd. Eng. Chem., 12, 270 (1940). J. D. H. Strickland, J. Am. Chem. Soc., 74, 868 (1952). C. 0. Ingamells, Anal. Chim. Acta, 52, 323 (1970). J. C. VanLoon and C. M. Parissis, Anal. Lett., 1, 519 (1968). N. H. Suhr and C. 0. Ingamells, Anal. Chem., 38, 730 (1966). E. Richardson and J. A. Waddams, Research (London),7, 542 (1954). L. Shapiro, J. Res. U.S. Geol. Surv., 2, 357 (1974). C. 0. Ingamells, Anal. Chem., 38, 1228 (1966). G. B. Alexander, J. Am. Chem. Soc., 76, 2094 (1954). C. A. Bennett and N. L. Franklin, "Statistical Analysis in Chemistry and the Chemical Industry", Wiley, New York, 1954, pp 171-177.
RECEIVEDfor review May 7,1975. Accepted September 15, 1975.
Evaluation of Sample Pretreatments for Mercury Determination Robert Litman, Harmon L. Finston, and Evan 1.Williams City University of N e w York, Chemistry Department, Brooklyn College, Brooklyn, New York 11210
Losses of ionic mercury have been observed during both digestion (up to 35%) and lyophilization (up to 8 0 % ) of iaboratory and environmental samples. in addition, high rates of adsorption onto polyethylene glass and Teflon surfaces have been measured at mercury [as Hg( NO&] concentrations of less than 1 ng/mi. Both these observations are consistent with a mechanism of reduction of Hg(ii) to the metal. In view of these observations, a method of anaiysls which minimizes sample handling is recommended.
The accurate determination of trace concentrations of mercury is a major analytical problem in view of the growing awareness of mercury and its consequences in the environment. Organomercury pollution which can result from bacterial activity ( I ) , or pesticide runoff, is of special importance because the concentration at which these compounds assume significant toxicity is one tenth that of ionic 2384
mercury. It is difficult at present to test for specific mercury compounds at the low concentrations encountered in environmental samples to g of mercury per gram of sample). Consequently, more emphasis has been placed on the determination of total mercury in environmental samples. Most modern methods of mercury analysis require sample preconcentration, digestion, or both, and subsequent determination by comparison with standards. Since the final state of the sample before analysis is often a solution, recent findings that mercury, at low concentrations, is lost because of absorption on various surfaces is of significance in the determination of mercury concentration. We have inferred from the findings of Feldman and Rook (2, 3) that adsorption of mercury onto these surfaces may be due to reduction of mercury to the metal. In experiments by these authors, mercury was maintained as Hg(I1) in strong oxidizing media such as dichromate or tetrachloroaurate(II1).
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
Hence, storage of mercury solutions of low concentration, even for short periods of time, without such stabilizing agents, may lead to significant adsorption. Adsorption is of particular importance in the various wet-digestion procedures (4, 5), now used prior to analysis, which typically dilute the mercury concentration by a factor of 100. A sample with a relatively high initial mercury concentration such that there is no significant loss due to adsorption is transformed upon 100-fold dilution into a sample for which there may be significant loss by adsorption. Rains and Menis (6) have demonstrated that loss of mercury during wet digestion can be as high as 30%. They were able to eliminate this loss by packing the condensing column with Raschig rings, which suggests that a possible mechanism is entrainment (7). Another approach is lowtemperature wet-ashing (30 " C ) in a persulfate-permanganate medium as suggested by Snell (8). We find that the most satisfactory procedure is the dry-combustion technique of Rook et al. ( 3 ) ,in which the sample is burned in a stream of oxygen and the combustion products are collected in a liquid-nitrogen condenser. Lyophilization is a common pretreatment which facilitates storage and helps concentrate the sample. Losses of mercury during lyophilization have been reported by Pillay (9). In contrast to this, La Fleur (10) and Friedman et al. (11) have reported that there is no loss in their samples during lyophilization. Losses of mercury from solutions of natural waters a t atmospheric pressure have been reported by Mackay (12). [Subsequent to submission of this manuscript to ANALYTICALCHEMISTRY,a study demonstrating loss in lyophilization of natural waters by Harrison, LaFleur, and Zoller appeared in this journal (13).] Heretofore, there has been no comprehensive study reported of the extent of loss in different sample media, a t different mercury concentrations, and at different pressures. We have carefully studied all the above procedures and have observed losses due to adsorption and losses which occur during the evaporation or sublimation of water from the samples. A method which obviates sample pretreatments and eliminates these possible causes of mercury loss is instrumental neutron-activation analysis, employing high-resolution y-ray spectroscopy as used by Friedman (11)and Finston (14). The preferred technique (11)is to count the four lg7Au x rays from the electron-capture decay of lg7Hgseparately and the mercury concentration is determined from the total count in each peak separately and also from the summed counts in all the peaks giving 5 values for the mercury concentration. Any discrepancy in the results of these determinations indicates the presence of interfering activities, in which case the combustion procedure of Rook, or some equally reliable radiochemical procedure, must be used to isolate the mercury. EXPERIMENTAL Tracer Solutions. A solution of 203Hg(N0& (2 mCi, ICN Isotopes Division) was diluted to 1 liter with 1 N nitric acid and 1.72 mg of H g ( N 0 3 ) ~was added to make the final mercury concentration 1.5 fig/ml. This solution was the stock solution for both adsorption and lyophilization studies. A solution of 59FeC13 (1 mCi, ICN Isotopes Div.) was diluted to 1 1. with 0.5 A4 HC1 and 1.02 mg Fe wire (previously dissolved in the concentrated HC1); final iron(111) concentration was 1.09 pg/ml. This solution was used only in the lyophilization experiment. Standards Preparation. Two sets of Hg standards were prepared; one by dilution of the stock solution with 1 N nitric acid and the other by dilution with deionized water. Aliquots of these diluted solutions were transferred to 10-ml glass volumetric flasks and sealed with snap caps and Parafilm. These flasks were counted and then reserved for subsequent adsorption studies. Standards
for activation analysis were prepared in the same manner; after the dilutions were made, 100-fil aliquots were sealed into quartz vials. Samples. Samples of fish and invertebrates were collected from Lower New York Bay. Sediment samples were taken from the area of the Atlantic Ocean just south of Atlantic Beach (New York Inner Continental Shelf). National Bureau of Standards Reference Materials 1571, 1577, and 1630 were used as obtained. In addition, portions of each sample, including the Standard Reference Materials, were lyophilized to determine the moisture content in order to correct the weights of irradiated samples to dry weight. Heckers unbleached flour was found to contain no mercury and was used, as available, as a representative matrix for digestion and lyophilization studies. Reagents. Nitric and sulfuric acids, mercuric nitrate, and all reagents necessary to prepare the samples were Baker Analyzed Reagents. Dimethylmercury was obtained from Eastman Organic Chemicals. All reagents were used as received without any further purification. Adsorption Studies. Procedure I . Tracer solutions of mercuric nitrate 1 N in nitric acid, were maintained in sealed volumetric flasks for various periods of time. Each flask was then weighed, vigorously shaken, counted, drained, reweighed, and finally recounted. Procedure II. This adsorption procedure employed large-volume flasks (made of Teflon, polyethylene, and glass) each of which contained 0.5 1. of mercuric nitrate tracer solution 1 N in nitric acid. Small aliquots (5 ml) of each solution were removed periodically (following vigorous agitation of the container) during the next three weeks and counted. Digestions. Wet Digestions. Simulated samples for digestion consisted of 1- to 2-gram samples of flour plus varying amounts of mercury tracer, corresponding to a total mercury content from 0.01 to 1 fig. Concentrated nitric acid was first added, followed by very slow addition of sulfuric acid to minimize the evolution of N204 fumes. A total of 20 ml of each acid was added and the final temperature was allowed to reach 60 "C. After the digestion, the clear solution was quantitatively transferred into a 100-ml volumetric flask, diluted to the mark with deionized water, and aliquots were counted. In the second digestion procedure, 5 to 10 ml of concentrated nitric acid was added to the simulated sample and the mixture stirred a t 0 "C for 5 minutes before careful addition of 20 ml of concentrated sulfuric acid and 15 ml of concentrated nitric acid. During the entire procedure, the reaction temperature was not allowed to exceed 30 "C. The post-digestion procedure was followed as in the above experiment. A third procedure was performed on an actual environmental sample using the permanganate-persulfate procedure ( 8 ) and in this case aliquots of the sample, the initial, and the final digestion mixture were compared for mercury content by neutron-activation analysis. Dry Combustion. The dry-combustion procedure ( 3 ) was applied to laboratory standards, sediment samples (wet and lyophilized) and NBS Standard Reference Materials following neutron irradiation: and also to tracer-doped flour. Approximately 30 mg of mercuric oxide was added to the sample as carrier, prior to combustion in the 0 2 stream. The combustion tube was flamed to red heat, in order to ensure that all the volatile oxidation products were flushed into the liquid-nitrogen condenser. These combustion products were then washed into a 50-ml flask (using 10 ml concentrated nitric acid) and warmed on a hot plate to be certain that all the mercury was dissolved. The excess acid was neutralized with ammonium hydroxide, the mercury precipitated as the sulfide using thioacetamide, and the precipitate was collected on a Millipore filter. Lyophilization. General Procedure. A VirTis lyophilization unit was used exclusively. There was a liquid-nitrogen cold trap for each individual sample in addition to the main trap of the unit which was maintained a t -50 "C. Samples were initially frozen in Dry Ice-isopropanol or in liquid nitrogen, connected to the unit, the pressure was reduced, and the samples were allowed to warm to room temperature over a period of 6 to 24 hours. A pressure of 0.1 Torr, as measured with a McLeod gauge, was established for samples which were subsequently irradiated. The gauge was disconnected after pressure measurement. Liquid Samples. Solutions of 203Hg(N03)2with carrier, of concentration from 1.5 fi/ml to 4.5 ng/ml, were lyophilized a t pressures ranging from 1.0 to 0.01 Torr, to determine if pressure affected the amount of mercury lost during a 6-8 hour lyophilization. A sample of synthetic seawater was prepared to conform to the
ANALYTICAL CHEMISTRY, VOL. 47,
NO. 14,
DECEMBER 1975
2385
j
1 1
40
I20 nwr,
Figure 1. Adsorption of
2bb 01
mercury onto glass from aqueous solutions 1
N in nitric acid. ( A ) 0.375fig/ml. (6)0.0151 fig/ml. (00.0003 fig/ml
ion concentration reported by Horne ( 1 5 ) with sufficient mercury tracer and carrier added to bring its concentration to 0.015 fig/ml. This sample was lyophilized in the same manner as described above. Solid Samples. Portions of flour were added to each of two round-bottom flasks followed by addition of 50 ml of deionized water to make a slurry. A freshly prepared solution of dimethylmercury (250 ml of a 1.46 X lo-' ppm solution total mercury = 36.5 fig) was added to one flask, and to the other was added mercuric nitrate (2.0 ml of a 9.13 ppm solution, total mercury, 18.26 fig) and 248 ml of water. Both flasks were stoppered and stirred for four hours at room temperature to assure homogeneity. The samples were filtered and aliquots taken of the filtrate and wet flour prior to lyophilization, and of the dried flour, and the material caught in the trap after lyophilization. Irradiation and Counting. Neutron Actiuation. Samples were sealed in quartz tubes, whose dimensions were 50 mm by 3 mm i.d. Possible loss of mercury in the sealing process (using an oxygenmethane torch) was checked by the addition of tracer solution t o 70% of the capacity of the quartz tubes, the open end of which was sealed with Parafilm and then the tubes were counted. The tube was then sealed with the torch without freezing the sample in liquid nitrogen, and recounted. Samples which were scorched during this procedure were not used in subsequent analysis. Instrumentation. Irradiated samples were counted with both 0.1% and 7% efficient Ge(Li) detectors and also with an intrinsicgermanium detector (for high-resolution, low energy spectra) and the data recorded with a Northern Scientific multichannel analyzer (model 630). Tracer samples were counted with a 7.6 cm X 7.6 cm NaI(T1)well detector and a single-channel analyzer. Irradiations. All irradiations were performed in the reflector region of the Brookhaven National Laboratories High Flux Beam Reactor in a thermal-neutron flux of 1 X l O I 4 n/cm2-sec.Samples were allowed to decay for one day prior to counting. RESULTS A N D DISCUSSION Adsorption. Good linearity of our standards as prepared from the tracer solution, by dilution both with 1 N nitric
acid and with deionized water, was observed. Thus, the linearity of the calibration standards is not affected by adsorption in the pH range between 1 and 4. However, if the mercury loss during the preparation of each of the successive concentrations was proportional to that concentration, the resultant curve could also yield a straight line, but displaced to reflect a lower specific activity. In order to show that this was not the case, we examined the adsorption of mercury onto glass surfaces as a function of time as seen in Figure 1. Using the first method of adsorption study, if no adsorption occurs, the amount of mercury left in the flask after draining will be directly proportional to the amount of solution left in the flask after draining. Solutions of concentration from 0.375 to 0.0003 pg/ml of mercuric nitrate 2368
0
110
5
10
15
20
25
30
Time (days1
cont1ct
Figure 2. Adsorption of mercury onto polyethylene and Teflon from aqueous solutions 1 N in nitric acid ( A ) Teflon, 1.5 fig/ml.(6)Polyethylene, 3.41 fig/ml. ( C ) Teflon, 0.15 fig/ml. (0) Teflon, 0.015 fig/ml. (6Teflon, 0.0015 fig/ml. ( F ) Polyethylene, 0.341 fig/ml.(G)Polyethylene, 0.068 fig/ml. Number of recorded counts for the 80-
lutions ranged from 10000 to 800000 total
showed that, in the first few hours after preparation, no adsorption takes place whereas, after a period of 1 week, the activity remaining in the drained flask is as much as 70 times greater than corresponds to the amount of solution left in the flask. From Figure 1, we can therefore conclude that adsorption occurs a t all concentrations but, because the number of available adsorption sites are limited, the effect is not noticeable at high concentration. Therefore, the percentage of mercury lost increases as the concentration decreases. I t is also shown that negligible adsorption takes place within the first few hours of contact which is greater than the time necessary for preparation of standards. The former of these two premises is further supported by the following adsorption study. In this second experiment, a container (Teflon, polyethylene, or glass) was used to store the mercury solutions for periods of 2-3 weeks. The amount of mercury adsorbed on the container walls reaches a maximum value as indicated by a decrease to constant tracer activity in the successive aliquots. At concentrations of mercury above 1pg/ml it was observed that the extent to which adsorption occurred was insignificant compared to the concentration of the solution. As can be seen in Figure 2, the least amount of mercury is adsorbed by Teflon, a greater amount by glass, and the most by polyethylene. We attempted to reduce the amount of mercury adsorbed onto glass by washing several new volumetric flasks with acid-dichromate solution. However, we added no dichromate to the solutions to be studied for adsorption. We observed that after one week, there was a maximum of only 12% adsorption a t the lowest mercury concentration. It is known that chromium is strongly adsorbed by glass surfaces (16), from which we infer that chromium (either as dichromate or as the reduced chromic ion) occupies the available adsorption sites on the surface. During this study, we found that only 60% of adsorbed mercury is removed after several washings with concentrated nitric acid. Several washings with acid-dichromate solution were necessary to completely remove the adsorbed mercury. Digestion. The classical methods of wet digestion (4, 5) which are still widely used, suffer from two inherent disadvantages: the concomitant dilution of the initial mercury concentration in the sample and the addition of large quantities of reagents which themselves contain trace mercury.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
Table 11. Lyophilization of Environmental and Laboratory Samples
Table I. Loss of Mercury During Destruction of Organic Material Method
Wet ashingc Wet ashingc Wet ashingd Wet ashingd Wet ashingd Permanganatepersulfate Dry ashinge
Maximyn temp, C
Mercury concn, Wmla
Mercury lost, %b
60 60 30 30 30
0.0075 0.075 0.75 0.075 0.0037
34.6 27.4 5.6 4.3
60-70 800
1.0 0.5
34 0 0 0
0.01 0.001
1.1
a This column represents the mercury concentration which would be present in the digestate assuming no loss. b Variance among replicate samples was less than 5%.c Wet ashing: Procedure of the Association of Official Analytical Chemists. d Digestion flask was cooled in an ice bath during reagent addition otherwise same as c. e Ref. 3.
Complete digestion of 1g of sample typically requires 60 to 80 g of acid. If these acids contain an average mercury concentration of 1 ng/g, there is a reagent contribution of 60 ng of mercury. If the sample itself contains only 10 ng of mercury, the determination requires subtraction of a very large reagent blank, resulting in large uncertainties in the final value for the mercury Concentration. It is also observed that there is a 3-5% loss, due to adsorption onto the digestion flask. The digestion of an actual environmental sample was monitored by removing aliquots of the digestion mixture at the beginning and a t the end of the digestion, and determining the amount of mercury in each by activation analysis. Our results show that during the digestion, 50% of the mercury was lost from a sample, which initially contained 10 pg Hg (1pg of which was the reagent contribution). Another factor which must be taken into account in the digestion procedure is possible loss of mercury by entrainment in the volatilized water; a loss not due to the volatility of the mercury salts. The results obtained by Menis (6)as well as our own results (summarized in Table I), suggested to us the possibility of loss by entrainment. Since lower digestion temperatures result in less water volatilized, this would favor less mercury loss due to entrainment, as we have shown. However, we performed digestions of flour samples following the same procedures using iron tracer (59FeC13with carrier), a t temperatures as high as 100 OC and found no loss of iron from the solution. In fact, no traces of iron activity were found in the open condenser a t any time, suggesting entrainment is not the reason for loss, Improved methods of wet digestion have been proposed (6, 17) which minimize or eliminate mercury loss but retain the inherent disadvantages of any wet digestion. The drycombustion procedure of Rook (14) was tested using tracer-doped flour a t several mercury concentrations. As shown in Table I, quantitative recovery was obtained. As used by us in conjunction with neutron activation analysis, this method does not suffer from the aforementioned disadvantages of wet digestion. Lyophilization. The effects of lyophilization on the loss of mercury reported in the literature (9-11) appear to warrant further investigation. In general, what can be said is that loss of mercury during lyophilization is possible; however, the loss may be dependent upon concentration, and it cannot be assumed that a variety of samples will behave identically under similar conditions.
Sample
Hg c o n t e n t (rug/g) Water c o n t e n t , Before lyAfter ly- Hg lost, % ophilization ophilization.
0.440 0.202 54 (CH,), Hg (on flour) . . . ... 0.0869 0.0470 45 Hg(NO,), (on flour) 0.213 8 Sediment No. l b 34 0.235 Sediment No. 26 20 0.126 0.0973 22 0.424 31 Sediment No. 3 b 44 0.615 2.5 71 Squid 78 8.8 74 7.8 2.3 70 Butterfish 5.1 59 Sea Cucumber 89 12.4 UUncertainty of replicate samples is 27% maximum. bMercury concentrations determined by x-ray counting following dry combustion of neutron-irradiated samples. ~-
A comparison of the vapor pressure of water (16)and that of dimethylmercury (18)at, -50 "C shows that the vapor pressure of the dimethylmercury is 23 times greater than that of water. Since dimethylmercury is known to form as a result of bacterial action in sediment of aquatic systems (I), we could expect this and other compounds of similar volatility to be lost in this process. The data in Table I1 demonstrate a definite loss of this compound during lyophilization of our fabricated laboratory samples. It is of interest to note that there are also large losses of ionic mercury, the volatility of which is negligible as compared with that of water. A possible explanation for this observation was the entrainment of mercury during the sublimation of the water from the sample. The higher the mercury concentration, the greater the quantity of mercury lost, but, for the lower concentrations, the loss is a larger fraction of the initial amount. This observation casts doubt that entrainment of mercury in water vapor was taking place. If this is indeed the mechanism for loss, this effect should also be observed for other elements such as iron. When a solution of 59FeC13,in 0.5 M HC1, was lyophilized, we obtained quantitative recovery of the iron in the sample container. This result clearly shows that entrainment is not the problem. Another possible mechanism for this loss is reduction of mercuric ion to the element, which can then be volatilized. M This is feasible for a mercury concentration of 5 X (1 ppm) and an N204 concentration of 1 X M , at pH 2, as calculated from the Nernst equation. Nitric acid and water form a low-boiling azeotrope, which means that, a t the low pressure used in the experiment, nitric acid would be lost leading to an increase in the pH of the lyophilized medium. The loss of mercury is consistent with this hypothesis because the lower the mercury concentration, the easier it is to keep it in the f 2 oxidation state, leading to a smaller loss of mercury at the lower concentrations, as observed. However, following this logic, any nonvolatile strong oxidant would keep mercury in the + 2 oxidation state, and hence would prevent loss of mercury metal. This was studied in separate experiments with dichromate, sulfuric acid and dichromate, sulfuric acid, and tetrachloroaurate(II1). In each case, the amount of mercury lost was equal to the amount lost when only nitric acid was used. Nevertheless, these observations do not negate the fact that mercury is being reduced and volatilized. This was proved by placing a silver wool plug in the line between the sample container and the trap. Quantitative recovery of the mercury lost from the sample container was achieved by amalgamation of the mercury metal on the silver wool plug: no mercury was found on the Nalgon tubing connecting sample flask to
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
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Table 111. Comparison of Values for Mercury Concentrations in Standard Reference Materials Bovine Liver
O r c h a r d Leaves (1577), ug/g
(1571),~g/g
NBS Certified value Instrumental value After separation
Channel
mercury standard. (B) NBS SRM 1630 (coal), before radio-
chemical separation
trap (as was the case in all the other experiments) or in the trap itself. Although it does not seem possible that the mercury would be reduced in such a strong oxidizing medium as dichromate, we must also remember that we are not dealing with a solution but with an aqueous solid, in which the electrochemical properties of the species will most certainly be different from that of a solution. The same mechanism can be inferred for loss of mercury in the digestion procedures. Reduction potentials, as calculated from the Nernst equation, in accordance with increased N204 concentration, reduced activities of acids, etc., indicate that reduction of Hg(I1) to the metal is possible. Filby has demonstrated (19) that an increase in pH a t constant mercury concentration decreases the amount of mercury lost during lyophilization. In our experiment with a synthetic seawater sample (pH 7) we have found a 30% loss, as opposed to a 60% loss for the same mercury concentration (0.015 pglml) at pH 1. Since homogenization of a sample often necessitates the addition of varying quantities of water (in order to ensure even distribution in the sample) the sample is going to approximate a solution, making possible losses similar to that observed in the experiments above. These samples represented only the result of a simulated situation and therefore it was necessary to determine whether the same results would be obtained for an actual environmental sample. Table I1 shows that in biological samples where the water content is high, the amount of mercury lost is correspondingly very high. Also, we would 2368
*
t 0.305 i 0.161 t
*
Flgure 3. The 68-keV region of two irradiated samples ( A ) Prepared
0.155
0.0145 t 0.002 0.200 f 0.021 0.018 0.003
C o a l (1630), Pglg
0.015 0.07
0.13 i 0.01
0.013
0.13 i 0.01
0.486
t
0.06
expect that mercury already incorporated into organisms would have reacted with protein in the body of the organism, thus reducing the volatility. Table I1 shows the results of a similar experiment in which sediment specimens were lyophilized. The amount of water in the sediment samples was much less than in the biological samples and we also find a smaller loss of mercury from the samples, even though the mercury concentration of these samples is much less than in the biological materials. Our results indicate that loss of mercury may be largely due to reduction of mercury to the metal followed by volatilization. Furthermore, if this problem exists for mercury then it might certainly exist for other elements, which are capable of reduction to a volatile form. We find that samples identical except for their mercury concentration lose mercury in proportion to the original concentration. Neutron-Activation Analysis. These results emphasize that minimal sample treatment will lead to the smallest chance for loss of mercury during preparation for analysis. Instrumental activation analysis obviates these pretreatments, but the x and y radiations of mercury may be obscured by radiations of other radionuclides also activated during the irradiation. Selenium, for example, a ubiquitous trace element in environmental samples, upon activation yields a 279-keV y ray that overlaps the 280-keV y ray of 203Hg.The most prominent peaks in the y-ray spectrum of neutron-activated mercury are the 68-keV x rays from the electron-capture decay of lg7Hg.A thin solid-state detector detects these x rays with high efficiency, and is relatively transparent to the higher-energy y rays, which is clearly advantageous for the determination of mercury. In fact, in the absence of interfering elements, we have been able to determine as little as 0.3 X g of mercury. Interfering elements are those with radiations which cannot be resolved from the x rays following lg7Hg decay. The poorer the resolution of the detector, the more the method is subject to interferences; this factor rules out the use of an ionization chamber, which may be more sensitive but has much poorer resolution than a thin solid-state detector. Loss of Hg during the sealing of quartz tubes, prior to irradiation, was monitored by sealing duplicate tubes of tracer solutions. This yielded no loss of mercury, even though the conditions, that is, filling of the tubes to 75% (normally less than 50%) of their capacity and not freezing the sample in liquid nitrogen prior to sealing, would tend to maximize loss. In a series of analyses of NBS Standard Reference Materials, using a Ge(Li) detector whose resolution at 1.3 MeV was 2.6 keV (FWHM), we obtained consistently higher values than those reported by the NBS. However, upon careful examination of the higher-energy y rays in the spectrum, we found that there were trace concentrations of La, Sm, Eu, Pt, and Gd which would give rise to y transitions in the energy range 65 to 80 keV that contributed to the peaks corresponding to the various transitions from the mercury decay. To eliminate and positively identify these interferences, we subjected our samples to the dry-combustion procedure of Rook et al., and subsequently counted the combustion products as well as the residue after combustion. Our re-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
sults from this procedure are compared with the results from our instrumental analysis and the NBS certified value in Table 111, and, as can be seen, are in agreement with the latter values. Careful examination of the y-ray spectrum and half-life determination of the y-ray peak (at 68.8 keV) in the residue indicates that the chief interference in the case of the Standard Reference Materials was 153Sm, produced by 152Sm(n,y).Equal quantities of samarium and mercury, upon activation would yield a ratio of activities of Sm/Hg = 32:1, on the basis of cross section, natural abundance of the isotopes, decay scheme, and half-life. The abundance of samarium in the Earth's crust is almost an order of magnitude greater (20) than that of mercury, and an environmental sample free of mercury pollution would give an activity ratio of Sm/Hg = 300:l. Figure 3 shows the 68-keV region of the y-ray spectrum of a prepared mercury standard and of SRM 1630, as counted with an intrinsic-germanium detector, before and after separation of mercury. Even though the resolution of the detector is 400 eV (FWHM a t 68 keV) the 68-keV x ray can be seen only as a small shoulder on the large peak due ~ ~ . higher-energy y ray (77.6 keV) to the y ray of 1 5 3 S The from a transition in Ig7Au is also seen in the standard, but is subject to interference from the 79-keV y rays of 169Gd and lg7Pt. CONCLUSIONS
We have shown that significant losses of mercury can occur during sample treatment and analysis, and we conclude that a method which can eliminate as many steps in trace-element determination as possible, will lead to the most accurate determination of mercury concentration in the sample. Neutron activation is such a method, affording good sensitivity, but the y spectrum (particularly the x-ray region) must be carefully scrutinized for interfering activities. Therefore, preliminary examination (using a high-resolution detector) of the y-ray spectrum of a sample must show the absence of interferences, otherwise radiochemical separation is required. In addition, we have provided evi-
dence which supports the hypothesis that mercuric ion is reduced to mercury metal during sample pretreatments such as lyophilization and digestion. Although Nernstequation calculations indicate the possibility of spontaneous reduction for the reaction conditions imposed, the fact that an equal amount of mercury is lost (as HgO) from nonvolatile oxidizing media, which theoretically maintain mercury as Hg(II), shows that a simple mechanism for reduction cannot be proposed at this time. LITERATURE CITED (1) J. Wood, F. Kennedy, and C. Rosen, Nature(London),220, 173 (1968). (2) C. Feldman, Anal. Chem., 46, 99 (1974). (3) H. Rook and J. Moody, "Stabilization and Determination of Nanogram Quantities of Mercury in Water", Proceedings of the Second International Conference on Nuclear Methods in Environmental Research, University of Missouri, Columbia, Mo., 1975; and H. Rook, T. Gills, and P. LaFleur, Anal. Chem., 44, 1114 (1972). (4) W. W. Scott, "Standard Methods of Chemical Analysis", D. Van Nostrand and Co., New York. 1925, Vol. 2. (5) W. Hatch and W. Ott, Anal. Chem., 40, 2085 (1968). (6) T. Rains and 0.Menis, J. Assoc. Off. Anal. Chem., 55, 1339 (1972). (7) R. Ginell, Department of Chemistry, Brooklyn College, private communication, 1974. (8) F. D. and C. T. Snell, "Colorimetric Determination of Elements", D. Van Nostrand and Co., New York, 1936. (9) K. K. Pillay et al., Anal. Chem., 43, 1419 (1971). (10) P. D. La Fleur, Anal. Chem., 45, 1534 (1973). (1 11 M. Friedman, E. Miller, and J. Tanner, Anal. Chem., 46, 236 (1974). (12) D. MacKay, Environ. Sci. Techno/.,7, 611 (1973). (13) S. Harrison, P. LaFleur, and W. Zoller, Anal. Chem., 47, 1685 (1975). (14) H. L. Finston and E. T. Williams, "Nuclear and RadiochemicaiTebhniques in Chemical Analysis", U S . At. Energy Comm. Tech. Rep. NYO3417-6, June 1968. (15) R. A. Horne. "Marine Chemistry", Wiley Interscience, New York, 1969. (16) R . C. Weast, "Handbook of Chemistry and Physics", The Chemical Rubber Co., Cleveland, Ohio, 1974. (17) C. Feldman, Anal. Chem., 46, 1606 (1974). (18) J. Thompson and W. Linnett, Trans. Faraday SOC.,32, 682 (1936). (19) R. Filby et al., "Role of NAA in the Study of Heavy Metal Pollution of a Lake River System", Second International Conference on Nuclear Methods in Environmental Research, Columbia, Mo., 1974. (20) A. F. Trotmen, "Comprehensive Inorganic Chemistry", Pergamon Press, New York, 1973, Vol. 3.
RECEIVEDfor review June 16, 1975. Accepted September 5 , 1975. This work was supported in part by the United States Energy Research and Development Administration under Contract No. E(11-1) 3126.
Atmospheric Pressure Ionization Mass Spectrometry: Corona Discharge Ion Source for Use in Liquid Chromatograph-Mass Spectrometer-Computer Analytical System D. 1. Carroll, 1. Dzidic, I?.N. Stillwell, K. D. Haegele, and E. C. Horning institute for Lipid Research, Baylor College of Medicine, Houston, Texas 77025
A corona source for a liquid chromatograph-mass spectrometer-computer analytlcal system is described. The performance was compared with that of the B3Nisource prevlously employed with direct injection of samples. Both Ion sources gave the same positive ions with solvents and test compounds. The corona source had a larger dynamic response range. A separation of several polynuclear hydrocarbons was demonstrated; API detection was compared with UV detection. Wlth isooctane as the solvent, the hydrocarbons formed MH' ions through solvent-mediated ion molecule reactions. Selective ion detection was used to monitor the elution of each hydrocarbon.
The technique of atmospheric pressure ionization (API) mass spectrometry, using a 63Ni ionization source, has been described (1, 2). A second ionization method, using a corona discharge, was later employed in a liquid chromatograph-mass spectrometer-computer (LC-MS-COM) analytical system (3, 4 ) . The primary purpose of the work described here was to compare the properties of the 63Ni and corona ionization sources, and to define the circumstances governing limiting sensitivity of detection. In the positive ion mode, both 63Ni and corona sources give essentially identical reactant ion spectra and lower limits of detection. The corona source, however, has approximately one hun-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
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