nation since, in such thin layers, phenomena such as recoil implantation (19), mixing, and initial selectivity (IO), which accompany the sputtering process, could be troublesome. ACKNOWLEDGMENT We thank F. J. de Boer for performing several analyses using the plasma torch, W. Tolksdorf and J. Kelly for providing the garnet and tin-silver samples, respectively, and C. van de Stolpe for his stimulating interest in this investigation. LITERATURE CITED (1) W. A. Pliskin and S. J. Zanin in "Handbook of Thin Film Technology", L. I. Maissel and R . Glang, Ed., McGraw-Hill, New York. 1970, Chap. 11. (2) C. C.Chang. Surf. Sci., 25, 53 (1971). (3) W. Reuter, Surf. Sci.,25, 80 (1971). (4) H. W. Werner, Vacuum, 24, 493 (1974). (5) C. A. Andersen and J. R . Hinthorne, Anal. Chem., 45, 1421 (1973). (6) C. W. White, D. L. Simms, and N. H. Tolk, Science, 177, 481 (1972).
(7) G. E . Thomas and E. E. de Kluizenaar. Acta Nectron.. 18, 63 (1975). (8) H. Oechsner and W. Gerhard, Phys. Lett. A, 40, 21 1 (1972). (9) J. W. Coburn and E. Kay, Appl. Phys. Lett., 19, 350 (1971). IO) H. W. Werner, to be published. 11) M. Kaminsky, "Atomic and Ionic Impact Phenomena on Metal Sur-
faces", Springer-Verlag. Berlin, 1965. 12) G. Carter and J. S. Colligon, "Ion Bombardment of Solids", Heinemann Educational Books, Ltd., London, 1968. 13) M. L. Tarng and G. K. Wehner, J. Appi. Phys., 42, 2449 (1971). 14) P. W. J. M. Boumans and F. J. de Boer, Spectrochim. Acta, Part B, 27, 391 (1972). (15) P. W. J. M. Boumans and F. J. de Boer, Proc. Anal. Div. Chem. SOC. (London). 12, 140 (1975). (16) P. W. J. M. Boumans and F. J. de Boer, Spectrochim. Acta, Part 6, 30, in press. (1975). (17) C. H. de Minjer and P. F. J. v.d. Boom, J. Electrochem. SOC., 120, 1644 (1973) (18) M. Hansen, "Constitution of Binary Alloys" McGraw-Hill, New York,
1958.
(19) C. 8. Kerkdijk and R . Kelly, Surf. Sci,, 47, 294 (1975).
RECEIVED
for review May 29, 1975. Accepted August 5 ,
1975.
Determination of Silicon in Glasses and Minerals by Atomic Absorption Spectrometry R. A. Burdo and W. M. Wise Research and Development Laboratories, Corning Glass Works, Corning, N. Y. 14830
Silicon in glasses and minerals is determined by atomic absorption spectrometry. Samples are fused with a sodlum carbonate-sodium borate flux and then dlssolved in an acidic molybdate solution to form a silico-molybdate complex. The presence of molybdate prevents the polymerization of slllca, buffers possible flame interferences, and provides an accuracy of 0.2 to 0.3 % absolute for samples containing 14 to 94% silica. The method considerably reduces the time required for silica analysis by classical gravimetric procedures.
There are a number of reported literature methods for the atomic absorption determination of silicon in high silica materials. Nearly all of the methods rely on a lithium metaborate fusion (1-4), a lithium carbonate-boric acid fusion (5, 6), a sodium peroxide fusion (7), or an HF-Teflon bomb attack (8, 9) for sample decomposition. However, most of these methods either have not been applied to glasses or lack sufficient accuracy for our purposes. In this study, a fusion technique employing a sodium carbonate-sodium borate flux is chosen for its speed, simplicity, and universality in glass decomposition. The problem of chemical interferences on the silicon absorption signal is investigated. The formation of a silico-molybdate complex is proposed as an effective means of eliminating such interferences, preventing the polymerization of silica, and providing excellent accuracy for silica analysis in comparison to classical gravimetric procedures.
EXPERIMENTAL Apparatus. A Varian Techtron AA-5 atomic absorption spectrometer was employed under the analytical conditions stated in Table I. Reagents. All chemicals are analytical reagent grade. Aqueous reagents are prepared in distilled, deionized water. All solutions are stored in plastic containers. Flux. Mix sodium carbonate (NaZC03) and sodium borate 2360
(NanBdOi) in equal proportions by weight. Both reagents are -200 mesh. Ammonium Molybdate Solution (39 g/l.). Dissolve 78 g of ammonium molybdate ((NH&jM07024*4H20) in about 700 ml of water, filter through a Whatman No. 41 fast filter into a 2-1. volumetric and dilute to volume with water. Diluting Solution. Dissolve 1 g of flux in 100 ml of 0.394 N nitric acid (1:39). Add 100 ml of molybdate solution and dilute to 500 ml with 0.126 N nitric acid (1:124). Discard solution after several days if a hard scale of hydrated molybdenum oxide forms on the container walls. Procedure. Sample Fusion. Hand grind the sample t o -100 mesh using a corundum mortar and pestle. Desiccate as required. Weigh 100 mg of sample (to the nearest 0.1 mg) and 1.0 g of flux into a 30-ml platinum crucible. Mix powders well, cap crucible with a platinum cover, and place directly in a muffle furnace a t 1000 "C for 30 to 40 minutes. Melt splattering is avoided by the use of anhydrous flux reagents. On removal of the crucible from the furnace, swirl the liquid melt onto the inner sides of the crucible and solidify the melt by half-immersing the crucible in an ice bath such that no water enters the crucible. The platinum cover should remain intact until the melt solidifies in order to prevent the loss of melt due to the possible ejection of small particles on rapid cooling. Melt Dissolution. Transfer 100 ml of molybdate solution and 100 ml of 0.394 N nitric acid to a 250-ml Teflon beaker containing a half-inch magnetic stir bar (Teflon-coated). Place a similar stir bar in the platinum crucible, immerse the crucible into the beaker, and begin magnetic stirring. No heat is required. The melt dissolves in 10 to 30 min, forming a yellow silico-molybdate solution. Immerse the platinum cover near the end of dissolution only if it shows evidence of melt splattering. Quantitatively transfer the yellow solution to a 500-ml volumetric (glass with plastic screw cap) and dilute to volume with 0.126 N nitric acid. Standard Solutions. Substitute 0.1000 g of pure silica powder (99.9+ %, -100 mesh) in the fusion procedure to yield a standard stock solution of 93.5 ,ug/ml silicon. Dilutions of the stock solution are made with the diluting solution already described. Solutions containing less than 40 wg/ml silicon may form 4 hard and nonadsorbing scale of hydrated molybdenum oxide an the container walls after 10 days depending on the silicon concentration and the fullness of the container. The scale is easily cleaned with ammonium hydroxide solution.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
'L I
Table 1. Instrumental Operating Conditions for Silicon Instrument: Varian Techtron AA-5 Flame: Nitrous oxi e-acetylene, slightly oxidizing Wavelength: 2516 Slit: 50 pm; dispersion: 3.3 nm/mm Readout: Digital, time-averaging Lamp current: 18 mA Scale expansion: 2X; concentration control at full (2.3X) Detection limit: 0.3 pg/ml (2 times standard deviation of noise) Concentration range employed: 10-100 pg/ml
1.0
W
N
3 05
l!
P
0
Figure 1.
Table 11. Effect of Foreign Cations on Silicon Absorption % C h a n g e in absorption Cation added
Li+
Kf Mg2+ Caa+ Sr2+ Nia+ cuz+ Zn2+ Cr3+ ~ 1 3 +
Fe3+ Ti''+ Zr4+
Molybdate absent
Molybdate present
+2.4 +0.8 +4.6 +5.9 +3.9 +4.7 +0.8 +0.8 +2.3 +3.8 +4.0 +4.0 +4.0
-0.1 -0.2 + 0.7 + 0.4 -0.2 -0.9
+0.1 -0.4 -0.8 +0.2
Precipitates Precipitates Precipitates
RESULTS AND DISCUSSION N a t u r e of Silico-Molybdate Solutions. Formation of the yellow silico-12-molybdate complex, (SiMo12040)~-, occurs in an optimum pH range of 1to 2 in the presence of only a slight excess of molybdate (10, 1 1 ) . In the dissolution procedure, sufficient acid is added to both neutralize the flux and lower the pH to 1.5. The correct amount of acid was empirically determined, as the presence of molybdate decidedly may alter the pH of the solution. The p H of representative standard and sample solutions falls in the range 1.05 to 1.10 after full dilution. The amount of molybdate present during melt dissolution is sufficient to complex 110 mg of pure silica. Some cations produce insoluble molybdates depending on the p H of the solution. At a pH of 1.1,barium and lead form precipitates which are somewhat more soluble a t a pH of 0.4. Acceptable analytical results can still be obtained in the presence of a precipitate by simply aspirating the supernatant after settling or filtering the precipitate before final dilution. Filtering is required only if the amount of precipitate is large and the solution is to be saved for several days before analysis. (Refer to section on accuracy.) The yellow silico-molybdate color begins to fade a t the onset of complex formation. This color degradation is attributed ( 1 1 ) to the gradual conversion of the initial p form of the complex to the more stable CY form which has a lower absorptivity. Figure 1 illustrates the approximately exponential decrease in the absorption of the silico-molybdate complex at 434 nm as a function of time after preparation (absorptions are normalized to unity a t 2 hr after preparation). Although colorimetric methods are obviously affected by this degradation, the atomic absorption signal for silicon is stable from the start and over a period of weeks. Freshly prepared standard solutions of silico-molybdate produced the same silicon flame sensitivity as those that had aged for 3 weeks. Silica Polymerization. The literature is replete with evidence of the erratic effects of silica polymerization. Ingamells (12) states that excess acid in the dissolution proce-
50 TIME ( H R S )
100
Relative decrease in silico-molybdate absorption with time
dure results in less complete solution of silica. Van Loon and Parissis ( 1 3 ) indicate that excessive stirring causes silica polymerization. Guest and MacPherson (7) and Medlin et al. ( 1 ) note that the stability of silicon standard solutions could be affected by aging and by the concentration of silicon, acid, and other species in the solution. Suhr and Ingamells (14) also mention the problem of silica polymerization with time. The remedy for the erratic stability of silicon solutions and signals lies in recognizing that, above a certain concentration limit, silica polymerizes at a rate that is somewhat unpredictable and dependent on pH and other solution characteristics. This limit is reported by Richardson and Waddams (15) to be about 180 pg/ml silica and by Shapiro ( 1 6 ) to be about 240 pg/ml silica in a LiB02-HN03 solution. The degree of polymerization is also related to the presence of depolymerizing agents such as molybdate (16, 1 7 ) , fluoride (16), and hydroxide (16). Further, dilution of a solution containing some polymerized silica results in gradual depolymerization (18). In view of the foregoing evidence, two guidelines can be established. First, silica in acid solutions of pH about 1 to 3 should not exceed a concentration of 240 yglml (110 yg/ml Si) and, second, the presence of a depolymerizing agent such as molybdate will enhance silica solubility. In this study, the final maximum concentration of silicon is limited to 100 yg/ml and the silico-molybdate complex is employed to prevent silica polymerization in the melt dissolution process where local and bulk silica concentrations far exceed the limit beyond which polymerization occurs. The effect of limiting the silica concentration and forming the molybdate complex is to stabilize the silicon absorption signal over long periods of time, as noted above. Interferences. Several authors have detected the interference of aluminum and other elements on the silicon absorption signal. Omang ( 5 ) reports that the aluminum interference can be masked by the addition of 1%lanthanum salt to samples and standards. Boar and Ingram ( 4 ) find that 1%tartaric acid is also an effective mask. However, Ingamells (12) states that the addition of lanthanum salts or EDTA does not completely eliminate such interferences and that calibration vs. natural samples of certified compositions yields the best results. Table I1 illustrates the effect of foreign cations on the silicon absorption signal in a sodium-carbonate-borate nitric acid medium containing 23 yg/ml Si in the presence and absence of molybdate. Solutions not containing molybdate were prepared in a manner similar to those containing molybdate so that pH and salt contents would be the same. The foreign cations were introduced as nitrate salts (except Ti) a t a concentration level of 200 yglml (8.7:l weight ratio to silicon). Solutions were aspirated under the conditions in Table I. In the absence of molybdate, most foreign cations caused an enhancement of the silicon absorption relative to that obtained in the same solution when no foreign cations were added. The effect of adding foreign cations to molyb-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
2361
STOICHIOMETRIC
OXIDIZING
I
REDUCING
"/ 200
m a
70
ACETYLENE
75
so
85
% Si02
FLOW (ARBITRARY UNITS)
Figure 2. Silicon absorption as a function of flame stochiometry ( 0 )Without molybdate. (0) With molybdate
date-containing solutions is, conversely, negligible in most cases. A major consideration is the possible aluminum interference, which is well-masked by molybdate. Note that iron, titanium, and zirconium form insoluble molybdates. No results are recorded in these situations, since these cations were added in such large and exaggerated excess that insufficient molybdate is present to complex all available ions. This is in contrast to the normal procedure in which sufficient molybdate is present to complex an entire sample of limited foreign ion content and limited precipitation capability. Instrumental Considerations. Stable burner operation can be adversely affected by the aspiration of solutions of high total salt content. It is best to limit the salt concentration to 1% unless special burners are used. A salt concentration of 1%is attained here by limiting the ammonium molybdate concentration to 0.7896, the flux concentration to 0.2%, and the final dilution volume to 500 ml. These requirements also limit the maximum silicon concentration to less than 100 kg/ml for a 100-mg sample. Stable burner operation is also aided, in the case of the molybdate medium, by the aspiration of a 0.1 M nitric acid solution (instead of water) between sample aspirations, a procedure which also prevents the formation of precipitates in the drain and atomizer systems. A determination of the optimum flame stoichiometry for silicon is aided by the graph of Figure 2 which plots silicon absorption response as a function of acetylene flow (at constant N20 flow) for both molybdate and non-molybdate solutions containing 63 pg/ml Si. In the figure, an arbitrary acetylene flow of 7.5 units corresponds to about 4 l./min, and an arbitrary absorption of 500 units corresponds to about 0.13 true absorbance unit. The silicon signal is enhanced by 10 to 25% in the presence of molybdate, depending on flame stoichiometry. Operation in a reducing flame produces the maximum signal but causes carbon and silicon carbide deposits a t the burner orifice which result in poor response stability. In practice, the flame is operated in a slightly oxidizing-to-stoichiometric mode with a minor loss in signal but a great gain in stability. In order to realize a precision in absorption measurements that is conducive to an accurate result, it is desirable to compare analyte and standard signals in the shortest possible time period. High precision in analyte-to-standard absorption ratios is obtained by alternating absorption measurements between the analyte solution and a single standard solution which matches the analyte solution in silicon concentration to within 6 pg/ml. A suitable concentration match is quickly found by briefly aspirating the analyte solution and comparing its absorption to that of a 2362
standard solution containing 50 pg/ml Si. When measurements are alternated between analyte and standard solutions, a solution of 0.1 N nitric acid is aspirated after each analyte or standard aspiration to improve stability and to define a background absorption value which is subtracted from the analyte or standard absorption. For all practical purposes, a 0.1 N nitric acid solution yields the same background absorption as the molybdate diluting solution (see Reagents). The average of any two successive analyte absorptions is then compared to the intervening standard absorption to yield an analyte-to-standard absorption ratio ( R ) .The percentage of silica in the original sample is then calculated according to the formula:
= (fi)(C)(V/W)(2.1393)(100)
(1)
where 8 is the average of several analyte-to-standard ratios; C is the concentration of the standard solution in HgI ml; V is the initial dilution volume in liters; W is the sample weight in milligrams; and 2.1393 is the gravimetric factor for the conversion of Si to Si02. Precision of Measurements and Calculations. The precision of the % Si02 calculation is affected by each of the variables in Equation 1. The expression for the estimation of the variance of the calculated value of % Si02 is derived by finding the total differential of Equation 1, squaring the result, and dropping insignificant terms to yield:
+
+
Var(% SiO2) = (kRV/W)Var(C) (kCV/W)2Var@) (kC8/W)2Var(V) (-kC8V/W2)2Var(W)
+
(2)
where Var means variance and k is the combination of constants from Equation 1 (k = 213.93). Although not immediately apparent, the variance of the average ratio, Var(8), is the most important term in Equation 2. The precision associated with the determination of any single ratio, R , can be improved by alternating analyte and standard measurements as mentioned, and by employing time-averaged signals. In this study, absorption measurements were time-averaged for 20 sec with the resulting absorption values being digitally displayed to three significant figures. The precision associated with a single determination of R , under these conditions and those stated in Table I, was 0.4% relative standard deviation (RSD) for solutions containing an average of 50 pg/ml Si. However, solutions of higher silicon concentration produced deviations for absorption ratios which were significantly lower than deviations for solutions of lower silicon concentration. The % RSD of the ratio R was 0.3% for solutions averaging 80 pg/ml Si, but increased to 0.5% for solutions averaging 20 pg/ml Si. The absorption ratio is close to unity when analyte and standard solutions are closely matched in silicon concentration, so that the standard deviation (SD) of the ratio is nearly equivalent to % RSD/100. Thus, the SD of the ratio R is 0.003, 0.004, and 0.005 units for solutions containing 80, 50, and 20 pg/ml Si, respectively. The SD of a collection of 8 values is expected to improve with the square root of the number of determinations included in each 8. In this case, four determinations of R are included in each 8 so that the SD of a collection of R values is expected to be 0.0015, 0.0020, and 0.0025 for solutions containing 80, 50, and 20 pglml Si, respectively. The SD of a 100-mg weight measurement was determined to be 0.05 mg. The SD of the initial dilution volume ( V = 0.5 1.) is estimated as 0.0001 1. The SD of the concentration C is affected mainly by initial weighing errors and secondarily by errors in dilutions of stock solutions. I t is estimated that C is known to a precision of 0.06% RSD. By substituting k = 213.93, C = 50.00, W = 100, V = 0.5, R = 1.00, Var(C) = (0.03)2,Var(R) = (0.002)2, Var(V) = (0.0001)2,and Var(W) = (0.05)2 in Equation 2, the variance
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
of % Si02 is calculated as 0.0133 which yields a SD of 0.115 in % SiOz. Similar calculations yield standard deviations of 0.146 and 0.056 for the 80 and 20 pg/ml Si levels, respectively. Solutions containing 80, 50, and 20 pglml Si correspond to original sample silica contents of 85.6, 53.5, and 21.4%, respectively. The above calculations assume analyte and standard solutions of matched or nearly equal silicon concentrations. If analyte and standard solutions are of significantly different silicon concentrations, then nonlinearity of signal response Table 111. Silica in ASTM Glasses by AAS % SiO, Sample t y p e
AASa
Gravb
Ca-Phosphate Zn-Fluoride Borosilicate Ba-Pb glass Pb glass
75.9 60.4 68.3 66.2 42.3
75.7 60.5 68.7 66.2 42.0
Absolute % difference
+0.2 -0.1
-0.4 0.0 +0.3 Av 0.20 a Triplicate average. b Average round-robin gravimetric values. Table IV. Silica in NHS Glasses by AAS % SiO, Sample
NBS89 NBS91 NBS 9 3
Type
Lead-barium Opal Borosilicate
AAS
Certified
65.6 67.8 80.3
65.4 67.5 80.6
Table V. Silica in Various Glasses by AAS 92 SiO, Sample type
AAS
Uncert.a
Zircon Pb-glass Al-Ti-Zr glass Ba-Ti-La glass
33.8 36.7 58.5 21.0
34.1 36.6 58.3
a
21.4
Absolute % difference
+0.2 +0.3 -0.3 Av 0.27
Absolute % difference
-0.3 +0.1 +0.2 -0.4 Av 0.25
Uncertified gravimetric value.
as a function of silicon concentration could introduce an additional error in the calculation of % SiO2. Under the conditions of this study, the relative silicon response (response per unit concentration) did not vary by more than 0.2 to 0.3% relative over a range of 15 pg/ml Si. A series of seven standards covering the range from 20 to 90 pg/ml Si provides an ability to match analyte and standard solutions to within 6 pglml, that is, to within 0.12% relative silicon response. An average match of 3 pg/ml would produce an average nonlinearity error of 0.06% relative. Although additional terms could be included in Equation 2 to account for this error, it is small enough to be neglected in the first approximation. Further, the error can be compensated by bracketing, that is, obtaining several ratios with a slightly high standard and several ratios with a slightly low standard. The errors considered up to this point are determinate errors. Indeterminate errors relating to sampling problems and chemical interference can be detected only on replkation of analyses and comparison of results for known materials. Sampling errors include the loss or gain of water or carbon dioxide between samplings as well as inhomogeneity in the sample itself. Chemical interference may not only bias the results, but the bias itself may be variable depending on flame stochiometry and the height of signal observation in the flame. Other errors such as contamination or loss of sample after weighing may also be important in some cases. Precision and Accuracy of Analyses. Table I11 lists silica results by the AAS-molybdate method for five ASTM round-robin glasses which were previously analyzed by gravimetric methods. For silica in the 42 to 76% range, the average absolute difference between the AAS values and the round-robin values is only 0.2% (mean error). Note that the calcium phosphate glass (1.6% P205) and the zinc fluoride glass (3.53% F) produce good results. The barium glass (11.2 % BaO) and the high lead glass (50.7% PbO) do not indicate a low bias even though some precipitate was carried to the final dilution. Table IV shows a mean error of 0.3% absolute for three NBS glasses including a lead glass which produced a precipitate. Acceptable results for other precipitate-producing glasses are recorded in Table V. The addition of several milliliters of concentrated sulfuric acid to the zircon sample (Table V) did not seriously affect the result. This is consis-
Table VI. Replication of Silica in NBS Samples by AAS
92 SiO,
Absolute % difference
Type
AAS
Av i SD
Certified
NBS-1A
Limestone (Ca, c, -41)
14.4 i 0.15
14.1
+0.3
NBS-70A
67.4
i
0.49
67.1
+0.3
NBS-78
Feldspar (AI, K) Refractory (Al, Ti)
21.2
i
0.10
20.7
+0.5
NBS-77
Refractory (Al, Ti)
32.8
2
0.21
32.4
+ 0.4
NBS-97A
Flint clay
14.3 14.4 14.6 67.0 67.7 21.1 21.3 21.2 32.6 32.9 33.0 43.6 43.5 48.6 48.8 68.5 68.5 93.8 93.8
43.6
i
0.07
43.7
-0.1
48.7 i 0.14
48.9
-0.2
68.5
i
0.0
68.7
-0.2
93.8
i
0.0
93.9
-0.1
Sample No.
NBS-98A NBS-99 NBS-102
(-41)
Plastic clay (-41)
Feldspar (AI, Na) Silica brick
Mean error
0.26
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
2363
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
