2410 ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979

a t 11 203 cm-' to 30784 cm-', leaving an energy gap of 31 533 cm-' which must be supplied thermally to ionize the atom. For this experiment kT is -1390 cm-' assuming a 2000-K flame. Use of the second dye laser tuned to 453.1 nm further excites those atoms excited by the first laser to within 9468

strated an ability to utilize an increased number of atomic lines for many elements, such interferences could become a prob- lem. However the probability of an overlap of a consecutive sequence of two lines of one element with that of another is greatly reduced.

cm-' of the 62 317 cm-' ionization limit, and the same 1000 ppm solution gives a strong signal with a signal to blank noise ratio of 5700.

Two points are noteworthy concerning the observed signals as a function of laser power: (1) the signal-to-noise ratios and consequently the detection limits improve with increasing laser power a t both XI and A,; (2) the measured enhancements for Li and Na are observed to decrease with increasing XI power (in both cases the transition at XI is relatively easy to optically saturate). Thus, the enhancement factor is not of fundamental significance and can be very large (>1000) a t low X1 powers. The measured enhancements depend upon laser power a t Xi and X2, the degree of optical and electrical saturation ( 5 ) involved, as well as on the fundamental properties of photon energy, ionization potential, and transition strengths of the lines.

Despite the large enhancements achieved, the detection limits obtained in this work using two lasers are, with the exception of sodium, inferior to those reported earlier (I) for LEI using single photon excitation with a frequency-doubled flashlamp-pumped dye laser. This observation is consistent with preliminary studies on how the different pulse-energy characteristics of the two lasers affect signal sensitivity.

The use of stepwise photoexcitation might also aid in dealing with the most troublesome interference problem which has been encountered in LEI spectrometry, namely ionization interference from matrix species. The presence of high con- centrations of easily ionizable matrix species has been shown to adversely affect measurement of the analyte signal ( I ) . Further improvement in sensitivity would allow a sample containing high levels of such species to be highly diluted, thus reducing the concentration of the matrix to a manageable level.

Stepwise excitation can also be a useful means of dealing with interferences arising from the overlap of spectral lines of the various elements in a sample. Since LEI has demon-

CONCLUSIONS Substantial enhancements of analytical sensitivities using

laser enhanced ionization can be realized through the use of two tunable dye lasers for stepwise photoexcitation. By in- creasing the energy level of the photoexcited level the proba- bility for thermal ionization is enhanced. As a result, the LEI determination of high ionization potential elements, which exhibit low sensitivity for single photon excitation, should now improve dramatically.

ACKNOWLEDGMENT The authors express their gratitude to Warren Gutheil of

Molectron Corporation for the loan of the second dye laser which made this work possible. The authors also thank J. C. Travis for helpful discussions.

LITERATURE CITED (1) G. C. Turk, J. C. Travis, J. A. DeVoe, and T. C. O'Haver, And. Chem.

51, 1890 (1979). (2) J. C. Travis, G. C. Turk, and R. 6. Green, Chapter 6 in "New Applications

of Lasers to Chemistry", G. M. Hieftje, Ed., ACS Symposium Series 85, American Chemical Society, Washington, D.C. 1978, p 91.

(3) G. C. Turk, J. C. Travis, J. R. DeVoe, and T. C. O'Haver, And Chern., 50, 817 (1976).

(4) R. B. Green, R. A. Keller, P. K. Schenck, J. C. Travis, and G. G. Luther, J . Am. Chem. Soc., 98, 8517 (1976).

(5) J. C. Travis, P. K . Schenck, G. C. Turk, and W. G. Mallard, Anal. Chem., 51, 1516 (1979).

(6) P. K. Schenck, W. G. Mallard, and K. C. Srnyth, in preparation. (7) P K. Schenck, W. G. Mallard, J. C. Travis, and K. C. Smyth, J . Chem.

Phys., 69, 5147 (1978).

RECEIVED for review June 11,1979. Accepted August 23,1979. In order to adequately describe experimental procedures, i t was occasionally necessary to identify commercial products by manufacturer's name or label. In no instance does such identification imply endorsement by the National Bureau of Standards nor does it imply that the particular products or equipment are necessarily the best available for that purpose.

Organic Acid Acidification of High Salt Solutions in Determination of Metals by Atomic Absorption Spect romet ry

Emmett G. Gooch" and Paula R. Roupe Analytical Services, Dow Corning Corporafion, P. 0. Box 1592, Midland, Michigan 48640

Sample preparation for the determination of silicon and other metals in silicon compounds can present a problem. Silicon may be present in a variety of forms ranging from refractories to organosilicon compounds. Most organosilicon compounds are solvent soluble; however, unless the molecular structure of the compound is known and can be matched to an appropriate standard material, atomic absorption analysis of these compounds for silicon is of limited value, since each structure has an individual response factor. Insoluble silicon compounds are normally converted to a soluble form before flame atomic absomtion analvsis can be undertaken. Manv

to discharge carbon dioxide ( I ) . Generally, hydrochloric acid is employed for acidification,

resulting in a high concentration of sodium chloride. When aspirated, these solutions often build up salt deposits in the burner slot and nebulizer, causing high noise levels and signal drift. Good reproducibility is thus difficult to obtain, espe- cially when several samples are to be analyzed in one series.

Acidification of these basic solutions with an organic acid, such as acetic acid, alleviates salt buildup and minimizes response drift. In addition, sensitivity for most metals is increased with the use of the organic acid. -

silicon compounds, such as poiy(dimethylsiloxane), cannot be quantitatively wet-ashed because silicon is lost through vola- tilization. Reaction of these various materials in a closed-cup bomb with sodium peroxide resolves these problems by pre- venting loss Of volatiles and converting all silicon present to sodium silicate. Acidification of the melt after dissolution in water is necessary to ensure complete solution of salts and

EXPERIMENTAL Apparatus. Measurements were carried out on a Perkin-Elmer

atomic absorption spectrophotometer, Model 603, equipped with nitrous oxide burner head, part number 040-0277, standard burner regulator assembly, part number 057-0262, corrosion-resistant nebulizer assembly, part number 303-0404, Perkin-Elmer model 56 recorder, and Perkin-Elmer hollow cathode lamps as needed.

0003-2700/79/0351-2410$01 0O:O C 1979 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979 2411

A B C D E F

F1GURE 3 &IL* * Crn I O

Figure 1. Closed-cup peroxide combustion bomb. (A) Assembled unit. (B) Steel compression nut: 41.3 mm, across flats; 28.6 mm deep. (C) Steel compression ring: 38.1 mm, across flats, 25.4 mm deep, 47.6-mm bore, 38.1-mm threads. (D) Nickel crucible: capacity, 15 g Na,O,; cavity, 17-mm diameter; 40 mm deep. (E) Lead gasket: 0 8 mm thick, cut to fit cap annular groove. (F) Nickel cap: 8 mm thick, 35.5-mm diameter. Annular groove, 5 mm wide, 1 mm deep

12" x 4"

RASFPLATE 12" x 12"

Figure 2. Diagram of protective bomb shield

The apparatus used for sample preparation was a nickel closed-cup fusion bomb with a capacity of 15 g of sodium peroxide, a blast burner, oxygen and natural gas, a protective shield, 600-mL nickel beakers with covers, 25-cm nickel tongs, and 500-mL volumetric flasks.

Reagents. All acids and metal salts were analytical reagent grade. The sodium peroxide, granular 20 mesh or finer, catalog number 1-9418, was purchased from J. T. Baker. The silicon standard was prepared from Dow Corning 200 Fluid (poly- (dimethylsiloxane), 37.85'70 silicon). Distilled, deionized water was used throughout.

Procedure. Samples and three standards (ranging from 2 to 200 ppm silicon) are prepared for the sodium peroxide closed cup fusion by the following procedure: Weigh 50-250 mg of sample into a $1 gelatin capsule. Place the closed capsule in the nickel bomb crucible and fill with 15 g of sodium peroxide. Settle the peroxide by tapping the crucible on the bench top and cover with crucible cap and lead gasket. Place the bomb in the closure nut and tighten with a wrench (Figure 1). Place the bomb in the protective heating rack (Figure 2) and fill the bomb head with water. Heat the crucible with a blast burner flame just touching the bottom of the crucible until it is cherry red, 1-1.5 cm up from

Figure 3. Absorbance traces of fusionate solutions, containing 100 ppm silicon, acidified with (A) acetic acid, (B) hydriodic acid, (C) nitric acid, and (D) hydrochloric acid and aspirated for 3 min. Solutions contained 0.77 M sodium salt and 1.17 M excess acid

Table I. Fusion Solutions Acidified with 1.94 M Acetic Acid

Sensitivity Increases of Various Metals in Bomb

absorbances

metal

AI Ca Cr c u Fe Si Zn

A, nm

309.3 422.7 358.0 324.7 248.3 251.6 213.9

concn, PPm 40.9

4.4 5.4

28.1 30.9

100.0 5.0

with hydro- chloric

acid

0.178 0.240 0.100 0.238 0.242 0.260 0.700

with acetic acid

0.212 0.277 0.118 0.281. 0.291 0.301 0.740

% increase

19.1 15.4 18.0 18.1 20.2 15 .8

5.7

the bottom. Adjust the flame so that heating is completed within 60 to 70 s. To avoid melting of the lead gasket, replenish water in the bomb head, if necessary, until heating is completed. Remove the bomb from the heating rack and quench in ice water until cool. Extreme caution should be used and adequate protective clothing, including full face shield, should be worn during sample fusion.

Disassemble the bomb and place the crucible in a 600-mL nickel beaker with enough distilled water to cover the crucible. Rinse any peroxide adhering to the crucible cap into the beaker with distilled water. Cover the beaker and allow the melt to react. After the solution is reacted and cooled, remove the crucible. Rinse with distilled water and add the rinsings to the beaker. To avoid precipitation of hydrous silica, adjust the volume of solution in the beaker to 3OCF350 mL before acidification. Acidify the com- bined solution with the appropriate acid. After the solution has cooled, transfer to a 500-mL volumetric flask, rinse the beaker, and dilute to volume. These solutions are stable for an indefinite period when stored in polyethylene bottles.

RESULTS AND DISCUSSIONS Typical silicon absorbances for the fusionate solutions con-

taining four different acids and high salt concentrations are shown in Figure 3. The signal for the hydrochloric acid solution is very erratic and unstable. As the solution is aspi- rated, sodium chloride builds u p in the burner slot and nebu- lizer causing decreased sensitivity (2). The silicon absorbances of the hydriodic and acetic acid solutions are very stable, while the noise level of the nitric acid solution is between the erratic hydrochloric acid and stable hydriodic acid solutions.

Burner stability appears to be a function of the solubility of the salt involved. As solubilities of the salts increase, the noise levels decrease. The greater burner stability observed when acetic acid is used, results in improved precision. When hydrochloric acid is used to acidify the fusionate solutions, the standard deviation for analysis of our silicon standard is 3% relative. With the use of acetic acid, the standard devi- ation is reduced to 1.2% relative.

Figure 3 also illustrates an enhancement effect. Of the four solutions, the acetic acid solution produces the greatest ab-

2412 ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979

Table 11. Neutralized Sodium Salt Solution vs. Silicon Absorbance of Same Solution with 1.17 M Excess Acid

Absorbance of 100 ppm Silicon in a 0.77 41

absorbance

at acid used neutralization

nitric 0.229 hydrochloric 0.234 formic 0.266 acetic 0.264 propionic 0.266 butyric 0.277

a t 1 .17 M excess

0.256 0.240 0.281 0.296 0.333 0.375

sorbance. As seen in Table I, a similar effect is observed with other metals. Sensitivity increases of 5-2070 were observed for other metals in organic acid fusionate solution. I t is pos- sible that an even greater increase could be obtained because the absorbances for the other metals may or may not have plateaued. The silicon absorbance did plateau with 55 mL (1.94 M) of acetic acid in the fusionate solutions.

In a further study of this enhancement effect, solutions of 0.77 M sodium hydroxide, containing 100 ppm silicon, were neutralized with nitric, hydrochloric, formic, acetic, propionic, and butyric acids. The solutions neutralized with the organic acids all showed approximately the same absorbance. The

absorbances of the solutions neutralized with nitric and hy- drochloric acid were lower.

The same basic solutions were then acidified with a 1.17 M excess of each of the same acids. The silicon absorbance increased with increasing molecular weight of the organic acid. An enhancement effect was also observed with the excess nitric acid, whereas excess hydrochloric acid had no significant effect on sensitivity (Table 11).

Surface tensions of the solutions were investigated as a source for the enhancement effect. As the molecular weight of the organic acid increased, surface tension of the solutions decreased. However, when a surfactant, Eastman Kodak, Photo-Flo 200 Solution, was added to the hydrochloric acid solution, its surface tension decreased by 50%; yet it had the same silicon absorbance.

Although the flame chemistry of this system is not under- stood, acidifying basic fusionates with organic acids was shown to be useful for alleviating signal drift and increasing sensi- tivity.

LITERATURE CITED (1) McHard, J. A,; Servais, P. C.: Clark, H. A. Anal. Chem. 1948, 20, 325. (2) Grove, E. L. "Analytical Emission Spectroscopy", Vol. I, Part 11; Marcel

Dekker: New York, 1972: Chapter 7, p 260.

RECEIVED for review April 19, 1979. Accepted September 18, 1979.

Modification of a Commercial Micrometer Hanging Mercury Drop Electrode

J. E. Bonelli,' H. E. Taylor, and R. K. Skogerboe'

U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colorado 80225

The hanging mercury drop electrode (HMDE) has found widespread use in electroanalytical chemistry, especially for anodic and cathodic stripping voltammetry. The introduction of the micrometer HMDE in the late 195O's, (ref. 1-5), in which mercury is displaced from a reservoir by a screw driven plunger to form renewed and reproducibly sized electrodes, was a significant advance in convenience and performance.

One of the most popular commercial versions of the mi- crometer HMDE is manufactured by Metrohm AG (Herisau, Switzerland), and is marketed in the United States by Sy- bron-Brinkmann Instruments Ltd., (Westbury, N.Y. 11590) and by EG&G Princeton Applied Research Corp. (Princeton, N.J. 08540) (Figure 1). The Metrohm E-410 HMDE employs a borosilicate glass mercury reservoir and capillary from which mercury is displaced by a screw driven plunger (see Figure 2). Although this electrode performs well when properly filled and assembled, it is difficult to fill the mercury reservoir and simultaneously exclude all air. The consequences of trapped air in the mercury reservoir are familiar to all who have used this electrode: excessive mercury thread withdrawal upon dislodging a drop, nonreproducible drop sizes, premature drop dislodgment, and, worst of all, changing electrode size during an analysis. Even a properly filled electrode soon admits air

Present address: Department of Chemistry. Colorado State University, Fort Collins, Colo. 80523.

to the mercury reservoir, necessitating frequent cleaning and refilling.

A simple and inexpensive modification to the Metrohm E-410 HMDE is described here to eliminate these problems. The Metrohm E-410 HMDE is modified to accommodate a new borosilicate glass mercury reservoir-capillary and screw driven plunger tip from the recently introduced EG&G Princeton Applied Research Corp. (PAR) Model 302 Universal Mercury Electrode (UME). The necessary parts are available from PAR a t low cost ($70).

The Model 302 UME is a combination dropping mercury electrode (DME) and HMDE. In the HMDE mode, the plunger is driven by a motor down the top tapered section of the mercury reservoir bore, displacing excess mercury and air, until a sealing gasket on the plunger contacts the walls of the reservoir. From this point downward, the reservoir bore is constant, and the plunger very effectively displaces mercury via the capillary to form new and highly reproducible electrode surfaces (Figure 2). In contrast, the original configuration of the Metrohm E-410 HMDE relies on an O-ring seal on the upper rim of the borosilicate glass mercury reservoir-capillary to contain the mercury and exclude air. Leaks are possible both between the plunger and O-ring and between the O-ring and reservoir rim.

The following replacement parts available from PAR are required to make the modification: One PAR G104 UME Capillary; one PAR 2517-0736-29 UME metering rod plunger.

This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society