750
Anal. Chem. 1981, 53, 750-751
Table 11. Comparison of Mg*+and Ca*+Concentrations in Pond Water (PW) and Filtered Blood Serum (FBS) Samples Measured by Ion Chromatography and by Atomic Absorption Spectrometry Mgz+concn, Ca2+concn, PPm PPm sample IC AAS IC AAS PW1 PW2 PW3
FBS, pH 1.05 FBS, pH 1.17
381 147 97 21 22
399 145 97 22 21
575 384 225 134 138
561 370 210 122 121
suppressor system, a peak amplification factor of about 20 (compared to 5 for the sulfate suppressor system) was observed with the postcolumn present. This large amplification factor does not imply greater sensitivity of the iodate system over the sulfate system. Rather, it resulted largely from the fact that in the absence of the postcolumn, peaks were very small. The OH--exchange secondary suppressor column was used only when analyzing for alkaline earth cations since transition-metal ions are precipitated in this column. In fact, this column may be used when analyzing mixed samples containing both alkaline earth cations and transition-metal cations to eliminate transition-metal ion peaks which may interfere with the Ca2+peak. Like the H+-exchangepostcolumn, this column served to increase the sensitivity of the method. However, unlike the H+ postcolumn, the OH- secondary suppressor operated by removing the background H+ conductivity, effectively lowering the base-line conductivity above which cation peaks were detected. This column gave a smoother, more stable base line than did the H+postcolumn. This latter observation confirmed the hypothesis ( I ) that much of the base line noise associated with these eluent-suppressor systems is a result of pH fluctuations. The cation retention times for the iodate system were similar to those for the sulfate system ( I ) except in the case of Cu2+which eluted with the other transition-metal cations at 8.2 min. The Cu2+peak was not broadened as it was with the sulfate suppressor system. The sensitivity in M/@ of the iodate system (Table I) was comparable to that of the sulfate system for all cations except Cu2+,for which the iodate system was approximately twice as sensitive as the sulfate system. By use of the iodate system, Ba2+could be determined along with the other alkaline earth cations. The retention time for Ba2+was 19 min. The Ba2+peaks were symmetrical and not much broader than those for Sr2+,although sensitivity for Ba2+
Fl7l-T5
0
10
minutes Flgure 1. Chromatogram of pond water sample no. 3 using 1.0 mM Pb(NO& eluent (pH 4), 30% pump rate (2.3mllmin), with 3 X 150 mm and 6 X 250 mm separator columns, 6 X 250 mm suppressor, and 3 X 150 mm postcolumn. Full scale is about 15 pa-’. The large
peak before Mg2+and Ca2+is due to monovalent cations.
was not as great as for the other alkaline earth cations (Table I\
1).
Blood serum samples, prepared as outlined previously ( I ) , as well as pond water samples were analyzed for Mg2+and Ca2+using the iodate system. The results of these analyses are given in Table I1 and a representative chromatogram appears in Figure 1. Good agreement was found between results obtained by ion chromatography and by atomic absorption spectrophotometry.
ACKNOWLEDGMENT We are greateful to S. Jerry Rehfeld of the Veteran’s Administration Hospital, San Francisco, CA, for helpful discussions and for providing filtered blood serum samples and to L. B. Merritt of Brigham Young University for providing pond water samples.
LITERATURE CITED (1) Nordmeyer, F. R.; Hansen, L. D.; Eatough, D. J.; Rollins, D. K; Lamb, J. D. Anal. Chem. 1980, 52, 852-856. (2) Pethybridge, A. d.; Prue, J. E. Trans. Faraday SOC.1967, 63, 2019-2033. (3) Anderson, K. P.; Snow, R. L. J . Chem. Educ. 1967, 44, 756-757.
RECEIVED for review October 14,1980. Accepted November 24, 1980.
Erlenmeyer Flask-Reflux Cap for Acld Sample Decomposition Darryl D. Slemer” and Harry G. Brinkley Exxon Nuclear Idaho Company, Box 2800, Idaho Falls, Idaho 8340 1
When acid decomposing organic or biological samples (e.g., filter papers) for subsequent chemical analysis, it is still common practice to recommend either watch glass covered beakers or Kjeldahl flasks as digestion vessels (I). The beaker-watch glass combination has the disadvantage that considerable sample and spattered acid tend to “hang up” and dry out on the watch glass and the rim of the beaker. This necessitates an awkward, time-consuming, and possibly dan0003-2700/81/0353-0750$01.25/0
gerous rinsing step to get everything back into the beaker before proceeding with the decomposition. Very gentle, prolonged, heating of the digestion vessel is often recommended instead of a fast, vigorous, sample attack, to avoid this extra work. The Kjeldahl flask system requires careful heating with a special heating apparatus and manifold assembly in order to avoid sample expulsion from the neck of the flask. This 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
Table I. Recovery Study of As I11 on Filter Paper time for method of digestion digestion, min
no. of samples
mean recovery,a %
re1 std dev, %
751
blank response/ sample response: %
flask-reflux cap 10 5 99.7 3 -4 2.2 beaker-watchglass 270 5 99.4 3.5 2.5 Based on comparison of the relative analytical responses of half of the digestate prepared in each instance to that of 1-pg “Blank response” spike of arsenic added directly to the hydride generation assembly (corrected for acid reagent blanks). refers to analytical response (peak height AAS signal) observed when unspiked filter paper is taken through each digestion procedure.
Figure 1. Reflux cap inserted into the neck of an Erlenmeyer flask: (A) lip or ridge to support reflux cap; (6)7 or 8 mm diameter vapor escape hole; (C) spout curved to touch side of flask at lower lip. generally makes it quite difficult to add any extra sample decomposition reagents to the flask without cooling and disassembly of the entire system. This paper describes the use of simple glass reflux caps with standard Erlenmeyer flasks for these decompositions. This system permits a considerable savings in time and effort on the part of the analyst. G. Frederick Smith described the use of these reflux caps in 1953 (2),but a new generation of analysts do not seem to be aware of their advantages. Figure 1shows a cap in the neck of an Erlenmeyer flask. Its important design features include an expanded ring to prevent the cap from falling into the flask, a curved spout which touches the wall of the flask, and a hole on the inside of the curve of the spout through which the bulk of the vapors generated can escape. This design prevents loss of droplets during the initial spattering (which usually occurs if the sample is rapidly heated) because there is no straight-line ballistic path out of the flask. Additional desirable features of the design include the fact that refluxing acid continuously rinses down both the walls of the flask and the lower surface of the cap and that any extra reagents required (e.g., hydrogen peroxide) can be added to the flask conveniently and safely a t any time by simply pipetting them into the reflux cap. The reflux cap-Erlenmeyer flask sample decomposition system permits very rapid sample preparation because the sample and acid mixture can be heated very vigorously without fear of loss of analyte. In our practice, the decomposition
reagents (usually nitric and sulfuric acids) are added to the sample, the cap put in place, and the flask placed directly into a preheated (maximum heat setting 350-400 “C) hotplate in a hood. If the sample shows evidence of charring as the last of the nitric acid boils away, more nitric acid is added dropwise until the solution is clear and colorless as dense, white, SO3 fumes fill the flask. If the final analytical procedure requires quantitative removal of the oxides of nitrogen, the cap can be removed for a minute or so after the sulfuric acid starts to quietly reflux down the sides of the flask. Then the cap is replaced and the flask is set onto a wire screen or asbestos pad to cool for a few minutes. The final dilution of the acid solution is made by adding the water (or other diluent) through the reflux cap, The cap prevents spatter loss as the solvent reacts with the concentrated sulfuric acid remaining in the flask. One-half gram samples of a wide variety of organic substances (including ham, tert-butyl phosphate, glycerol, peanuts, filter paper, mayonaise, and bread) can be easily and completely mineralized in less than 10 min by using 125-mL flasks, 1 mL of 18 M sulfuric acid, and from 2 to 5 mL of 12 M nitric acid. A recovery study of 2-pg As I11 “spikes” pipeted onto 5.5-cm disks of Whatman No. 40 filter paper revealed that no measurable fraction of arsenic is lost by use of this digestion procedure in lieu of the 4.5-h sample decomposition recipe recommended by the manufacturers of the analytical equipment used for the final arsenic determination (1). The results of the study are detailed in Table I. A Varian Techtron AA6 atomic absorption spectrometer with a Model 64 hydride generation accessory was used for the arsenic determinations. These caps are not standard items in the laboratory glassware catalogs available to these writers but, because no dimension is critical, they are easily made in the laboratory from borosilicate glass tubing.
LITERATURE CITED (1)
Stratton, A. J.; Routh, M. W. “Determination of Airborne Arsenic for Industrial Hygiene Purposes”,Technical Note No. AA4; Varian Tech-
tron Instrument Group: Palo Alto, CA, June 1980. (2) Smith, G.
Frederick Anal.
Chlm. Acta 1053, 8, 397-421.
RECEIVED for review November 4, 1980. Accepted December 22, 1980.
