2015
Anal. Chem. 1985, 57,2015-2016
and triple distilled water. For flushing, a manual command activates the nebulizer until the cell is filled to 2-3 cm above the regular level. There are two backup routes from the atmosphere to the cell causing diffusion of oxygen into the solution. The diffusion route was checked by introducing a solution of 0.1 M KC1 into the cell and recording the reduction waves of oxygen after 1h. The results are shown in Figure 7 . Judging from this figure the rate is about l % / h OF the total amount of oxygen that can be present in air-saturated solution. Further Developments. The use of the SMDE (EG&G PARC Model 303) and a motor-driven RDE is now under inspection. The replacement of one electrode by another is, clearly, rather simple. The use of Vycor frit instead of Sauereisen paste for reducing memory effects is now under inspection. The feasibility of using the automated cell as a monitoring device is being tested. For this purpose, the sample tubing of the nebulizer is inserted in a plating bath. A small filter
is introduced at the end of the tubing, and successive polarograms of the plating solution are obtained by the automated system.
ACKNOWLEDGMENT The author wishes to thank Jordan Valley Applied Radiation, Ltd. (Israel), and the American-Israeli Binational Foundation (Bird-F) for their support. LITERATURE CITED (1) DeAngelis, T. P.; Heineman, W. R. Anal. Chem. 1978, 4 8 , 2262-2263. (2) Freiha, B. A,; Wang, J. Anal. Chem. 1982, 5 4 , 334-336. (3) Wang, J.; Ouziel, B.; Yarnitzky, Ch.; Ariel, M. Anal. Chim. Acta 1878, 102, 99-112. (4) Samuelsson, R.; O'Dea, J.; Osteryoung, J. Anal. Chem. 1984, 5 2 , 2215-2216. (5) Yarnitzky, Ch.; Ouziel, E. Anal. Chem. 1978, 48, 2024-2025. (6) Borman, S. A. Anal. Chem. 1982, 5 4 , 698A-705A.
RECEIVED for review October 9, 1984. Resubmitted April 12, 1985. Accepted April 12, 1985.
Two-Valve Injector To Minimize Nebulizer Memory for Flow Injection Atomic Absorption Spectrometry James M. Harnly* and Gary R. Beecher
U.S. Department of Agriculture, Agricultural Research Service, Beltsville H u m a n Nutrition Research Center, Nutrient Composition Laboratory, Beltsville, Maryland 20705 A single line flow injection system was used for sample introduction for flame atomic absorption spectrometry ( U S ) using a conventional, pneumatic nebulizer. Introduction of samples with high elemental concentrations resulted in contamination of the nebulizer which was not removed during the fill-clean cycle. This paper will describe an alternate, two-valve system which significantly reduced the nebulizer memory effects. The initial flow injector consisted of a standard single line configuration (Figure 1)with a six-port rotary valve (Model No. AH60, Valco Instrument Co., Houston, TX). Pump 1was a variable flow positive displacement minipump (Milton Roy Co., Riviera Beach, FL) operated at flows of 3.2,1.6,0.8, and 0.4 mL min-l. The AAS was a laboratory-built prototype which has previously been described ( I , 2). This instrument is capable of multielement determinations and calibration over 5 to 6 orders of magnitude for each element. In this study, urine was analyzed for K (- 1900 pg mL-l), Mg ( 140 pg mL-l), Na ( 2400 pg mL-l), and Zn (-0.7 pug mL-') either directly or diluted by a factor of 10 with 5% HNOB acid. Reduced flow rates were used to increase the atomization efficiency (3) and maximize the peak area. In both cases, the memory for Na was significant following the introduction of the urine samples. Figure 2 shows the nebulizer memory for the direct determination of 500 MLof urine at a flow rate of 1.6 mL min-l. Data acquisition was triggered when the sample loop was switched into the main stream and continued for 18 s. Injection of the diluted urine (1to 10 in 5% HNOJ reduced the memory effect by approximately 40%. In each case, five to seven atomizationsof the blank were necessary before the base line returned to its original level. Increasing the injection flow rate by 50% and 100% did not significantly reduce the nebulizer memory. Direct aspiration (with a flow rate of 9.0 mL min-l with the injection system removed) of the same sample,
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FILL-CLEAN CYCLE
PUMP 1
w
AUTO-SAMPLER
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ATOMIZE CYCLE PUMP 1
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Figure 1. Schematic diagram of a conventional two-pump, one-valve flow injector.
I:
J
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URINE
BLANKS-
Figure 2. Absorbance traces for the determination of Na (-2400 p g mL-') in a 500-pL urine sample using the two-pump, one-valve flow injector.
This article not subject to U S . Copyright. Published 1985 by the American Chemical Society
2018
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985 FILLCLEAN CYCLE
PUMP 1
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WFy7
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Flgure 3. Schematic diagram for a threepump, two-valve flow injector.
for the same duration, resulted in negligible nebulizer memory with the blank returning to its previous value on the second atomization after the sample. To increase the solution flow rate through the nebulizer during the fill-clean cycle, a second six-port valve and a third pump were added (Figure 3). This resulted in three pumps for each of the three separate functions. Pump 2 filled the sample loop (Figure 3, fill-clean cycle), pump 3 cleaned the nebulizer (Figure 3, fill-clean cycle), and pump 1 delivered the sample to the nebulizer (Figure 3, atomize cycle). Pump 3 was adjusted to a flow rate of approximately 10 mL min-’, exceeding the direct aspiration rate (with the injector system removed). This produced a significant reduction in the
BLANK URINE BLANKS+ Flgure 4. Absorbance traces for the determination of Na (-2400 kg mL-’) in a 500-WLurine sample using the three-pump, two-valve flow injector.
nebulizer memory as shown in Figure 4. Like Figure 2, Figure 4 shows the atomization of 500 MLof undiluted urine at a flow rate of 1.6 mL min-l. The blank signal has returned to its previous value after two atomizations, a reduction of the nebulizer memory by greater than a factor of 10.
LITERATURE CITED (1) Harnly, J. M.; O’Haver, T. C.; Golden, 6.; Wolf, W. R. Anal. Chem. 1979, 57, 2007. (2) Harnly, J. M.; Miller-Ihli, N. J.; O’Haver, T. C. J . Autorn. Chem. 1982, 4 , 54. (3) Wolf, W. R.; Stewart, K. K. Anal. Chem. 1979, 57, 1201.
RECEIVED for review March 12,1985. Accepted April 22,1985. Mention of trademark or proprietary products does not constitute a quarantee or warranty of the product by the U S . Department of Agriculture and does not imply their approval to the exclusion of other products that may also be suitable.
CORRECTION Comparison of Hydrocarbon Composition in Complex Coal-Liquid Samples by Liquid Chromatography and Field-Ionization Mass Spectrometry Todd W. Allen, Robert J. Hurtubise, and Howard F. Silver (Anal. Chem. 1985,57, 666-671). The weight percent of the double bond fractions for the Wyodak coal liquid data in Table I was taken from Boduszynski et al. (Anal. Chem. 1983,55, 225). In addition, mol % data from the Wyodak sample was taken from the previous reference and Boduszynski et al. (Anal. Chem. 1983,55,232). These data were converted to wt % data for the Wyodak sample in Figures 1, 2, 3, and 4 and Table 111. We inadvertently gave the impression, for instance, in our conclusions, that the HPLC-FIMS method discussed in the paper was developed by the present authors while the original combination of HPLC-FIMS for the characterization of hydrocarbons in coal liquids was first introduced by Boduszynski et al. (Anal. Chem. 1983,55, 225 and 232). Work by Boduszynski et al. (Fuel 1984, 63, 93 (ref 24)) is mistakenly referred in the text to St. John et al. “Organic Chemistry of Coal”; Larson, J. W., Ed.; American Chemical Society: Washington, DC, 1978; ACS Symp. Ser. No. 71, Chapter 17 (ref 23).
