816
Anal. Chem. 1983, 55, 810-812
separate from the nonvolatiles.
LITERATURE CITED
ACKNOWLEDGMENT Coal sample supplied by Pennsylvania State University Coal Data Base Sample No. PSOC-371. Registry No. butane, 106-97-8; pentane, 109-66-0.
(1) Stenhagen, E., Abrahamsson, S., Mclafferty, F. W., Eds. ”Registry of Mass Spectral Data”; Wlley: New York, 1974; Vol. 1.
RECEIVED for review June 10, 1982. Resubmitted August 5, 1982. Accepted January 5,1983. Supported in part by D.0.E .. Contract DEF 79ET 6003.
Removal of Oxygen from Solvents for Llquid Chromatography with Electrochemical Detectlon Fred Senftleber,* Daniel Bowling, and M. Sonla Stahr Depafiment of Chemistry, Murray State University, Murray, Kenrucky 4207 1
In the development and application of electrochemical detection for use in liquid chromatographic procedures, the oxidative mode of detection has dominated the literature to date (1-4). High background current from the reduction of oxygen impurities in the mobile phase has been a major deterrent to the use of reductive detection (4,5).T o decrease the magnitude of this background current, investigators have commonly employed one of three approaches. The most popular technique is to remove dissolved oxygen from the eluent solvent with an inert gas purge (6-9). Although moderately effective, this technique is beset with several problems. Time requirements can be excessive. Lund et al. (IO),for example, suggest bubbling argon through the mobile phase for at least 4 h before introducing the eluent into the chromatograph. Voluminous inert gas consumption and loss of eluent through evaporation can be additional problems. When mixed solvents or salt solutions are employed, solvent evaporation can lead to significant change in the mobile phase composition. Other investigators (11,12) have suggested continuously refluxing the mobile phase as a means to remove dissolved oxygen. In our laboratory, we have not found this technique to be entirely successful. Significant background currents were generally still observed. Recently, Hanekamp et al. (5)introduced a two-electrode on-line “scrubber” to electrochemically remove oxygen and other reducible impurities from the eluent solvent. This system appears to be very efficient but requires a stable high current potentiostat for operation. Ironically, to minimize current levels, the authors suggest that a nitrogen purge be used to “preclean” oxygen from the solvent. In choosing a mobile phase for use in this system, care must be exercised to ensure that products from the oxidation and reduction processes in the scrubber do not react chemically with sample molecules subsequently injected into the eluent stream. In this paper an apparatus is described which overcomes the disadvantages associated with other mobile phase deoxygenating techniques. By use of a series of alternating evacuation-refdl cycles, the system minimizes the consumption of inert gas, the evaporation of solvent, and the formation of bubbles in the detector due to outgassing. A typical deaeration procedure using the present system requires less than 15 min.
EXPERIMENTAL SECTION System Description. A schematic representation of the apparatus is given in Figure 1. The main body of the system (A), constructed from borosilicate glass, consists of two halves joined via 25 mm i.d. O-ring joints (e) to provide a greaseless vacuumtight seal (Ace Glass, O-ring Seal T Joints, Catalog No. 7648, work equally as well). The bottom section of the apparatus is a 500-mL Erlenmeyer-shaped flask (f) equipped with a Teflon-coated magnetic bar (i). The top half of the system contains vacuum
and inert gas inlet ports (a) and (b), respectively, and a Teflon needle valve (d) through which mobile phase can be removed. The oblique bore, three-way stopcock (c) prevents opening of the system to the vacuum and inert gas ports simultaneously. Connection between the apparatus and a liquid chromatograph is made with lIs in. stainless steel tubing (not shown). A 1/4 X lIs in. stainless steel reducing union (Swagelok Fitting 400-6-2) employing a Teflon ferrule on the 1/4 in. side provides an airtight connection between the stainless steel tubing and the glass outlet of the needle valve. Section (B) of the system serves as a safety bubbler. Through the incorporation of a liquid-seal check valve (n), air and bubbler oil cannot be inadvertently drawn into the system. For correct operation, the oil level in the bubbler must be above the seat on the valve. System Operation. The apparatus is first assembled as discussed above and purged with an inert gas. No eluent is present in flask (f) during this procedure. By opening needle valve (d) and correctly positioning stopcock (c), one can permit inert gas to flow through the system displacing the air in the stainless steel tubing connecting the apparatus to the liquid chromatograph. In our laboratory where a Waters Associates (Melford MA) Model M6000A chromatographic pump is used, the inert gas is also permitted to purge the input manifold and solvent takeoff valve on the pump. After closing stopcock (c), needle valve (d), and the solvent takeoff valve, flask (f) is removed and the system is ready for operation. Approximately 300 mL of filtered mobile phase is placed in the flask (f) and the flask is reconnected to the apparatus. With the magnetic bar stirring, stopcock (c) is positioned to evacuate the system. The magnitude of the vacuum and its duration are described in the Results and Discussion section. After the appropriate time, stopcock (c) is turned to permit inert gas to enter the flask. The system is then cycled alternately between vacuum and inert gas until the desired oxygen level is attained. To start the solvent into the liquid chromatograph, temporarily cover outlet (1) on the safety bubbler and open needle valve (d) on the apparatus. When the solvent takeoff valve on the liquid chromatograph is opened, a positive inert gas pressure forces the mobile phase to the chromatograph’s input manifold. Uncover safety bubbler outlet (1) and adjust the inert gas flow to a rate sufficient to keep a blanket over the solvent. Instrumentation. Electrochemical experiments were performed with an IBM Model EC/225 voltammetric analyzer equipped with a Bausch & Lomb Series 100 Omnigraphic X-Y recorder. Materials. Argon and nitrogen employed in the experiments contained less than 5 ppm and 10 ppm oxygen, respectively. Aqueous solutions were prepared with doubly distilled water. All chemicals were reagent grade and used without further purification.
RESULTS AND DISCUSSION The effectiveness of the present system to remove dissolved oxygen from an eluent solvent was demonstrated through two
0003-2700/83/0355-0810$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4,APRIL 1983
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I
B
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VOLTS vs SCE
Figure : System evaluation: linear sweep voltammogram pH 7.8, 0.10 M phosphate solution: scan rate, 200 mV/s; (a) before deaeration: (b) after 1.0 mln of evacuation, O2concentration = 15.7% of curve (a); (c)after 2.0 mln of evacuation, O2concentration = 9.5% of curve (a): (d) after 4.0 min of evacuation, O2concentration = 2.9% of curve (a).
Figure 1. (A) Solvent deaeration apparatus: (a) size 1819 ground-glass bail joint (connection to vacuum source): (b) size 1819 ground-glass
socket joint (connection to bubbler); (c)three-way 3 mm ground-glass stopcock: (d) Teflon needle valve, 0-3 mm: (e) O-ring joint, 25 mm: (f) 500-mL flask: (9) Teflon tubing, in. 0.d.: (h) stainless steel fllter, 2 pm porosity: (I) Teflon-coated magnetic stirring bar. (B) Safety bubbler: (j) size 18/9 groundglass ball joint (connectionto deaeration apparatus): (k) inert gas inlet tube: (I) bubbler outlet tube: (m) oil level: (n) ground-glass liquid-seal check valve. sets of experiments. In the first series, the oxygen in a pH 7.8, 0.10 M sodium phosphate solution was monitored as a function of the evacuation time in the deaeration cycle. Each experiment involved placing a 75-mL aliquot of phosphate solution into the apparatus, evacuating to 45 torr pressure with a water aspirator for a set period of time, and backfilling the apparatus with argon. A portion of the solution was then transferred through the needle valve on the apparatus to a three-electrode electrochemical cell using a positive argon pressure developed by temporarily covering the outlet tube (1, Figure 1)on the safety bubbler. To minimize the possible introduction of oxygen from other sources, we excluded a chromatographic column and pump from the evaluation system. Oxygen levels in the solvent were measured a t a mercury drop electrode using linear sweep voltammetry. The voltammetric technique allowed us to discriminate between oxygen and other possible reducible impurities in the test solutions. Results from a typical series of experiments are shown in Figure 2. As illustrated by the reduction in peak current, both the oxygen level in the phosphate solution and the rate of oxygen removal were a function of the evacuation time. After the apparatus was evacuated for 1 min, the oxygen level decreased to about 15.7% (curve b) of its original value (curve a). Increasing the time to 2.0 and 4.0 min resulted in a reduction of the level to 9.5% (curve c) and 2.9% (curve d), respectively. Although higher percentages of oxygen were removed by using the longer evacuation times, the greatest reduction in oxygen concentration per unit time was achieved during the first minute. A second series of experiments involved repeatedly cycling a single 300-mL aliquot of phosphate solution through the evacuation-refill sequence. Each evacuation period was 1.0
min in length and nitrogen was used as the inert refill gas. After the system was cycled once, the oxygen level in the phosphate solution was found to be 20.5% of the original concentration. This somewhat higher concentration, as compared with the 15.7% observed in the first series of experiments (Figure 2, curve b), can probably be attributed to the larger volume of solvent used in this experiment. The use of nitrogen instead of argon as the refill gas should have had little, if any, influence on the efficiency of the system. Cycling the system a second time reduced the oxygen concentration in the solution to 4.3% of its original level. The third cycle decreased the level to 1.6% and the fourth cycle to 0.3%. The removal of dissolved oxygen by repeatedly cycling the system was a cumulative process. Each cycle reduced the oxygen concentration to an average 24% of the level existing before that cycle. After five consecutive 1-min evacuation-refill cycles, the current measured in the voltammogram was indistinguishable from background. The ability of the present system to deoxygenate the eluent solvent can be examined in terms of Henry’s law. According to this law, the solubility of oxygen in the mobile phase is directly proportional to the partial pressure of oxygen above the solvent. Through the use of the evacuation-inert gas refill cycle, the present apparatus reduces the oxygen pressure above the mobile phase, and thus the oxygen solubility, to a fraction of that existing before the cycle. The most effective use of the system, as illustrated by our experiments, is achieved through repetitive cycling of the apparatus rather than by employing a single lengthy evacuation period. Four 1-min cycles, for example, reduced the oxygen level in the phosphate solution to 0.3% of its original concentration as compared with a reduction to 2.9% using one 4-min evacuation. If one assumes an initial dissolved oxygen concentration of 2.5 X mol L-l (13) and a reduction to 24% in each cycle, four 1-min cycles would reduce the oxygen level in the solution to less than 1 x lo4 M. With additional cycles, assuming a leak-free system and the use of an inert gas containing less than 0.1 ppm 02,it should be possible to reduce the oxygen concentration in the solvent to the nanomolar level, a level well below the detection limit of the voltammetric technique utilized in the present study. The apparatus described in the present paper has been successfully employed in deaerating mobile phases which have been utilized in the liquid chromatographic separation and reductive electrochemical detection of derivatized polyamines (14). The system is efficient, relatively inexpensive, and minimizes the solvent evaporation and inert gas consumption
812
Anal. Chem. 1083, 55, 812-813
difficulties experienced with previous techniques described in the literature. In addition, the rapid deaerating capabilities of the present apparatus allows for the rapid change of solvents during method developments. Registry No. Oxygen, 7782-44-7.
LITERATURE CITED (1) Heineman, William R.; Klsslnger, Peter T. Anal. Chem. 1978, 50,
186R-175R. (2) Heineman, William R.; Klsslnger. Peter T. Anal. Chem. 1880, 52,
-
138R-151R . _.. . - .. ..
(3) Majors, Ronald E.; Barth, Howard G.; Lochmiiller. Charles H. Anal. Chem. 1982, 54, 323R-363R. (4) Ryan, Michael D.; Wllson, George s.Anal. Chem. 1982, 54, 2 0 ~ 27R. ( 5 ) Hanekamp, H. B.; Vwgt, W. H.; Bos, P.; Frel, R. W. Anal. Chlm. Acta 1980, 118, 81-86. (6) MacCrehan, W. A.; Durst, R. A. Anal. Chem. 1978, 50, 2108-2112.
(7) Eggll, R.; Asper, R. Anal. Chlm. Acta 1978, 101, 253-259. (8) Hanekamp, H. R.; Voogt, W. H.; Bos, P.; Frei, R. W. Anal. Lett. 1979, 12. 175-189. (9) Funk, Max. 0.; Keller, Maggle 8.; Levison, Bruce Anal. Chem. 1980, 52. 771-773.
(IO) Lund, Walter; Hannlsdal, Merete; Grelbrokk, Tyge J . Chromatogr. 1979, 173, 249-281. (11) Brunt, K.; Bruins, C. H. P. J . ChrQmatOgr. 1978, 161, 310-314. (12) Michel. L.; Zatza, A. Anal. Chlm. Acta 1979. 105. 109-117. (13) Lange, Norbet Adolph, Ed., "Handbook of Chemistry", 9th ed.; Handbook Publishers: Sandusky, OH, 1956;p 1091. (14)Senftleber, Fred; Bowling, Daniel; Stahr, Sonla 33rd Annual Pittsburgh Conference, March 8, lg82,Atlantic City, NJ, paper 077.
RECEIVED for review August 3, 1982. Accepted December 10, 1982. This work was funded by the Committee for Institutional Studies and Research, Murray State University, under Grant Numbers 454 and 212724.
Determination of Lead In Plant Ash by X-ray Fluorescence Spectrometry Mark L. Dletz and Stanford L. Tackett" Department of Chemistry, Indlana Unlverslv of Pennsylvanla, Indiana, Pennsyhanla 15705
The use of X-ray fluorescence spectrometry for the analysis of plant tissue is well established. First introduced in 1958 by Brandt and Lazar (1)for the determination of manganese, cobalt, zinc, and molybdenum, the technique has since been expanded to permit the determination of a number of major and trace elements in plant material. At the same time, a variety of sampling methods have been introduced, thereby permitting the material to be analyzed in any one of several forms. Typically, plant tissue is analyzed as a compressed disk or pellet of the dried material (2-5). Alternately, a sample may be analyzed in the form of a loosely packed dry powder (6),as an aqueous solution (7), or as an ash, after deposition upon a filter paper disk (8). All of these methods, however, suffer from shortcomings which make them not entirely satisfactory. For example, not all types of samples may be compressed into self-supporting disks or pellets. In such cases, binders such as cellulose must be added to increase the rigidity of the sample disk. This, however, often precludes further use of the sample (9). Loose packing of dry samples, while simple and rapid, is generally not precise enough for quantitative work (3). Aqueous solution methods can involve tedious sample preparation steps, such as digestion of samples in nitric acid (7). Lastly, analysis of ash after deposition on a paper disk offers an accuracy of only k 1 5 % for many elements (8). In conjunction with our studies of the uptake of heavy metals by plants, we have sought a simple technique for use with plant tissue specimens not readily converted to selfsupporting disks or pellets, a technique more precise and accurate than loose-packed dry sample or deposited ash methods, but without the tedious sample preparation associated with aqueous solution methods. The present paper describes such a technique. Specifically, it describes a new sample holder and illustrates its use in the determination of lead in spinach leaves. EXPERIMENTAL SECTION Apparatus. The sample holder consisted of an XRF polyethylene sample cup (Chemplex Industries, Inc.) modified by the authors by fitting it with a core of cast acrylic monomer into which a depression (1.50 cm, diameter and 0.46 cm. maximum depth) had been bored with a radius generator. Samples were held in this depression by a thin Mylar film cover secured with a polyethylene ring. The holder is shown in Figure 1. Analyses were performed with a Picker solid-state X-ray fluorescence spectrometer comprised of a Model 6238 X-ray
geenrator, a Model 6239 spectrodiffractometer,and a Model 6245 radiation analyzer. A tungsten target X-ray tube operated at 50 kV and 30 mA was used in conjunction with a LiF diffracting crystal and a scintillation detector. Instrument settings were those recommended as optimum by the manufacturer (10). Dry ashing of plant material took place in a Hoskins FD 204 C electric furnace. Reagents. Deionized water was used in the preparation of all solutions. The stock solutions of lead were prepared by dilution of a 1000 wg/mL lead reference solution (Fisher Scientific Co.). The ashing aid was prepared by addition of concentrated sulfuric acid (Ultrex, J. T. Baker Chemical Co.) to an equal volume of deionized water. Procedure. Oven-dried spinach leaf samples were prepared for analysis by dry ashing overnight at 430 OC (11). A 2-mL aliquot of the ashing aid was added to each sample prior to ashing to ensure complete recovery of lead (12)and to reduce the amount of unoxidized carbon remaining. Once prepared, a 0.3OOO-g portion of each sample ash was weighed into the sample holder and the intensity of its Pb La1 emission measured by accumulating counts for a period of 120 s at 33.92O 20. During measurement, samples were rotated at a rate of 30 rpm. Each sample was counted three times, and the emission intensities were averaged, Background radiation was measured by counting at two values of 20, one on either side of the Pb La1 peak. The background count for the sample was then determined from these values by interpolation. This background was subtracted from the average peak count rate to obtain the net count rate for the sample. The lead content of each was then determined from a calibration curve obtained by evaluation of a series of ash standards. These standards were prepared by addition of various aliquots of a 100 pg/mL lead stock solution to weighed portions of commercially obtained spinach. The method used for the ashing and for the evaluation of the standards was identical with that described for the samples.
RESULTS AND DISCUSSION For evaluation of the accuracy of the lead determinations performed by using the new sample holder, the lead contents of a series of spinach samples which had been spiked with various aliquots of a 500 pg/mL lead stock solution were determined. The aliquot added was, in each case, chosen so that the lead content of the ash would be less than 1.00% by weight, as above this the calibration curve is nonlinear. The results of these determinations are presented in Table I. It can be readily seen from these results that there is good agreement between the amounts of added lead and those determined experimentally over the entire range of concentrations examined.
0003-2700/83/0355-0812$01.50/00 1983 American Chemical Society
