Anal. Chem. 1984,56,1033-1034
obtained by the two methods of detection for benomyl as 2-AB at a 1 ppm level or greater. The RTP detector lacked the sensitivity to determine lower levels due to solvent or crop extract interferences. R T P was found quite useful for conf i i a t i o n of benomyl residues at the actionable levels of 7 ppm for apples and 10 ppm for grapes. Registry No. &map, 132-66-1;alanap sodium salt, 132-67-2; asulam, 3337-71-1; 2-aminobenzimidazole, 934-32-7; benomyl, 17804-35-2; carbaryl, 63-25-2; coroxon, 321-54-0; coumaphos, 56-72-4; devrinol, 15299-99-7;diphenylamine, 122-39-4; mobam, 1079-33-0;morestan, 2439-01-2; warfarin, 81-81-2; carbophenothion, 786-19-6; diquat, 2764-72-9; paraquat, 4685-14-7; phenmedipham, 13684-63-4;pyrolan, 87-47-8; dexon, 140-56-7.
LITERATURE CITED ( I ) Dalterio, R. A,; Hurtublse, R. J. Anal. Chem. 1982, 54, 224. (2) Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1980, 52,656. (3) Parker, R. T.; Freedlander, R. S.; Schulman, E. M.; Dunlap, R. B. Anal. Chem. 1879, 51, 1921. (4) Schulman, E. M.; Parker, R. T. J. Phys. Chem. 1977, 87, 1932. (5) Cline Love, L. J.; Skrilec, M.; Habarta, J. G. Anal. Chem. 1980, 52, 754. (6) Von Wandruszka, R. M. A.; Hurtubise, R. J. Anal. Chem. 1977, 49, 2164. (7) Paynter, R. T.; Wellons, S.L.; Winefordner, J. D. Anal. Chem. 1974, 46, 736. ( 8 ) Parker, R. T.; Freedlander, R. S.; Dunlap, R. B. Anal. Chim. Acta 1960, 719, 189. (9) Parker, R. T.; Freedlander, R. S.;Dunlap, R. 8. Anal. Chim. Acta 1980. .- - -, 120 -- , 1
(IO) Seybold, P. G.; White, W. Anal. Chem. 1975, 47, 1199. (11) Aaron, J. J.; Winefordner, J. D. Analusls 1979, 7, 168. (12) Moye, H. A.; Winefordner, J. D. J. Agrlc. FoodChem. 1965, 73,516.
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(13) “Pesticide Analytical Manual”: Volume I,Methods Which Detect Muitlple Resldues; U S . Department of Health and Human Services, Food and Drug Admlnistration; revised April 1983. (14) “Pesticlde Analytical Manual”; Volume 11, Method for Individual Residues; U S . Department of Health and Human Services, Food and Drug Admlnlstration; Manufacturer’s Petition for Registered Pesticides, revised Sept 1983. (15) Miller, J. N. Trends Anal. Chem. 1981, 1 , 31. (16) Aaron, J. J.; Kaieel, E. M.; Winefordner, J. D. J. Agric. Food Chem. 1879, 27, 1233. (17) Schulman, E. M.; Walling, C. J. Phys. Chem. 1973, 77, 902. (18) Kwasnik, J. M.A. Thesis, State University College, Buffalo, NY, 1980. (19) Jakovljevic, I.M. Anal. Chem. 1977, 49, 2048. (20) VoDinh, T.; LueYen, E.; Winefordner, J. D. Talanta 1977, 24, 146. (21) Aly, Osman M.; El-Dib, M. A. Water Res. 1971, 5, 1171. (22) Mallet, V. N.; Volpe, Y. Anal. Chim. Acta 1978, 97,415. (23) Gray, W. F.; Pomerantz, I. H.; Ross, R. D. J. Heterocycl. Chem. 1972, 9, 707. (24) Noegel, K. A.; Mobay Chemical Corp., personal communication, 1961. (25) Pease, H. L.; Gardiner, J. A. J. Agric. Food Chem. 1868, 17, 267.
Joseph J. Vannelli U.S. Department of Health and Human Services Food and Drug Administration Buffalo, New York 14202 E. M. Schulman* Department of Chemistry State University College a t Buffalo Buffalo, New York 14222
RECEIVED for review September 19,1983. Accepted February 3, 1984. Abstracted in part from the thesis submitted by Joseph J. Vannelli to the Department of Chemistry, SUCB in partial fulfillment of the requirements for the M.A.
Silicone Rubber Tubing for Elimination of Background Conductivity in Anion Chromatography Sir: The purpose of this correspondence is to point out the potential utility of the silicone rubber tubing in the vacuum flask “deoxygenator“ described by Reim (1) for eliminating background signals in ion chromatography. In the popular ion chromatographic (IC) analysis technique originally described by Small, Stevens, and Bauman (Z),dilute solutions of sodium carbonate and/or sodium hydrogen carbonate are often used as the eluent for anion determinations. After the separation of sample anions by an anion exchange “separator” column, the mobile phase passes through an H-form, cation exchange, “suppressor” column (or “hollow fiber”) which substitutes hydronium ions for all of the cations originally in both the sample aliquot and the eluent. A typical solution passing through the conductivity detector used with these instruments consists of sub-part-per-million concentrations of the acids formed from the sample anions in water containing 5 mM/L of dissolved C02. Even though carbonic acid is a “weak” acid (pKl = 6.45 (3)),ita relatively large concentration causes the background conductivity signal to be typically greater-often lox greater-than that of the analyte ions. Typical IC detection systems incorporate a “backing” or “zero” control to null out this base line offset. However, the existence of the background signal is nevertheless objectionable for t&ee reasons: First, any perturbation in the dissolved C 0 2 concentration causes an annoying shift in the base line from which and@ signals must be measured (e.g., the “water dip”). Second, in any form of instrumental analysis, the subtraction of large base lines causes an increased “noise” level in the net analytical response which is detrimental to the detection capability. Third and most important, the high background signal makes eluent strength programming (e.g., 0003-2700/84/0356-1033$01 SO10
start an elution with 1 mM/L NaHCO, and finish with 10 mM/L Na2C03)impractical. In principle, a practical eluent strength programming system would offer the same advantages in IC that solvent programming does in HPLC (or temperature programming does in GC), Le., a greatly enhanced range of analytes determinable in a reasonable time with a single set of experimental parameters. The oxygen removal efficiency of Reim’s device (1) is approximately 99% for aqueous solvents. One factor disfavors while another favors its potential efficiency for removing C02 relative to 02.The unfavorable factor is the higher solubility of C02 in water. The Henry’s law coefficient K in the equation X = P / K (where X is the mole fraction and P is the partial pressure (in torr) of the gas) is 1.08 X lo6 for C 0 2 vs. 2.95 X lo7 for 02 a t 20 “C (4). The favorable factor is that the permeability of CO2 through silicone rubber is 5.6 times that of 0 2 (5). Warming the solution while it is in the silicone rubber “postsuppressor” tubing should both speed the kinetics of the reaction (HzC03 C02 H20) supplying the gas to the donor side of the membrane and simultaneously reduce ita solubility in water. With Reim’s original system this could be accomplished by incorporating a “warm finger” (around which the tubing is wrapped) in the vacuum flask. A more practical approach is to eliminate the vacuum system entirely and simply immerse the coil of tubing in a warm, gently agitated, solution of a dilute base (e.g., 0.1 M KOH) which serves as the “sink” for the acidic gas. Some preliminary results seen with a simple example of the latter type of system are listed in Table I. In these experiments, “standard eluent” (0.003 M NaHC03/0.0024 M
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+
0 1984 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
Table 1. Carbonic Acid Removal with a 3.6-m Silicone Rubber Tubing, “Postsuppressor”a,b , c conductivity, PS
experimental conditions
1.67
water, no eluent, no postsuppressor std eluent, no postsuppressor as above, postsuppressor in KOH 23 “C as above, postsuppressor in KOH 34 “C as above, postsuppressor in KOH 49 “C as above, postsuppressor in KOH 56 “C as above, postsuppressor in KOH 70 “C as above, postsuppressor in KOH 73 “C as above, postsuppressor in KOH 79 “C
19.8 4.77 4.06 3.00 2.67 2.03 1.87 1.61
22/ 2 11
% background
EO
73 87 93
‘1
a /I
I;
-
- “I
I
I
I
98 99 100T
%background reduction= (1- [(cond- 1.67)/(19.81.67)])X 100. A Dionex Model 2000i ion chromatograph was used with an eluent flow rate of 2.0 mL/min for these experiments. The 3.6-m postsuppressor added an indicated 25 psi of backpressure and 0.24 mL of “dead volume” to the system. The distilled water used in the first experiment probably had some carbonic wid in it.
i
-
95
a
21
A C
reduction
2 MINUTES
4
Figure 2. Responses seen for acetate both with (A) and without (B) the “postsuppressor”; same conditions as for Figure 1.
carbonic acid usually present. The usual case, therefore, is that the carbonic acid (background) conductance simultaneously goes down when that of the analyte species goes up and only the algebraic difference is actually seen as the ”analyte signal”. Another aspect of this phenomenon is demonstrated in Figure 2. Here, the weak-acid analyte’s (7.5 ppm acetate) conductance response is enhanced by a factor of 400% by the “postsuppressor”. Normally, the dissociation of the acetic acid formed in the suppressor column is strongly suppressed by concomitant carbonic acid. Calculatiomindicate that even greater signal enhancements are expected for anions of acids with still higher pK, values. In our opinion silicone rubber tubing is superior to the expanded poly(tetrafluoroethy1ene) (Gore-Tex) tubing previously studied for this application (6). The fundamental practical limitation of presently available forms of Gore-Tex tubing is that only very limited pressure differentials (about 10-12 psi with new, clean tubing-much less with dirty or previously wetted tubing) can be applied before it leaks. This severely limits the type and number of devices (e.g., detectors, fittings, additional columns, etc.) which can follow the “postsuppressor”. At the present time, we are unable to rapidly investigate all of the practical ramifications of this idea because of the pressure of other work. However, the potential benefits of being able to apply “solvent strength programming” to IC may encourage interested readers of this journal to give the idea a thorough examination in their own laboratories.
4pvLA 1
MINUTES Figure 1. Determination of fluoride and chloride with (A) and without (B) a “postsuppressor”: standard eluent at 2 mllmin; Dionex No. 57618 separator column; Dionex No. 27802 anion suppressor column; “postsuppressor” tubing in 7 8 *C, 0.1 M KOH.
Na2C03)was pumped first through a conventional suppressor column and then through 3.6 m of the same silicone rubber tubing (0.31 mm i.d. by 0.6 mm 0.d.) used by Reim. This tubing was immersed in a gradually warmed solution of 0.1 M KOH contained in a large beaker situated on a hot plate. The data indicate that essentially all of the carbonic acid is removed at higher temperatures. Figure 1shows ion chromatograms obtained both with (A) and without (B) this “postsuppressor”. The sample consisted of 0.4ppm F(-)and 1.5 ppm Cl(-) (as sodium salts) in distilled water. Changes brought about by the “postsuppressor” include a drop in the base line signal from 20.3 FS to 1.68 wS, complete elimination of the “water dip”, and a 45% enhancement of the analyte response seen for both ions. The increased sensitivity is due to the fact that all strong-acid analyte ions cause significant suppression in the degree of ionization of the
LITERATURE CITED (1) Reim, R. E. Anal. Chern. 1983, 55, 1188-1191. (2) Small, H.; Stevens, T.; Bauman, W. Anal. Chem. 1975, 4 7 , 180 1-1809. (3) Laitinen, H. A,; Harris, W. E. “Chemical Analysis-2nd Ed.”; McGrawHlll: New York, 1975; p 111. (4) “Handbook of Chemistry and Physics-54th Ed.”; Chemical Rubber Co.: Cleveland, OH, 1963; p 1708. (5) Perry, R. H.; Chilton, C. H. “Chemicals Engineers Handbook-5th Ed.”; McGraw-Hill: New York, 1973; pp 17-35. (6) Sunden, T.; Cedergren, A,; Siemer, D. Anal. Chem., in press.
Darryl D. Siemer* Vergil J. Johnson Exxon Nuclear Idaho Co. Box 2800 Idaho Falls, Idaho 83402 RECEIVED for review November 17, 1983. Accepted January 1, 1984.
