Copper(l1) Salts as Amine Abstractors in Gas Chromatography Colin D. Chriswell," Larry D. Kissinger, and James S. Fritz Ames Laboratory, E.R.D.A. and Department of Chemistry, lowa State University, Ames, lowa 500 1 1
An effective amine abstractor for use In gas chromatography was prepared by coating Chromosorb G AW/DMCS with copper( II) chloride. Other abstractor materials prepared were less effective. Pie-columns containing copper( II) chloride coated Chromosorb G retained ail amines tested and had negligible effects on the separation of other types of compounds on analytical columns. Attractive properties of amine abstractors were demonstrated by separating pyridine solutlons of alcohols, ketones, halogenated hydrocarbons, alkanes, alkenes, and aromatic hydrocarbonson appropriate analytical columns with and without the use of amine abstractor precolumns. Applications of the abstractor were demonstrated In methods for determining volatlle components in coal, on modifications of the resin sorption method for determinlng part-per-billion concentrations of organic impurities in water, and for determinlng impurities In reagent grade pyridine.
In gas chromatography a short column containing a material that completely retains a given type of sample constituent has sometimes been used. often called "abstractor" or "subtractor" columns. For example, a short section of boric acid at the end of a conventional gas chromatographic column removes alcohol peaks from a gas chromatogram ( I ) . Other solvents have been reported for removal of aldehydes (I,2 ) , ketones (2),epoxides ( 2 ) ,carboxylic acids (2),and certain hydrocarbons ( 3 ) .Fryka and Pospisil ( 4 ) used phosphoric acid as a subtracting agent for nitrogen bases in reaction gas chromatography. Leathard and Shurlock have a chapter on abstractors for gas chromatography ( 5 ) . It would be particularly useful to have an abstractor column of sufficient capacity and selectivity to eliminate the solvent peak from a gas chromatogram without affecting any of the other sample peaks. In gas chromatography, solvent peaks often cover up the peaks of other sample constituents. In this work, it is shown that amines are retained completely and selectively on a short pre-column containing copper(I1) chloride coated onto Chromosorb G AW/DMCS. Pyridine, which is an excellent solvent for many organic substances, can be used as the solvent for chromatographic samples, and the normally large pyridine solvent peak is completely eliminated by use of the pre-column. This abstractor column also removes other amines from the carrier gas stream and thus avoids interference from amines t h a t often form broad, tailing peaks in gas chromatographic procedures.
EXPERIMENTAL Apparatus and Reagents. A Hewlett-Packard Model 5711A gas chromatograph equipped with dual flame ionization detectors and Tracor Model 550 gas chromatograph equipped with dual flame ionization detectors were used in this work. Six-foot stainless steel analytical columns, I/s-in. o.d., (10% SE-30 on 100/120 mesh Chromosorb G AW/DMCS) were used for the separation of alkanes, alkenes, aromatic compounds, substances extracted from coal and pyridine impurities. Two-foot stainless steel analytical columns, %-in. o.d., packed with 80/100 mesh Amberlite XAD-2 resin were used for the separation of halogenated hydrocarbons, alcohols, and ketones. (Note: Amberlite XAD-2 resin is a divinyl benzene cross-linked polystyrene porous polymer that is apparently identical with Chromosorb 102.)
Chromosorb G AW/DMCS was coated by evaporating aqueous solutions containing 2.5 g of metal salts (copper(I1)chloride dihydrate; copper(I1) sulfate pentahydrate; zinc(I1) chloride; and nickel(I1) chloride hexahydrate) to near dryness on 7.5 g of Chromosorb using a rotary evaporator. The resulting paste-like material was wet with acetone (copper or zinc salts) or methanol (nickel salts) and reevaporated to apparent dryness. The coated support was then dried at 115 "C for 1h, sieved, and 80/100 mesh fraction packed into columns or injection port inserts and conditioned at 220 "C before use. Cation exchange resins having capacities of 0.72 mequiv/g, 1.50 mequiv/g, and 2.55 mequiv/g were prepared by reacting SO/lOO mesh Amberlite XAD-2 resin with concentrated sulfuric acid at 150 "C for 20,35, and 60 min, respectively. These resins and the commercially available Amberlite A-15 and IRC-50 cation exchange resins were all thoroughly washed with distilled water and methanol and placed in the copper(I1) form by passing a solution of cuprous sulfate through them. The copper(I1) form resins were again washed with distilled water and methanol, dried, sieved, and the 8O/lOO mesh fractions packed into columns. The nickel(I1) form of the 2.55 mequiv/g sulfonated XAD-2 resin was prepared in a similar manner. Copper(I1) chloride was coated on Amberlite XAD-2 resin by evaporation of acetone solutions. The coated resin was air dried, packed into columns, and conditioned at 220 "C for 1 h. Water triple distilled in quartz was used to prepare samples of organic compounds in water. All other chemicals were of reagent or the highest purity available. Analytical Methods Utilizing Amine Abstractors. Preliminary work was performed on three different analytical methods to demonstrate some of the potential uses of amine abstractors. In each method a 1.0-ft stainless steel column, %-in.o.d., packed with Chromosorb G AWDMCS coated with copper(I1) chloride was used as an abstractor pre-column in series with an appropriate analytical column. Method for Determining Volatile Components i n Coal. Ground coal (100/120 mesh) was extracted with pyridine and an aliquot of the pyridine extract was chromatographed using the amine abstractor in series with the SE-30 column. The column oven temperature was programmed from 120 to 160 "C at a rate of 32 "C/min with a final hold at 160 "C. The amount of coal extracted, the volume of extraction solvent, and the injection size can all be varied to give the desired response for the substances extracted from coal. A 2-g sample of Iowa coal containing -0.01% of volatiles was extracted with 75 ml of pyridine and a 5-pl aliquot was chromatographed at an attenuation of 10
x 8.
Method f o r Determining Impurities i n Pyridine. A sample of pyridine was injected into a gas chromatograph fitted with an abstractor column in series with the SE-30 column. The temperature was held a t 150 "C. The injection size depends on the concentration of impurities in the pyridine but generally a 2 - ~injection 1 was used. Modified Resin Sorption Method for Determining Organic I m purities in Drinking Water. The following modifications were made in the resin sorption method for determining organic impurities at part-per-billion concentrations in water ( 6 ) :a) A 6.0-in. X 6-mm 0.d. glass column was used instead of the larger column previously reported. b) Sorbed organicswere eluted with 4.0 ml of pyridine instead of 25 ml of ethyl ether; c) the drying and evaporative concentration steps were omitted; d) a 10-pl aliquot of the pyridine extract was injected into a gas chromatograph fitted with an abstractor column in series with a SE-30 analytical column. Tests of these modifications were performed on synthetic samples containing 100 parts per billion each of benzene, toluene, ethylbenzene, isopropylbenzene, n-butylbenzene, and naphthalene in 11. of triple distilled water.
RESULTS The effectiveness of abstractor materials in retaining pyridine was determined by installing 1.0-ft, Ys-in. o.d., stainless steel columns packed with the test material in the gas chromatograph. Repeated 2 4 injections of pyridine a t increasing ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
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Table I. Substances Not Affected by Abstractor Columns Packed with Copper(I1) Chloride on Chromosorb G AWI DMCS n-Pentane n-Hexane n-Heptane n-Octane n-Decane n-Undecane n-Tridecane Methanol" Ethanol 1-Propanol 1-Butanol 1-Pentanol Acetone Butanone 2-Pentanone 3-Methyl-2-pentanone Dichloromethane Chloroform Carbon tetrachloride 1-Bromobutane rn-Dichlorobenzene
Benzene Toluene Ethylbenzene Isopropylbenzene n-Butylbenzene Naphthalene Hexene Heptene Octene Acetonitrile Acetaldehyde Benzaldehyde Nitrobenzene Phenol o-Cresol 1,4-Dioxane Benzyl alcohol Isopropyl ether Ethyl benzoate Diethyl ether Ethyl acetate
Retention time and peak shape not affected, peak height decreased. temperatures were made until a temperature was reached a t which pyridine was no longer retained. Of the abstractor materials evaluated, only copper(I1) chloride and sulfate coated on Chromosorb G A W D M C S were effective in retaining amines without affecting the chromatographic behavior of non-amines. T h e chloride salt is preferred to sulfate because copper(I1) sulfate is reported t o undergo a slow decomposition a t temperatures above 200 "C (7).Copper(I1) chloride on Chromosorb G AW (not treated with DMCS) retained pyridine well but caused much tailing of olefins, halogenated alkanes, and ketones. This tailing indicates t h a t active sites on the support are accessible even after coating with the copper salt. Chromosorb W AC/DMCS coated with phosphoric acid ( 4 ) had a significantly lower capacity for pyridine than the copper-coated resin. T h e acidcoated Chromosorb also caused a slight broadening of alcohol and ketone peaks. Nickel( 11) chloride coated on Chromosorb caused nonamines, particularily alkenes, to tail. Using Chromosorb G AWIDMCS coated with ZnCl2 gave no peak for pyridine at isothermal temperatures up to 160 "C. Difficulty in maintenance of a stable baseline during programming of the ZnCl2 column was observed and indicates that a background "bleed" of pyridine is present when this material is used. Gopper(I1) chloride sorbed on XAD-2 resin and the copper(I1) form of IRC-50 cation exchange resin were ineffective in retaining amines. The nickel(I1) form of sulfonated XAD-2 resin completely retained amines up to the decomposition temperature of the resin, but most non-amines tailed significantly and some alcohols and alkenes were completely retained. The copper(I1) forms of sulfonated XAD-2 resins and A-15 resin retained amines effectively. Pyridine was eluted only at 275 "C and diethylamine was not eluted u p to the resin decomposition temperature. However, these resins also caused non-amines to tail significantly. The retention of amines by the abstractor columns depends on stable complexes of low volatility being formed. I t was expected that some sterically hindered amines would not be retained by the abstractor column, and in fact, di-n-butylamine was less effectively retained than other amines tested. 1124
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Figure 1. Chromatogram showing the effect of abstractor columns on the separation of normal alkanes in pyridine (a) (A) pentane: (B) hexane: (C) heptane: (D) octane: (E) nonane; (F) decane. Column: 6 4 X Ya-in.: 10% SE-30 on 100/120 mesh Chromsorb G AW/DMCS. Temperature: 70 to 90 "C at 5 'C/min, initial temperature hold 2 min, final temperature hold 2 min: carrier gas flow rate 35 ml/min. Sample size 2 ~ 1 . Chromatogram(a) with 7 In. X '/&In. abstractor pre-column containing 100/120 mesh Chromosorb G AWIDMCS coated with copper(l1)chloride. Chromatogram ( b ) same conditions as in (a),but without the pre-column
Aniline, n-butylamine, diethylamine, triethylamine, and cyclohexylamine were all completely retained u p t o temperatures in excess of 140 "C. Pyridine was retained u p t o a temperature of 180 "C, and di-n-butylamine was retained u p t o only 120 "C. The effect of abstractor columns on compounds not containing the amine functional group was determined by injecting solutions containing 0.1% of the test compound in pyridine into a gas chromatograph containing a 1.0-ft, vs-in. o.d., stainless steel column packed with the abstractor material in series with an appropriate analytical column. T h e chromatographic behavior of a compound determined in this way was compared with t h a t of the pure compound chromatographed under identical conditions with t h e abstractor precolumn removed. Copper(I1) chloride coated abstractor columns have a negligible effect on the chromatographic behavior of nearly all non-amines tested (Compounds tested listed in Table I). Only methanol was significantly affected by the abstractor pre-column. Methanol peaks were decreased in intensity, although the retention times and peak shapes were not significantly altered. Retention times of other compounds were lengthened slightly owing t o the greater total column length. T h e slightly longer retention times do, of course, slightly reduce peak heights. The peaks obtained using the abstractor are reproducible and the detector response linearity is not affected. The capacity of abstractor pre-columns packed with CODper(I1) chloride coated Chromosorb was determined by making repeated 2 4 injections until pyridine was eluted. This procedure was repeated using 1.0- and 2.0-ft columns, Ys-in. o.d., and 7.0 in., lh-in. 0.d. injection port inserts. The capacity of abstractor columns is affected by temperature and, of course, by the volume of the abstractor material. A 1.0-ft, $',-in. o.d., column effectively retained 30 ~1 of pyridine a t 150 "C. Lower temperatures and longer or larger columns have higher capacities. Thermally desorbing retained amines can result in a partial regeneration of spent abstractor columns. Regeneration a t 220 "C for 1h of a column having an original capacity of 30 ~1 resulted in regaining 10 p1 of ca-
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Figure 2. Chromatogram showing the effect of amine abstractor columns on the separation of aromatic compounds (a) (A) benzene; (B) pyridine: (C) rrbutylbenzene:(D) naphthalene. (b) (A) benzene; (B) toluene; (C) ethyibenzene, (D) isopropylbenzene: (E) n-butylbenzene: (F) naphthalene. Column: 6-ft X y&in.: 10% SEQO on 100/120 mesh Chromosorb G AW/DMCS: Temperature, 120 to 160 OC at 16 OCImin; carrier gas flow rate 20 ml/min; Sample size 1 wl containing about 0.2% of each aromatic compound in pyridine. Chromatogram (a) without amine abstractor pre-column. Chromatogram (b)with 1-ft X '/8-in. abstractor pre-column containing 100/120mesh Chromosorb G AWIDMCS coated with copper(l1) chloride
pacity. Although partial regeneration is possible, it is more convenient to replace t h e abstractor material periodically. Applications of Amine Abstractor Columns. T h e effectiveness of copper(I1) chloride coated abstractor columns in removing interfering solvent peaks from samples of various compositions is shown in Figures 1 through 3. In each case, the pyridine solvent seriously interfered with one or more sample components and the abstractor pre-column effectively removed the pyridine and thus the interference. Similar results were obtained for mixtures of alkenes, alcohols, and chlorinated alkanes in pyridine. Although the potential uses of amine abstractors are many and diverse, preliminary work was done on three methods t o demonstrate some of the applications where solvent elimination would be desirable. The method for determining volatile components in coal was applied to samples of Iowa coal and components normally obliterated by the solvent peak were readily resolved (See Figure 4). None of the volatile components in coal were identified since this experiment was only intended t o demonstrate a potential use of amine abstractors. Because amines are the only substances retained by these abstractor columns, peaks appearing in t h e chromatograms
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Figure 3. Chromatograms showing the effect of amine abstractor columns on the separation of ketones in pyridine (a) (A) acetone; (B) butanone; (C)pyridine. (b)(A) acetone; (B) butanone; (C) 2-pentanone: (D) 4-methyl-I-pentanone. Column: 2-R X '/e-in.; 80/100mesh Amberlite XAD-2 resin: temperature 150 OC; carrier gas flow rate 32 ml/min; Sample size. 1 FI containing about 0.2% of each ketone in pyridine
of reagent grade amines will be due t o non-amine impurities. Thus, amine abstractors can be used to determine such impurities. This technique was applied to samples of different brands of pyridine. Benzene and toluene were detected in all samples at concentrations ranging up t o 50 and 160 partsper-million, respectively. The resin sorption method for determining organic impurities in drinking water has been proved t o be extremely effective (6).The method basically consists of sorbing neutral, hydrophobic organic compounds from water on a column packed with Amberlite XAD-2 resin, eluting the sorbed organic compounds with diethyl ether, drying the ether and concentrating the organics further by evaporation of the ether, and determining the organics by gas chromatography. This method is not effective for determining some low boiling contaminants in water; these are either lost during the evaporative concentration step or are not resolved from t h e ether solvent peak. An attempt was made t o overcome these disadvantages by using pyridine as the elution solvent and retaining it on an abstractor pre-column. Because smaller amounts of pyridine were required to elute organics from XAD-2 columns and larger injections could be made into t h e gas chromatograph, no sensitivity was lost by omitting the evaporative concentration step. Water is miscible with pyridine. However, it does not appear on chromatograms; therefore, the drying step could ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
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An obvious fault of the modified procedure is that it is not applicable when amines are the contaminants in water. A second limitation is that organic compounds not eluted from the abstractor a t temperatures below t h a t where pyridine is eluted will be lost.
CONCLUSIONS Amine abstractor pre-columns have a number of potentially attractive uses. Further studies on the use of pyridine as an elution solvent in the resin sorption method may lead to a more widely applicable method. In addition, it appears possible to use a suitable amine as an extraction solvent for materials such as industrial waste water and then use amine abstractor columns to prevent interferences of the extraction solvent with the gas chromatographic peaks of extracted substances. It might also be feasible to use amine abstractors to simplify the chromatograms of biological fluids by retaining components having amine functional groups. In such a use, one application might be to screen the blood or urine of industrial workers exposed to potentially harmful substances such as benzene. The work reported in this paper has been aimed a t removing amines from samples in order to eliminate their interference with other sample components. Abstractor columns might also be used as a method for concentrating trace quantities of amines. Such use would require a more effective means of eluting amines than the thermal desorption method used for regenerating columns.
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Figure 4. Chromatogram showing effect of abstractor column on volatile components extracted from coal (a) Without amine abstractor pre-column, (b)with amine abstractor pre-column
also be eliminated. A test of this procedure was made on a sample to which was added 100 parts-per-billion *each of benzene, toluene, ethylbenzene, isopropylbenzene, n-butylbenzene, and naphthalene, and quantitative recovery of all compounds was attained. Despite these encouraging results, these modifications do introduce some significant drawbacks.
B. A. Bieri, M. Beroza, and W. T. Ashton, Mikrochim. Acta, 1967, 637. R. R. Allen, Anal. Chem., 38, 1287 (1966). R. Rowan, Anal. Chem., 33, 658 (1961). J. Fryka and J. Pospisil, J. Chromatogr., 67, 366 (1972). D. A. Leathard and 8 . C. Shurlock, "ldentlflcatlon Techniques in Gas Chromatography", Wiley-interscience, London, 1970, p 66. (6) G. A. Junk, J. J. Richard, M. D. Grieser, D. Witiak, J. L. Witiak, M. D. Arguello. R. Vick, H. J. Svec, J. S. Fritz, and G. V. Calder, J. Chromatogr.. 99, 745 (1974). (7) "Handbook of Chemistry and Physlcs, 51st Edition", Chemical Rubber Publishing Co., Cleveland, Ohio, 1970-71, p B-89.
(1) (2) (3) (4) (5)
RECEIVEDfor review October 24, 1975. Accepted April 8, 1976. Appreciation is expressed to the National Science Foundation (Grant No. GP-32526) for partial financial support.
Theoretical and Experimental Characterization of Flow FieldFlow Fractionation J. Calvin Glddings," Frank J. Yang, and Marcus N. Myers Department of Chemistry, University of Utah, Salt Lake City, Utah 84 1 12
The interplay of diffusion and field-induced drift in field-flow fractionation (FFF) is discussed in order to define the speciflc place of flow FFF among FFF methods. Retention and plate height equations are then developed, based on the general theory of FFF. Approximations and graphical presentations are given for the more complicated plate-helght terms. Experimental tests of the theory are reported using nine different proteins and six distinct sizes of polystyrene latex beads. Excellent agreement is found between experimental and theoretical retentlon parameters, and it is shown that diffusion 1126
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
coefficients can be measured using observed retention values. Observed plate helght parameters do not agree well with theory, as Is also typical of other FFF systems.
Field-flow fractionation (FFF or F3) consists of a group of techniques in which some field or gradient forces solute into the semistagnant regions near the wall of a narrow flow tube or channel (1-3). A steady state is established in which solute is driven toward the wall a t velocity U and driven away from
