from Table 11,the relative standard deviation increases considerably if the difference between the equivalent weights of the counterions decreases. This is, of course, due to the simultaneous*decreaseof the weight difference of the sample in the two ionic forms (see Table I). Similarly, for the anion exchange resin AG 21-K, the relative standard deviation increases if the ion pair Cl-/N03- is used instead of Cl-flOs(Table 11). Comparison of the precision of the titration method with that of the gravimetric method shows that the precision of the latter is either of the same order of magnitude or somewhat better, provided that counterion pairs of sufficiently different equivalent weights are used. After completion of the manuscript, it came to our attention that Ratechinskij and Saldachse in a Russian conference report ( 3 )proposed a similar gravimetric approach for the determination of ion exchange capacities. However, no experimental data on ion exchange capacities thus determined were reported.
ion exchange process-simple, has the same precision as the standard titration method in the sample weight range (1-0.1 g) investigated, and requires a minimum of laboratory equipment. It is especially suited for ion exchangers, the ion exchange capacity of which cannot be determined by the titration method, since conversion to their H+ form is-as e.g. for many clay minerals-not possible. Compared to the titration method, the gravimetric method is less time consuming, since the difference weighing requires little time and the drying of the two aliquots is performed simultaneously. Besides that, the gravimetric method should be suited for investigating large sample numbers, because a considerable number of drying processes can be carried out at a time.
ACKNOWLEDGMENT The technical assistance of M. Haimerl is gratefully acknowledged. LITERATURE CITED (1) K. Bunzl and B. Sansoni, Chem.-lng. Tech., 47, 925 (197% Engl. (2)F. Helfferich, "Ion Exchange", McGraw-Hill Book Co., New York, N.Y.,
CONCLUSION An alternative to the titration method for the determination of the pure ion exchange capacity has been presented. The gravimetric method is-provided that the ion exchanger is thermally stable and that no complications occur during the
1962. (3) W. W. Ratechinskij and K. M . Saldachse, Teor. lonnogo Obmena Khromatogr., Tr. Vses. Nauchn.-Tekh. Konf., V . V . Rachinskii, Ed., Izd. Nauka, Moscow, USSR, p 154 (1965), Russ.
RECEIVEDfor review June 8, 1976. Accepted August 30, 1976.
Low Resolution Glass Capillary Column for Gas Chromatography I?. G. Einig" and J. L. MacDonald Ralston Purina Company, Checkerboard Square, St. Louis, Mo. 63188
Because capillary columns can resolve components of very complex mixtures, they are used in laboratories where such separations are needed. Packed columns are still used extensively where less demanding separations are generally encountered even though the same resolution obtained for any given peak pair on the packed column can usually be achieved in a shorter time using a capillary column. In 1972,Verzele et al. ( 1 ) summarized the generally recognized advantages and disadvantages associated with the use of these columns and stated that "many gas chromatographers indeed feel that the advantages of open tubular columns do not outweigh their disadvantages." Since then, technological improvements have been made in both the columns and gas chromatographs, but the columns are still expensive, require more careful handling than packed columns, and have a lower maximum allowable operating temperature than equivalent packed columns. In 1970, Bossart (2) was granted a patent for his idea of chemically bonding a phase to the inner surface of a glass capillary column to increase its thermal stability. He did not present any applications for this column. Three years later, the first application of a bonded phase capillary column was published by Jonsson et al. (3) for the analysis of amino acids. Others (4-6)have used this technique to form a bonded sub-layer to improve wettability of the glass and adhesion of a subsequently coated liquid phase. This paper describes a laboratory preparation of a bonded phase borosilicate glass capillary column and demonstrates its usefulness for separations encountered in routine laboratory analyses. The glass column was prepared by etching it with an acetic acidhydrofluoric acid mixture that roughened the inner surface and increased the surface area. The surface
was activated by heating the column in a dry nitrogen atmosphere and octadecyltrichlorosilane (ODTCS) was dynamically coated on it. Some of the chloro groups of the ODTCS reacted with hydroxyl groups of the glass column to form an octadecylsilane (ODS) bonded to the capillary wall. Any unreacted chloro groups remaining on the bonded material were hydrolyzed by passing moist nitrogen through the column and, after a short conditioning period, the column was ready for use.
EXPERIMENTAL Apparatus. A Perkin-Elmer 900B gas chromatograph (GC) equipped with dual flame ionization detectors, linear temperature programmer, and heated capillary injector with splitter was used for the hydrocarbon analyses. Hydrogen and air flow rates were 30 ml/min and 300 ml/min, respectively. Prepurified nitrogen carrier gas was varied from 1to 4 ml/min while make-up gas was maintained a t 40 ml/min. A Perkin-Elmer 3920 gas chromatograph equipped with a 63Ni electron capture detector and heated capillary injector with splitter was used for the analysis of chlorinated compounds. Prepurified nitrogen was used as both the carrier and make-up gas. The carrier gas flow was varied from 0.3 to 4 ml/min while the make-up gas was maintained a t 30 ml/min. Reagents. The etchant solution was a 1:l w/w ratio of 35% hydrofluoric acid/glacial acetic acid. The column coating solution was Dow Corning 2-1215 silane (ODTCS) diluted with an equal volume of toluene. Procedure. A borosilicate glass capillary, 20 m X 0.5 mm i.d., was connected with polyethylene tubing to a water aspirator via a 50-ml polyethylene trap. All solutions used in the Column preparation procedure were drawn through the capillary using reduced pressure. About 25 ml each of chloroform, acetone, and methanol in that order were used to wash the glass initially. The column was dried for about 15 min by drawing air through it after which the etching process was begun. Sulfuric acid (30%in water) was drawn into the capillary until
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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A
f b
c
0
3
6 min
0
Figure 1. Effect of ODTCS treatment
5
15 min
10
Figure 3. Separation of chlorinated pesticides
Separation of hydrocarbon mixture (a)hexane, (b)decane, (c)naphthalene prior to treatment (A) and after treatment (e).Nitrogen flow rate was 2 ml/min and oven temperature was 65 OC (A) and 85 O C (B)
Peak (a),lindane; (b),heptachlor: (c),aldrin: (d), heptachlor epoxide: (e), dieldrin:
(0,p,p'-DDE;(g),endrin: (h),unknown;(I), p,p'-DDT; 01, unknown. Nitrogen flow rate was 2 ml/min and oven temperature was 200 O C
c
Table I. Estimates of the Mean and Standard Deviation for Three Analyses of a Pesticide Mixture Lindane Heptachlor Aldrin Heptachlor epoxide Dieldrin p,p'-DDE Endrin p,p'-DDT
a
d
-
0
2
4 min
Figure 2. Separation of Cs hydrocarbons
Peak (a),2,4-dimethylhexane(bp 11 1 OC):(b),trans-l,4-dimethylcyclohexane (bp 120 "C): (c),ci.-1,2dimethylcyclohexane (bp 129 OC)and ethylcyclohexane (bp 132 OC): (a), cycloctane (bp 149 "C).Nitrogen flow rate was 4 ml/min and oven temperature was 55 OC a 15-cm length of column was filled. An air gap of 3 to 4 cm was followed by etchant solution in the next 15-cm length of column and again followed by another 3- to 4-cm air gap. This process was repeated alternating sulfuric acid and etchant solution until the first 1.5 m of the column was filled. This solution was then drawn through
X, mm
SD, mm
142
2.1 2.8 2.1
177
245 127 139 184 54
143
1.4
0.7 2.1 0.7 2.1
the entire length of the column before any more solution was added. After it had passed completely through the column, the entire procedure was repeated two more times after which several plugs of 30% HzS04 were drawn through to remove any reaction products. Methanol was drawn through the column for about 15 min to remove the HzS04. During the etching procedure, it was necessary to keep the solution moving a t all times to avoid a build-up of fluorosilicate that would permanently clog the column. Momentary blockage was quickly dissipated by applying about 25 psig nitrogen pressure until the solution began to move rapidly. The column was connected to the injection port of a GC and heated for 2 h at 150 "C with a 2 ml/min nitrogen flow rate to drive off surface water but not dehydrate silanol groups (7).The column was removed from the GC, reconnected to the water aspirator, and 3-4 cm of toluene was drawn into the capillary immediately followed by the ODTCS column coating solution until the column was half filled. This solution was slowly drawn through the column, a Cas04 drying tube was attached to the open end, and the solvent was evaporated. The column was reconnected to the injection port of a GC and nitrogen, moistened by passing through a 10-cm pre-column of wet glass wool, flowed through the column for 2 h a t 50 "C to hydrolyze unreacted chloro groups. The pre-column was removed and the oven tempera-
2282 * ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
4.0
3.0
z
0r
a
1.0
1.o
1.0
2.0
3.0
F L O W R A T E (rnlhnin)
Figure 4. Resolution vs. flow rate as a function of injector split ratio (E, 1O:l; +, 25:l; 0,1OO:l)for an injection of 10 ng each of aldrin and dieldrin (9) ture programmed to 250 " C at 0.5'/min with a 2 ml/min dry nitrogen flow rate.
RESULTS AND DISCUSSION The initial test for the column was a comparison of its resolving power before and after ODTCS treatment. The column preparation procedure was interrupted after the column had been cleaned, etched, and dried. The column was mounted in a GC and a mixture of hexane, decane, and naphthalene was injected. As shown in Figure lA, no separation was obtained. The column was removed from the GC and the final stage of column preparation, ODTCS treatment, was performed. The column was again mounted in a GC and the hydrocarbon mixture reinjected. Figure 1B shows the complete separation of the three components in this mixture by the ODS layer, a separation that had not been achieved by the same column without the ODs. The ability of the column to separate Cg hydrocarbons is demonstrated in Figure 2. Five compounds ranging in boiling point from 111 to 149 "C were resolved into four peaks and eluted in the order of increasing boiling point. The column failed to separate cis-1,2-dimethylcyclohexane(bp 129 "C) and ethylcyclohexane (bp 132 "C). The column exhibited moderate polarity in the separation of eight chlorinated pesticides (Figure 3). Although no attempt has been made to determine the column polarity according to the standard Rohrschneider/McReynolds constants, the resolution achieved for dieldrin and p,p'-DDE was the same as that obtained with the moderately polar stationary phase, 1:lw/w 1oo/o DC-200/15%QF-1, recommended for the analysis of pesticides in the Food and Drug Administration Pesticide Manual (8). According to this same source, these two pesticides do not separate on a non-polar phase such as DC-200. Table I summarizes the results of three separate injections of this pesticide mixture on the bonded phase capillary column using a nitrogen flow rate of 2 ml/min and an oven temperature of 200 "C. The peak height of each of the pesticide peaks was measured, and the total of the three injections was averaged. The low standard deviations demonstrate that this chromatographic system is reproducible. The thermal stability of the column was measured by temperature programming from 50 to 325 "C at 2"/min and
20 FLOW R A T E (ml/minl
3rJ
Figure 5. Retention time of dieldrin vs. flow rate at an injection split ratio of 1OO:l
monitoring the bleed with a flame ionization detector at a sensitivity of 1 x AFS. A detectable baseline rise started at 300 "C and become quite steep at 325 "C. For this reason, 300 "C was established as the maximum temperature for isothermal operation. The column was then subjected to 385 "C for about 1h after which an analysis of four pesticides was attempted. Although the same chromatographic conditions used to obtain Figure 3 were used for this analysis, the results were very different. Heptachlor was reduced in size and appeared as a shoulder on the front of aldrin while dieldrin and p,p'-DDE were completely merged. This column deterioration was attributed to thermal decomposition of the ODS surface and so the column was re-treated in an attempt to restore its earlier performance. The column was removed from the chromatograph and reconnected to a water aspirator. Chloroform was drawn through the column to wash away organic residues and was followed by a methanol rinse. The column was dried by evaporating residual solvent under vacuum. Since this column was already etched, the etching portion of the column preparation procedure was deleted and the retreatment began by drawing several plugs of 30% HzS04 into the column. After the acid had passed through, methanol was drawn through for about 15 min to wash away any remaining acid. As described in the Experimental section, the column was dried, treated with ODTCS, and conditioned. When this was completed, the mixture of four pesticides was reinjected and the resulting peaks appeared the same as in Figure 3. This demonstrated that a column, whose performance had deteriorated, could be easily restored to its original condition. Figure 4 describes the effect of flow rate and sample size on the resolution of aldrin and dieldrin (9).One ~1 of a solution containing 10 pg/ml of both aldrin and dieldrin was chromatographed at 205 "C with Na as carrier gas. Three injector split ratios of about l O : l , 25:1, and 1OO:l were used to vary the amount injected into the column. Resolution was markedly decreased with increasing flow rate a t split ratios of 1 O : l and 25:1, but at a split ratio of 1OO:l the decrease in resolution was minimal. As shown in Figure 5, this effect can be used to significantly shorten analysis times. Figure 5 demonstrates the change in retention time of dieldrin with flow rate when the 1 O O : l split ratio was used. Increasing the flow rate from 0.5 ml/min to 2 ml/min reduced the retention time of dieldrin from 20 to 5 min with little effect on the resolution of aldrin and dieldrin.
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