Gas Chromatographic Analysis of Aromatic Hydrocarbons with Modified Bentonite Columns Effect of Bentone 34 Concentration on Performance of Packed Columns
I
4 A
2
0
Figure 1 .
Chromatogram of aromatic hydrocarbons on column made from new batch of Bentone 34
Column packing: 1 1.54% D C - 2 0 0 / 5 0 0 ond 1 1.54% Bentone 34. o-xylene, 7. isopropyl benzene
SIR: The use of modified Bentone 34 (dimethyldioctadecylammonium bentonite) packed columns for the separation of the xylene isomers has been the subject of numerous publications (1, 2, 4-6). Of these five publications, owever, only two ( 2 , 6) have shown the separation of isopropyl benzene and o-xylene. Because isopropyl benzene is present in almost all xylene mixtures, this separation is as important as the analysis of the xylene isomers. To maintain the above separations, we have found it necessary to reoptimize the weight per cent of silicone oil and Bentone 34 used in our columns when a new batch of Bentone is used. The activity of each batch of Bentone appears to be different.
Peaks: 1 . benzene, 2. toluene, 3. ethyl benzene, 4. p-xylene, 5. m-xylene, 6.
IIIO-~AYPS
3
0.5
I
ti
10-8AMPS-
EXPERIMENTAL
h Perkin-Elmer
Model 800 gas chromatograph equipped with a differential flame ionization detector was used for these studies. The recorder was a Leeds & Northrup Speedomax Model G with a full-scale span of 5 mv. rill of the tubing used for fabricating the columns was 15-feet X '/*-inch 0.d. x 0.085-inch i.d. stainless steel. The various packings used for carrying 1160
ANALYTICAL CHEMISTRY
0.1 X I O - ~ A M P S
Figure 2.
Chromatogram of aromatic hydrocarbons.
Bentone 34 same as Figure 1
Column packing: 16.1 6% DC-200/500 and 3.03% Bentone 34. Peakr: 1 . benzene, 2. toluene, 3. ethyl benzene, 4. p-xylene, 5. m-xylene, 6. o-xylene, 7. isopropyl benzene, 8. n-propyl benzene
Table 1.
Effect of Liquid Phase and Bentone on Column Efficiency
Weight 7c Weight (yo DC-2OO/lOO Bentone 34 19.42 16.16 12.63 12.25 11.54
2.91 3.03 3.16 6.12 11.54
n
HETP (mm.)
6750 7220 7950 9495 10390
0.7 0.6 0.6 0.5 0.4
out the analyses contained DC-200/100 methyl silicone oil ( n o w Corning Corp., Midland, Slich.), Bentone 34 (Kational Lead Raroid Division, Houston, Texas) and nonacid washed chromosorb W (Johns-Manville Corp., Xew York, N. Y.). The various ratios that were used are listed in Table I. The operating temperatures for the column, injector, and detector were loo', 215", and 175' C., respertively. -4 carrier gas flow rate of 12.7 ml. per minute (measured a t column outlet) was used in all cases. The column materials were made using the conventional slurry technique. After packing, the column was coiled and then preconditioned at 140' C. for 30 minutes.
R p-xylenem-xylene 0.96 1.53 2.12 4.14 3.98
R m-xyleneo-xylene 1.9 0.9 0.6
=0
0
I? ._ o-xyleneisopropyl benzene 5.7 3.1 2.5 1.4 0.9
culated for the n-propylbenzene peak along with the resolution values calculated for the following peak pairs : p-xylene and m-xylene; m-xylene and o-xylene; and o-xylene and isopropyl benzene. The equations used for the calculations correspond to the internationally accepted nomenclature (3). Using this data it is possible to predict that the best column for the analysis of these materials would be one containing 16.16% silicone oil and 3.03% Bentone 34. Figure 2 shows the chromatogram obtained on this column.
DISCUSSION
LITERATURE CITED
Our early work on the modification of Bentone 34 showed that columns made with equal weights of silicone oil and Bentone 34 gave the best results. Later results obtained on a column made in the same manner but with a new batch of Bentone 34 gave the results shown in Figure 1. T o find the correct ratio of silicone oil to Bentone 34, five different column materials were made. Table I shows the weight per cent of the silicone oil and Bentone 34 used in each column. Also shown in this table are the number of theoretical plates and the HETP values obtained for each column cal-
(1) Cieplinski, E. W., Gas Chromatography A plications, No. GC-DS-002,
Perkin-Eker Corp., Norwalk, Conn., 1964. (2) Mortimer, J. V.,Gent, P. L., Nature 197, 789-90 (1963). (3) Pure A p p l . Chem. 8, 553-62 (1964). (4)Spencer, S. F., ANAL.CHEM.35, 592 (1963). (5) Van Der Stricht, M., Van Rysselberge, J., J . Gas Chromatog. 1, No. 8, 29-33 (1963). (6) Van Rysselberge, J., Van Der Stricht, M., Nature 193, 1281-2 (1962). W. CIEPLINSKI~ EDWARD Perkin-Elmer Corp. Xorwalk, Conn. Present address, Olin Research Center, New Haven, Conn.
Preparation of High Purity Acetonitrile SIR: The favorable dielectric, solvent, and optical properties of acetonitrile (MeCN) have resulted in its wide use in spectrophotometric and electrochemical experiments and in peptide chemistry. Practical grade MeCN generally contains, as impurities, water, unsaturated nitriles, toluene, and various aldehydes and amines. The reagent material has higher chemical purity but may actually have poorer optical qualities and is not suitable for electrochemical use. Similarly, commercially available spectro grade MeC N does not show optimum optical or electrochemical properties. The generally accepted method for purifying MeCN is that given by Weissberger ( 4 ) . I t involves successive refluxing with PzOb and Na2C03, followed by careful distillation through a good rectifying column. MeCN of high purity was prepared with the use of this method by Janz and Danyluk (S), who refluxed the MeCK with PzOr six times for 36 hours each time before continuing with the NazCOJ treatment and distillation. MeCN prepared by us with this method has a boiling point of 81.7' C. at 763 mm., contains less than 0,0570 water, and is a satisfactory voltammetric solvent. GLC analysis does reveal, however, a substantial amount
(ca. 0.5%) of a n impurity which apparently forms a n azeotrope with MeCN and which is not removed until the greater part of the charge has been distilled over. Other procedures listed by Weissberger include azeotropic distillation with methylene chloride, benzene, or trichloroethylene. These methods are only intended to remove water and could not produce solvent with generally satisfactory optical or electrochemical properties. Purification procedures intended to produce solvent for use for reductions at the mercury electrode have been described by Coetzee et al. ( I ) . Of the several variations described, that recommended for general use involves stirring with calcium hydride for 48 hours followed by fractionation from P ~ o s ,with a final fractionation from CaH2. This procedure is inadequate when the solvent is intended for optical or anodic electrochemical use because no provision is made for removal of small amounts of aromatic hydrocarbons which absorb in the 260- to 280-mp region and which are oxidized at the platinum electrode. We suggest the following procedure for purifying MeCN: place 800 ml. of practical grade MeCN in a 1-liter round-
bottom flask, add 10 grams of anhydrous NazC03and 15grams of KMn04. Distill a t 5 to 10 ml./min. into a receiver protected from atmospheric moisture. Make the distillate slightly acidic with concentrated sulfuric acid. Decant from precipitated ammonium sulfate and distill through a 30-plate rectifying column at 10 ml/hr. with a reflux ratio of 20: 1. A small forecut, typically 40 ml., must be discarded. The resulting MeCK has a boiling point of 82.0' C. at 763 mm. GLC examination of solvent prepared by this procedure revealed two impurities amounting to less than about 0.1% which appear to be unreactive a t a platinum anode. The water content was less than 0.01%. This method appears to offer definite advantages when compared with the procedures decribed above. It produces solvent of generally higher purity, it is rapid, only about one day being required before usable solvent is obtained, and it is much less wasteful of solvent. Solvent prepared by this procedure has been used within the potential range of $2.3 volts, with NaC104 a t a platinum electrode, to -2.7 volts, with tetraethylammonium bromide at a mercury klectrode. Potentials are expressed relative to the aqueous S.C.E. MeCN purified by the K M n 0 4 proVOL. 37, NO. 9, AUGUST 1965
* 1161
