Preparation of Porous Layer Open Tubular Columns by the Dynamic Method J. G.Nikelly Department of Chemistry, Philadelphia College of Pharmacy and Science, Philadelphia, Pa. 19104
To DATE nearly all porous layer open tubular (PLOT) columns are made either by static coating procedures (I, 2) or by a combination of static and dvnamic nrocedures (3). These ~
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methods require expertise and facilities such as a drawing apparatus and special furnace that are not normall,y available .n m e oramary 1 . ~ - ~~ _,. ~1iaoorarory; -~ ,s ~ i anaiyricai consequently, PLOT columns are possibly not used as widely as would he expected from their advantages over packed or wall-coated open tubular columns. Some PLOT columns have also been made by the dynamic procedure hut have received limited use. Kaiser has made glass PLOT columns by mixing the coating mixture in a reservoir with a magnetic stirrer (4). Others have made adsorption type PLOT columns using silica (5) or molecular sieves (6),but these columns were limited to the analysis of hydrocarbons and fixed gases, respectively. The present paper describes a dynamic coating procedure which is similar t o that used for making wall-coated open tubular columns. The procedure is therefore relatively simple in that the required equipment can he usually found in the analytical laboratory and the procedure can be followed very easily by anyone accustomed t o making his own gas chromatographic columns. Briefly, the technique consists of passing under gas pressure a plug of coating material through coiled capillary tubing. The coating material is a suspension of the solid support in a solution of the liquid phase and volatile solvent. The gas flow is continued until all the solvent is removed leaving a uniform porous layer of liquid phase on the inside walls of the tubing. The analytical properties and applications of the prepared columns (sample capacity, permeability, efficiency, and stability) are approximately the same as those of the commerical PLOT columns and are dc:scribed inthe succeeding Note (7). . I ~
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Figure 1. Column coating apparatus .
EXI'WIMENTAL
Materials. Mainly type 304 stainless steel tubing was used (Superior Tube, Norristown, Pa.). Type 316 stainless as well as copper were used initially hut type 304 is preferable because of lower cost and suitability of surface properties. The column dimensions were '/la in. 0.d. X 0.020 or 0.030 in.i.d. X 25or50ftinlength. Ordinary, non-acid washed, flux-calcined (white) diatomaceous earth, such as Johns-Manville Chromosorh W, was used as the solid support after pulverizing and sizing as described below. Apparatus. The reservoir or filling tube for introducing the coating mixture (plug) into the column tubing is shown in Figure 1. It consists of a 4-in. length of '/,-in. metal tubing connected at one end to the column tubing through a l/*-in. (1) I. Halasz and C. Horvath, ANAL.CHEM., 35,499 (1963). (2) I. Halasz and C. Horvath, US.Patent 3,295,296 (1967). (3) D. W. Grant, . I . Gas Chromalogr., 6, 16 (1968). (4) R. Kaiser, Chromntogmaphia,1-2, 34, (1966). (5) R. D. Schwartz, D. J. Brasseaux, and G. R. Shoemake, ANAL. CHEM., 35,496 (1963). (6) J. E. Purcell, Nature, 201, 1321 (1964). (7) J. G. Nikelly, ANAL.CHEM., 44, 625 (1972).
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to '/ls-in. Swagelok redunng union (Crawford kittmg company, Cat. No. 400-6-1). At the other end, it is connected to the outlet of a nitrogen or air gas regulator through a Swagelok Quick-Connect fitting (Cat. No. 400-QC-6). Since the gas delivery pressure required for some of the column coating procedures exceeds 70 psig, a high pressure regulator is recommended rather than t h e ordinar) regulators found in the analytiral Inboratory. Preparation of Solid Support. About 2 grams of material are pulverized ior ahout 3 to 5 minutes a i t h a mortar and pestle pretierahly ofagateor procelain. Next the finely divided material is rized by surpending it in about 40 ml of relatively dry acetone using a 50-1111 beaker and discarding the fraction that settles in the first 2 to 3 minutes. The suspcnded material is recovered by allowing it to settle over a period of a few hours or by evaporating otf the acetone. The recovered fraction is then dried at 110 OC for ahout an hour kind stored under desiccation. Coating Mixture (Suspension). For coating a typical PLOT column, (50 i t x 0.02-in. 1.d.). ahout 1.5 grsms of suspension are needed. This is prepared by mixing in a small glass btopperrd-vial 0.3 gram of solid suppon and 1.2 grams of liquidphawsolution havinga concentrationoi2 to l0'x. 'Tubing. A 5O.ft section of tubing coiled into a suitable shape and connected to the filling tube is flushed with a few milliliters of 1 solution of liquid phase in chloroform or other high density solvent. The tuhing is further flushed with air or nitrogen at a pressure of IO to 20 prig for ahour 30 seconds after [he above I %, solution emerges. Coating Step. With the aid of t i capillary pipet, the suspension is added to the tilling t u k and this is connected to the gas supply regulated at 20 prig or higher. Light tapping
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Table I. Typical Composition of Coating Mixtures A, grams B, grams Mixture Solid Support 0.3 0.7 Liquid Phase 0.1 0.1 Solvent, CHCh 1.0 2.7 Table 11. Characteristics of Typical PLOT Columns Prepared by the Dynamic Coating Procedure 50 f t 50 f t Column length 0.02 in. 0.03 in. Tubing inner diameter 3 . 1 ml 10.5 ml Column volume Geometric surface area of 240 cm2 360 cm2 column" 30-40 km 10-20 Fm Thickness of porous layerb DEGS SE-30 Liquid phase 60 mg 45 mg Volume of liquid phase 50 210 Phase ratioc 6 x 10-5 cm2 15 X cmz Permeability constant a Calculated from tubing dimensions. b Estimated from tubing dimensions and the volume of support and liquid phase retained in the column in the coating step. c Inner tubing volume divided by liquid phase volume.
of the column is sometimes necessary to ensure continuous flow of the plug. After the excess suspension emerges (in about 3 minutes), the pressure is reduced to 1 or 2 psig and the gas flow is continued for 'I2hour. Column Conditioning. The column is conditioned in the same way as with other open tubular columns. RESULTS AND DISCUSSION
In the course of this work, it became evident that the main problem in making satisfactory PLOT columns by the dynamic (plug) method is determining and achieving the practical conditions that maximize column length, surface area (roughness factor), and amount of liquid phase while minimizing column bore, liquid film thickness, and incidence of column plugging during the coating step. Column length was fixed at 50 ft because tubing is commonly available in this size and also because it is sufficient for generating 15,000 plates (with plate height = 1 mm) which is adequate for most separations, assuming capacity factors typical of PLOT columns. For reasons of tubing availability, compromise of column efficiency L;S. capacity, and ease of coating, the inside diameter of the tubing was chosen to be 0.02 or 0.03 in. Solid Support. Various support materials were considered such as Bentone 34, the diatomaceous earth supports Chromosorb R6470-1, Chromosorb P, Chromosorb W, and silica (TLC grade). To evaluate these materials each support was dried, reduced to a very small particle size (usually less than 10 pm diameter), combined with a solution of Carbowax 1540, and then used in coating one or more columns. The columns were compared by measuring the resolution of MeOH and EtOH. Although this pair has a favorable relative retention (1.26), it is suitable for evaluating PLOT column properties because of the relatively low capacity ratio. Regardless of the type of support used, the best results (nearly complete MeOH/EtOH resolution on a 50-ft column) were obtained with the finer sized particles. Consequently, Chromosorb W was used in most of this work mainly because it is more easily reduced to less than 2 pm size and owing to its low hydroscopicity forms stable suspensions in chloroform-liquid phase solutions. The physicochemical prop624
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Figure 2. Liquid phase loading as a function of average linear coating rate, ii, and composition of coating mixture Curve A , 10 Carbowax 1540 Curve B , 5 Carbowax 1540
erties of the finely divided Chromosorb W are similar to those of Chromosorb R6470-1 used in the commercial PLOT columns (8). Coating Mixture. Three interrelated conditions must be considered in preparing the coating suspensions : Plug size, choice and concentration of solvent (plug fluidity), and the concentration of liquid phase. The optimum quantity of coating suspension was 1.5-3.0 grams. This quantity is just sufficient to coat the tubing with very little excess emerging with the final nitrogen breakthrough. A larger plug size causes a non-uniform (intermittent) travel rate and, sometimes, column plugging. Of course, a smaller plug size causes incomplete coating of the column surface. Carbon tetrachloride and ethylene bromide were used initially as solvents for the liquid phase, but chloroform was used in most cases because of its high density and suitability as a solvent for most liquid phases. The optimum concentration of solvent was about * j 3 of the total weight of suspension. This is the minimum concentration of solvent needed to prevent column plugging during the coating step, and at the same time it produces sufficient thickness of porous layer and, consequently, sufficient capacity ratio. In addition to plug fluidity, the inner surface of the tubing must be precoated with solvent or liquid phase-solvent solution in order to prevent plugging ( 4 ) . The optimum concentration of liquid phase is to l j 3 of the solid support or 3 to 7 % of the coating mixture. This concentration results in columns that have a phase ratio, p, of about 50. Optimal quantities and compositions of typical coating mixtures are shown in Table I. The respective liquid phases for A and B were DEGS and SE-30. Mixture A was used to coat a 0.02-in. i.d. column and mixture B to coat a 0.03-in. i.d. column. Coating Procedure. It was found practical to push the plug of coating mixture through the tubing at constant applied pressure rather than at constant linear travel rate. Depending on column parameters, the usual applied pressure was 20 to 80 psig. Air was sometimes used in place of nitrogen and, contrary to what may be expected, no liquid phase oxidation was observed. The average linear coating rate (column length divided by the duration of the coating step) was 10-20 cmjsec. At lower rates the porous layer was too thin or incomplete while at higher rates frequent column plugging was encountered. In general, the liquid phase loading increases with increasing
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(8) L. S. Ettre. J. E. Purcell, and K. Billeb, J . C/iromtogr., 24, 335
(1966).
The phase ratio for columns made with silicone liquid phases is generally higher than 100 because, owing to their viscosity, solutions of such phases are usually used at lower concentrations to permit satisfactory coating rates. The second column in Table I1 has a high phase ratio not only because the liquid phase is SE-30, but also because the column diameter is 0.03 instead of the more common 0.02411. i.d. The permeability constants in Table I1 were calculated from pressure drop, carrier viscosity, column length, and average linear flow rate (11). As expected, the constants represent a significant decrease from the theoretical values (= r*/8,where r = radius of the empty tube, in cm).
travel rate but the latter seems to be less critical than in the case of wall-coated open tubular columns (9). Apparently, the liquid phase loading or thickness of the porous layer depends largely on the composition of fluidity of the coating mixture as shown in Figure 2. The amount of liquid phase shown as weight per unit column length, was determined by flushing the column with a volatile solvent, followed by separation of the liquid phase by decanting and evaporating the solvent and weighing the recovered liquid phase. Column Characteristics. Typical column characteristics, some of which are listed in Table 11, are very similar to those of PLOT columns made by the static method ( I , 2, 8, IO).
RECEIVED
for review August 30, 1971. Accepted October
5, 1971.
(9) R. Kaiser, “Gas Phase Chromatography,” Vol. 2, Butterworths, Washington, D.C., 1963, p 47. (10) L. S . Ettre, J. E. Purcell, and K. Billeb, Separation Sci., 1, 777 (1966).
(11) L. S . Ettre, “Open Tubular Columns In Gas Chromatography,” Plenum Press, New York, N.Y., 1965, p 40.
Evaluation and Applications of Porous Layer Open Tubular Columns Made by the Dynamic Method J . G. Nikelly Department of Chemistry, Philadelphia College of Pharmacy and Science, Philadelphia, Pa. 19104 and Cooke (3). The retention volume of methane was used as the nonretained component. Carrier gas flow rates were controlled with a precision valve (Micro-Flow Valve, Cat. No. 151, Matheson Gas Products, East Rutherford, N. J.) and measured with conventional flow-tube meters that were calibrated against a soap bubble flow meter for each column size, i.e., for each back pressure. The average linear gas velocity a, was calculated from the equation
ALMOST ALL PLOT COLUMNS are made by the static rather than the dynamic (plug) method. Recently, however, there has been interest in the dynamic method because it can be easily used in the ordinary gas chromatography laboratory ( 1 , 2). Several PLOT columns were made by the dynamic method in the author’s laboratory using Carbowax 1540, SE-30, DEGS, and Apiezon L-Igepal Co 880. In the present paper these columns were evaluated for efficiency, capacity, and other properties of practical analytical interest. Several analytical applications are also presented. EXPERIMENTAL
Apparatus. Two F1D gas chromatographs were used in this work: an F & M Scientific (Hewlett-Packard) Model 609 and a Gow-Mac, Model 69-700. The instruments were equipped with flame ionization detectors and were used in conjunction with 1-mV, 1-sec potentiometer recorders. In order to reduce the extracolumn (instrumental) volume of the Model 609, the original inlet was replaced with an oncolumn inlet which was adapted for 1/18-in. columns (Cat. No. 3511, Carle Instruments, Inc., Fullerton, Calif.). Also, the fitting for the column outlet was modified with a 1/16-in. adaptor that provides a 6-cm long, low volume extension into the detector block. The Model 69-700 was also adapted to ‘/I6 0.d. columns. 0.d. glass Samples were injected into a short section of tubing which was inserted in the injection port and connected to the column with a low volume stainless steel reducing union. A Teflon (Du Pont) reducing ferrule (Cat. No. 200/100, Chemical Research Services, Inc., Addison, Ill.) was used at the column outlet allowing it to extend 4 cm into the detector housing. Procedure. The instrumental dead volume, excluding the column volume, was determined by the procedure of Karger (1) R . Kaiser, Chromatographia, 1-2, 34 (1968). (2) J. G. Nikelly, ANAL.CHEW,44,623 (1972).
ti = Litm (1) where L is the column length and t , is the retention time for methane. Dynamically coated PLOT columns were prepared as discussed in the preceding Note (2). The columns were of 1Il6-in.0.d. and 0.02- or 0.03-in. i.d. in 2 5 , 50-, and 100-ft. lengths. The 100-ft lengths were made by joining 50-ft sections with Swagelok unions. Sample injections were made without a stream splitter. Regular microliter syringes were used or special syringes capable of introducing into the column as little as 0.01 111 volumes of liquid samples (Pressure-Lok Mini-Injector, Precision Sampling Corporation, Baton Rouge, La.).
RESULTS AND DISCUSSION
Flow Rate and Column Efficiency. Van Deemter plots for two dynamically coated PLOT columns are shown in Figure 1. The curves are fairly flat and have a minimum HETP between 15 and 25 cm/sec average linear flow rate or 2 to 3 cc/min. These are similar to the flow rates that are typical for some PLOT columns made by the static method ( 4 , 5). These relatively low optimum flow rates generally (3) B. L. Karger and W. D. Cooke, ibid.,36,991 (1964). (4) L. S. Ettre, J. E. Purcell, and K. Billeb, Separ. Sci., 1, 777 (1966). (5) L. S . Ettre, J. E. Purcell, and K. Billeb, J. Chromatogr., 24, 335 (1966).
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