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. 351 1, 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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31
I
2
I. 0 IO
-
20
30
U, CM I S E C
Figure 1. Plot of HETP us. average linear carrier velocity, ii, for n-propanol Column, 50-ft X 0.02-in. i.d. Carbowax 1540. Sample volume, 0.01 pl. Curvl , phase ratio = 50. Curve 2, phase ratio = 90
RETENTION TIME, MIN.
Figure 3. Separation of benzene homologs Column, 50-ft X 0.02-in. i.d. with Apiezon-L (80 %) and Igepal CO-880 (2073. Temp., 60 “C. Flow rate, 2 ml/min. Sample volume, 0.05 PI. Peaks: benzene (l), toluene (2), ethylbenzene (3), p-xylene and rn-xylene (4), oxylene (5)
RETENTION
TIME,
MIN.
Figure 2. Separation of lower primary alcohols Column, 46-ft X 0.02-in. i.d. with Carbowax 1540 liquid phase. Temp., 60 “C. Flow rate, 2 ml/min. Sample volume, 0.05 pl. Peaks 1-4 correspond to the respective alcohols
mean more analysis time for the same degree of resolution; however, this can be offset by the relatively high partition ratio k . For propanol at 60 “C column temperature, k was approximately 4. The minimum values for HETP in Figure 1 are somewhat higher than those reported in the literature. At least in part, this may be attributed to instrumental dead volume which was estimated to be about 0.5 ml. The contribution of extra column volume to HETP was also shown by the essentially negligible increase in the HETP minimum when two 25-ft columns were joined with a regular Swagelok union rather than a low volume (“zero volume”) union. Furthermore, there was no significant difference in HETP values between 0.03-in. and 0.02-in. i.d. columns. Comparison of the two curves in Figure 1 indicates the expected increase in column efficiency gained by going to lower liquid phase loadings ( 4 ) . Indeed, an SE-30 column which was made with a low liquid phase loading (6 G 300) 626
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J
0
2
4
RETENTION
6 TIME,
8
IO
12
MIN.
Figure 4. Separation of methyl esters Conditions are given in Table 11. Peaks: solvent (l), palmitoleate (2), stearate (3), oleate (4), linoleate (5), behenate (6), arachidonate (7)
had a minimum HETP value below 1 mm; however, the reduction in capacity ratio which accompanies the reduction in liquid phase loading, made this column useful only for the analysis of high boiling compounds (high k values). Capacity. Sample capacity was measured by injecting increasing volumes of a single component, toluene, and determining the sample size above which the peak height is
1
I
'
5
Table I. Column Capacity Shown as Sample Volume us. Normalized Peak Height
6
Sample volume, p1
Area, cma
Height, cm
Height/area
0.01 0.02 0.10
4.20 8.16 40.3
22.5 44.0 198.3
5.36 5.40 4.92
Table 11. Separation Data for Methyl Esters on A DEGS PLOT Column Column length, ft 50
R E T E N T I O N TIME,
MIN.
Figure 5. Separation of benzene homologs Column, 50-ft X 0.03-in. i.d. with SE-30. Column temp, 50 "C. Flow rate, 4 ml/min. Peak identification as in Figure 3
no longer linear with the volume of injected sample. The volume of the injected sample was calibrated against the peak area which was recorded using a high chart speed to reduce the error. The results are shown in Table I. Only three sample sizes were used but this is sufficient to show that the column tested (Apiezon L and Igepal Co-880, 0.02-in. i.d. X 50-ft length) has a sample capacity of between 0.02 and 0.1 ~ 1 . For 0.03-in i.d. columns, the capacity was 2 to 3 times higher. Since sample capacity depends on many other parameters including liquid phase loading, capacity ratio, and column length, all of which were not considered here, the data in Table I represent only a typical column. As in the case of the static method, dynamically coated PLOT columns can probably be made with a wide range of sample capacity. Furthermore, sample capacity may also be defined in terms of resolution of a pair representing a difficult separation and it has been shown that, according to this definition, sample capacity is more than double the value determined from peak height linearity (5). Stability. Some of these columns have been used intermittently for over a year with no detectable deterioration or aging. Bending the column ends several times in order to fit different gas chromatographs also seems to have no deleterious effects on the column performance. One of the columns was shipped by mail with no adverse effects. Selected Applications. Typical examples of separations are presented in Figures 2 through 5. The separation of
Column id., in. Column temp., 'C Phase ratio Partition ratio" Flow rate, ml/min No. of theoretical plates" HETP mma Retention time," min. Relative retentionb Resolutionb a Methyl oleate. b Methyl oleate/methyl stearate.
0.03 150 60 5.3 2.0 12,500 1.2 4.8 1.13 1.o
lower primary alcohols in Figure 2 shows complete resolution of the MeOH-EtOH pair which may be considered a difficult separation because of the low capacity ratio. In the case of wall-coated columns, more than 100 feet of column length would be required for this separation. Data from the separation of methyl esters on a DEGS column (Figure 4) are also presented in Table I1 in order to provide some comparison with corresponding data for other column types (4, 6). Specifically the important stearateoleate separation which required 12 minutes for complete resolution using a carefully selected and specially preconditioned packed column is accomplished in about 5 minutes on the PLOT column. It appears, however, that the same separation is even faster on a PLOT column reported by Ettre (4). In that instance, the oleate-stearate separation has a resolution of 3.4 in 18 minutes which means that for a resolution of 1, the analysis time would be in the order of two or three minutes. CONCLUSIONS
In terms of efficiency and capacity, dynamically coated PLOT columns are nearly equivalent to those made by the static method. Compared to wall-coated columns, the dynamically coated PLOT columns may be better because of their higher capacity ratios. RECEIVED for review August 30, 1971. Accepted October 5,1971. (6) "Gas-Chrom Newsletter," Nov./Dec. 1970, Applied Science Laboratories, Inc., State College, Pa., 1970, p 4.
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