Advantages of a Dual Column (Packed and Capillary Columns in Series) for Gas Chromatographic Analysis John Q. Walker and Clarence J. Wolf McDonnell Douglas Research Laboratories, Mc Donne11 Douglas Corp., St. Louis, Mo. 63166
ALTHOUGH IN THE PAST 10 years many advances have been made in the field of gas chromatography, separations are still limited by the particular column chosen for the analysis. Gas chromatographic columns of many different diameters, lengths, stationary and/or liquid phases, etc., have been described in the technical literature for a great many complex separations. However, it is difficult to obtain a single gas chromatographic column which is capable of separating a mixture of components whose boiling points range from - 160 to 350 “C. Specific single columns are available for the separation of either low or high boiling mixtures. For example, Beckham and Libers ( I ) have described a separation of the low boiling point hydrocarbons methane (bp - 161 “C) through cyclopentene (bp 49 “C). The separation of a high boiling hydrocarbon mixture [n-octane (bp 49 “C) through ndocosane (bp 327 “C)] was discussed by Ettre (2). Jacobs (3) described a single column, a support-coated open tubular column containing Dow Corning DC200 silicone oil, which when temperature programmed from - 55 to 140 “Cis capable of separating C1 to Clahydrocarbons. Other workers (4, 5 ) have used two columns containing a switching valve and/or utilizing backflushing techniques coupled with subambient programming to separate hydrocarbon mixtures boiling from - 161 to 213 “C (1-dodecene). We have constructed a special column which consists of two columns of different internal diameters connected in series to separate complex mixtures (C, to Czo)with a wide boiling point range without the use of subambient temperature programming, backflushing, or other carrier gas switching techniques. Two columns of different internal diameter or packing cannot be connected in a random fashion and still possess good separation capability (6). Both the height equivalent to a theoretical plate (HETP) and the column resolution (R) depend on several factors such as column length, velocity of the carrier gas, and average pressure in the column. Therefore, to prepare a useful dual column composed of two different columns, the length of each section and the carrier gas velocity must be carefully chosen. Dual columns with short ‘/*-inch (0.075-inch i d . ) packed columns followed by a support coated open tubular (SCOT) column are described for the separation of a mixture with a wide boiling point range. The experimental parameters required to optimize the separation of a given set of compounds are presented and discussed. EXPERIMENTAL
A gas chromatograph (Hewlett-Packard, Model 5750) equipped with both dual thermal conductivity and dual flame (1) R. D. Beckham and R. Libers, J . Gas Chrornarogr., 6, 188 (1968). (2) L. S. Ettre. “Open Tubular Columns in Gas Chromatography,” Plenum Press, New York, 1965, pp 123-132. (3) E. S. Jacobs. A ~ A LCHEM., . 38,47 (1966). (4) D. J. McEwen, ibid., 1047 (1966). (_ 5 ) ,L. J. PaDa. D. L. Dinsel, and W. C. Harris, J. Gas Chrornatogr., 6, 270 (1968). (6) J. Q. Walker, ANAL.CHEM., 40, 226 (1968). 1652
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ionization detectors (FID) was used. In all experiments the carrier gas was helium. The chromatograms were recorded on a 1-mV strip chart recorder (Moseley). The Hz and air flow rates into the FID were 20 and 275 cm3/min,respectively. Liquid samples were injected directly with a 1.0-pl syringe (Hamilton No. 7101) into a 0.029-inch i.d. on-column injection adaptor. Gas samples were introduced with a gas sample valve (Aerograph No. 57-000034-00) equipped with Viton quad rings. The area and elution time of each peak in the analyses were measured with a digital integrator (Infotronics CRS-100). Eight-inch stainless steel columns (0.075-inch i.d.), prepared in 5-, lo-, and 20-foot lengths, contained lOO/lSO mesh Porasil “B” (Waters Associates, Inc.). SCOT columns containing polydimethyl silicones (OV-101) were obtained from the Perkin-Elmer Corp. in lengths of 50, 100, 150, and 200 feet. All columns were preconditioned for 16 hours at 160 “C with a helium flow of 5 cm3/min. RESULTS AND DISCUSSION
The efficiency of all lengths of each column was determined as a function of carrier gas flow rate. The efficiency ( N ) was calculated from the observed peak width and retention time of n-nonane according to the relation: (retention time)2 N = 16(peak width)z The optimum flow rate for each length of column is equal to that flow rate yielding the highest efficiency. The optimum flow rate (OFR) as a function of column length (L) for the Porasil “B” 0.075-inch i.d. packed columns was found to fit the empirical equation, OFRporasii
=
0.22 L
(2)
where the flow rate is a cm3/min and the length is in meters. A similar linear equation was determined relating the optimum flow rate to the length of a 0.02-inch i.d. SCOT column: OFRSCOT = 0.025 L
(3)
Since the optimum flow rate in both a 5-foot packed column and a 50-foot SCOT is 3.5 cm3/min, a dual column system containing the 5-foot packed column in front of the 50-foot SCOT column was constructed. The optimum flow rate in this column should correspond to either a 10-foot packed or a 100-foot SCOT column, Le., 7 cm3/min. The height equivalent to a theoretical plate (HETP) is a quantity of fundamental interest and is related to the column length (L) and efficiency ( N ) by: HETP
L N
= -
(4)
The separation capability, i.e., the resolution (R), of the column for the separation of a mixture is of prime interest when selecting a column for a particular analysis. The resolution R of two compounds 1 and 2, can be defined in
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
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-
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Figure 1. Height equivalent to a theoretical plate ( H ) and resolution (R) as a function of the average carrier gas velocity ( E ) for a 5-ft by 0.075-inch i.d. packed column. H is determined from the elution of n-nonane, and R is based on the separation of n-nonane from 3methyloctane
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Figure 3. Height equivalent to a theoretical plate (H) and resolution ( R ) as a function of the average carrier gas velocity (U) for a dual column consisting of a 5-ft packed column in front of a 50-ft SCOT column. H is determined from the elution of n-nonane, and R is based on the separation of n-nonane from 3-methyloctane
Table I. Pressure Variation in the 5-foot Packed Column, 50-Foot SCOT Column, and Dual Column Flow Rates in Each Column Correspond to Maximum Efficiency for the Particular Column Length Used Flow rate, Column cm3/min ~ i , , l , t psi Poutlet psi F , psi 5-ft packed 3.5 29.5 14.5 23 14.5 21 50-ft SCOT 3.5 24.5 Dual 7.5 35 14.5 26 packed ... 35 25 32 SCOT ... 25 14.5 21
I:
b
0
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Figure 2. Height equivalent to a theoretical plate ( H ) and resolution (R) as a function of the average carrier gas velocity (U) for a 50-ft by 0.02-inch i.d. SCOT column. H is determined from the elution of n-nonane, and R is based on the separation of n-nonane from 3methyloctane
terms of their respective retention times ( t r ) and the width ( W ) at the base of the peak (7).
Both HETP and resolution as a function of the average carrier gas velocity (G) for the 5-foot packed, the %foot SCOT column, and the tandem column of 5-foot packed-50foot SCOT columns are shown in Figures 1, 2, and 3, respectively. The average carrier gas velocity (equal to the column length divided by the elution time of inert compound, CHI) is used because of its fundamental importance in the Van Deemter equation (8). The resolution shown in these figures is calculated according to Equation 5 for the separation of 3-methyloctane from n-nonane. (7) W. E. Harris and H. W. Habgood, in "Programmed Temperature Gas Chromatography," John Wiley and Sons, Inc., New York, 1966, Equation 5.22, p 121. (8) J. J. Van Deernter, F. J. Zuiderweg, and A. Klinkenberg, Chem. Etig. Sci., 5, 271 (1956).
With the 5-fOOt packed column (Figure l), the minimum of HETP with carrier gas velocity occurs over a broad flat region extending from a equal to 5 cmjsec (3.5 cm3/min)to 10 cmjsec (7.0 cm3/min)while the resolution is essentially constant in the same gas velocity interval. The minimum in HETP with the %-foot SCOT column (see Figure 2) occurs at an average carrier gas velocity of 16 cmjsec (corresponding to a flow rate of 3.5 cm3/min)and is rather sharp. The resolution, however, decreases monotonically from a maximum of 11.2 to 3.6, The dual column (see Figure 3) exhibits a minimum HETP with an average carrier gas velocity of 30 cm/sec (flow rate 7.5 cm3/min) while the resolution decreases slowly when the carrier gas velocity increases from 15 to 88 cmjsec. In fact, the resolution varies only 5 % while the average gas velocity changes by a factor of 6. It is important to note that two apparent unrelated variables are actually changed when the columns are operated in the dual mode. The average carrier gas velocity is different in each section of the column because of the different diameters; and the average pressure in each section of the column is different than when the same section of the column is used alone. The inlet and outlet pressures required to achieve an outlet flow rate of 3.5 cm3/min with the 5-foot packed and 50-foot SCOT columns are shown in Table I. In addition, the inlet and outlet pressure at the 5-fOOt packed-50-foot SCOT column function required for an outlet flow rate of 7.5 cm3jmin in the dual column are also shown. The average pressure in all three columns and in the separate sections of the dual column are listed in Column 5 of Table I.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
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Time (min)
Figure 4. Chromatogram showing separation of hydrocarbon products from pyrolysis of phytane using the dual column consisting of a 5-ft packed column in front of a 50-ft SCOT column. The column temperature was programmed from 50 to 200 “C at a rate of 10 deg/min
The average pressure is equal to the outlet pressure (Pout) divided by the compressibility factor, j , where
packed column when used in the dual column has a higher efficiency than when used alone. The complete separation of the hydrocarbons from C1to CI8 cannot be performed with the SCOT column alone. The hydrocarbons C1,Cp,and C3appear as a single peak and even C4(butene) and Cs (pentene) are poorly resolved even when the initial temperature of the column was below ambient. However, with the dual column, good separation of the light hydrocarbon gases was obtained with the initial column temperature as high as 50 O C . A chromatogram obtained with the dual column for the separation of the products from the vapor phase pyrolysis of the phytane (2,6,10,14-tetramethylhexadecane, bp 323 “C) is shown in Figure 4. Here, CH,, CzH4,and C2H6 are separated to the base line, and even a trace of C2Hz is partially resolved. The apparatus and experimental procedure used in the pyrolysis experiment were described previously (10). In general, the use of dual columns can be applied to the analysis of many mixtures other than reported here. For example, a dual column consisting of the packed Porasil “B” and SCOT OV-101 columns yields symmetrical peaks for all aliphatic hydrocarbons. However, aromatic hydrocarbons form nonsymmetrical peaks with excessive tailing. This can be eliminated by using a dual column consisting of a 5-foot packed Porasil “C” (rather than Porasil “B”) column in series with the 50-foot SCOT OV-101 column. When two columns are joined to form a new dual column, it is desirable to choose the length of each column in such a manner as to correspond to the maximum efficiency for the particular flow rate used. The flow rate should be selected on the basis of resolution and analysis time.
We have previously shown (9) that an increase in the average pressure in the column decreases HETP, so that the 5-foot
RECEIVED for review May 22,1970. Accepted August 10,1970. This research was conducted under the McDonnell Douglas Independent Research and Development Program.
(9) J. Q.Walker, J. D. Kelley, and C. J. Wolf, Hydrocarbon Process., 47, (4) 288 (1968).
(10) D. L. Fanter, J. Q. Walker, and C. J. Wolf, ANAL.CHEM., 40, 2168 (1968).
Autocatalysis of the Kinetic Wave of Acetylacetone in Acetonitrile Solvent Thomas E. Neal and Royce W. Murray Department of Chemistry, University of North Carolina, Chapel Hill,N . C . 27514
THE ELECTROCHEMICAL REDUCTION of several aromatic 0diketones in DMSO solvent, has been investigated by Buchta and Evans ( I ) . The detailed experiments performed on one case, dibenzoylmethane, demonstrate a reduction to the radical anion followed by protonation by unreduced Pdiketone and coupling to form the pinacol. The pinacol subsequently, and on a relatively slow time scale, undergoes an autocatalyzed decomposition. Recent experiments in these laboratories on the reduction of acetylacetone in acetonitrile solvent have shown that this aliphatic P-diketone also exhibits an unusual electrochemical reaction but unlike that of the dibenzoylmethane case. Reduction waves for acetylacetone (1) R. C . Buchta and D. H. Evans, ANAL.CHEM., 40, 2181 (1968). 1654
have been reported in aqueous media (2-4), but other reports (5,6) note an inability to observe reduction in a variety of electrolytes. EXPERIMENTAL
Instrumentation. The solid-state potentiostat constructed for these experiments employed positive feedback control (7)
(2) G. Semerano and A. Chisini, Gazz. Chim. Itul., 66, 504 (1963). (3) A. Winkel and G . Proske, Ber., 69, 1917 (1936). (4) I. Tachi, Mem. Coll. Agr., Kyoto Imp. Unio., 42, 1 (1938). ( 5 ) H. Adkins and F. W. Cox, J . Amer. Chem. Soc., 60, 1151 ( 1938). (6) S. Harrison, Collect. Czech. Chem. Commun., 15, 818 (1950). (7) G. A. Lauer and R. A. Osteryoung, ANAL. CHEM.,38, 1106 (1966).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
