Gas Chromatographic Columns with Adjustable Separation Characteristics David R. Deans and Ian Scott
IC/Ltd., Petrochemicals Division, Billingham, Teesside, U . K . Where two gas chromatographic columns, containing different stationary phases are connected in series and used as if they were one column, it is shown, theoretically, that by artificially changing the ratio of the gas velocities in the two columns the relative retention times of eluted compounds can be changed. A simple apparatus is described in which the pressure at the junction between the two columns can be independently controlled. By adjustment of this pressure the separation characteristics of the columns can be changed. Two practical examples of the system in operation are given: in each, the separation of a hydrocarbon miiture, which is normally difficult to obtain, is optimized by moving the peaks relatively to one another.
The objective of this work was to produce gas chromatographic columns where the relative retention times of compounds, eluted from the columns, can be changed simply by turning a knob on the control panel of the gas chromatograph. It has been common practice, almost since the advent of gas chromatography, to use a mixture of stationary phases to obtain apeparation of compounds that has proved difficult to obtain on a single stationary phase, e . g . , James and Martin ( I ) . In some cases, the stationary phases are mixed homogeneously in one column. In other cases, the stationary phases are put separately into two or more columns and the columns connected in series. The use of two or more columns, with different stationary phases, in series, is particularly advantageous when some form of flow switching, e.g., heart cutting or back flushing, is used between the columns. This is frequently practiced in on-line process chromatography. Prediction of the best lengths of columns containing different stationary phases from retention data for the individual stationary phases is very difficult, from the practical point of view. This is because it is necessary t o predict the retention time of each individual compound on each column. Prediction of relative retention times and number of theoretical plates is not sufficient. This is made appar-
ent by the formula for overall relative retention times (adjusted) for two compounds A and B on two columns 1 and 2 in series
The reproducibility of retention times for nominally identical columns under nominally identical operating conditions is, in the authors’ experience, not sufficiently good to enable the optimum separation to be obtained immediately from predicted column lengths. We have used the computer programmes of Peterson and Pine (2) to predict the columns required for given separations. While the predictions are within the reproducibility of the columns, final adjustment of the ratio of the columns is usually necessary. It is obviously desirable that the effective ratio of two or more stationary phases used in series should be easily adjustable without recourse to repacking or shortening columns.
THEORY The theory and practice of adjusting the contribution of columns to mixed systems will be described in terms of pressure control. I t is possible to achieve similar results using flow control but, in the authors’ experience, flow controllers give rise to a number of problems including relatively poor precision of quantitative analysis, compared with that achieved using pressure control. Some of the reasons for this have been described previously (3). Consider two columns in series where the flow of carrier gas is controlled by an inlet pressure controller as shown in Figure 1. At the junction of the two columns, the pressure P, is determined by the inlet and outlet pressures P, and Po and the relative restriction to flow offered by the two columns. The flow of gas out of the first column is, of course, equal to the flow of gas entering the second column. If Pi and Po are maintained constant and P, is altered by some external means, then the flow of gas from the first column will no longer be equal to that into the second column. If Pj is altered then the gas hold up time (2) R. M . Peterson and C. S. F. Pine, Chrornatographia. 5 , 146-155
(1) A . T. James and A . J. P. Martin. Brit. Med. Bull (1954).
10(3), 170-176
(1972). (3) D. R. Deans, “Gas Chromatography 1968,” C. L. A . Harbourne, Ed., Institute of Petroleum, London, p 447. A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 7, J U N E 1973
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~I
PGIP I
t~ is also altered on each column. If P, is increased then, t ~ ( 1on ) the first column is increased and t ~ i 2 on 1 the second column is decreased and vice versa. The formulas for the gas hold up times on the two columns in series are:
COLUMN
e
COLUMN
DEICCCTW PO
(3) The adjusted retention time of a compound is defined by the equation t’R
= tM
x (KIP)
(4)
where K is the partition coefficient and P is the phase ratio. From Equations 1 and 4, the relative retention time for two compounds on two columns in series is given by
Figure 1. Two columns in series
%9
INJECTION POINT
COLUMN
!2
DETECTOR
Figure 2. Two columns in series with the junction pressure controllable above the natural pressure
By differentiating r with respect to t . ~ , ~ ) / t ~and ( 1 put1 ting this equal to zero, it is possible to show that the relative retention time r of the two compounds is only independent of t ~ ( ~ ) / t .when ~ ( 1 )both K , A , and Krsi are equal to zero on one of the columns, that is when both compounds are unretarded by one of the columns. Or when K[A,I)= K(B,I,and K ( , ~ , 2= 1 K ( B , ~that I , is when there is no resolution between the compounds on either column. Or when K,AJIIP(II = K , A , P I / Pand ~ K,B,lI/P,ll= K f B , 2 , / P ( 2 ,that , is when the resolution between the compounds is exactly the same on both columns. Thus, theoretically it should be possible to change the relative retention times of compounds, using two columns of different characteristics in series, by changing the ratio of the gas hold up times on the two columns. This is readily achieved by altering artificially, the pressure a t the junction between the two columns.
EXPERIMENTAL
Lrl DETECTOR
Figure 3. Two columns in series with the junction pressure controllable both above and below the natural pressure 1138
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Apparatus. Figure 2 shows, diagrammatically, the apparatus used to raise the pressure a t the junction between the two columns. With pressure regulator PR2 off, pressure regulator PR1 is set in the normal way to give approximately the required flow through the columns. Under these conditions it is possible to read t h e “natural” junction pressure on pressure gauge PG2. T o alter the relative retention times, PR2 is set so t h a t PG2 shows a higher reading t h a n its “natural” pressure. This increased pressure must, of course, always be lower than t h a t shown on PG1. When PG2 reads above the “natural” pressure additional carrier gas is being supplied to column 2 through PR2. Figure 3 shows the form of the apparatus used when the junction pressure is to be lowered. Again, the flow through the columns is set in the normal way by adjusting P R 1 with PR2 and the needle valve NV turned off. To lower the junction pressure the needle valve is opened partially until the reading on PG2 drops just below t h e required pressure and t h e required pressure is set by adjusting PR2. The setting of the needle valve is not critical as, providing PR2 is supplying some carrier gas ( t h a t is, it is “on control”), the restriction of the needle valve determines
PG2
:
I
Figure 4. Heart cutting system used with two columns in series
1% I
.
Figure 6. hapntha sample neart Jbnction presshre 21 psig
C L ~to
. . ..
remove non-aromatcs.
Pea< 1 oenzene: 2 . to ,em. 3 etnyl oeizene 4 m - an0 z-x, ene 5. sopropy oenzene 6. ormoxy e i e . 7. n'-propy benzene. 8 3- an3 4-erny IO ,me' 9 mesit, e i e . 10 2-eti11 :o ,em: 1 1 pse-oo c-mene 6
Figure 5. Naphtha sample run straight through two columns in series
neither the junction pressure, nor the quantity of gas split off from column 1. If the needle valve is opened further the only result is that more gas has to be supplied by PR2 t o maintain the required pressure; this extra gas goes out through the needle valve. The only part of the system that has t o go in the chromatographic oven is the Tee piece a t the junction of the two columns, with a pipe connection (usually ya-in. outside diameter) to the pressure controller outside the oven. In most chromatographs, this system is easy and cheap to install.
PRACTICAL EXAMPLES Aromatics in Naphtha. The analysis of naphthas (in this example, a mixture of Cq to CS paraffins, naphthenes, and aromatics) for individual aromatics is difficult, even using a high efficiency column, because of the large numbers of non-aromatic isomers in the higher carbon number groups that obscure the aromatics. T h e normal approach is t o use a polar liquid phase which will retard the aromatics so t h a t they are eluted after the non-aromatics. However, those phases giving the best retardation of aromatics with respect t o non-aromatics do not generally produce a satisfactory separation of the aromatics themselves and it is therefore necessary to add a second column, usually relatively non-polar, to obtain a n adequate separation of the individual aromatics. If the non-aromatics are vented a t the end of the first column, by heart cutting, so that they do not go on to the second column and interfere with the subsequent separation of the aromatics, the job of the first column is to retard all the aromatics behind the last eluted naphthene and paraffin. The job of the second column is to accept the aromatics in the order and a t the time intervals determined by the first column and produce a final separation of the individual aromatics. Cyanoethylformamide was chosen as the stationary phase for the first column because of its aromatic retarding characteristics. The computer program previously mentioned was used to select OV17 as the second stationary phase and to suggest the quantities of the phases to use. Figure 4 shows the flow diagram of the apparatus A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 7, J U N E 1973
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Figure 8.
C4
hydrocarbons. Junction pressure 29 psig
Peak 1, propylene; 2, isobutane; 3, propadiene: 4 , n-butane; 5 , 2-methyl propene; 6, isobutene; 7, trans-butene-2: 8, butadiene 1 : 3 , 9, cis-butene-2; 10, methyl acetylene; 11, butadiene 1 : 2
Figure 7. Naphtha sample heart cut to remove non-aromatics Junction pressure 23 5 psig
used for the analysis. Venting of the paraffins and naphthenes a t the end of the first column was achieved by opening and closing on/off valve C a t the appropriate times. This technique of heart cutting has been described previously ( 4 ) . The chromatographic conditions used were: First column, 1.5-m X 2.4-mm i.d., stainless steel packed with 22.5% w/w CEF on 100-120 B.S. mesh acid washed Chromosorb P. Second column, 6.0-m X 2.4-mm i.d., stainless steel packed with 3% w/w OV 17 on 60-80 B.S. mesh, acid washed, dimethyl chlorosilane treated Chromosorb G. Column oven, 90 “C; vaporizer, 200 “C; carrier gas, nitrogen; inlet pressure, 30 psig (2.07 bar); detector, flame ionization, and sample, 1.5 pl naphtha. Figure 5 shows the chromatogram obtained without venting the non-aromatics a t the end of the first column. Figure 6 shows the chromatogram obtained using the same experimental conditions but “cutting” the non-aromatics out of the system a t the end of the first column. It can be seen that peak 5 (isopropyl benzene) is not separated from peak 6 (0-xylene). Examination of’the retention data for these two compounds on CEF and OV 17 indicated that to move peak 5 forward into the gap between peaks 4 and 6, the influence of the OV 17 column should be reduced. As discussed earlier, this can be achieved by raising the junction pressure. The natural junction pressure was 21 psig (1.45 bar) and by raising it to 23.5 psig (1.62 bar) the optimum separation was achieved, peak 5 moving forward into the available space between peaks 4 and 6, see Figure 7. C4 Hydrocarbons. Another analysis which has always posed something of a problem involves the separation of ( 4 ) D. R Deans, Chromatographla 1, 18-22 (1968)
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Figure 9.
C4
hydrocarbons. Junction pressure 30 psig
the C4 hydrocarbons. This is because the stationary phases which separate 2-methyl propene and butene-1 tend to leave unresolved other components in the sample and vice versa. For example, phenylisocyanate/Porasil C (Durapak) which separates 2-methylpropene and butene-l, elutes propylene with isobutane whereas on squalane, propylene and isobutene are separated, but 2-methyl propene and butene-1 overlap. Unfortunately, it is not possible to produce a combination of these two phases which will give complete separations, as further coincidences occur among the other C4 hydrocarbons; however, complete separation was achieved using a three-column system as detailed:
First column, 3-m X 2.4-mm i.d. stainless steel packed with 25% w/w Squalane on 60-80 BS mesh, acia washed Chromosorb P. Second column, 4-m X 2.4-mm i.d. stainless steel packed with 80-100 BS mesh phenylisocyanate, Porasil C(Durapak). Third column, 10-m x 2.1-mm i.d. stainless steel packed with 30% w/w 1.2.3-trid2-cyanoeth0xy)propane on 60-80 BS mesh, acid washed Chromosorb P. Column oven, 50 "C; carrier gas, hydrogen: inlet pressure, 35 psig (2.41 bar); detector, flame ionization; and sample, 50 p1 Cq vapor. In this three-column system, the pressure at the junction of the first s n d second column was adjustable. Figure 8 shows the separation obtained before adjusting the junction pressure, which was initially a t 29 psig (2.00 bar). Under these conditions, peaks 3 and 8 (propadiene and butadiene, 1 and 3, respectively) required moving forward to obtain the best separation. As both these compounds were unresolved from compounds boiling higher than themselves, the forward movement was achieved by increasing the effect of the squalane column. Raising the junction pressure from 29 psig to 30 psig (2.00 to 2.07 bar) caused peaks 3 and 8 to move forward to the positions shown in Figure 9.
DISCUSSION Artificially altering the junction pressure changes the gas velocity in both columns. It is important, in order that acceptable efficiency, in terms of peak width, be maintained, that the gas velocity in either column does not drop much below its optimum value. In practice this is not often a problem as it is usual to work a t appreciably higher velocities than the optimum for maximum efficiency. In designing columns for use with this system. it is normally preferable to select those that have a fairly flat HETP against velocity curve above.the optimum.
The essence of the system described is that relative velocities in two different columns in series should be adjustable simply. This can be achieved by other means than the adjustment of the junction pressure. Once a junction pressure controller or junction flow controller is incorporated, then alteration of the inlet pressure or flow while maintaining the junction pressure or flow constant will equally change the relative velocities in the two columns. Similarly, adjustments to both a t the same time can sometimes be useful. Whichever system is used it is perfectly practicable to have a knob on the control panel which effectively adjusts the separation characteristics of the columns in the oven.
NOMENCLATURE j = Pressure correction factor k = Column permeability K = Partition Coefficient t , =~ Gas hold up time
t,? = Gross retention time t ' H = Adjusted retention time For the above symbols subscripts A and B refer to compounds A and B. Subscripts 1 and 2 refer to columns 1 and 2 .
P, = Column inlet pressure P, = Pressure a t the junction of two columns Po = Column outlet pressure r = Relative retention time (adjusted) 7 = Viscosity of the carrier gas Received for review November 22, 1972. Accepted January 15, 1973.
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