Albert M. Koury a n d Jon F. Parcher The University of Mississippi University, MS 38677
Mixed Stationary Liquid Phases for Gas-Liquid Chromatography
Anyone who wishes to separate the components of a complex mixture by gas-liquid chromatography is immediately faced with the critical choice of a stationary liquid phase. There are a larze number and wide varietv of these stationarv phases available and the selection of the best one for the separation of a given mixture is often difficult. There are some gcmeml rules and qualitative guides i m the ielecfiun uf a statimar\, i,hasc; howcvrr, t h r choirc is ot't1.n made, f n m . liq~~iel . . previous experience, intuition, or just plain guessing. I t is possible to use two or more liquid phases in the same column. In this case, the polarity or separating power of the column will he a function of the composition of the binary liquid phase. If the components of the mixed liquid differ markedly in polarity, the polarity of the mixed column can he varied over the full range by changing the composition. The liquid phase selection problem then becomes a choice of how much of each liquid phase to use rather than which one to use. These mixed liquid phases can he utilized as mixed bed columns in which each support particle is coated with a single pure stationary phase and the particles are mixed, or they can he utilized as blended columns with each support particle coated with a binary liquid phase (1,2).It has been shown that these two techniques produce equivalent columns for mixtures of sqnalane and dinonylphthalate (3,4). A mixed bed column is much simpler to prepare, and it is the most common technique for preparing columns with two stationary liquid nhases. Pnrnell (3, 4) has recently developed a technique for determining the optimum composition of a mixed bed column for the separation of a particular mixture. This method is based on the interpretation of "window" diagrams.
umn. This means that the relative retention volumes will also vary linearly with composition to a first approximation. Example The retention volumes of three solutes A, B, and C are 3330, 967, and 3330 on a nonpolar liquid phase and 954,995, and 3330, respectively, on a polar liquid phase. The first step i n . the production of a window diagram is to calculate the relative retention ratios. These ajj values can be used to generate a window plot by graphing the data in the form r u j M i x = rui,N~mpulsr + ((y..Poler
- (yjjNonpolar) Chpuler far ruOM'"2 1
(1)
is the volume (or weight) fraction of the polar component < 1,the reciprocal of the liquid phase. If the calculated of aij, can he graphed in the same form using nji instead of nij for the pure liquid phases. Figure 1 is a plot of the data and the heavy lines indicate the minimum alpha value, a * as a function of composition. In this example, the components cannot he s e ~ a r a t e don either the pola; or nonpola; column. However, amixed bed or hlended column with equal proportions of the two uhases . . should readily separate this three component mixture because aij will be a t least 2 for all solute pairs. $,,,I,,
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Window Diagrams
One way to quantitate the selectivity of a column or liquid ohase is hv means of the relative retention ratios.. a;;. .,. of all possible solute pairs. orij is defined as the ratio of the adjusted retention volume or time of component i to the retention volume or time of component j. If an nij value for any solute pair in a mixture is close to unitv then the separation of this pair of solutes will he imperfect. If we choose B e ij pairs such that a,. 2 1, then the limiting factor in the separation of the mixture will he the resolution of the solute pair for which aij is a minimum. If this minimum u value is designated as n*, window diagrams are plots of a * as a function of liquid phase composition (3.4). Purnell(3,4) has shown that for normal chromatographic liquid phases, the partition coefficient (adjusted retention volume per ml of stationary phase) is a linear function of composition of a binary stationary phase or a mixed bed col-
Figure 1. Window diagram for example data set.
Volume 56. Number 9, September 1979 1 823
Laboratory Experiment for instrumental Analysis The preparation of a mixed liquid phase column is a good exercise for an undergraduate instrumental analysis course. This experiment consists of three procedures. (1) Measurement of the retention times of each component of a specific mixtu~eon a gc column containing a pure nonpolar stationary liquid phase and the same measurement on a column with a Dure polar liquid ptiase.
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lutes. (3) Preparation of a mixed bed column with the optimum eomposition and the (hopefully) complete resolution of the mixture an this column.
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Procedure Each group of students is grven 5-8 pure solute components and two packed columns. One of the columns should contain a nonpolar liquid phase and the other should contain a polar liquid phase. The column sizes and liquid loading should be the same fur both columns. The initial procedure is to measure the retention time of each component in the mixture on each column. Only the d a t i v e retention parameters are calculated so there is no need to measure the flaw.rate o r pressures. A graphof a i versus volume fraction of the polar liquid phase can then be constructed for each solute pair. This can be carried out by hand for a small number of components; however, aRlatively simple computer program' can be used for the routine calculations and graphic display. The use of a small computer with a graphic display terminal adds another dimension to the experiment and reduces the time required for repetitive calculations.
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BLENDED GW
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Figure 2. Chromatogram of a test mixture at 60°C
Results Figure 2 is the chromatogram of a test mixture on OV-101 and OV-25. The OV-101column cannot separate dioxane from n-heptane or iodobutane from n-octane. Benzene and n-octane arenot separated on OV-25, so neither of these columns would he satisfactory for the test mixture. The window diagram for this system is shown in Figure 3. There are three significant maxima in the ol* versus eomposition curve a t comoositions of 0.26.. 0.61,. and 0.85 with a* values of 1.19, 1.23, and 1.30, respectively. A mixed bed or hlended column with acomposition of 85% OV-25 and 15% OV-101 should give a reasonable separation of all of the components in this particular mixture a t 60°C. A column of approximately this composition (81% OV-25) was prepared from the packings in the original OV-101 and OV-25 columns. The chromatogram of this same mixture on the blended column is also shown in Figure 2. All seven components of the mixture are resolved on this column, although there is partial overlap of the benzene and octane peaks. The minimum resolution observed was 1.02 and the observed n* value of 1.260 (henzenelactane) was in gbad agreement with the n' value (1.257) predicted from the window diagram generated from the relative retention data on pure OV-101 and OV-25. After completion of the experiment, the original cuated packings can be recovered if the liquid phases are coated on solid supports of different sizes. If one of the original colrimns contains 60180 mesh support and the other contains 100/120mesh support, the coated supports can he segregated by sieving (5). ~~~
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Discussion This experiment is relatitely stmpleand yet the student e m learn n great deal of practical clmmarugraphy. 'The prublem involves the srparatim of cumpler mixture; and the practical prcpnratiun d n A sample program is available from the authors upon request.
624 / Journal of ChemicalEducation
Figure 3.Window diagram for the test mixture
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oacked column. Yet the student is not reouired to coat or analvze a cz,lumn pnrk~ngand the experimental mearurrmtnt
