a window diagram for key component analysis in on-line gas chromatography

A Window Diagram for Key Component Analysis in On-Line Gas Chromatography Richard Villalobos OnLine Analytics, Box 1742, Duxbury, MA 02332-1742, USA

Key Words: GC Window diagrams Optimization of mixed phases On-line chromatography

Summary

Window diagrams that optimize for the separation of only one or a few components in a complex mixture are applied to on-line process analysis when speed of analysis is more important than a complete separation of all components in the sample. A window diagram based on retention indexes is the most useful for quickly evaluating the feasibility of a given pair of phases. The one with the most useful output is one based on partition ratios, as this can be used directly with the columns in hand. A PC-based spreadsheet program with integral specific retention volume data for the common liquid phases is described as a tool for selecting the optimum ratio of lengths for the columns in hand.

1 Introduction

In the practice of gas chromatography, the use of mixed dissimilar phases has long been recognized [1,2] as the easiest and most effective way of maximizing separation factors in the analysis of complex multicomponent mixtures. Window diagrams [31 provide a simple, convenient way of determining the optimum ratio of the two phases for a given separation, given that much of the needed data is available in the literature or can be quickly obtained with a few simple measurements. The question remaining is how best to convert the data into a form that can be manipulated by a window diagram calculation, or, more importantly, to produce results in a form that can be used directly to construct a column system which performs the desired separation from the columns on hand. The present work describes our efforts at providing a practical answer to this question.

2 Theory

The window diagram program previously described by the authors [4,5,6] was modified by adding a data base of specific retention volumes for the reference n-paraffins for the common liquid phases. The data base takes retention indexes that have been experimentally determined on the columns in hand and converts them into partition ratios.

Specific retention volume data for each phase is stored in the form:

In Vgn = + bn/T

where V,, is the specific retention volume of reference n-paraffin of carbon number n on the given liquid phase; Tis the temperature [K]; a is the intercept coefficient; and b is the slope coefficient.

A set of data for each liquid phase includes a value of In Vg for each n-paraffin reference hydrocarbon through carbon number 16. A complete set of data is included for each of the phases

I. High Resol. Chromatogr

OV-1, OV-11, OV-17, OV-22, and OV-25 calculated from the data of Chien, Furio, Kopecni, and Laub [7,8], and for Carbowax 20M from in-house data. Partition ratios are calculated for each solute by comparison with the phase-specific V,s using the relations given by Said [9].

From the column dimensions and operating conditions partition ratios are calculated that are specific to the columns on which the data were obtained, using the relations given by Said [9]. The calculated window diagram can then be used directly to select the optimum ‘effective’ lengths of the columns to be series-coupled to produce the desired separation. The column optimization program [6] evaluates the contributions of the carrier gas com- pressibility at the specified temperature and calculates the actual lengths and carrier gas pressure to give the desired resolution at the specified operating conditions, A particularly useful facet of this form is that retention index data from the literature or from in-house data can be used to identify hypothetical columns as probable candidates for a practical solution to the problem at hand.

3 Experimental

All experimental work was done with a Hewlett-Packard Model 5890 gas chromatograph equipped with capillary splitter inlet and FID detector. The response time of the detector and electron- ics was assumed to be 100 ms, for purposes of calculating peak width and optimum resolution [ 101. Column head pressures were obtained with a separate pressure gauge with syringe needle attached for insertion into the sample inlet. Retention indexes were obtained on the actual columns that were later cut into shorter lengths and coupled in series. To eliminate film thickness as a source of error, the true film thickness in each case was determined by comparison with specific retention volume data in the literature [7,8] or the in-house data base. All work reported here was done at 50 “C with helium camer gas.

Column lengths, retention times and peak widths were calculated with the computer program described in reference 4 using the length ratios determined from the window diagram. The examples that follow were all calculated from the data shown in Table 1.

4 Basis for Window Diagrams

All window diagrams are based on some type of retention data that describe the relative retention of the solutes on each of the given phases. Measures of relative retention that can be (and have

VOL. 18, JUNE 1995 343

A Window Diagram for Key Component Analysis in On-Line GC

Table 1. Retention indexes of solutes in window diagrams.

No. Solute Retention index @ 50 "C

C O I U ~ 1') Column 2"'

1 Vinyl chloride 386 603

2 Ethyl chloride 44 I 668

3 1.1-Dichloroethylene 513 144

4 1.1-Dichloroethane 564 901

5 Chloroform 604 1037

6 1,2-Dichloroethane 632 1090

7 1,1,l-Trichloroethane 637 898

8 Trichlo~oethylene 688 1019

9 Bromodichloromethane 686 1174

10 1,1,2-Trichloroethane 744 1282

11 Chlorobenzene 827 1224

12 Dibromochloromethane 770 1310

13 Tecrachloroethylene 799 1044

a) column 1: 30 m x 0.32 mm i.d. X 0.96 pm OV-I; column 2: 30 m X 0.32 X 0.54 pm Carbowax 20M

been) used are based on either general retention characteristics- such as retention index, specific retention volume, partition coefficient, and relative retention time - or retention characteristics that are specific to a column of stated dimensions (length, diame- ter, film thickness). Column-specific characteristics include: partition ratio, retention volume per unit length, and retention time (pressure corrected) per unit length. The simplest format in which the retention data for a window diagram can be presented is the relative retention graph, as shown in Figure 1, which is based on retention indexes. The window diagram is generated by calculating the difference in retention index units between adjacent solutes and taking the smallest value for that percent of phase 2. This is repeated at increments of one percent for phase 2 from 0 to 100 %, and the values plotted as retention index units difference vs percent of phase 2. A graph of the results is shown in Figure 2. The peak maxima denote those percent ratios of the two phases at which the smallest difference in retention indexes between any two consecutively eluting compounds is greatest. The larger the difference, the easier the separation. Conversely, at percent ratios of the two phases that result in exact coelution of any two compounds the difference in retention indexes is zero and the value of the graph goes to zero, as indicated by the valleys in the graph. It is obvious that one wants to use those ratios of the two phases that correspond to the locations of the maxima, and avoid those that correspond to those of the valley minima. (It should be noted that because the calculation in these examples is performed at one-percent intervals, the exact ratio at which coelution occurs may not be a whole-number value. The minimum values at the bottom of the valleys in that case will be greater than zero.)

0 20 40 60 80 mo PERCPlT CARWIWAX 20 M ws O W

Figure 1. Relative retention graph based on retention indexes.

0 20 40 80 IM

PERCENT CARBMYAX 20 M YL O W

Figure 2. Window diagram based on retention indexes.

This form of the window diagram has the advantage of requiring the least experimentation and calculation and thereby can quickly provide a yes/no answer as to the feasibility of a separation and, very approximately, the degree of difficulty to be encountered with those phases. However, because the relation of retention index to time or retention volume is not linear [3], this type of window diagram is not at all useful in predicting the ratio of the lengths of columns that will produce the equivalent ratio.

A far more accurate prediction of actual column length ratios is given by a window diagram based on partition ratios obtained on the columns that are to be used in the procedure, as will be described below.

5 Construction of Window Diagrams for Key Component Analysis

The objective of a conventional window diagram is to identify the optimum ratio of the two columns (phases) that will separate all the components of the sample. A window diagram program of this form was previously tested and experimentally confirmed [4] , For on-line measurements wherein the objective is to control the process, it is usually more important to measure one or two

344 VOL. 18, JUNE 1995 J. High Resol. Chromatogr.


0 20 40 60 80 100

PERCENT CARBOWAX 20 M ve.0V-1 - LOWER; SEPARATE ALL - UPPER ONLY 6 L 13

Figure 3. Window diagram based on partition rations for separating all components (lower curve) and for separating only key components (upper curve).

key components in the fastest time possible, than to obtain a complete analysis of the process sample. A window diagram to support this strategy will differ from the conventional in that only the key components are designated as ‘must separate’. The conventional diagram is constructed by calculating a, the separation factor, for all pairs of solutes, for a given ratio of the two phases, and taking the smallest a for that ratio. For key component analysis, only a’s for those pairs of solutes that include a key component are calculated; a’s for all other pairs of solutes that do not include key components are ignored and do not enter into the calculations.

The spreadsheet program was modified to do this calculation in parallel with a conventional diagram for separating all solutes. The program was written so that any number of components may be designated as key components. Figure 3 shows the resulting window diagram with two designated key components, trichlo- roethylene (no. 8) and tetrachloroethylene (no. 13). The upper curve is that for separating key components only, and the lower is for separating all components. The latter shows a maximum of a = 1.17 at 54 % Carbowax 20M. The ‘key components only’ curve shows the largest maximum of CY = 1.19 at 19 % and a second maximum of a = 1.18 at 54 % of Carbowax 20M. Using these values the data were input to the computer model described in reference 4, to calculate the retention times and peak widths for separating all components on 54 % Carbowax, with a minimum resolution of 3.3. A computer-generated chromatogram is shown in Figure 4. Columns cut to the calculated lengths and run at the calculated operating conditions gave the experimental chromatogram shown in Figure 5. The calculation was repeated for 19 % Carbowax 20M to separate and measure only the two key components, nos. 8 and 13, also with a minimum resolution of R = 3.3, This resulted in a significantly faster analysis, as shown in the computer-generated chromatogram in Figure 6.

Columns were cut to the computer-calculated length and run under the calculated conditions. The experimental chromatogram is shown in Figure 7. The relative retennon times agree well with the predicted times, although there are some small differences in the actual times observed. This can be attributed to probable errors in the temperature- and pressure-measuring elements of the chromatograph. However, the overall resultr were as pre-

O O C W I, 46%. 0x4. E6 m. 0.32 mn. aS6 un COLUH 2.54% :CARmWAX ZJM. 131 m, 0 32 m. 0 M m

H E L M CARRIE& IS 35 paip 7.0 ml

I 11

10 S

2 5 s 12 13 I 4 7 03 S

1 n -I 200

TIME. s E m m

Figure 4. Computer-generated chromatogram for columns to separate all components with a minimum resolution of R = 3.3 (from reference 4).

COLUMN I: O W . 8.6 rn X 0.32 mrn X 0.96 micron

COLUMN 2: CWPOM. 13.1 m X 0.32 mm X 0.54 micron

5OC HELIUM. 7 ml/m

t i

Figure 5. Experimental chromatogram with columns cut to dimensions calculated by the computer program (from reference 4).

COLUMN I . 81% : OV-I. 6.9 m. 0.32 mm. 096 urn COLUMN 2.19% : CARBOWAX 20M. 21 m. 0.32 M. 0.54 m

HELW CARRIER: 9.5 psig. 7.0 ml I1

0 20 40 60

TIME. SECDW.5

Figure 6. Computer-generated chromatogram for columns calculated for separating only two key components, with column system with 19 % of Column l, as determined from Figure 3.

J. High Rcsol. Chromatogr. VOL. IS, JUNE 1995 345


1 CfXUW 1.BIX: OV-I. 6.9 m. 0.32 nun. 0.96 vn aXW 2.19 % :CARBOWAX 20M. 2 1 m. 0.32 mm. 0.54 un

ELIUM CARR4ER. 7.0 rnl/m

13 9

1 I I I , I 1

0 20 40 60 80 IM1

TIME. SECONDS

Figure 7. Experimental chromatogram with columns and conditions as pre-

1.45

1.4

1.35 ’ 1.3 : 1.25

2 ii $ 115

i 11

lLO5

PERCENT CARBOWAX 20 M VB. OV-l _- LOWER MUSURE ALL - UPPER ONLY NO. 9

Figure 8. Window diagam for separating only one key component, hromodi- chloromethane.

dicted by the model. It should also be noted that components other than the ‘key’ components are at least as well separated as the ‘key’ components; this is, however, merely fortuitous, as the columns are calculated to ensure separation of the designated components only. This incidental benefit, although unintended, will usually be the case.

A window diagram calculated for only one key component, no. 9, dibromochloromethane, is shown in Figure 8. The computer- generated chromatogram for 26 % Carbowax 20 M is shown in Figure 9. In this case the analysis time has again been greatly reduced to less than 30 s. This predicted performance was also confirmed experimentally with columns cut to the computer-calculated length and run at the prescribed conditions, as shown in Figure 10. As in the previous example, other components are also sufficiently well separated coincidentally with the designated ,key’ component.

! !

COLUMN I 74% OV-1 20 rn 0 32 m 096 urn t.442 2846.CARBOWAX2DM. 077 m 032 mm 0 5 4 urn

0 4 8 12 16 20 24 28

TIME. SEOIQDS

Figure 9. Computer-generated chromatogram for measuring only one key component with column system containing 26 % Column 2, as determined from Figure 8

I COLUMN I. 74% : OV-1.2.0 m. 032 rnm. 0.96 wn COLUMN 2. =%a CAREOWAX 2oM. 0.75 rn. 0.32 mm. 0.54 un

HELIUM CARRIER. 7.0 ml/m c

0 4 8 12 16 20 24 28 32

TIME, SECOWDS

Figure 10. Experimental chromatogram with columns and conditions as prescribed in Figure 9.

6 Discussion and Conclusion

In the world of process control, on-line gas chromatographic analysis, as embodied in process gas chromatographs, is the most widely used analytical method for the analysis of complex hydrocarbon mixtures. For effective process control, speed and frequency of analysis is often more advantageous than a complete analysis of the process sample. Faster speeds can almost always be realized if only those ‘key’ components are measured that are needed for control of the process, and the analytical system designed with that purpose in mind. A spreadsheet-based window diagram program has been developed that facilitates the selection of phases and optimizes their ratios in the design of column systems for measuring only one or a few key components. This prograni has been proven to significantly reduce the time and effort in the design of gas chromatographic column systems for that purpose.

346 VOL. 18, JUNE 1995 I. High Resol. Chromatogr.

A floppy disk copy of this spreadsheet program in Lotus 123 format can be obtained at no cost by written request to the author at the above address.

Acknowledgment

T h e author expresses his thanks to the Foxboro Company for permission to use portions of the data presented in this article.

References [l]

[Z]

W.H. McFadden, Anal. Chem. 30 (1958) 479

G.J. Price in: J.C. Giddings, E. Crushka, and P.R. Brown (Eds) Advances in Chroma- toyaphy, Volume 28, Marcel Dekker, Inc., New York, USA (1989) p. 113 ff.

A Window Diagram for Key Component Analysis i n On-Line GC

131 R.J. Lalib mid H.J. Pumell, J. Chromatogr. 112 (1975) 71-79

[4] R. Villalobos and R. Annino, HRC 13 (1990) 7 W 7 7 3 .

151 R. Villalobos ‘and R. Annino, in: Proc. of ISA Calgary 1989 Symposium, April 19x9, Instrument Society of America, Research Triangle, NC, USA, 5 7 4 7 .

R. Villalobos and R. Amino, J. High Resol. Chromatogr. 14 (1991) 681686

C.F. Chicn. D.L. Furio, M.M. Kopecni, and R.J. Lauh, J. High Resol. Chromatogr. 6 (1983) 577-580.

C.F. Chien, D.L. Furio, M.M Kopecni, and R.J. I.auh, J High Resol. Chromatogr. 6 (1983) 669-679.

A.S. Said, Theory and Mathematics of Chromatography, Hiithig, Heidelberg, Germany (1981) 146.

[6]

171

181

[9]

l l0J R. Villalobos and R. Annino, J. High Resol. Clromatogr. 12 (1989) 149-160. Ms received: February 7, 1995

Accepted: March 15, 1995

I. High Resol. Chromntogr. VOL. 18, JUNE 1995 347

a window diagram for key component analysis in on-line gas chromatography

Documents