computer-aided selection and optimization of chromatographic columns and conditions for multicolumn...

Computer-Aided Selection and Optimization of Chromatographic Columns and Conditions for

Multicolumn Analysis Procedures Richard Villalobos and Raymond Annino* The Foxboro Co., Foxbom, MA 02035, USA

1 Introduction Serial- and parallel-coupled columns have been used in process gas chromatography almost as long as packed columns have been in existence. This practice has been continued with capillary columns [l-31 Multiport valves or pressure balancing schemes are used to reverse the flow through one column segment (backflushing strongly retained components to vent or a detector), or to redirect components which are not separated on one column into another (trap-and-store, dual-column- with bypass, or heart-cut). Also, in the case of packed columns, mixtures of pack- ings containing different stationary phases have been used to obtain the proper selectivity to enable a particular separation to be completed within the time constraints of the analytical problem. In the case of capillary columns, the practical use of this technique is with serially connected columns of the appropriate length.

Design of column configurations and operating conditions for a new application is a tjme-consuming, labor intensive, and ex- pensive process involving, in the end, "cut and try" procedures which may have to be repeated a number of times before the "optimized" configuration, operating parameters, and valve-timing are obtained.

It has been our intention to reduce the time and expertise required for this task by developing computer-aided column design procedures which could be used by the moderately trained technician to an- swer application requests in a timely fashion. Furthermore, in order to make the program widely applicable, it was designed to support a simple, single oven,

isothermal analysis procedure which can be utilized by all laboratories without having to modify their present instrumentation

We have, therefore, directed our research toward this end 141, and the basis for a single column optiinization program has been described in detail [5l Recently, we

also presented the results obtained with an optimization program developed for serially connected columns containjng different stationary phases [ 6 ] , and the extension of this program to the more general problem of multicolumn design [7 ] .

The purpose of this paper is to present the basis for the above mentioned work, as well as furnishing further documentation regarding the validity of computer-aided design procedures.

Since, in process GC applications, column temperature is often an analysis restraint (because of "bubble point" or "dew point" restraints on sample temperature and pressure), it was decided toleave this as an input variable rather than one of the optimized variables.

2 Theory

2.1 Selectivity Tuning

Adjusting stationary phase selectivity by mixing stationary phases of different polarities was first investigated over 25 years ago [8, 91. The idea was given a new lease on life by the work of Purnell and Laub [ lo , 111 whose window diagram procedure provided an excellent way to com- pile the selectivity information about the two phases in a manner which simplified the selection of the optimum mixture for the separation.

The limited gradations in selectivity which are currently available with cross-linked stationary phases are further encourage- ment to the use of multicolumn configurations for the solution of complex separation problems.

764 Journal of High Resolution Chromatography 0 1990 Dr. Alfred Huethig Publishers

lnstrumentat ion

0.09 I

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

I I I I 1 I 1 I I I 0 20 40 60 80 100

P m CAWOWAX 20M

Figure 1

Plot of retention volume per length of column versus volume percent of Carbowax 20M in ov-1.

1.1

1.09

1 .OB

1.07

1.06

1.05

1.04

1.03

1.02

1.01

1

0 20 40 60

PER- WRBoWAx 2ou

Figure 2

Window diagram constructed from the data summarized in Figure I.

100

There are several means of achieving phase mixtures [12], but the simplest method in terms of capillary chromatography is to couple columns in series, with each column containing a different stationary phase, and with the lengths chosen to reflect the desired percentage composition of the final mixture. Previous attempts to develop computer-aided optimization procedures for serially connected columns have, however, met with only limited suc- cess 113-151; only approximate solutions were obtained, and the final combination of columns had to be fine tuned by trial and error. The primary difficulty, as pointed out by PurneIl and his colleagues [16-181, is that the actuallengths of the columns to be used will not be in simple proportion to the window-diagram-stipulated volume fraction of stationary phase, but will vary depending on the pressure drop experi- enced by the two columns. This is readily apparent if one constructs the window diagram as a function of the capacity factor. One must adjust column lengths so that the capacity factors of the sample components are in the required ratio. This IS accomplished by adjusting the average velocities in the two columns to be in the required ratio.

Since window diagrams are an indispens- able tool for designing serially coupled columns of desired specificity, they are included as part of the computer-aided design program. The fundamental data used for their construction is stored in a database prepared both from literature data 119-221 and our own experience. The information can be presented in terms of experimentally determined retention volume per unit length of column (to avoid correcting for the retention time depen- dence on film thickness and column diameter of the selected columns) versus the mole fraction or percent of the selected stationary phases as shown in Figure 1. The values for the pair most difficult to separate are calculated from these data and used to construct the window diagram shown in Figure 2.

As explained previously, because of carrier gas compressibility, the uncorrected length obtained from this window diagram will not equal the actual length to be used as calculated by the program. Moreover, it should also be recognized that the solution to the problem holds for only one value of input pressure. Thus, there is a need for an efficient and accurate method for deter- mining not only the optimum lengths required for the analysis, but also the input pressure to be used.

Journal of High Resolution Chromatography VOL. 13, NOVEMBER 1990 765

Instrumentation

2.2 Computer Model - Single Column

The previously described computer-aided design program for optimizing conditions for single column analysis consists essen- tially of two parts: a model describing the performance of a capillary column of given inside diameter, film thickness, length, etc. ; and a procedure for using t h s model to obtain optimized solutions to the separation problem which satisfy the restraints of analysis time and minimum detection limits.

The present work utilizes the same model, the extended Golay-Giddings equation of Gaspar et al. 1231, shown below, to provide solutions to problems involving serially coupled cohimns.

h = B / i i + ( C 1 + C , ) i i + D $ (1)

where:

B = c, =

c, =

11 =

12 =

P = k = D = D, =

4 =

k = df =

d, =

L = PI =

Po = h =

2D, j1 12 2kdf" I [3(1 + k)' Di] [(Ilk2 + 6k + 1) dC2 jl] / [ 96( 1 + k2) D, j2] 9 ( P - 1) (P' - 2) I 18(P - 1)l 3(P - 1) I [2(P - 1)l P11 Po ( t r - tm) / tm r2 I L ( l + k)2 solute diffusion coefficient in the gas phase solute diffusion Coefficient in the liquid phase capacity factor stationary phase film thickness column inner diameter length of column inlet pressure outlet pressure height of a theoretical plate

11 and 12 = carrier gas compressibility factors

t, = elution time of solute tm = elution time of an unretained com

ponent, and z = system time constant

This equation accurately predicts the performance of a capillary gas chromatographic system (rather than lust the column) as a function of flow rate

The flow rate, or rather the average linear velocity, a, is, in turn, a function of the permeability B,, the inlet and outlet pressures (p, and po respectively), the column length L, and the carrier gas viscosity q , as shown below

ti = [B, 12 Po(p2 - 111 / 2qL (2 )

B, = dCZ I 32 for capillary columns, and d, = internal column diameter.

Using this model, together with the retention times calculated from the known spe- cific retention volumes on the selected stationary phase (Eqs (3) and (4)) yields a remarkably accurate chromatogram at any selected input pressure (flow rate).

v, = Vm(l + k) (3)

t, = t,(l + k) (4)

where:

V, = dead volume of the column. V, = retention volume, and

2.3 Computer Model - Multicolumn

Mayfield and Chesler I241 have examined three methods which have been proposed for calculating the effective capacity factor of a series-column combination. No defi- nite conclusion was reached as to the accuracy of the best of the three. The work did not include that of Purnell et al. [25] which was published at the end of the study.

Our solution consists simply of a model for each column with the outlet flow from one connected to the input of the other. As can be seen from Eq. (2), however, in order to calculate the flow rates in the two columns it is necessary to know the junction pressure, i.e. the input pressure of column 2 and the outlet pressure of column 1.

Under conditions of equal mass flow in both columns, the junction pressure, p(x), IS calculated according to the following equation:

p(x) = po[P2 + X(1 - PZ)11'2 (5)

where p(x) is the pressure at a fractional &stance, x, from the front of the column (assumed to be a capillary of uniform internal diameter). In the simplest case, that of equal inside diameters, x can be set to the fractional distance measured from the beginning of the first column to the junction. This is not, however, the case for coupling columns of differing internal diameters, a not uncommon method of effecting desired selectivities.

The application of columns of unequal diameter is accommodated by recognizing that the same head pressure will cause equal mass flow through lengths of column which are related as follows:

L1 I Lz = ( d l / d2)4 ( 6 )

For computationalpurposes, then, the program replaces the upstream column with a

ameter as the downstream one and a length Ll(pscudo), equal to the original upstream column length, L1, times the ratio (dz/d1)4, where dl is the internal diameter of the original upstream column and d2 is the diameter of the downstream column.

During the first phase of the calculations, the input pressure is adjusted by the program to yield the desired flow rate (user input).

The junction pressure is then calculated at the fractional length x, of the total length comprising both pseudo and upstream columns. Once this pressure is obtained, the pseudo column is discarded and the average flow rate for each column is calculated according to Eq. (5) (usingp(x) for the outlet pressure of column 1 and the inlet pressure of column Z), followed by a calculation of the dead time in each of the columns. Alternatively, the junction pressure can be specified by the user, in which case the pseudo column calculation is bypassed by the program.

For those applications where optimization and valve timing are the primary objectives (e.g.$ when to backflush or heart-cut, etc.) this operation is repeated for any desired increment in the input pressure (flow rate) to generate a table of complete chromatograms for each column in the series; from these the desired operating conditions may be selected.

In the case of optimization of column lengths to yield the effective phase ratios prescribed by the window diagram, an iterative adjustment of relative column lengths, calculation of intercolumn pressure, and calculation of dead times are made by the program until the ratio of dead times in the two columns is the same as that prescribed by the window diagram. The complete chromatogram is then calculated using Eq. (4) to compute retention times, and the model to calculate peak widths.

Further Optimization is accomplished by the program by incrementing the input pressure, repeating the above procedure, and selecting the best pressure and relative column lengths to obtain the fastest analysis under the prescribed resolution restraints and total column length. When analysis times are too long and the resolution too high a final optimization step is used to adjust the total column length and optimize the pressure.

Thus, the final optimized parameters fur- where: pseudo column of the same internal di- nished by the program include the actual

766 VOL. 13, NOVEMBER 1990 Journal of High Resolution Chromatography

Instrumentation

lengths that must be used to yield the desired phase ratio obtained from the window diagram constructed at the analysis temperature, as well as the input pressure which must be used to obtain the desired performance,

3 Experimental

3.1 Apparatus

Chromatographic analysis was performed with a Varian 3700 chromatograph. For multicolumn experiments requiring verfi- cation of the chromatographic performance of the first column in a series, TC detection was employed, using a detector, built in-house, with an internal volume of less than one microliter, and a time constant of 0.2 ms. This detector was operated in the constant resistance mode and the overall system time constant was 1.2 ms.

t VENT

t Figure 3

Pneumatic configuration used for selectivity tuning by adjusting junction pressure. PRI, PR2 = pressure regulators, BPR = back pressure regulator (0-20 psig), CI,C2 = columns 1 and 2, P I ,P2 = Mensor pressure gauges,Vl , V2, V3 = Whitey on/off ball valves, NVI = Nupro needle valve, D = detector.

Records were made using an oscillograph- ic type recorder (Brush 2400, Gould Inc., Cleveland, OH, USA), an Hewlett Packard 3650 Integrator/Recorder, or data acquisi- tion hardware (ISAAC 91A, Cyborg Inc, Boston, Ma, USA). Time constants appropriate to the equipment (10,100 or 250 ms.) were used in the model.

3.2 Column Sets

No special demands were placed on the supplier of the columns used in this research In some cases, however, the specified internal diameter, or film thickness, of the columns did not correspond to that calculated from measurements of its permeability and the retention indices of

Table 1

Comparison of theoretical and experimentally determined retention times for serially connected columns of different diameters containing stabilized OV-1 as a stationary phase.

A

Component Retention time isla) b, c,

Theor. Obs . Error

Unretained (air) 2-Propanol Pentane Vinyl acetate Methyl acrylate Hexane Vinyl isobutyl ether Vinyl propionate Ethyl acrylate Methyl methacrylate

202.05 224.82 228.67 248.03 262.49 262.99 295.71 308.95 320.09 336.77

198.4 222.4 226.0 248.2 264.1 264.1 300.4 311.3 327.2 345.2

3.7 2.4 2.7 0.2 1.6 1.1 5.3 2.3 7.1 8.4

B Component Retention time [sId) e , *)

Theor. Obs. Error

Unretained (air) 2-Propanol Pentane Vinyl acetate Methyl acrylate Hexane Vinyl isobutyl ether Vinyl propionate Ethyl acrylate Methyl methacrylate Heptane

95.44 108.93 111.21 122.70 131.27 131.57 150.96 158.81 165.42 175.31 177.99

94.4 107.6 110.5 122.0 130.3 131.9 151.2 157.3 166.1 176.2 179.9

0.04 0.3 0.7 0.7 1.0 0.3 0.2 1.7 0.7 0.9 1.9

Input pressure = 29.4 psig, t = 60 "C. b, Column 1 = 27.28 m x 0.32 mm 1 d., dt = 0.25 ym. c, Column 2 = 19.24 m x 0.165 mm i.d.. df = 0.40 ym. dl Input pressure = 29.66 psig, t = 60 "C. e, Column 1 = 19.24 m x 0 165 mm i .d., df = 0.40 ym. f] Column 2 = 27.28 m x 0.32 m m i.d., df = 0.25 pm.

known probes The columns' internal diameters and film thicknesses reported herein have been adjusted from the manu- facturers' specifications to those obtained experimentally

3.3 Junction Pressure Adjustments

The configuration used for junction pressure adjustments is shown in Figure 3. The equilibrium junction pressure is read with V1 open and V2, V3 closed. For junction pressures above equilibrium valve, V2 is closed and V1, V3 open. The needle valve, NV1, is adjusted to give the same junction pressure with PR2 set to the same pressuIe as PR1. Junction pressure is raised by changing the PR2 regulator set- ting. If a suitable range pressure regulator is available, the needle valve can be elim-

inated and all adjustments can be made with the regulator For ]unction pressures below the equilibrium value, V3 IS closed and V1, V2 open The backpressure regu lator is then adlusted to the required pressure

3.4 Pressure Measurement

Since accurate pressure measurements are extremely important for verification of program output vs experimental results, a Mensor Model 11600 digital pressure gauge (Mensor Corp , Houston, TX, USA) with an accuracy of 0 04 % and readability of 0 001 psi was used throughout the research

3.5 Computer Requirements

The version of the column design program that was used was written in the Lotus 123

JournaLof High Resolution Chromatography VOL. 13, NOVEMBER 1990 767

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Table 2

Comparison of predicted and observed retention times for intercolumn and system chromatograms.

Component Retention time Isla) Column Ib) Theor. Obs. Theor. Obs.

Retention time Is] Column 1 + Column 2"

Unretained 45.04 45.8 52.61 53.0 Acetone 54.82 55.3 66.90 66.5 2-Propanol 55.48 55.3 73.12 72.3 Acrylonitrile 56.85 55.8 80.36 80.5 Vinyl acetate 66.13 66.1 81.03 81.1 Methyl acrylate 72.76 73.5 91.15 90.1 Vinyl isobutyl ether 87.99 88.5 99.27 99.0 Vinyl propionate 94.06 93.5 113.94 111.5 Ethyl acrylate 99.17 100.0 122.56 121.3 Methyl methacrylate 106.82 107.6 131.63 130.0

a) PI = 13.5 psig, t = 60 "C. F = 2.59 ml/min. b, Column 1 = 25.0 m x 2 5 mm i.d., d, = 0.5 Fm GB-1. c, Column 2 = 6.0 m x .25 mm i.d., dc = 0.5 wm Carbowax 20M.

Table 3

Series-coupled columns optimized for length at a flow rate of 2.6 ml/min and operated at 2.6 mumin@.

Time Is] Width Is] Resolution No. Component Theor. Obs. Theor. Ohs. Theor. Obs

Unretained (air) 1. Acetone 2. 2-Propanol 3. Acrylonitrile 4. Vinyl acetate 5. Ethyl acetate 6. Methyl acrylate 7. Vinyl isobutyl ether 8. Vinyl propionate 9. Ethyl acrylate

10. Methyl methacrylate

50.36 63 66 68.92 75.12 77.31 83.13 86.45 95.48

108.54 116.45 125.12

52.2 0.63 0.70 64.7 0.81 0.89 96.5 0.86 0.97 75.2 0.92 1.13 78.2 0.95 0.97 84.2 1.02 0.99 87.1 1.05 0.99 96.5 1.17 1.28

107.4 1.33 1.43 116.5 1.43 1.57 124.9 1.54 1.70

18.48 15.7 6.28 5.2 6.97 5.4 2.36 2.8 5.89 6.1 3.20 2.9 8.13 8.3

10.43 8.0 5.74 6.0 5.85 5.1

a) See Figure 4 for descnption of experimental conditions.

environment and was calculation-intensive. It ran best with a coprocessor and a fast microprocessor, such as a 16 Mhz 2861386. All possible PC configurations, froma640KRAMXTtoa lM386wereused during the course of this research.

4 Results

4.1 Prediction of Retention Times

That the program can accurately predict the retention times of components eluting from serially coupled columns of different diameters was verified by experiments with a 0.18 mm i.d. column (nominal i.d. - actual i.d. determined experimentally from permeability measurements to be 0.166 mm) and 0.25 and 0.32 mm i.d. columns. Similarly, in separate experiments, the accuracy of the intercolumn chromatograms was verified. The results tabulated in Tables 1 and 2 illustrate the close correspondence between predicted and experimental results which can be ex- pected using this model.

4.2 Selectivity Tuning

Optimum Lengths of Serially Coupled Columns

A number of experiments has been performed to verify the ability of the program to design an acceptable column combination for the analysis of a sample whose components could not be separated on either column. For example, for the list of compounds shown in Table 3, neither OV-1 nor Carbowax 20M is acceptable for the analysis, since the pairs of compounds vinyl isobutyl ether/acetone, ethyl ace- tatehinyl acetate, 2-propanol/methyl acrylate, and ethyl acrylate/acrylonitrile are not separated on the Carbowax column, and the pairs acetonela-propanol, and ethyl acetate/methyl acrylate are not separated on OV-1. Since the conflicts are not the same on the two phases, it would appear, however, that an appropriate mixture of the two will work.

The retention volume/metre of column vs percent Carbowax plot, shown in Figure I , and the window diagram (Figure 2), constructed from critical a values, indicate a 12.5 % CarbowaxiOV-l mixture to be the best. This information, when input to the program. yielded the results shown in Figure 4 and the computer-generated chromatograms shown in Figure 5. In this case, the toggle switch which allows selectivity tuning by adjusting junction pressure was turned off. The choice of station-


lnstrumentat ion

- DSE SELBCTIVITT TWNG? 2 l = Y E s , 2 = m , Nh NA 0.00 ERR JtRICTIOti PRBssUpE m BE USED 10.00 p i g Nh NA 0.00 EE

ary phases for columns 1 and 2, the carrier gas, 0.125 as the optimumfractionallength obtained from the window diagram, the total column length, column diameters and film thicknesses, temperature of analysis, sample size, minimum resolution required, and the desired flow rate were entered as input.

The program then calculated the input pressure to be used for the analysis, and the lengths of columns which had to be used to operate effectively at the desired stationary phase ratio. Notice that (for

Figure 5

Computer-generated chromatograms produced for the conditions outlined in Figure 4.

reasons discussed previously) the 25 and 5 metre lengths which were calculated by the program are not 0.875 and 0.125 of 30 metres originally entered by the user. The program also calculated and output retention times, peak widths, resolution, and minimum detection levels for each component of the sample mixture.

The experimentally obtained chromatogram produced by the column combination recommended above of 25 metre GB-1 and 5.0 metre of CW 20M, operated at the recommended pressure of 13.19 psig, is shown in Figure 6, and the retention times, peak widths, and system resolution are compared with predicted values in Table 3. The chromatograms obtained for this column combination operated at other input pressures are shown in Figures 7 and 8 and the results are compared with predicted values in Tables 4 and 5. In all

SECONDS

Figure 6

Experimentally produced chromatogram with column combination and conditions specified in Figure 4.

4.5 , 1 I 6 I

0 20 40 60 80 100 1 20

n m SECONDS


lnstrumentat ion

1 I

I 3

I I I I I I I I I 30 42 54 66 78

SECONDS

Figure 7

Experimentally produced chromatogram with column combination specified in Figure 4 but operated at 21.5 psig input pressure (5.0 ml/min flow rate).

1 I I 3

I I I I I I I I I I 15 21 27 33 39

SECONDS

Figure 8

Experimentally produced chromatogram with column combination specified in Figure 4, but operated at 44 psig input pressure (15.0 rnl/min flow rate).

cases, peaks are identified by number in the order listed in these tables, with acetone as number 1.

4.3 Junction Pressure Adjustments

The method of Deans and Scott [26] has been recently reviewed and finely tuned by Hinshaw and Ettre [27, 281. In this procedure, the separation characteristics of serially coupled columns can be altered by adjusting the junction pressure of the combination so that the ratio of the flow rates through the two columns is changed.

The results shown in Tables 6-8 illustrate the close correspondence between experimentally derived results and model predictions (with toggle switch in the "on" position, see line 2 of Figure 4) for experiments below, at, and greater than, the equilibrium junction pressure. Also of interest is the ability of the program, in this situation, to rule out quickly this method of selectivity tuning for this analytical problem. Peak movement is much too sensitive to junction pressure for this procedure to

Table 4

Series-coupled columns optimized for length at a flow rate of 2.6 ml/min and operated at 5.0 rnl/rnina).

~~

Time [sl Width [s] Resolution No Component Theor Obs Theor Obs Theor Obs



3201 336 40 29 41 5 43.31 44 3 4691 4 7 7 48 89 50 1 52 67 54 1 54 58 55 8 60 89 62 3 68 74 69 0 73 61 747 79 11 80 1

0.39 0.42 0.60 0.57 0.64 0.73 0.69 0.73 0.70 0.73 0.75 0.83 0.78 0.83 0.86 0.92 0.98 1.0 1.04 1.2 1.12 1.2

~~~

16 59 16.0 4.80 4.3 5.38 4.7 2.89 3.3 5.18 5.1 247 2 0 7.70 7.4 8.46 7.0 4.79 5.2 5.07 4.5

See Figure 4 for description of experimental conditions

Table 5

Series-coupled columns optimized for length at a flow rate of 2.6 mWmin and operated at 15.0 mVmina).

~

Width [s] Resolution Time [sl No Component Theor Obs Theor Obs Theor Obs



16.36 17.0 20.46 21.0 21.76 22.1 23.36 23.7 24.81 25.3 26.79 27.3 27.62 28.0 31.27 31.6 34.95 34.7 37.32 37.5 40.13 40.2

0.22 0.45 0.50 0.51 0.53 0.48 0.56 0.51 0.56 0.51 0.59 0.56 0.61 0.56 0.66 0.61 0.74 0.74 0.77 0.82 0.82 0.87

11.37 8.3 2.52 2.2 2.92 3.2 2.61 3.1 3.47 3.7 1.37 1.3 5.79 6.2 5.26 5.0 3.14 3.6 3.51 3.2

a) See Figure 4 for description of experimental conditions

Table 6

Comparison of retention times and peak widths of serially coupled columns for a junction pressure of 3.0 psige) b, @.

Component

~ ~~~~

Time [s] Width [s] Resolution Theor. Obs. Theor. Obs. Theor. Obs.

Unretained (air) Acetone Vinyl acetate Vinyl isobutyl ether Propanol Ethyl acetate Methyl acrylate Acrylonitrile Vinyl propionate Ethyl acrylate Methyl methacrylate

61.7 82.23 99.25 99.62

100.91 102.09 113.79 117.75 131.54 146.17 155.46

61.5 82.2 1.75 1.76 99.5 1.91 1.84 99.5 1.74 1.84

101.5 2.08 1.75 102.9 1.90 1.88 113.9 2.07 2.01 118.7 2.35 2.45 131.3 2.17 2.13 146.6 2.35 2.48 155.9 2.43 2.56

9.25 9.34 0.20 0.00 0.68 1.11 0.59 0.50 5.89 5.91 1.79 2.10 6.11 5.50 6.49 6.63 3.89 3.72

a) Pi = 20.9 psig. t = 60 "C. b, Column 1 = 27.6 m x 0.25 mm, di = 0.42 pm, GB-1.

Column 2 = 11.0 m x 0.25 mm, df = 0.42 pm, CW 20M


Instrumentation

Table 7

Comparison of retention times and peak widths of serially coupled columns for a junction pressure of 7.65 psigal b, @.

Component Time [s] Width [s] Resolution Theor. Obs. Theor. Obs. Theor. Obs.

Unretained (air) Acetone Propanol Vinyl acetate Acrylonitrile Ethyl acetate Methyl acrylate Vinyl isobutyl ether Vinyl propionate Ethyl acrylate Methyl methacrylate

57.76 72.1 79.92 85.93 8'7.46 90.93 95.85 99.78

116.24 125.24 133.72

57.4 71.2 1.42 1.42 79.7 1.52 1.43 86.2 1.51 1.50 87.4 1.61 1.44 90.4 1.52 1.44 96.2 1.58 1.56

100.6 1.55 1.56 115.6 1.72 1.75 125.9 1.81 1.90 134.3 1.88 2.00

5.31 5.96 3.97 4.43 0.98 1.24 2.21 1.94 3.17 3.86 2.51 2.82

10.09 9.06 5.11 5.64 4.60 4.30

a) P, = 19 85 pslg t = 60 "C b, Column 1 = 27 6 m x 0 25 mm dt - 0 42 pm GB-1 ') Column 2 = 11 0 m x 0 25 mm. dc = 0 42 pm, CW 20M

Table 8

Comparison of retention times and peak widths of serially coupled columns for a junction pressure of 9.57 psiga) b) c).

Component Time [s] Width [s] Resolution Theor. Obs. Theor. Obs. Theor. Obs.

satisfy the robustness constraint that is imperative for process control analysis.

4.4 Series Column Combination with

To illustrate the utility of the program for the design of systems utilizing column switching or backflushing, we considered the above case for an analysis where it was not necessary to measure acrylonitrile, ethyl acrylate, and methyl methacrylate. These compounds can therefore be separated as a group on the first column and discarded, thus simplifying the separation of the rest of the sample.

To optimize the system for the separation of the remaining compounds, another window diagram was generated without these three compounds in the sample (Figure 9). Two favorable windows were evident at 19 % and 33 % Carbowax. Noting from the retention indices that acrylonitrile, ethyl acrylate, and methyl methacrylate elute last from the Carbowax column. the test

Backflush to Vent

Unretained (air) 64.06 64.1 Acetone 78.83 78.4 1.43 1.38 Propanol 85.34 85.4 1.50 1.50 4.45 4.86 Acrylonitrile 91.86 92.2 1.56 1.40 4.26 4.68 Vinyl acetate 93.70 94.3 1.52 1.50 1.20 1.44 Ethyl acetate 99.76 99.8 1.55 1.40 3.95 3.79 Methyl acrylate 103.87 104.6 1.59 1.55 2.62 3.25 Figure g Vinyl isobutyl ether 112.07 113.1 1.62 1.70 5.11 5.23 Vinyl propionate 127.31 126.8 1.76 1.75 Ethyl acrylate Methyl methacrylate 145.49 146.8 1.94 2.00 4.93 4.65 eliminated from the samde.

9.01 7.94 Window diagram constructed from data summarized in Figure 1, but with acryloni-

136.24 137.5 1.86 2.00 4.94 5.71 trile, ethyl acrylate, and methyl methacrylate

P, = 20 0 pslg, t = 60 "C b1 Column 1 = 27 6 m x 0 25 mm, df = 0 42 urn, GB-1 'I Column 2 = 11 0 m x 0 25 mm, df = 0 42 ym. CW 20M

was run with the Carbowax column upstream so that these three compounds could be discarded by backflushing. The complete sample was included in the computer simulation in order to evaluate the separation, on column 1, of the unwanted compounds from the rest of the sample. It was quickly noted from the computer- generated chromatograms (Figures 10 and 11) that the 33 % Carbowax combination apparently gives a somewhat better separation and, from the standpoint of valve timing, would be easier to set up. This was confirmed from the printout of retention times and resolution. Ethyl acetate and vinyl propionate were separated by 9.4 s ( R = 7.5) on the 33 % Carbowax column and 5.4 s ( R = 5.4) on the 19 % Carbowax combination.

Journal of High Resolution Chromatography VOL. 13. NOVEMRFR 1990 771

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6

5

4

8 IL: 3 [ n

2

1

0

0 20 40 60 80 100 1 20 140 160

nME. SECONM

Figure 10

Computer-generated chromatograms using column lengths of 5.3 m x 0.25 mm i.d., d, = 0.5 pm, CW 20M, and 29.7 m x 0.25 mm i.d., df = 0.5 pm OV-I to give an effective 19 % CarbowaxlOV-I combination. (Pi = 14.82, F = 2.60 mllmin, t = 60 "C).

1 5 2 air 7 4 6 8 I0 OUTPUT, COLUMN 1

(3) 1 2 4 5 7 6

OUTPUT, SYSTEM

1

0

n u t SECONDS

Figure 11

Computer-generated chromatograms using column lengths of 9.5 m x 0.25 mm i.d.., df = 0.5 pm, CW 20M, and 25.5 m x 0.25 mm i.d., g = 0.5 pm, OV-1 to give an effective 33 O/O

CarbowaxlOV-1 combination (Carrier = Ht, Pi = 14.8, F = 2.60 mllmin, t = 60 "C).

Note that the computer-generated chromatograms simulate the backflushing procedure, that is, all compounds are included in the chromatogram produced by the first Column, but are absent from the output of the second. 4.5 SeriesColumn CoAbination with

Backflush to Detector

Instead of discarding the backflushed components, it is sometimes desirable to determine their concentration as a lumped sum by passing them, in a parallel path, through the same detector as is used to measure the rest of the sample. It is necessary, therefore, to select a backflush time so that the backflushed peak will not appear at the detector at the same time as the rest of the sample is eluting from column 2. By proper timing of the backflush valve, the backflush peak can be caused to elute before, after, or between adjacent peaks eluting from the second column - not a particularly simple timing problem. The computer-generated chromatogram shown in Figure 12 was obtained from a system designed to give a total aromatic measurement with separation of the c 5 - C ~ non-aromatic compounds on the second column. Carbowax 20M was chosen as the stationary phase for the first column because of its known selectivity for aromatic vs non-aromatic compounds. All of the non-aromatics were eluted before benzene on the first column, and were further separated on the OV-1 column. With a thin film Carbowax column, a fast separation of the non- aromatics from the aromatics was obtained, and it was possible to backflush the aromatics through the detector before the lightest of the non-aromatics appeared from the rather heavily loaded second column.

5 Discussion Excellent correspondence of predicted vs experimental results has been demon- strated for the series-column optimization model. As with all models, the accuracy of the results depends critically on the accuracy of the input. Although we have obtained acceptable results using literature retention index data and manufactur- ers' stated column specifications, it is recommended that for critical or difficult separations, input data be experimentally determined on the columns which are to be used. This eliminates the effects slight variations in film thickness, column diameter, stationary phase properties, and oven temperature profiles might have on the accuracy of the predictions.


instrumentation

500

4

5.5

3

d 2-5

0 E 1.5

w U 2 P

1

0.5

0 . . 400 Jennin;, J. Chromatogr. 239 (1982) 39 0

Figure 12

&? f h ~j

ild

air

rn

MWm

100

z b c d o I

Analytical Chemistry and Applied Spectro- scopy, March 6. 1989, Atlanta, GA, USA.

R. Villalobos and R . Annino. "Series Cou- pled Capillary Columns in Process Gas Chromatography- Theory and Practice" ISA Calgary '89 Symposium Proceedings, pp 57-68, April 3-5, 1989.

(81 H. J . Maier and 0. C. Karpathy, J. Chroma- togr. 8 (1962) 308.

191 G. P. Hildebrsnd and C N. Reilley, Anal. Chem. 36 (1964) 47

[lo] R. J. Laub and J . H. Purnell, J. Chromatogr. 112 (1975) 71.

1111 R. J. h u b and J. H. Purnell, Anal. Chem. 48 (1976) 1720.

[121 P. Sandra, F. David, M. Proot, M. Diricks, M. Verstappe, and M. Verzele, HRC & CC 8 (1985) 782.

1131 D. F. Inoraham. C. F Schoemaker. and W.

!7]

mi+ SECONDS 1141 G. Takeoka, H. M Rlchard, M. Mehran, and W. Jennmgs, HRC & CC 6 (1983) 145.

(151 M F Mehran, W J Cooper, and W Jen-

I161 J H Purnell and P S W~liiams, HRC & CC 6

[17] J H PurneIIandP S Williams J Chroma-

nings. HRC & CC 7 (1984) 215

(1983) 569

Computer-generated chromatograms using column lengths of 11.7 m x 0.25 mm i.d., df = 0.1 pm,CW20M,and48.3mx.25mmi.d.,df=0.5pm,0V-l (Carrier=H~,Pi=22.3,Fs2.80ml/min, t = 80 "C, a = n-butane, b = n-pentane, c = n-hexane, d = cyclohexane, e = n-heptane, f = benzene, g = n-octane, h = toluene, i = ethylbenzene, j = n-nonane, k = p-xylene, I = rn-xylene, m = o-xylene).

togr 292 (1984) 197

The behavior of serially connected column References

111 combinations can change radically as a result of changes in the input pressure. This effect is the result of a complex combination of changes in effective stationary phase ratios (owing to changes in [31 the ratio of average velocity in the 1 ~ 1 columns), and the usual variations in h

151 which occur as velocity is changed. The series-column model accurately portrays

161 this behavior and affords an efficient method of manipulating variables and eva- luating the various column combinations to produce the optimum solution.

F. Muller. Amer. Lab. 15 (1983) 94.

R. Villalobos and R. Annino, Anal. Instrm. 21 (1985) 73.

S. A. Webb, Adv. Instrum.. 40 (1985) 93.

R. Annino and R. Villalobos, Adv. Instrum.. 41 (1986) 383.

R. Villalobos and R. Annino, HRC 12 (1989) 149. R. Villalobos and R. Annino, "Computer Aided Design of Serially Connected Columns With Different Stationary Phases" presented at the Pittsburg Conference on

[18] J . H. Purnell, M. Rodriguez, and P. S. Wil- liams, J Chromatogr.. 358 (1986) 39.

[19] C. F. Chien. M. M. Kopecni, and R. Laub, HRC & CC 4 (1981) 531.

1201 Idem. Ibid 6 (1983) 577.

1211 Idem, Ibid 6 (1983) 669.

[22)R. Laub, HRC & CC 10 (1987) 565

12.31 G. Gaspar, R. Annino, C. Vidal-Madjar, and G. Guiochon, Anal. Chem. 50 (1978) 1512.

(241 H. T. Mayfield and S. N. Cheder, HRC & CC

(251 J. H. Purnell e t al., loc. cit.

[ZS] D. R. Deans and I. Scott, Anal. Chem. 45

[27] J. V. Hinshaw Jr and L. S. Ettre, Chromato-

1281 J. V. Hinshaw Jr and L. S. Ettre, Chromato-

8 (1985) 595.

(1973) 1137.

graphia 21 (1986) 561.

graphia 21 (1986) 669.


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