Construction of an All-GlassFused Silica, Zero Dead Volume Multidimensional Gas Chromatographic System Using Press-Fit Connectors: Preliminary Results Carlos lbaiiez Analytical Research Department, Lucta S.A., P.O. Box 11 12, Barcelona 08080, Spain

Key Words: Multidimensional gas chromatography Capillary columns Press-fit connectors Glass splitters

Summary A multidimensional gas chromatographic system has been con- structed using commercially available press-fit connectors, glass splitters, and fused silica lines. In this new design all the lines are continuously purged with carrier gas, ensuring no dead volumes in the chromatographic path. Some preliminary results and practical considerations are presented.

1 introduction

Some flavours and fragrances are very difficult to analyze because of the large number of components they contain In such circum- stances the separation power of multidimensional gas chroma- tographic techniques is extremely useful [l-41 Using MDGC, a first separation is performed on one chromatographic column and a heart cut from this separation is taken to a second column of dfferent polarity where the components of interest are separated agan At the end of the second column high punty peaks are obtained, completely separated from the rest of the mxture

We have developed in our laboratory a new, double oven, multidi- mensional system using pressure changes based on Deansswitch- ing (51 The MDGC is constructed using three glass, press-fit sphtters, appropriately mounted in the oven The pressure switch is a valve located outside the oven and totally independent of the chromatographic path A simlar system, installed at the end of the second column drives final high purity peaks to a detector, sniffing port, or a trap where they are isolated for later spectroscopic analy- sis

2 Experimental

2.1 Test mixtures

The non-polar Grob [6] test mixture (Supelco, ## 4-7300) contamed 500 pg/ml of each of 2-octanone, octanol, 2,6-dimethylanihne, 2,6- dimethylphenol, and Clo- C13 hydrocarbons in dichloromethane The polar Grob test mixture (Supelco, # 4-7302) contained 500 p g / d of each of 2-octanone, 1-octanol, 2,6-dimethylanihne, 2,6- dimethyl- phenol, and C15-cZO hydrocarbons in dichloromethane

2.2 Gas chromatography

The first chromatograph was an HP 5710A fitted with a 25 m x 0 32 mm i d capillary column coated with a 0 52 pm film of Ultra-1 polydimethylsiloxane Inlections were performed manually in spht mode (spht ratio 1 65) Inlector and detector were operated at 250 "C and the auxihary port (transfer hne) at 200 "C Hehum was used as carrier gas

The second chromatograph was a Vanan Aerograph 1200 fitted w t h a 25 m x 0 32 mrn i d capillary column coated with a 0 3 pm

film of HP-20M (equivalent to Carbowax 20M). The detector was operated at 250 "C

2.3 System Description

The MDGC system compnsed four Porter 8286 pressure regulators, one Rheodyne 7010 valve, one Valco 4-C4UWT valve 6 Hewlett- Packard glass sphtters for 250 pm capillary columns Chrompack Universal Qmck- Seal glass connectors, and J&W glass unions

The system was prepared by connecting the first column inlet to the first GC inlector and the exit of this column to the first glass sphtter It could be connected directly to the sphtter or better to a premously installed glass union, t h s enabled the column to be changed without touching the MDGC system The glass union was connected to the glass sphtter by 10 cm x 0 25 mm i d fused sihca tubing, as shown in Figure 1 The opposite arms of the sphtter were connected to two further glass sphtters, by simlar pieces of tubing One of these sphtters was connected to the first FID by fused sihca tubing and a glass union, the other to the second column (in a

Figure 1 Schematic diagram of complete MDGC system: 1, split injector; 2, first GC column; 3, first MDGC system; 4, second column; 5, second MDGC system; 6, external valves and pressure regulators; 7, first FID; 8, second FID; 9, trap.

manner identical to that to the first column (again in order that the column could be removed without the need to touch the system) The second column entered the second oven through a heated auxlhary port in the wall of the chromatograph The two other free arms of the sphtters were connected to the first valve, positioned outside the oven, by fused sihca tubing, as shown in Figures 1 and

552 VOL. 16, SEPTEMBER 1993 Journal of High Resolution Chromatography

An All-Glass/Fused Silica MD-GC System

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sphtters in series up to the first detector without any changes in peak shape Figure 3 shows the results obtamed from inlection of 1 pl of the polar Grob test mixture on to the polydimethylsiloxane column (nght), and a heart cut (black bar on the upper part of the nght hand chromatogram) to the second, HP-20M, column (left) System performance and inertness is demonstrated by the peaks obtained from 8 ng of 2,6-dimethylanihne and 2,6 dimethylphenol (peaks indicated with arrows) which had passed through two col- umns, 4 sphtters and 6 glass connectors without any increase in band broademng

Figure 2 Schematic diagram of operation of multidimensional system. In the figure on the left the column eluate exits through the left (splitters C and A); in the figure on the right, with thevalveswitched, theeluateexitsthroughtheright(sp1itters C and 6).

2. Two pressure regulators were connected to this valve. A similar system was positioned inside the second oven at the exit of the second column, as shown in Figure 1.

When two systems are in series it is necessary, to ensure good pressure balance and correct switching, to choose carefully the correct length and diameter of the fused shca lines, R1 and R2, whch act as restrictors. A 5 cm length of 0 11 mm i.d. fused skca tubing was used as R1 and a 60 cm length of 0.20 mm i.d. fused skca tubing as R2. When only one system is used, restrictors are not necessary. Lengths of 0.53 mm fused shca tubing (2 m) were used for the hnes between the valves and the multidimensional system; the remaining connections were made with 0.25 mm i d tubing. Exit tubes to the second FID and the trap were 30 cm long. With this configuration, the correct pressures for obtaining ade- quate flows and good switching were: first column inlet, 150 kPa; first valve, 130 and 110 kPa; second valve, 30 and 20 kPa.

A schematic drawing of the system is shown in Figure 2. The upper part of the diagram on the left shows two helium lines (dotted hnes), with the pressure regulators at different pressures, and the valve, all situated outside the oven. The rest of the components are inside the oven. With this configuration, pressures at points B and C are hgher that that at A. Column eluate (the continuous line in the lower part of the diagram) exits through the left hand side of the system (i.e. through splitter A). If the valve is switched (diagram on the right) the situation is the opposite: the pressures at points A and C are higher than that at B, and the column eluate exits through the right hand side (splitter B) Carrier gas always purges all lines and connectors and there is no possibility of dead volume.

3 Results and Discussion

The inertness and performance of the chromatographic path were tested by inlection of 1 pl of the non- polar Grob test mxture [6] there are no apparent differences between the chromatography obtamed from the first column connected directly to the first FID vvlthout any connectors, as in normal GC, and that obtained after inlection of the same sample into the MDGC system without heart cutting (the GC pressures were adjusted to furnish sirmlar retention times) The sample passes through 2 press- fit connectors and 2 "Y"

. -2- I Figure 3 Chromatograms obtained from 1 FI polar Grob test mixture: first oven tem- perature, 220°C; second oven temperature, 80 "C then programmed (after cut) at 4 "Imin to 220 "C; injector pressure, 150 kPa; MDGC pressure, 130 kPa and 110 kPa; attenuation, 8 x 10; A, 2,6-dimethylaniline; B, 2,6- dimethylphenol.

Figure 4 Chromatogram obtained from 1 pI floral fragrance: first oven temperature programmed from 80 to 250 "C at 4 Ymin; second oven temperature from 80 to 220 "C at 4 "/min; other conditions as for Figure 3.

Figure 4 shows the chromatogram of a complex floral fragrance Methyl isoeugenol (Figure 5) is not resolved under the standard conditions employed in our laboratory the compound has a reten- tion time very close to those of rmraldyl acetate on HP-20M and p- ionone on polydimethylsdoxane All three compounds are present in large amounts in thls sample Temperature program optimzation is very difficult because of the large number of peaks present in the fragrance

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An All-GlasdFused Silica MD-GC System

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Methyl- \

isoeugenol

0

Miraldyl acetate

Figure 5 Chemical structures of p-ionone, methyl isoeugenol, and miraldyl acetate.

Figure 6 Chromatograms obtained from 1 kl floral fragrance: first oven temperature programmed from 80 to 250 "C at 4 "Imin; second oven temperature, 80 "C then programmed (after cut) at 4 "Imin to 220 "C; pressures as for Figure 4; attenuation, 8 x 10.

With our MDGC system separation is easy (Figure 6) The p-ionone and methyl isoeugenol peaks (only) from the polydimethylsiloxane column are cut and switched (black bar at right) to the HP-20M column (left) on which the retention times of the two compounds

are very different and there is no miraldyl acetate to interfere The methyl isoeugenol peak can be identified by inlecting standards under the same conditions, or by collecting the peak from the second column for subsequent identification by other methods (sniffing, spectroscopic analysis etc)

3.1 Practical Considerations

If the system is to be completely leak free it is necessary to cut the ends of the tubes with an appropnate tool and to ensure they are pressed sufficiently tightly agamst the connectors With a httle practice it is possible to achieve gas-tight seals (8 91 Leaks may be detected by inlecting isopropanol between the wall of the external tube and that of the internal connector If the leak is close to the detector a large isopropanol peak w d regster on the integrator If not, bubbles are easlly observed inside the connector In such circumstances it is necessary to disconnect, re-cut and reconnect the tubing excess isopropanol disappears when the oven is heated A small piece of fused sihca may occasionally remain inside the connector, makmg it impossible to reuse the latter Although a new umon must then be used the defective connector may be regener- ated by the method of Wesen and Mu [9] with excellent results

4 Conclusion

We have developed in our laboratory a new, double oven, multidi- mensional system operated by means of pressure changes The man difference between this system and those avalable commer- cially is that all the hnes and connectors are constructed from glass or fused sihca tubing, and continuously purged with carrier gas Ths ensures system inertness and the absence of dead volume

Some prehmnary results related to system performance, inertness, and troubleshooting (reuse of press-fit connectors, leak control) are presented

References 111 P A Rodnguezand C L Eddy J Chromatog S a 24 (1986) 18

121 D W Wnqht K 0 Mahler and L B Baiiard J Chromatogr Sci 24 (1986) 60

131 A Bernreuther N Christoph and P Schreier J Chromatogr 481 (1989) 363

[4] C Bjcchi and A Pisciotta J Chromatog 508 (1990) 341

[51 DR Deans Chromatographa 1 (1968) 18

161 K Groband G Grob Chromatographla4 (1971) 421

171 ER Rohwerand V Retonus HRC & CC 9 (1986) 295

181 C Wesen and HuihnqMu HRC 15 (1992) 136

Ms received May 28, 1993 Accepted July 28, 1993

554 VOL. 16, SEPTEMBER 1993 Journal of High Resolution Chromatography