Short Communications 10674

Determination of Gas Hold-Up Time in Open Tubular Columns

M. Larsson Department of Analytical and Marine Chemistry, Chalmers University of Technology and University of Goteborg, S-412 96 Goteborg, Sweden

Key Words:

Gas chromatography, GC Column evaluation Thick film columns Narrow bore columns

1 Introduction

An adequate technique for measurement of the gas hold-up time is a prerequisite forthe proper determination of several important column parameters in chromatography. Various methods presented for determining gas hold-up time in gas chromatogra- phic columns have been extensively reviewed and discussed by Smith eta/. [l], including both direct and indirect measurements as well as theoretical methods. In the development of capillary columns with small diameters, open tubular fused silica columns of 50 pn i.d. and below, coated with immobilized silicone gum phases (PS-255andSE-54), have been studied by LCandGC [2,3]. Evaluation of stationary phase characteristics included retention measurements by gas chromatography [2,3]. However, difficul- ties appeared in finding asuitable method forthe determination of the gas hold-up time (to) forthese narrow bore columns, mainlyfor two reasons. First, the length of the capillaries was only two meters, so to measured only a few seconds. Therefore, a simple split injection of an unretained component with a syringe will give rise to some slight degree of uncertainty in the injection time. Secondly, columns were prepared with different stationary phase film thicknesses, yielding p values (mobile to stationary phase volume ratio) in the range of 6 to 500. The compound used for to measurements, butane, was found to be retained on thick film columns at the column test temperature, 90°C. This report describes a technique that can be used for hold-up time measurements on short, thick film columns.

2 Experimental

A Carlo Erba Fractovap 21 01 gas chromatograph, equipped with a splitlsplitless injector and a flame ionization detector (FID) was used. A special pressure regulator was installed to obtain hydrogen carrier gas pressures up to 15 atm. The dead-time marker butane was injected with a 10 pI gas-tight syringe (Hamilton Bonaduz AG, Switzerland). The split flow rate was set at 120 rnllmin. The FID signal was recorded on a Perkin-Elmer 56 recorder with a chart speed of 12-24 cmh in . Preparation of narrow bore columns has been described in detail [2,3].

3 Results and Discussion

3.1 GC Measurement of Gas Hold-Up Time

The uncertainty introduced in the injection time by manual handling of a syringe was eliminatm by the use of an alternative technique. Instead of measuring the elution time for an unretained

peak, the time needed for emptying the column was measured. The present method slightly resembles a method presented by Hilrni [4]. In that work, the carrier gas was presaturated with a low volatility solvent (e.g. n-decane at a column temperature of 3OoC). A negative air peak was recorded by a flame ionization detector. The hold-up time was then obtained from the retention time of the air peakafter correction forthe vapour pressure of the solvent 141.

The present procedure is illustrated by a typical recording shown in Figure 1. Keeping the split vent closed, the sample is injected (point A). The injector and column is thereby filled with the gas sample, which is detected by the FID (point B). When a steady signal decrease (owing to dilution of the sample with the carrier gas) is observed, the split valve is opened (point C). The time from this event to the distinct decrease in the detector signal (point D), which reflects the replacement of sample in the column by pure carrier gas, determines the gas hold-up time. This method (referred to as the “splitless” method) was evaluated on a long, thin film column by comparison with split injection. Good agreement was observed (Table I).

I

Figure 1

FID recording of a hold-up time measurement on a 2 m X 50 pm open tubular column. The time for emptying the column (from C to D) corresponds to to.

0 1987 Dr. Alfred Huethig Publishers Journal of High Resolution Chromatography &Chromatography Communications 357

Short Communications 10674/10675

Table 1

Evaluation of the present method for determination of to, per- formed on a 25 m X 0.32 mm X 0.2 pn OV-1 column.

Consequently, the hold-up time at 90°C is obtained from

to (90°C) = to (22OOC) . qgo/q220 = t o (22OOC) 0.81 2 (2)

Experimental to values (s) Extrapolated to value split injection “splitless” from 220” to 90°

goo 220” 90”

39.1 39.2 47.8 38.8

_.

St.dev (%) 0.3 0.4 0.5 n = 5

__

3.2 Extrapolation of to from 22OOC to 90°C

In order to avoid retention of butane on thick film columns, to measurements were performed at 220OC. Assuming laminar flow, the dead-time is proportional to carrier gas viscosity. The viscosity of hydrogen (q) is related to the temperature, T (K), by the formula derived by Ettre [5]:

q = 83.5 (T/273)0.680 (1)

The validity of this expression was examined with to measure- ments using the “splitless” technique on the 0.32 mm i.d. column at 90°C and 22OoC, respectively. As can be seen from the data in Table 1. the experimentally obtained to value at 90°C is close to the to value extrapolated from 220°C by use of eq. (2).

References

R. J . Smith, J . K. Haken, and M. S. Wainwright, J. Chrornatogr (1 985) 95.

A. Farbrot, S. Folestad, and M. Larsson, HRC & CC 9 (1 986) 11 7.

334

S. Folestad, 5. Josefsson, and M. Larsson, J. Chrornatogr. 391 (1987) 347.

A. K. Hilrni, J. Chrornatogr. 17 (1965) 407.

L. S. Ettre, Chromatographia 18 (1984) 243.

Ms received: April 13, 1987

Gas-Chromatographic Analysis of Carbonyl Sulfide in Propylene at Levels Lower than 50 ppb, with the Helium Ionization Detector

G. Frisina Hirnont ltalia S.p.A. “G. Natta” Research Center, 44100 -

Key Words:

Gas chromatography, GC Carbonyl sulfide, carbon oxide sulfide, COS Propylene Helium ionization detector, HID

Ferrara, Italy

1 Introduction

The recent development and application of Ziegler-Natta cata- lysts for olefin polymerization has required greater and greater purity of the monomers used. In fact, impurities such as CO and COS have proved to be poisons for these catalysts, even when they are present insub-ppm concentrations inthemonomer.Thus analytical procedures with sensitivities reaching or exceeding the 50 ppb limit have become a must for monitoring monomer quality.

Traditional chemical-volumetric methods have long been used for COS detection. However, since their response time is very long and the determination of the color change point at very low concentration levelsquite critical, their use is no longeradvisable.

More recently this problem has been approached using gas chro- matography (GC) with the various ultra-sensitive detectors now commercially available. Among these, the “Flame Photometric Detector” (FPD) has been most widely used for the specific deter- mination of COS [l-41. However, its sensitivity limit is around 100 ppb.

10675

The use of the “Hall Conductivity Detector” [5], which is reported to be at least 10 times more sensitive than FPD [7], has opened a new door toward resolving the problem [6-71, In propylene, for example, COS has been detected at a sensitivity of 10 ppb [8].

Recently, a special detector, specific for sulfur compounds and based on emission spectroscopy, the “Microwave Induced Plasma Detector”, has also been suggested. With this detector it is possible to obtain a 50 ppb sensitivity limit for COS [9].

Finally, the “Helium Ionization Detector” (HID), long used by this laboratory for CO determination at ppb level in the same type of monomers, was reconsidered in both its traditional and its modified form [lo-121. It is recommended for sub-ppm detection of various atmospheric pollutants such as COS and H2S [I 11.

This paper describes the method used in this laboratory for the easy routine analysis of less than 10 ppb of COS in propylene, using a commercial instrument equipped with a helium ionization detector [13-141.

358 Journal of High Resolution Chromatography & Chromatography Communications 0 1987 Dr. Alfred Huethig Publishers