Gas-sampling valve

There are several types of sample introduction systems available for GC analysis. These include gas sampling valves, split and splitless injectors, on-column injection systems, programmed-temperature injectors, and concentrating devices. The sample introduction device used depends on the application. 

Sampling for gas phase chromatographic analysis can be readily achieved by a grease-free, rotatable, multiple-ported gas sampling valve. Various commercial units of this sort are available. However, the author s laboratory preferred to fabricate to its own design (Y. Shimoyama, 1970) the result is a valve containing only stainless steel and PTFE and specifications... 

The other two were connected in a circuit containing a bellows-type gas circulating pump (Metal Bellows Corp. Model MB-21) to a gas sampling valve mounted in a Carle AGC-311 gas chromatograph. The latter is equipped with two columns, Porapak (80 N, 20 Q,... 

Activity Measurements. A mlcrocatalytic reactor Interfaced by a six-way gas sampling valve with a gas chromatograph was used to study the vapor phase oxidation of methanol at 150°C. The feed gas consisted of 72, 24 and 4% by volume of nitrogen, oxygen and methanol vapour, respectively. For each run about 0.3 g of catalyst was used and the products were analysed with a 10% carbowax 20M column (2m long). The observed major products... 

VPO catalyst selectivity is tested by both fixed-bed microreactor measurements and by pulsed microreactor measurements. In the former, the rate constants are measured in a microreactor on about 1 g of catalyst at temperature between 360 and 390 °C in a 1.5% butane/air environment. The pulsed microreactor evaluations are carried out by injecting 0.05 ml pulses of butane using a gas-sampling valve over about 0.5 g of catalyst in a microreactor heated to about 380 °C. /i-butane conversion and selectivity to maleic anhydride (MA)... 

Sample injector. A device for introducing liquid or gas samples into the chromatograph. The sample is introduced directly into the carrier gas stream (e.g., by syringe) or into a chamber temporarily isolated from the system by valves which can be changed so as to instantaneously switch the gas stream through the chamber (gas sampling valve). 

Due to the ease of reproducing injection volumes of gas with a gas-sampling valve and the difficulty of appyling the technique of internal standardization discussed below for gas samples, external standardization is the preferred approach to the analysis of gas samples. For these reasons considerable attention will be given to the preparation of gas standards and the problems associated with gas analysis. In many cases this touches also on the area of trace analysis since much of the gas analysis done today is the analysis of trace components in an air matrix. 

Standards are available today in small pressurized cans that are extremely convenient to use with a gas-sampling valve for injection. Again supplier reliability with verification are a must. 

GAS SAMPLE VALVE. With all the problems associated with syringe injection of gas samples, it is not surprising that a more accurate way of injecting gas samples has been found. This system makes use of a gas sampling valve. There are a number of these valves on the market using either rotary or push-pull actuation and interchangeable volumes are standard. A schematic for a rotary valve is shown in Figure 4.15. In the load position the volume of the valve is connected to the in and out load ports. 

Figure 4.15. Flow schematic of a typical rotary gas sample valve.

Figure 6.8. Injection of gas samples by means of a gas sampling valve.

For the most sensitive operation with gas-sampling valves, flows and pressures must be very well balanced in the system. 

Temperature programmed desorption (TPD) of C02 (5 °/min, flow of He, 15 ml/min) was carried out on a conventional flow apparatus. In a typical experiment, 0.29 g of the catalyst were activated as above reported, then the system was cooled to 25°C and approximately 2 10 5 mol of Co2 were injected by means of a gas sampling valve. After degassing in flow of helium for 60 min the amount of the irreversibly adsorbed C02 was determined with an on-line g.l.c. equipped with a thermal conductivity detector,... 

The gas bags were connected directly to the gas sampling valve. Replicate injections gave good precision. However, this technique required calculation of the dilution factor between the reservoir and the gas sampling valve and preparation of several standard concentrations of vinyl acetate in helium. 

The pulse technique is in many ways similar to the flow technique except that the adsorbate is introduced by adding pulses (e.g. from a gas sample valve) into the carrier gas. The pulse volume is chosen so that the first few pulses will be completely adsorbed. Further pulses are introduced until no more gas is adsorbed. The quantity of gas adsorbed is calculated by summing up the amounts adsorbed in the successive pulses. This technique is only applicable for strongly retained adsorbates. 

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