Valve headspace

Figure 8.26(A) is an example of a valve type interface [329]. Helium carrier gas is provided to the headspace saiq)ler and is split into two flow paths. One path is flow-controlled and provides a constant flow of carrier gas which passes from the headspace unit through the heated transfer line to the gas chromatograph. The second flow path is pressure-regulated and, in the standby mode, the seunple loop and seuapling needle are flushed continuously by the helium flow. At a time determined by the operator, the sampling needle pierces the septum and helium pressurizes the headspace vial to any desired pressure. The headspace gas is then allowed to vent through the sample loop. Once filled, the sample loop is placed in series with the normal carrier gas flow and its contents are driv Bbhrough the heated...

Figure 8.26 A, valve diagram of the Hewlett-Packard 19395A Beadspace Sampler. B, Headspace sampling with constant headspace pressure, as carried out with the Perkin-Elmer HS-lOO Automatic Headspace Sampler.

Principles and Characteristics Extraction or dissolution methods are usually followed by a separation technique prior to subsequent analysis or detection. While coupling of a sample preparation and a chromatographic separation technique is well established (Section 7.1), hyphenation to spectroscopic analysis is more novel and limited. By elimination of the chromatographic column from the sequence precol-umn-column-postcolumn, essentially a chemical sensor remains which ensures short total analysis times (1-2 min). Examples are headspace analysis via a sampling valve or direct injection of vapours into a mass spectrometer (TD-MS see also Section 6.4). In... 

Figure 20.4—Static mode of headspace sample analysis. The sampling phial is pressurised with the carrier gas of the chromatograph. After equilibrium, a small volume of the gas containing the volatile compounds is inserted into a sample loop. Rotation of the six-way valve allows introduction of the sample into the injector of the chromatograph. Consequently, this set-up combines sample preparation with sample introduction into the chromatographic column. (Reproduced by permission of Tekmar.)...

The measurement of carbonation is carried out using a device similar to that shown as Figure 7.4. It consists of a jig in which the container can be restrained, and a piercer attached to a pressure gauge. The container is placed in the jig, and is first of all pierced and, then shaken before the pressure is measured. The release valve is opened until the pressure gauge reads zero and all the gas has been exhausted from the container headspace. The release valve is then closed and the container shaken again. The pressure is re-taken. The container is released from the jig and the temperature of the contents measured. A carbonation chart is then used to determine the volumes of carbonation. 

Once the pressures are equated, the gas valve is closed and the liquid valve opened. Flow commences, stopping when the pressures are equated. A short settling period is then allowed before the liquid valve is closed and the gas within the container headspace is snifted (vented) off. This settling period, and subsequent sniffing are required because otherwise, when the container is lowered from file filler bowl, the pressurised gas within the headspace will be exposed to the atmosphere. This would result in severe fobbing of the product... 

Chromatographic System (See Chromatography, Appendix HA.) Chromatographic conditions may vary depending on the type of headspace unit used. Use a gas chromatograph equipped with a headspace sampler, flame ionization detector, backflush valve, 1-mL gas sample loop, and a 1-m x 3.2-mm (id) nickel precolumn and a 6-m x 3.2-mm (id) nickel analytical column, or equivalent, containing 60- to 80-mesh... 

Figure 2. Apparatus for dynamic headspace sampling of volatiles, released from intact apples. 1 = high purity purge air, 2 dessicators, 3 = thermostated waterbath (18°C), 4 = fine metering valve, 5 = Tenax adsorber, 6 = flow meter, 7 = wet-testmeter.

The liquid or solid sample is placed in a vial, which is sealed with a septum and heated to a predetermined temperature for a period of time. Equilibrium between the sample and vapor phase is then established and a portion of the volatiles in the gas phase (headspace) is subsequently injected onto the column. Several different methods have been used to transfer headspace volatiles into the GC, from manual withdrawal that uses a gas syringe, to sophisticated automatic sampling that involves transfer lines, and valves that lead directly onto the column. 

At the end of the sampling period, the valve is re-opened and the normal carrier gas flow resumed. In this way, the sample is introduced onto the column without using a gas syringe, thus avoiding fractionation due to pressure changes in the syringe. Since the system is closed, no headspace is lost during the transfer. The equipment requires no multiport valves, which reduces the number of components that come into contact with delicate samples. 

Turn off the pump 5 min after a stable bed has been achieved. Close the bottom valve and switch the inlet valve to waste. Expect the resin bed to rebound higher than the packed bed height as soon as the flow is stopped. Lower the adapter gradually to eliminate the headspace and compress the resin to the final bed height. 

Samples were contained in glass flasks of either the configuration of a serum bottle (100 ml) capped with a Mininert valve or of a specially-designed Erlenmeyer flask (125 ml) constructed with a screwtop and fitted with a sidearm and a Teflon stopcock. The latter flask was capped with a silicone septum held with an open-topped screw-cap, and allowed syringe-sampling as well as evacuation. Samples for headspace analysis were equilibrated at 30°C. Calibration curves were calculated as log (area) versus log (mass of methanethiol or dimethyl disulfide). 

The HS-GC-MIP system consists of a modified HS-6 semi-automated headspace sampler (6) (Perkin Elmer) mounted on a F M-700 gas chromatograph. The outlet of the GC-column is connected to a heated four-way valve for solvent ventilation (Valeo GC-T). From there a heated transfer tube (150°C) is connected to a quartz capillary (2 mm i.d.) that is centered in a 1/4-wave Evenson-type cavity (Electro Medical Supplies, Model 214L). The microwave generator (Electro Medical Supplies, Microtron 200) is operated at 75 W, providing a maximum signal to noise ratio. Emission measurements are carried out with a Perkin-Elmer AAS-403 at 253.7 nm. 

The direct headspace (HS) technique has been used to determine VOCs in water samples. This method overcomes comphcations associated with the sample matrix and can be applied to a wide range of concentrations. HS requires little sample preparation. Salt is usually added to improve the partitioning into the gas phase and the sample is heated to temperatures of about 50-60 °C, which enhances the volatilisation of the analyte, increasing the efficiency of the extraction process and consequently the sensitivity. Typically, the HS is directly sampled with a p,L-lock valve-gastight syringe and injected into the GC. This method has been used for the determination of the isotope composition of MTBE, ETBE and TAME, reporting detection limits of 3-6 mgL for 5 C and 8-20 mgL for the 3 H [37-40]. 

Figure 3.13. Schematic diagram of the balanced pressure sampling system for automated splitless static headspace gas chromatography with cryogenic trapping. V = solenoid valve in the carrier gas (CG) line V2 = solenoid valve for the purge gas and V3 solenoid valve for the cooling gas. (From ref. [142] Elsevier).

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