Carrier gas stream

The thermal sweeper is a commercial product licensed to Zoex Corporation, Lincoln, NA, USA (16). The sweeper incorporates a slotted heater (operated at about 100 °C above the oven temperature) which passes over the capillary column (normally an intermediate thicker film column is used in this region as an accumulator zone). Eigure 4.3 is a schematic diagram of how the instrumental arrangement may be considered. The greater temperature of the rotating sweeper forces the solute which has been retained in the phase in the accumulator section to be volatilized out of the phase into the carrier gas stream, and then bunched up and brought forward... 

The detector. The function of the detector, which is situated at the exit of the separation column, is to sense and measure the small amounts of the separated components present in the carrier gas stream leaving the column. The output from the detector is fed to a recorder which produces a pen-trace called a chromatogram (Fig. 9.1fr). The choice of detector will depend on factors such as the concentration level to be measured and the nature of the separated components. The detectors most widely used in gas chromatography are the thermal conductivity, flame-ionisation and electron-capture detectors, and a brief description of these will be given. For more detailed descriptions of these and other detectors more specialised texts should be consulted.67 69... 

Thermal conductivity detector. The most important of the bulk physical property detectors is the thermal conductivity detector (TCD) which is a universal, non-destructive, concentration-sensitive detector. The TCD was one of the earliest routine detectors and thermal conductivity cells or katharometers are still widely used in gas chromatography. These detectors employ a heated metal filament or a thermistor (a semiconductor of fused metal oxides) to sense changes in the thermal conductivity of the carrier gas stream. Helium and hydrogen are the best carrier gases to use in conjunction with this type of detector since their thermal conductivities are much higher than any other gases on safety grounds helium is preferred because of its inertness. 

Step- and impulse-response methods. Chemisorption can conveniently be measured under flow conditions using transient techniques, in particular, step-response and impulse-response measurements. After pretreatment, pulses of probe gas are injected into a carrier gas stream passing through the reactor that contains the pre-treated. sample. The response is detected at the reactor exit. 

The traditional (Rosin, et al., 1932) mechanistic approach equates the time necessary for a particle to settle at a Stokes law velocity across the width of a cyclone s inlet duct, to the available residence time of the carrier gas stream in its number of spiral traverses within the barrel. With reference to Fig. 1, this permits solving for the smallest particle size able to cross the entire width and reach the wall in the available time. 

Only when the very contamination-sensitive electron-capture detector is used is it necessary to provide separate gas streams, one for the reaction and stripping part of the system, the other for the carrier gas stream of the column and detector. Otherwise, the same gas stream can be used to strip the hydrides from solution and carry them into the detector, which greatly simplifies the apparatus. This is of considerable significance, as each additional surface and joint in the apparatus increases the possibility of irreversible adsorption of the sensitive hydrides, and thus is a potential contributor to analytical error. The... 

Up to 30% hydrogen can be added to the carrier gas stream of a helium microwave plasma torch, in the determination of arsenic, bismuth and tin. The argon torch accepts up to 20% hydrogen LOD about 2.5 pg Sn/L, with linear dynamic range over 3 orders of magnitude23. 

Headspace analysis involves examination of the vapours derived from a sample by warming in a pressurized partially filled and sealed container. After equilibration under controlled conditions, the proportions of volatile sample components in the vapours of the headspace are representative of those in the bulk sample. The system, which is usually automated to ensure satisfactory reproducibility, consists of a thermostatically heated compartment in which batches of samples can be equilibrated, and a means of introducing small volumes of the headspace vapours under positive pressure into the carrier-gas stream for injection into the chromatograph (Figure 4.25). The technique is particularly useful for samples that are mixtures of volatile and non-volatile components such as residual monomers in polymers, flavours and perfumes, and solvents or alcohol in blood samples. Sensitivity can be improved by combining headspace analysis with thermal desorption whereby the sample vapours are first passed through an adsorption tube to pre-concentrate them prior to analysis. 

As the name implies, the sample is introduced into the mass spectrometer as a gas (Nier 1940). There are two types of sources, the classic viscous flow source and the continuous flow source. The viscous flow source typically consists of two identical inlet systems that are coupled to the mass spectrometer by a change-over valve, which allows rapid switching for comparison of isotope ratios measured for sample and standard gases. In the continuous flow source, samples gas is introduced as a bubble in a non-reactive carrier gas stream. 

Radon-222 may be transported with a carrier gas into an ionization chamber and its alpha particles counted. Short-lived isotopes in a carrier gas stream are measured this way using a flow-type ionization chamber. 

The aldol condensation/hydrogenation reaction was carried out in a continuous flow microreactor. The catalysts (0.5 g) were reduced in situ in a flow of H2 at atmospheric pressure at 723 K for 1 h for the palladium systems and 2 h for the nickel systems. The liquid reactant, acetone (Fisher Scientific HPLC grade >99.99%), was pumped via a Gilson HPLC 307 pump at 5 mL hr into the carrier gas stream of H2 (50 cm min ) (BOC high purity) where it entered a heated chamber and was volatilised. The carrier gas and reactant then entered the reactor containing the catalyst. The reactor was run at 6 bar pressure and at reaction temperatures between 373 and 673 K. Samples were collected in a cooled drop out tank and analyzed by a Thermoquest GC-MS fitted with a CP-Sil 5CB column... 

Figure 19.3 schematically describes in more detail the transport phenomena occurring during pervaporation. First, solutes partition into the membrane material according to the thermodynamic equilibrium at the liquid-membrane interface (Fig. 19.3a), followed by diffusion across the membrane material owing to the concentration gradient (Fig. 19.3b). A vacuum or carrier gas stream promotes then continuous desorption of the molecules reaching the permeate side of the membrane (Fig. 19.3c), maintaining in this way a concentration gradient across the membrane and hence a continuous transmembrane flux of compounds.

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). 

VENTED SEPTUM INJECTORS. A more radical injector modification is available. It prevents gas which has had contact with the septum from entering into the column. Figure 6.11 illustrates such a system in which the gas contacting the septum is vented by a bypass pipe. The carrier gas stream is split, the main portion going to the column while a small portion moves past the septum and... 

At the Mellon Institute he applied l4C tracers to examine the behavior of intermediates in Fischer-Tropsch synthesis over iron catalysts. By adding small amounts of radioactively labeled compounds to the CO/H2 synthesis gas mixtures, he was able to prove that some of these compounds (e.g., small alcohols) are involved in the initiation step of the chain growth process that leads to larger hydrocarbon products. It was during this era that his associates first placed a catalytic reactor into the carrier gas stream of a gas chromatograph and developed the microcatalytic pulse reactor, which is now a standard piece of equipment for mechanistic studies with labeled molecules. While at Mellon Institute Emmett began editing his comprehensive set of seven volumes called Catalysis, which he continued at Hopkins. 

Fig. 4. Minimum requirements for a gas chromatographic system include (I) a column which contains the substrate or stationary phase. 12) a supply nt inert carrier gas (moving phase) which is continually passed through the columns. (3) a means lor maintaining pressure and flow constant. (4) a means of admitting or injecting the sample into die carrier gas stream. (5) a detector which senses the sample components as they elute, and >) a display (recorder). The carrier gas may be any gas that does not react with the sample nr adversely artect the detector. Helium, hydrogen, nitrogen, and argon are often used...

An inlet system for quantitative work must present a representative sample to the carrier gas stream in the chromatograph. This can be a problem with highly... 

Indirectly heated process — An externally fired rotary dryer volatilizes the water and organics from the contaminated media into an inert carrier gas stream. The carrier gas is later treated to remove or recover the contaminants. 

Figure 1. Tube furnace laser vaporization apparatus used by Haufler et al. (1991) to produce high yields of C 0F. The 532 nm doubled Nd YAG pulse vaporizes C from the rotating graphite target into the inert carrier gas stream. The fullerenes condense just outside the oven on the tube wall.

Sampling is performed with two high pressure switching valves with internal volume. 5 pi (for the upper phase) and. 2 pi for the lower or middle phases. The samples are directly depressurized into a He carrier gas stream and analyzed with a Perkin Elmer Sigma 2 Gas Chromatograph, using a Porapak Q column supplied by Supelco. The response factors for the materials used were found to be close to those reported by Dietz (2). Typical reproducibility of the analysis is .003 in mole fraction, with somewhat larger deviations for the gas-phase compositions at low pressures. 

He Pulses in Nitrogen Figure 1. illustrates a typical response curve (continuous line) obtained for a 1 second pulse of He in a nitrogen carrier gas stream. Using the moment method (12), the effective diffusion coefficient for He is 0.137 cm /sec. To compare this with steady state data, it is necessary to make use of equation (3). 

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