Carrier gases switching

Helium is recommended as carrier and makeup gas Response and sensitivity is based on difference between relative molecular mass of analyte and that of the carrier gas approximate calibration can be done on the basis of relative density The sensing elements (hot wires) never touch sample, thus making GADE suitable for the analysis of corrosive analytes such as acid gases gold-sheathed tungsten wires are most common Best used with SFe as a carrier gas, switched with nitrogen when analyses are required Detector can be sensitive to vibrations and should be isolated on a cushioned base Ultimate sensitivity depends on the number of C-H bonds on analyte... 

In this way, the liquid can be transferred at a speed corresponding to the evaporation speed. The fraction to be analysed is contained in a loop (see Eigure 2.5), connected to a switching valve. By opening the valve, the sample in the loop is driven by the carrier gas into the GC unit (8), instead of the LC pump. An early vapour exit is usually placed after a few metres of the deactivated precolumn (9) and a short piece (3-4 m) of the main column (retaining precolumn). This valve is opened during solvent evaporation in order to reduce the amount of solvent that would reach the detector, and at the same time, to increase the solvent evaporation rate (6). 

The non-intrusive manipulation of carrier gas effluent between two columns clearly has significant advantages in two-dimensional GC. In addition, a pressure-driven switch between the columns introduces no extra band broadening to an eluting peak. 

The only disadvantage to the use of hydrogen as a carrier gas Is the real or perceived explosion hazard from leaks within the column oven. Experience has shoim that the conditions required for a catastrophic explosion may never be achieved in practice.. However, commercially available gas sensors will automatically switch off the column oven and carrier gas flow at air-hydrogen mixtures well below the explosion threshold limit [143]. 

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. 

Clearly there is a switch from MO to MIBK when hydrogen was made the carrier gas. The yield of isophorone dropped as at this temperature the hydrogenation intercepted the MO before it could undergo another aldol condensation. At 673 K the aldol condensation reaction became kinetically more competitive with the hydrogenation reaction. 

Consider that the flow through a packed column is suddenly switched from pure carrier gas to a mixture of carrier gas and adsorbate vapor at concentration c. As the mixture traverses the distance dl along the column, adsorption will cause the concentration to decrease by dc. Expressing the concentration as weight W of adsorbate per unit volume of mixture, gives the weight lost by the flow in the column length dl as... 

Isotherms of type I are characterized by a continuously decreasing slope which often becomes zero. Therefore, the adsorbate will move more rapidly through the column at higher concentrations. When the flow is switched from a mixture to pure carrier gas, the elution profile or chromatogram will appear as shown in Fig. 16.1, when plotted on a strip chart recorder. 

In Fig. 16.1, Pq is the signal produced by saturated vapor, is the time of switch-over to pure carrier gas. At time tj the signal corresponds to the pressure P and the horizontally shaded portion of the curve is proportional to the quantity of adsorbate remaining in the column. By successive integrations of the area under the curve along the line SPF, the quantity adsorbed at various relative pressures can be calculated. 

Type III isotherms possess a continuously increasing slope. Therefore, the adsorbate propagates more rapidly at lower concentrations. The corresponding elution profile when the flow is switched from pure carrier gas to a mixture of carrier and adsorbate is shown in Fig. 16.2. 

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). 

Even the sample loop should be controlled to the same pressure as the column side to minimize pressure upsets when switching the valve. Frazer et al. (9) found that pressure and flow regulation on all inlets as well as the appropriate micrometering valves (variable restrictors) used to dynamically balance the carrier gas flow and pressure allowed them to switch a 20 foot by 1/8-inch column in or out of a multivalve, multicolumn system with a resulting change in the baseline signal of less than 5 iV. 

Figure 10.3 Gas chromatograms of a cold-pressed lemon oil obtained (a) with an SE-52 column in the stand-by position and (b) with the same column showing the five heart-cuts (c) shows the GC-GC chiral chromatogram of the transferred components. The asterisks in (b) indicate electric spikes coming from the valve switching. The conditions were as follows SE-52 pre-column, 30 m, 0.32 mm i.d., 0.40-0.45 gm film thickness carrier gas He, 90 KPa (stand-by position) and 170 KPa (cut position) oven temperature, 45 °C (6 min)-240 °C at 2 °C/min diethyl-terf-butyl-/ -cyclodextrin column, 25 m X 0.25 mm i.d., 0.25 p,m film thickness carrier gas He, 110 KPa (stand-by position) and 5 KPa (cut position) oven temperature, 45 °C (6 min), rising to 90 °C (10 min) at 2 °C/min, and then to 230 °C at 2 °C/min. Reprinted from Journal of High Resolution Chromatography, 22, L. Mondello et al., Multidimensional capillary GC-GC for the analysis of real complex samples. Part IV. Enantiomeric distribution of monoterpene hydrocarbons and monoterpene alcohols of lemon oils , pp. 350-356, 1999, with permission from Wiley-VCH.

Under carefully optimized conditions (internal flow rates, temperature, switch time, feed concentration and enantiomeric excess ee) the enantio-selective SMB-GC pilot unit furnished a total of 20 g of each enantiomer of enflurane with an enantiomeric excess of 96.6 % (time requirement not known) with N2 as carrier gas (Biressi et al., 2002b). Noteworthy is the low particle size of the non-acid-washed Chromosorb A (NAW) used ( 0.6 mm). Enflurane was introduced in the gaseous form (saturated in N2). The SMB-GC unit produced the largest amounts of single enflurane enantiomers presently available for testing of potential differences in biological activity. No comparison of throughput between the batchwise vs. the SMB process is as yet available. 

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. 

The stainless steel micro reactor (figure 2) is constructed for catalyst pellet sizes of 0.175 to 0.20 mm. The reactor exit is connected via 0.9 m stainless steel capillary (i.d. 0.2 mm) to the analysing unit. The reactor and part of the capillary is mounted in an electric oven. A continuous stream of carrier gas passes the four way valve, then the catalyst bed, and flows via a stainless steel capillary into the detector. The carrier gas can be switched to pulse gas with the four way valve. The pressure in the reactor is determined by the resistance of flow in the capillary. The pressure difference between the carrier gas and the pulse gas is measured with a differential pressure detector. During the experiment the gas velocities of the carrier and the pulse gas are equal. The gasses are regulated by mass flow controllers. The gases used in the experiments were of a high purity. 

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