Carrier gas volume

The separated components emerging from a GC column are present in pico-gram amounts in the carrier gas stream. If the column eluant is to be coupled to a mass spectrometer then the volume flow-rate of carrier gas should be minimised for a given separation to achieve high sensitivities in the ion source and reduce pumping requirements. GC parameters are selected to obtain symmetrical sharp peaks which are eluted in the minimum carrier gas volume and with the best peak height to width ratio that can be achieved. Clearly, the components need to be resolved, as co-eluting peaks would not produce pure mass spectra which could then be compared to a library of reference spectra. A number of factors need to be considered to optimise the GC system ... 

The control of carrier gas composition is done by the detector unit at the end of the column. The course of time dependent concentration can be monitored via an amplifier and penwriter by the operator. The integrator calculates the area beneath the different peaks. The detection of concentration within the carrier gas has to be without any distortion or delay. Additionally it should be mentioned, that the detector volume should not exceed that carrier gas volume, referring to the characteristic width of the smallest peak. 

Because the chromatographic column is under pressure, the carrier gas volume is small at the high-pressure inlet, but expands during passage through the column as the pressure decreases. TTiis topic is discussed in Chapter 2. 

To characterize a substance, in addition to the retention times as defined above, the corresponding carrier gas volumes required for the appearance of the peak maximum can also be used. Thus on the abscissa in Figure 1.3 (p. 19) tlie retention volume is plotted instead of retention time it is given by the product of carrier gas flow rate and time. The flow rate of the carrier gas Fa is measured, at ambient temperature Ta and pressure at the column outlet by a soap bubble flowmeter the pressure of water apour in these conditions is pi. The flow rate, Fq, of carrier gas at the outlet pressure and temperature Tc of the column is ... 

Gives the carrier gas volume required for elution of a given component. 

The column is swept continuously by a carrier gas such as helium, hydrogen, nitrogen or argon. The sample is injected into the head of the column where it is vaporized and picked up by the carrier gas. In packed columns, the injected volume is on the order of a microliter, whereas in a capillary column a flow divider (split) is installed at the head of the column and only a tiny fraction of the volume injected, about one per cent, is carried into the column. The different components migrate through the length of the column by a continuous succession of equilibria between the stationary and mobile phases. The components are held up by their attraction for the stationary phase and their vaporization temperatures. 

Here z denotes distance measured along the tube from the injection point and f is the volume flow rate of carrier gas, per unit total cross-section. 

The simplest mode of IGC is the infinite dilution mode , effected when the adsorbing species is present at very low concentration in a non-adsorbing carrier gas. Under such conditions, the adsorption may be assumed to be sub-monolayer, and if one assumes in addition that the surface is energetically homogeneous with respect to the adsorption (often an acceptable assumption for dispersion-force-only adsorbates), the isotherm will be linear (Henry s Law), i.e. the amount adsorbed will be linearly dependent on the partial saturation of the gas. The proportionality factor is the adsorption equilibrium constant, which is the ratio of the volume of gas adsorbed per unit area of solid to its relative saturation in the carrier. The quantity measured experimentally is the relative retention volume, Vn, for a gas sample injected into the column. It is the volume of carrier gas required to completely elute the sample, relative to the amount required to elute a non-adsorbing probe, i.e. 

Fig. 17. A schematic of the alkane line obtained by inverse gas chromatography (IGC) measurements. The relative retention volume of carrier gas required to elute a series of alkane probe gases is plotted against the molar area of the probe times the. square root of its surface tension. The slope of the plot is yielding the dispersion component of the surface energy of...

Together with this solvent effect, another effect, called phase soaking, occurs in the retention gap technique if a large volume of solvent vapour has saturated the carrier gas, the properties of the stationary phase can be altered by swelling (thicker apparent film), a change in the viscosity or changed polarity. The consequence is that the column shows an increased retention power, which can be used to better retain the most volatile components. 

Gas holdup Vm is the volume of carrier gas that passes through the column to elute an unretained substance, such as argon or methane. The time required is tm. 

The purge activation time (or the sample transfer time) depends on the sample solvent and carrier gas flow relative to the volume of the injection port liner and the boiling points of the sample components. For most applications, a purge activation time of 50-120 sec is better than 25-50 sec. Early activation results in the loss of sample, while late activation results in peak tailing. A more accurate method of determining purge activation time is to divide the volume of the injector liner by the flow rate (F) of the carrier gas and multiply this value by 1.5 or 2.0. (Do not use a packed liner.)... 

Air leaks are another source of trouble in the MS. A simple method of leak detection is to squirt a small volume of acetone on flanges and other areas where leaks could occur. Caution is advised not to use this procedure near hot surfaces because of the flammability of acetone. A second way to test for small leaks is to tune the MS to m/z 40 and to use argon to test for leaks. The m/z 40 peak will increase if argon enters the source. Helium m/z 4) is a better choice, except when helium carrier gas is used in conjunction with the GC. A small stream of the gas is aimed at all seals where a leak can occur. If a leak is detected at a seal, it can sometimes be stopped by tightening the seal, but it is better to replace the seal than to overtighten it. 

Dead volume The total volume in the system that is swept by the carrier gas. 

Liquids. Many of the CVD reactants are liquid at room temperature, They must be heated to their evaporation temperature and transported into the reaction chamber by a carrier gas, which may be an inert gas such as argon, or another reactant such as hydrogen. If the vapor pressure of the liquid reactant is known, its partial pressure can be calculated and controlled by controlling the rate of flow and the volume of the carrier gas. 

Oxyfluorfen column, fused-silica capillary column coated with cross-linked methyl silicone (25 m x 0.3-mm i.d., 0.52- am film thickness) temperature, column 200 °C (1 min), 10°Cmin to 250 °C (5 min), inlet and detector 250 and 300 °C, respectively gas flow rates, N2 carrier gas 30mLmin , N2 makeup gas 30mLmin H2 3.5mLmin" air llOmLmin injection volume, 2 p.L. ... 

Injection volume Carrier gas flow rate Transfer line temperature Ionization mode Detector calibration Acquisition type Acquisition masses... 

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