GC columns

If interfering peaks hinder sample quantitation on the gas chromatogram, better resolution could be obtained by using an FFAP or DB-17 gas chromatographic column. 

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Clinical Analysis Clinical, pharmaceutical, and forensic labs make frequent use of gas chromatography for the analysis of drugs. Because the sample s matrix is often incompatible with the GC column, analytes generally must be isolated by extraction. Figure 12.25b shows how gas chromatography can be used in monitoring blood alcohol levels. 

Capillary Tubes Figure 12.42 shows a cross section of a typical capillary tube. Most capillary tubes are made from fused silica coated with a 20-35-)J,m layer of poly-imide to give it mechanical strength. The inner diameter is typically 25-75 )J,m, which is smaller than that for a capillary GC column, with an outer diameter of 200-375 )J,m. 

The adjusted retention times for octane, toluene, and nonane on a particular GC column are 15.98 min, 17.73 min, and 20.42 min, respectively. What is the retention index for all three compounds ... 

Successive spectra taken during elution of a substance from a GC column. The first spectrum obtained by El shows many fragment ions, while the second, obtained by Cl, gives molecular mass information. 

Schematic diagram showing the injection of a mixture of four substances (A, B, C, D) onto a GC column, foliowed by their separation into individuai components, their detection, and the dispiay (gas chromatogram) of the separated materiais emerging at different times from the coiumn.

The effluent from a GC column is already in the gas phase and needs only to be mixed with argon makeup gas before passage into the flame. Precautions need to be taken to divert temporarily the GC flow when the first solvent peak emerges because it contains far too much material for the plasma to withstand. 

The oil possesses moderate antibacterial and strong antifungal properties. Thus the appHcation of the oil to the cmst of cheese could serve to prevent the formation of mycotoxia ia the cheese. The optical purity of the carvone ia caraway has been determined usiag a chiral gc column (72). It was found to be (i )(+) = 97.64% and (5 )(—) = 2.36%. 

The most common chromatogram in the distilled spirits industry is the fusel oil content. This consists of / -propyl alcohol, isobutyl alcohol, and isoamyl alcohol. Other common peaks are ethyl acetate, acetaldehyde, and methanol. The gc columns may be steel, copper, or glass packed column or capillary columns. Additional analyses include deterrninations of esters, total acids, fixed acids, volatile acids, soHds or extracts (used to determine... 

More sophisticated GC columns and techniques perform more detailed separations of mixtures of hydrocarbons into discrete groups. Table 12-3... 

Figure 4. Graph Showing Change of Pressure Along a GC Column for Different Inlet/Outlet Pressure Ratios...

Consider the (n)th theoretical plate in a GC column, as depicted in Figure 21. The properties of the plate are defined as follows. 

Consider the heat balance of the cell. However, as opposed to the GC column, the heat capacity of the liquid mobile phase in an in LC column is relatively large. Consequently, heat convected from the cell by the mobile phase must also be taken into account. It follows that... 

Effect of Mobile Phase Compressibility On the HETP Equation for a Packed GC Column... 

Thus, the complete HETP equation for a packed GC column that takes into account the compressibility of the carrier gas will be... 

Equation (15) gives the variance per unit length of a GC column in terms of the outlet pressure (atmospheric) the outlet velocity and physical and physicochemical properties of the column, packing, and phases and is independent of the inlet pressure. However, equation (13) is the recommended form for HETP measurements as the inlet pressure of a column is usually known, and is the less complex form of the HETP expression. 

Now, consider a packed GC column due to the turbulence resulting from the rapid changes in direction of the gas velocity as it winds its tortuous path between the... 

It is seen that, for GC packed columns operated under the conditions assumed, the two factors contributing to dispersion by resistance to mass transfer are of the same order of magnitude. Consequently, equations (20) and (21) cannot be simplified and must be used in their existing form for all optimization procedures using packed GC columns. If the conditions differ significantly from those assumed, then by using the same procedure the possibility of modifying expressions (20) and (21) can be reexamined. 

Equations (2) and (4) allow the permissible extra-column dispersion to be calculated for a range of capillary and packed columns. To allow comparison, data was included for a GC column, in addition to LC columns. The results are shown in Table 1. 

The packed GC column has a value for (og) of about 55 pi, whereas the high efficiency microbore LC column only 0.23 pi. It is clear that problems of extracolumn dispersion with packed GC columns are not very severe. However, shorter GC capillary columns with small diameters will have a very poor tolerance to extracolumn dispersion. In the same way, short microbore LC columns packed with small... 

For convenience the Van Deemter equation for the LC column will be generally used in the following argument. However, when GC columns are considered the equations will be appropriately modified, where... 

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