Stationary-phase bleed

In high temperature (HT)-GC-MS and PyGC-MS experiments, special attention should be given to the stability of the column. GC columns can lose some of the stationary phase ( bleeding ) when heated up to the maximum operating temperature the thicker the stationary phase, the more column bleed may be expected. When coupled on-line to a mass spectrometer, the stationary phase may foul the ion source, which leads to rapid decay in sensitivity and detection of usually siloxane-related mass peaks at m/z 207, 281, 355, etc. HTGC-MS coupling was discussed by ref. [218]. 

Immobilized Chirasil-Dex phases are resistant to stationary phase bleeding, compatible with solvent input and are insensitive to temperature shock. The immobilization of Chirasil-Dex was a prerequisite for extending the scope of enantiomer separation to involatile racemates using supercritical fluid chromatography130 and was utilized in the separation of 7-chloro-2,3,4,5-te-trahydro-l-methyl-5-phenyl-l, 4-benzodiazepin-2(l//)-one (dihydrodiazepam) (Figure 17). 

Capillary columns, to be suitable to HT-HRGC, must be extremely robust and must be coated with a thin film of the stationary phase with the purpose of reducing the retention of the less volatile compounds and preventing stationary-phase bleed at high temperatures [7]. 

In general, the nature of the analyte determines the choice of stationary phase. For example, for the separation of organochlorine and pyrethroid pesticides, a nonpolar stationary phase such as DB-1 (or OV-1) is recommended. For the separation of somewhat more polar compounds, such as organophosphorus compounds, OV-17 (or DB-1701) can be applied. In addition, for confirmation purposes, the use of two columns with distinct stationary-phase polarities (e.g., DB-1 and DB-1701) is certainly required. A polar stationary phase (e.g., DB-wax) is suitable for the more polar compounds such as methamidofos, but its application to some detection modes is limited due to stationary-phase bleeding. 

The use of highly effective capillary columns is essential for high-resolution Pyr-GC-MS, because pyrolysis products of polymers are generally very complex mixtures. The bonded-phase fused-silica columns are especially effective for Pyr-GC-MS because of their low level of stationary-phase bleed at elevated column temperatures. 

Noise present in the detector signal may have two components, long-term noise and short-term noise. The former causes a slow baseline wander measured over a 1 h period and may be attributed to fluctuations in temperature, column stationary phase bleed, flow rate variation, or pneumatic leaks. Short-term noise is observed as small, sharp spikes of shorter duration than component peaks and usually arises in the detector. Most integrators smooth the signal so that noise is not apparent unless a direct plot mode is selected. It is important to establish the mean noise level, the baseline, in order to determine the limit of detection. The time period of a peak is most conveniently described by the peak width at half height and the noise, N, is measured as the variation between maxima and minima of the noise peaks over the time period. The contribution of noise to the total component signal should be less than 1% (Figure 5.17). 

The aim of baseline correction is to separate the analyte signal of interest from signal which arises due to changes in mobile phase composition or stationary phase bleed and signal due to electronic noise. Several baseline correction methods have been proposed in literature. 

The ECD is one of the most sensitive of GC detectors, especially if argon replaces nitrogen as the carrier gas, but traces of air, oxygen or water in the gas, liquid stationary phase bleeding from the colunm or residues from halogenated solvents used in sample preparation are detrimental to its performance, and its linear range is very limited. Quantitative analysis is difficiilt because response is dependent on solute structure. 

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