PerkinElmer TORION T-9, Руководство пользователя

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32 . Torion T-9 User’s Guide
What is low thermal mass (LTM) gas chromatography?
In microcolumn gas chromatography (GC), volatile chemicals separate as they travel through a
capillary column. Separation is dependent upon the equilibrium distribution of each chemical
between a liquid coating on the capillary wall and the carrier gas flowing through the capillary. An
equilibrium constant called the partition coefficient, K, governs the separation of chemicals and is a
ratio of the concentration of the target chemical in the stationary phase coating versus the mobile
phase (carrier gas). The affinity of a chemical for the stationary phase is a function of temperature
and determines the chemical’s partitioning between the two phases.
phase stationary in volume
phase mobile in volume
phase mobile in mass
phase stationary in mass
phase mobile in ion concentrat
phase stationary in ion concentrat
K
× = =
The partition coefficient of a chemical is related to its retention time, t
R
. The retention time is the
time a chemical takes to travel through the column. The typical representation of a GC analysis is a
plot of detector response as a function of time, called a chromatogram (see below). The time at the
center of the near Gaussian peak of a resolved chemical is its retention time, and the area under the
peak is related to the amount of chemical extracted and may be corresponded to the amount of
chemical in the sample.
A chemical that is volatile within the temperature range of the GC method parameters will eventually
exit the column (assuming the compound is not irreversibly adsorbed to the capillary coating). If the
same method parameters are always used, a specific chemical will have the same retention time.
Tentative identification of a chemical can be made by comparing its retention time with retention
times of known standards. Changing the temperature of the capillary column will change the
retention times of compounds being analyzed. Usually, the temperature of the column is increased
during an analysis to reduce the time of the analysis. This is typically done with the capillary column
installed in a convectively heated GC oven.
The degree of separation of two chemicals that exit the capillary column at the detector is called
resolution. Two chromatographic peaks are completely resolved when the baseline signal is present
between them; otherwise, they are only partially resolved.
In chromatographic systems with detectors like flame ionization that does not produce selective data
retention time and peak separation are critical to successful peak identification and quantitation.
Peaks with similar retention times may be resolved by reducing their peak widths or by changing
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3
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0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Time (min)
2.0
1. Dimethyl methyl phosphonate (DMMP)
2. 2-Chloroethyl ethyl sulfide (1/2 HD)
3. Diethylmalonate (DEM)
4. Methyl salicylate (MES)
5. 2-Chloroacetophenone (CN)
t
R