Application
News
No. G302A
Gas Chromatography
An Examination of the Effects of High/Low Column
Cooling Rates Using Nexis
TM
GC-2030
LAAN-A-GC-E067A
Gas chromatography is a technique for separating
components within a sample by utilizing the adsorption and
desorption of components from a column when it is heated.
The adsorption and desorption differ according to the
column's liquid phase type. After analysis, the column must
be cooled down to the initial temperature, but the rate at
which the column is cooled may affect the column's liquid
phase. Until now, priority was placed on shortening cooling
times in order to shorten analysis cycles. However, the Nexis
GC-2030 now allows operators to select a column cooling rate
of their choice. By selecting the optimum cooling rate, the life
time of the column can be maximized.
This article compares the baseline noise and the S/N ratio of
peaks according to different cooling rates when using the
Nexis GC-2030 cooling program.
A. Hashimoto
Column Liquid Phase
Fig. 1 shows the liquid phase model of a column coated with
methyl phenyl polysiloxane. Each of the individual figures
show the liquid phase structure in differing states: Fig. 1-1 at
a high temperature, Fig. 1-2 at a low temperature, and Fig. 1-3
after rapid cooling.
At a high temperature, molecular mobility is increased and
the liquid phase expands whereas at a low temperature the
molecular mobility is suppressed and the liquid phase
contracts. If the temperature change from high to low is
gradual, the liquid phase expands and contracts without
damaging its structure. However, if the column oven is cooled
rapidly, the liquid phase is damaged as shown in Fig. 1-3 in
which side chains with a large molecular structure in the
liquid phase polymer are cut off.
The following sections study the affects of this damage on
analysis results with respect to the stability and noise of the
baseline and S/N ratios.
Baselines
Table 1 lists the gas chromatography (GC) analytical
conditions used in this study and Fig. 2 shows the enlarged
chromatogram of nine compounds. Using methanol as a
solvent, a sample was prepared so that it contains the nine
compounds indicated in Fig. 2 each at a concentration of
1 vol%. This sample solution was analyzed continuously
100 times using the analytical conditions listed in Table 1
with differing column cooling rates. Fig. 3 shows an enlarged
view of the baseline of the 100th analysis with a low cooling
rate (red line) and that with a high cooling rate (black line).
Looking at Fig. 3, we can see that the noise of the baseline is
greater with the high cooling rate compared to that with the
low cooling rate. This is because side chains in the column's
liquid phase are cut off when using a high cooling rate and
those chains are detected when the column is heated the
next time around.
This result indicates that the cooling rate affects both the
liquid phase of the column and the analysis results.
Table 1 GC Analytical Conditions
Model : Nexis GC-2030/AOC-20i
Injection Mode : Split mode
Injection Volume : 1.0 μL
Split Ratio : 1:50
Injection Temp. : 250 °C
Carrier Gas : He
Carrier Gas Control : Constant linear velocity (30 cm/sec)
Column : SH-50 (30 m × 0.32 mm I.D., 1.00 μm)
*1
Column Temp. : 40 °C (0 min.) - 4 °C/min. - 280 °C (0 min.)
Detector : FID
Detector Temp. : 320 °C
Detector Gas : He 32.0 mL/min, Air 200 mL/min
Makeup Gas : He (24 mL/min)
*1 P/N: 227-36167-01
Fig. 1 Model Diagrams of the Column Liquid Phase
Fig. 1-1 Model of Liquid Phase Structure at High Temperature
Fig. 1-2 Model of Liquid Phase Structure at Low Temperature
Fig. 1-3 Model of Liquid Phase Structure After Rapid Cooling
