Source Apportionment of Common Peak Variation in Gas Chromatography

　　In the daily quantitative analysis of chromatography，the appearance of chromatographic peak variation or ghost peak not only seriously affects the quantitative accuracy, but also makes the analysis unable to move on. For that reason, we conducted an analysis for common types (15 types) of peak shape variation, after which we concluded possible reasons for inexperienced chromatographic workers’ reference. The peak variant we discuss here is deviation or distortion of certain chromatographic peaks or superfluous peaks appeared when we are making comparisons between them and known chromatograms once recorded. Or, as to the chromatographic methods which have been already set up, abnormal peaks like that cannot been separated, trailing peaks or asymmetrical peaks, appeared when there is no requirements or out of alternatives, do not belong to the above peak variations caused by equipment failure, lack of experience or operational errors. Otherwise, the original analysis method needs to be re-examined or modified. In addition, it should be pointed out: abnormal peaks including trailing peaks, peaks that have not been well separated or peaks with deviation or distortion, which are caused by random installation or use of chromatographic columns without evaluation do not belong to above peak variations either. Obviously, it seems to be beyond a common chromatographic analysist’s power to analyze the strengths and weaknesses of chromatographic columns in normal working conditions. It is recommended to do the following work before you raise doubts about the appearance of peak variations, get down to search for possible causes and eliminate inapplicable methods.
　　
　　Check carefully to confirm whether the operation conditions are consistent with the analytical method requirements.
　　
　　Compared with the standard chromatograms stored after analysis to see if there is a real problem.
　　
　　Carefully observe the working state of instrument or equipment to see whether there is any peak failure caused by operational errors.
　　
　　Analyze the following 3 kinds of abnormal peak shapes, find the possible causes and methods to eliminate them.
　　
　　1.Step peak:
　　
　　(1) TCD hot wire is corroded by elements such as halogen, oxygen and sulfur in the sample;
　　
　　(2) sudden change of gas flow, such as sudden leakage of injection pad, obstruction of gas path, etc.
　　
　　(3) record chromatographic peak device failure, such as the looseness of the bracing wire.
　　
　　2. Negative peak:
　　
　　(1) TCD use nitrogen (N2) as carrier gas. When the concentration of the component in nitrogen(N2) is different and the heat conduction value is nonlinear, the negative peak may appear, which sometimes can be overcome by changing the gas flow rate or the injection volume.
　　
　　(2) When ECD is operated, the negative peak occurs due to the large injection volume. This is because that the sensitivity will be greatly reduced after the transformation of the working principle from electron capture to ionization detection.
　　
　　(3) When FID is operated, solvents of low ionization efficiency, such as CS2, or impurities can reduce the output baseline with a relatively high primordial flow, ending up showing a negative peak.
　　
　　(4)When FID is operated, the negative peak will occur if the sample is not polarized.
　　
　　(5) When the NPD and FPD are operated, the solvent or some components will show a negative peak if the air flow ratio is not suitable.
　　
　　3. "N" or "W" peak:
　　
　　(1)When TCD is operated, with nitrogen used as carrier gas, the “N”or "W" peak will appear due to the non-linearity of thermal conductivity.
　　
　　(2)When FID is operated, the ionization efficiency of the sample solvent is low, such as CS2, or the air flow ratio is not good enough .
　　
　　(3) In ECD operation, the detector is polluted, the content of solvent peak or the components to be tested is higher, or pulse power supply is defective.