How to get higher sensitivity in gas chromatography

　　(1) Solvent focusing can achieve good sensitivity without sacrificing resolution;

　　(2) Select a suitable chromatographic column. Gas chromatography is often used to detect target compounds with very low content, such as drugs of abuse as environmental pollutants.

　　This is a major challenge for gas chromatographers. Here we provide two methods to solve this problem.

　　1. Solvent Concentration Method

　　The purpose of solvent concentration is to obtain sharp peaks without using split injection, because only in this way can good sensitivity and good separation be obtained.

　　Solvent concentration is produced by two methods.

　　The first is when the liquid sample evaporates during splitless injection and then cools and condenses on the cooler chromatographic column. Because the volume of the carrier gas is much larger than that of the liquid, when the sample is cooled and condensed, it is concentrated in a small area of ​​the chromatographic column.

　　The second method is used in splitless injection and direct injection. This method requires adjusting the column oven. The difference from the former is that the solvent and the analytical compound must be cooled and condensed on the cooler chromatographic column at the same time during injection. These condensed samples form a "sample flooded zone" on the surface of the chromatographic column. As the solvent gradually evaporates, the area of ​​the sample flooded zone gradually shrinks, and the concentration of the analyte in the "sample flooded zone" gradually increases. When the solvent is completely evaporated, the analyte condenses on a very small area of ​​the column surface.

　　So no matter which method is used, the analyte or the solvent and the analyte must condense on a small area of ​​the column.

　　Factors that affect the rate at which the sample condenses on the column include:

　　The initial column temperature of the chromatographic column, the volatility of the solvent, and the proportion of the stationary phase of the chromatographic column.

　　This document discusses in detail the effect of each factor on solvent concentration and how to change the initial column temperature to obtain sharp peaks.

　　Here is a summary of the key points of "solvent concentration":

　　The initial temperature of the chromatographic column is basically the easiest and fastest method to achieve. Generally speaking, the lower the initial temperature, the better the analyte condenses. It is recommended that the initial temperature be set about 50 degrees Celsius lower than the boiling point of the fastest analyte, and the time this temperature is maintained is preferably the non-divided holding time.

　　The second important factor is the "Phase Ratio" of the chromatographic column. The lower the stationary phase ratio, the thicker the film thickness. The thicker the film, the easier it is for the sample and solvent to dissolve in the stationary phase. To change this factor, you can only try different columns. This is a method used when changing the initial temperature is not possible. (The next gas phase lecture series will specifically introduce the "Phase Ratio")

　　The last factor that can be changed is the sample itself. Of course, most of the time, the sample itself cannot be changed. However, we can use different solvents to increase the effect. For example, the greater the difference between the boiling point and the initial temperature of the solvent, the better. For example, when the initial temperature of the column is 30 degrees Celsius, the sample solvent used is ethyl acetate (boiling point 77.1 degrees Celsius) rather than dichloromethane (boiling point 39 degrees Celsius).

　　2. Choose the right gas chromatography column

　　When a chromatographic method needs to achieve low detection limits and high sensitivity, we must choose low-bleed columns and inert procedures. Column bleed refers to some low-molecular-weight stationary phase fragments (mass spectrometer ions 355, 281, 207, 73). These low-molecular-weight fragments may be due to impurities in the stationary phase or the stationary phase cracking at high temperatures. The most obvious chromatographic manifestation is the gradual upward drift of the baseline.

　　In addition, the molten silica surface activity of the chromatographic column is too strong and needs to be deactivated during production to avoid secondary reactions between the analyte and the molten silica, which can cause peak tailing or sample adsorption (partial sample loss).

　　In order to avoid these two problems, Phenomenex provides a self-crosslinking (Engineering Self-Cross-Linking ESC) technology. The gas chromatography column obtained by this special technology not only has very low column bleed, but also greatly reduces secondary reactions. Some representative columns of this type include ZB-1ms, ZB-5MSi, ZB-5ms, etc.