Many samples are anomalous when analyzed. The most common is the analysis of solvent samples. The special case is the analysis of water samples. From the point of view of gas chromatography. It is known that water is not an ideal solvent, mainly due to the following reasons: 1 it has a large evaporation expansion volume; 2 water wettability and solubility are poor in many stationary phases; 3 water will affect a certain The normal detection of these detectors can cause chemical damage to the stationary phase of the column. In the usual chromatographic solvents, water has the largest vaporized expansion volume, see table.
Usually, the volume of the liner of the chromatograph injector is about 200-900 μl. When 1 μl of water is used, the vapor volume after vaporization (about 1010 μl) will expand and overflow the liner, which is called back pouring. It will cause the vaporized sample to return to the carrier gas and the purge gas path. Since the temperature of the purge gas path of the carrier gas is much lower than that of the gasification chamber, the sample will condense here and be blown into the analysis system by gas in a later analysis. Form a ghost peak.
Avoid methods that increase liner volume, reduce injection volume, reduce injector temperature, increase injector pressure, and increase carrier gas flow rate to reduce backflow.
Water enters the column and the form of the water is destructive to the stationary phase of the column. Because the surface energy of water is very high, and the surface energy of most of the capillary column stationary phase is low, the water has poor wettability to the stationary phase, and a smooth solvent film cannot be formed on the column wall to uniformly flow through the chromatogram. Columns, which form droplets, cause column performance to deteriorate. Due to this poor wettability of water and the higher boiling point of other solvents, usually at a lower column temperature, a part of the water flows through the column in a liquid state, so that the solute with good solubility in water is also Will exhibit band broadening, in extreme cases, showing chromatographic peak splitting.
When injected on a column, non-volatile compounds, such as water-soluble salts, are also carried into the column by liquid water, contaminating the column and the analytical system.
Water can also cause detector problems: for example, water can cause FID and FPD to extinguish fires; when entering larger water samples, in order to avoid fire extinguishing of the detector, the hydrogen flow can be increased to help stabilize the flame at the expense of sensitivity; To reduce the sensitivity of the ECD, to avoid the effects of water, a thick liquid membrane column can be used to keep the analyzed components long enough to ensure that the ECD performance may be restored after the water flows through the detector.
A more serious problem is that water can cause degradation of many stationary phases, directly damaging the performance of the column. In the chromatographic analysis, the chromatographic peak separation performance is degraded, the base flow is unstable, and the noise is increased.
Therefore, care must be taken when entering the water sample and samples with large water content. This also occurs in the case of solvent analysis. Typically, the analysis of trace organic extracts, whether using dichloromethane or carbon disulfide as a solvent, when the injection of 1 μl, the volume expansion is about 3001 μl, when the injection cannula volume is less than 300 μl, it is easy to form a back irrigation. Therefore, no matter what sample, the amount of injection must be compatible with the volume of the cannula in the injector. In this respect, various types of instruments are equipped with a variety of different types of injection cannulas for selection; A large amount of solvent also forms a washing action on the stationary phase, directly destroying the performance of the column. In the chromatographic analysis, it reflects the retention time advance, the chromatographic peak separation performance decreases, the base flow is unstable, and the noise increases. Therefore, the choice of solvent and injection volume must be taken into account when analyzing dilute solution samples.