The reason of high blank value in gas chromatography
1. The effect of silica gel pad on the injection port
① The quality of the injection needle is not good, the edge is not smooth enough, and the silica gel pad is easy to be damaged.
② The silica gel pad is used too many times or aged for a long time, the quality of silica gel pad is poor, the material elasticity is poor, and the silica gel pad debris enters the vaporization chamber.
③ The silica gel pad is polluted. For example, the sample drops attached to the needle are absorbed by the silica gel pad during sample injection, and then enter the chromatographic column after a little desorption in the subsequent analysis process.
02. Contamination of sample inlet, detector or liner and diverter plate
① No regular maintenance of chromatograph for a long time;
② Some sediments and substances with high boiling point are often accumulated in the vaporization chamber and liner. If encountering some substances with high boiling point for analysis, when the temperature of the sample inlet rises, the accumulated substances will escape, resulting in a high blank value;
③ The nonvolatile material in the groove of the detector and the shunt plate will slowly accumulate, resulting in a high blank value.
03. Influence of chromatographic column
① The chromatographic column is not fully aged, the column temperature is too low and the aging is not sufficient, and the liquid phase is lost due to too high temperature.
② The chromatographic column is used for a long time. The column has its own adsorption characteristics, high concentration samples or high boiling point substances remain in the column, and the column head is polluted. Moreover, the capillary column needs a high sensitivity detector because of its low load, which is easy to cause a high blank value.
04. Reagent pollution
The reagent is contaminated or impure in the process of pretreatment before injection.
It is worth noting that the laboratory passing the metrological certification, due to the regular verification of testing instruments and glass measuring instruments, as well as the adoption of comprehensive quality management measures, if the measured blank experimental value is too large, it is generally due to the lack of chemical purity, deterioration or contamination of the prepared test solution and other reasons. The test solution shall be reconstituted and the batch of samples shall be retested after rework until the blank test value is qualified.