The factors affecting retention time drift are not only mobile phase

When we do liquid chromatography, we will encounter some problems, such as peak retention time drift. This is a headache. There are many factors causing the peak retention time drift, such as whether the mobile phase is fully balanced, whether the choice of mobile phase system and stationary phase is appropriate, whether there is solvent effect, etc. How to solve when these effects are eliminated and the problem still exists?

A Case study:

Citric acid compounds are easily soluble in water. Using the acid system as the mobile phase on the reversed-phase C18 column, the peak retention and peak type are in good condition, but the peak retention time moves forward with the increase of injection times.

As shown in Figure 1:

1

The phenomenon still exists when the mobile phase is adjusted and the chromatographic column is replaced
As shown in Figure 2:

2

Adjust the mobile phase and replace the chromatographic column. The problem still exists, indicating that the phenomenon is not caused by the mobile phase system or chromatographic column. It is preliminarily inferred that the problem is from the sample itself.

According to the analysis of the structure of the substance, it is an acidic compound. When water is used as its sample dissolving solvent, its molecular state cannot be well maintained, because the pH value of water is neutral or even weakly alkaline. In such an environment, the chromophore in the compound is affected by other parts of its connected molecules and the solvent, which makes its absorption peak position move, resulting in the unstable state of the sample after dissolution, Therefore, we need to improve its solution stability.

Using mobile phase (pH3.0) as sample dissolving solvent, the problem of peak retention time drift is improved, but the effect is not ideal.

As shown in Figure 3:

3

Use phosphoric acid water (ph2.0) as sample dissolving solvent,
The problem of peak retention time drift is solved.

As shown in Figure 4:

4

Conclusion

1. The influence factors of peak retention time are not only the mobile phase and the stationary phase. When these external influence factors are excluded, there is still the problem of RT drift, which can be considered from the perspective of the sample itself.
   
2. The state of the dissolved sample is related to the retention time of the sample, the stability of the peak area, and whether the peak shape is good. When choosing a solvent, it is necessary to fully consider these factors.

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