Effect of solvent
The main reason for the prevalence of solvent effect in TCM samples is the solvent. Most TCM components need to be dissolved in pure organic phase or a high proportion of organic phase. However, in liquid phase analysis, due to the need for proper retention time, a high proportion of water is used, resulting in a large polarity difference between the mobile phase and the sample solvent. Some will appear liquid chromatography of different degrees of advance, thus affecting the detection. In this case, we can solve the problem by reducing the injection volume, using a larger pre-column volume instrument, or changing the testing conditions.
Example 1: Reducing the sample size can effectively avoid solvent effect
Project: Chinese Pharmacopoeia 2020 Edition 1 – Determination of content in cymbidium
Mobile phase: methanol-water =25-75
Reference solution: Take an appropriate amount of hesperidin reference solution and add methanol to make a reference solution containing hesperidin 0.1mg per 1mL.
The mobile phase was only 25% methanol, and the control was dissolved in pure methanol. The polarity difference between the two was large. When the sample was injected according to the standard requirement of 10μL, there was a forward delay phenomenon.
Example 2: Test with a large column front volume instrument
Item: Determination of content of Diyuhuaijiao pill, Part 1 of Chinese Pharmacopoeia 2020 Edition
Mobile phase: Methanol-acetonitrile -0.1% phosphoric acid =11-10-79
Reference solution: Take an appropriate amount of locacosin reference solution and add methanol to make a solution containing 25μg per 1mL.
Note: The sample solution was not fully mixed with the mobile phase, and the peak had obvious forward delay.
Note: The sample solution was thoroughly mixed with the mobile phase, and the forward phase phenomenon was eliminated.