How to Improve Unsatisfactory Purification Effect of Solid Phase Extraction?

Solid phase extraction is a sample pretreatment method based on chromatographic separation, and it is a relatively mature sample pretreatment method that has been used in recent years. Today, HAWACH will discuss how to improve the unsatisfactory purification effect of solid phase extraction. It is recommended to consider the following points.

1. Improvement of purification mode
Generally speaking, the mode of retaining the target compound is better than the mode of retaining the impurities. If the item being analyzed is a certain type of compound analysis, it is better to use the purification mode of retaining the target compound. For example, for the same analysis of carbendazim and thiabendazole (a type of alkaline pesticide) in vegetables, the use of cation exchange columns can specifically retain these pesticides, so that they are basically completely separated from interferences, and using normal phase adsorbent or graphitized carbon black to retain interferences can only remove the main interferences, and there are still many interferences remaining.

In addition, if more than one SPE cartridge can be used for the purification of the target compound, an adsorbent with good selectivity should be selected in terms of selectivity: ion exchange > normal phase > reversed phase.

2. Optimization of leacheate and eluent
For reversed-phase mode and normal-phase mode, the elution intensity of the leacheate should be increased and the elution intensity of the eluent should be reduced without affecting the recovery rate.

For reversed-phase mode, the target compound can be loaded in an aqueous solution, and elute with 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100% methanol aqueous solution in sequence, collect the eluate and analyze separately, establish the elution curve, find out the solvent system when the target compound is eluted.

The content of methanol in the eluate should be slightly lower than the solvent system when the target compound is just eluted, and the methanol content in the eluent should be slightly higher than the solvent system when the target compound is completely eluted. When pure methanol cannot elute the target compound, the elution curve of the methanol-methyl tert-butyl ether mixed solution should be tested: For some ionic compounds that are neither soluble in water nor soluble in methanol, acid or base can be added to the solvent system.

For normal phase systems, load the target compound’s n-hexane solution, then use 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100% n-hexane methyl tert-butyl ether solution for elution in sequence, separately collect the eluate for analysis, establish an elution curve, and find out the solvent system when the target compound is eluted. The content of methyl tert-butyl ether in the leacheate should be slightly lower than the solvent system when the target compound is just eluted, and the content of methyl tert-butyl ether in the eluent should be slightly higher than the solvent system when the target compound is just completely eluted.

3. Properly treat the SPE cartridge
The adsorbent in the SPE cartridge may contain a small amount of interferences. Effective activation can prevent these interferences from entering the eluent. The activation solution should select a solvent system with strong elution strength throughout the operation; in reversed phase mode, it mainly involves 0%-100% methanol aqueous solution from sample solution to eluent, so the activation solution should be pure methanol. When the target compound needs to be eluted with a methanol-methyl tert-butyl ether mixture, the activation solution should be methyl tert-butyl ether.

In the normal phase mode, it mainly involves 0%-100% n-hexane-methyl tert-butyl ether solution from the sample solution to the eluent, so the activation solution should be pure methyl tert-butyl ether.