Solid Phase Extraction Frequently Asked Questions and Answers(part 3)
Recyclability is poorly reproducible
Before the sample is loaded, the bed is dry and reactivated and balanced.
The adsorbent is loosely packed, and the solution is short-circuited. Replace the product, or squeeze the bed with a small stick.
The sample fluid flow rate is too fast. The flow rate is reduced. Generally, when the flow rate is lower than 5 mL/min, the result is stable.
purification effect is not ideal
A purification mode improvement
In general, the mode of retaining the target compound is better than the mode of retaining impurities. If the item being analyzed is analyzed for a certain compound or a certain type of compound, the purification mode of the target compound is retained; for a simple example, the same It is the analysis of carbendazim and thiabendazole (a class of alkaline pesticides) in vegetables. The cation exchange column can be used to specifically retain these pesticides, so that they are completely separated from the interferents, and the normal phase adsorbent or graphite is used. The operation of retaining the interference by the carbon black can only remove the main interferent, and there are still many interferents remaining.
In addition, if more than one SPE column can be used for the purification of the target compound, a selective adsorbent should be selected; in terms of selectivity, ion exchange > normal phase > reverse phase.
Optimization of B eluent and eluent
For the reversed phase mode and the normal phase mode, the elution strength of the eluent should be increased and the elution strength of the eluent should be reduced without affecting the recovery rate.
For the reverse phase mode, the aqueous solution of the target compound can be loaded, and then 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, aqueous methanol solution is sequentially used. Elution, separately collect the eluent analysis, establish a elution curve, and find the solvent system when the target compound is eluted. The methanol content in the eluent should be slightly lower than the solvent system when the target compound is just eluted. The content of methanol 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 which are neither soluble in water nor soluble in methanol, an acid or a base may be added to the solvent system.
For a normal phase system, the target compound can be loaded with a solution of n-hexane, followed by 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, The solution was eluted with n-hexane methyl tert-butyl ether solution, and the eluent was collected for analysis. The elution curve was established to find out the solvent system when the target compound was eluted. The content of methyl tert-butyl ether in the eluent should be slightly Below the solvent system when the target compound is just eluted, the methyl tert-butyl ether content in the eluent should be slightly higher than the solvent system when the target compound is completely eluted.
C Reasonable handling of SPE columns
There may be a small amount of interfering substances in the adsorbent in the SPE column. Effective activation can prevent these interfering substances from entering the eluent. The activation solution should select a solvent system with strong elution intensity throughout the operation; in the reverse phase mode, from the sample solution to The eluent mainly involves 0%-methanol aqueous solution, so the activation solution should be pure methanol. When the target compound needs to be eluted with methanol-methyl tert-butyl ether mixture, the activation solution should be selected as methyl tert-butyl ether; In the reversed-phase mode, the 0%-n-hexane-methyl tert-butyl ether solution is mainly involved from the sample solution to the eluent, so the activation solution should be selected as pure methyl tert-butyl ether.
Flow rate is too low
The sample solution contains more particulate impurities, blocking the sieve plate. Centrifuge or filter the sample solution before loading.
Sample solution viscosity is too high Dilute the sample with a suitable solvent
The SPE column bed is too high and the resistance to liquid is too high. Use a solid phase extraction instrument and decompress the liquid outlet.
Pressurize at the liquid inlet