Choosing a suitable SPE cartridge generally considers two aspects: the choice of adsorbent and cartridges specifications. Appropriate adsorptive packing can make the interfering substance and the target component get good separation and meet the recovery requirements. However, the quality of the compound that the adsorbent can adsorb is limited. Once the sample volume is too large, “overload” will occur, making the target “penetrate” the SPE cartridges without being retained.
Choose according to the nature of the substrate: The adsorbent can be selected mainly according to the properties of the target compound and the properties of the matrix. The choice of adsorbent can be roughly selected in the above two ways. For specific examples, the choice can be made based on the principle of “distinguishing the target compound from the main interferer”.
After selecting the appropriate adsorbent, it is also necessary to select the appropriate SPE cartridge specifications according to the sample volume required for the verification. Because the quality of the compound that the adsorbent of the extraction cartridges can absorb is limited, once this value is exceeded, the target substance cannot be effectively retained, which greatly reduces the elution effect. Therefore, it is necessary to select the appropriate cartridge specifications according to the number of components in the sample.
Regarding the common problems in the use of extraction cartridges, there are generally low recovery rates, poor reproducibility, poor purification results, and flow rates that are too fast or too slow. We will explain them one by one below.
The low recovery rate of SPE cartridges
Reason analysis: ① The target substance is insufficiently retained on the packing. Judgment method: add standard to the blank solvent, and recover the sample and eluent. If the actual detection concentration exceeds 5% of the content, the retention is insufficient. If the SPE cartridges are not suitable, choose stronger retention SPE cartridges.
If the elution strength of the sample loading solution or eluent is too strong, the elution strength of the sample loading solution or eluent will be reduced. The loading volume/concentration is too high, causing overload. Reduce the loading concentration or increase the amount of filler. In the process of operation, if the cartridge is “dried up”, the activation/equilibration must be performed again to ensure that the cartridge will not be “dried up” before the end of elution.
② The target elution is not complete. Judgment method: add standard to blank solvent, recover sample liquid and eluent, but no target is detected; but recover eluent, the recovery rate is not high.
If the small cartridge retention is too strong, select SPE cartridges with slightly weaker retention. If the elution capacity is too strong, increase the elution intensity. If the elution volume is too small, increase the elution volume.
③ Other reasons. Judgment method: add standard to the blank solvent, recover the sample solution, the eluent does not detect the target; recover the eluent, the recovery rate is normal.
Insufficient extraction of pretreatment requires changing the extraction solvent. The target substance is unstable, volatilized, or decomposed, change the extraction and transfer method, reduce the heating temperature, and the compound that is greatly affected by the pH can be added to the buffer salt to adjust the pH. The complexity of the pretreatment process requires simplification of the pretreatment process.
If the target compound is combined with impurities, change the appropriate method to remove the impurities. The interference effect of impurities is too much, change the pretreatment method, or compare with the matrix spiked substance. Too many impurities, exceeding the maximum retention of the small cartridges, change the pretreatment method, reduce the loading amount, or increase the amount of the small cartridges packing.
Reasons and countermeasures of too fast or too slow flow.
Causes of slow flow
If the particle size selected for SPE is too small, select SPE with large particle size. After the sample is pretreated, there are still suspended insoluble, and freeze and centrifuge before loading. The viscosity of the sample is too high and the sample is diluted. The SPE resistance is too large, and the liquid cannot drop by gravity. A manual solid-phase extraction cartridge device should be used to adjust the flow rate by adjusting the check valve, the valve, and the negative pressure.
Reasons for too fast flow rate
If the particle size selected for SPE is too large, select SPE with a small particle size. The flow rate is still very fast under the action of gravity. Use a manual SPE device to adjust the check valve. When pumping negative pressure, the flow rate is too fast, adjust the check valve and valve, and control the pressure of the vacuum gauge to be constant within a certain range.