Solid phase extraction (SPE) is a pre-treatment method to separate and purify samples by using the difference of the selective adsorption capacity of different components in samples. Solid phase extraction (SPE) is the most commonly used technology in food detection. Some common problems and solutions in SPE are listed below.
The recovery rate of the target compound is low (the target compound is not or partially adsorbed on the SPE column)
Reason 1: The SPE column is not well pretreated
Solution: Reverse column: Treat the column with methanol, isopropanol, or ethanol, and then treat the column with the solvent that dilutes the sample. Take care not to let the SPE column dry out.
Reason 2: Improper polarity of SPE column
Solution: Select the SPE column with obvious selectivity for the target compound.
Reason 3: The target compound has a much greater affinity for the sample solution than for the SPE column
Solution: Change polarity or pH of sample solution to reduce affinity of target compound in sample solution.
Reason 4: When a large volume of water sample passes through the SPE column, the reversed-phase column packing loses the methanol left by the column pretreatment
Solution: Add 1%-2% methanol or isopropanol or acetonitrile to the sample solution.
Low recovery rate of target compound (Target compound is not eluded on SPE column)
Reason 1: The polarity of the SPE column is improper
Solution: Choose other SPE columns with low polarity or weak selectivity.
Reason 2: The elution solvent is not strong enough to elute the target compound from the SPE column
Solution: Change the pH of the elution solvent to increase its affinity for the target compound.
Reason 3: The elution solvent volume is too small
Solution 1: Increase the solvent volume.
Reason 4: The target compound is irreversibly adsorbed on the SPE carrier
Solution 2: reverse phase: choose the carrier with weak hydrophobicity. If we used C18, we’ll change it to C8,C2,CN.
Cation exchange: Benzene sulfonic acid replaced by carboxylic acid.
Anion exchange: Replace tertiary amines with primary and secondary amines
Poor extraction reproducibility
Reason 1: the SPE column was dried before adding the sample
Solution: Re-perform SPE pretreatment.
Reason 2: The SPE column capacity exceeds the threshold
Solution: Reduce sample quantity or choose large capacity column.
Reason 3: The sample flow rate through the column is too fast
Solution: Reduce the flow rate.
Reason 4: The eluent flow rate was too fast
Solution: Permeate the column with eluent before applying external force. Two 5ml elutions may be more effective than one 10ml elution.
Reason 5: The solubility of the target compound in the sample was too large, so the sample passed through the column at the same time as the sample and was not retained
Solution: Change the solubility of the target compound by changing the sample polarity or pH.
Reason 6: SPE columns are treated with polar solvents and the eluents are incompatible with non-polar solvents
Solution: Dry the SPE column before using a non-polar solvent.
Reason 7: The solvent used to wash impurities is too strong, and part of the target compound and impurities are eluted from the SPE column at the same time. The amount of target compound lost in this step depends on the flow rate of the washing solvent, the characteristics of the SPE and the volume of the washing solvent.
Solution: Reduce the strength of the washing solvent.
Reason 8: The eluent volume is too small
Solution: increase the volume of eluent
When extracting with a reversed-phase SPE column, there is water in the eluted fraction
Reason: The SPE column was not dried well before the target compound was eluted
Solution: Dry the SPE column with nitrogen or air: Use 20~100μL containing 60%-90% methanol-water to remove the residual moisture on the SPE column.
The eluted fraction contains interferences
Reason 1: the interfering substance and the target compound are eluted at the same time
Solution 1: wash the interferents out of the SPE column with medium polarity solvent before eluting the target compound. Two or more solvents may be mixed to achieve different polarities.
Solution 2: select SPE column with higher affinity for target compounds and lower affinity for interferents.
Reason 2: interferents come from SPE column
Solution 1: Use two SPE columns with different polarities to remove interferents. Such as reversed-phase column and ion exchange column or silica gel column.
Solution 2: Wash the SPE column with eluting solvent before column pretreatment.
SPE column flow rate decreased or blocked
Reason 1: There are too many particles in the sample
Solution: Filter or centrifuge the sample.
Reason 2: The viscosity of the sample solution is too high
Solution: Dilute the sample with solvent.
Extraction of nonpolar target compounds from solid samples by reversed-phase column
Reason 1: The target compound is not in liquid solution
Solution: Homogenize the sample with methanol, isopropanol or ethanol. Then centrifuge and filter, and then dilute the clear solution with water to an aqueous solution with a water content of 70% – 90%.
Extraction of target compounds from solid samples by normal phase column
Reason: The target compound is not in liquid solution
Solution: use non-polar solvent (such as n-hexane, petroleum ether, chloroform, etc.) slurry.
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