There are a wide variety of sample substrates, such as vegetables, fruits, meat, eggs, milk, water, soil, rubber, textiles, etc., whether it is food or environmental samples, the degree of complexity is also different. Simple extraction can be used as a pre-processing method for samples with general complexity, but special pre-processing is required for particularly complex samples, such as using SPE cartridges for further purification to separate the target and impurities, and then using the chromatographic instrument.
When using the SPE pre-processing, the most annoying and maddening problems encountered by the analysis and testing engineers are whether the flow rate is slow, uneven flow rate, blocked, or unable to go down. How to solve it? Hawach would like to talk about it and also welcome to send Hawach your valuable suggestions.
The basic operation of SPE includes five steps: activation, equilibration, sample loading, rinsing, and elution. The problem is nothing more than these steps. Generally, reverse-phase or ion-exchange cartridges use methanol and water for activation and equilibrium, respectively, while normal-phase cartridges use petroleum ether or n-hexane to activate equilibrium.
1) The packing is too tight and the flow rate is too slow
You can increase the flow rate by pressurizing, or you can purchase products from manufacturers that have strict quality control on the flow rate. The Hawach experiment has a strict quality control range for the flow rate of each batch of SPE cartridges and cartridges outside the quality control range will be rejected.
2) The particle size of the filler is too small, resulting in a slower flow rate
For example, under the same activation solvent, a florist of 100-200 mesh will be slower than that of 60-100 mesh. You can select the appropriate size of the small cartridge according to the sample matrix type and usage habits.
3) Difference in cartridge tube volume
500mg 3mL; 500mg 6mL, the height of the filler formed by these two specifications is different. Obviously, 500mg 6mL has a large diameter, a low filler height, and a relatively fast flow rate. Just like the morning peak of a subway station, there are many gates, fast passing speed, and few gates. Naturally, it is congested, with a slow passing rate. The appropriate column can be selected according to the sample matrix type and usage habits.
4) Wrong order of the added solvent
For reversed-phase small columns, if the water is accidentally used as the activation solvent, the flow rate will be very slow. Because water has poor wettability for hydrophobic reversed-phase packing, it is accidentally activated by water as methanol.
5) The activation and equilibrium solvents are immiscible
For example, if methylene chloride is activated and the water balances the small column, everyone will find that after water is added, the flow rate will be slow enough to make you crash. This is caused by the poor miscibility of water and dichloromethane, which can be solved by adding the transition solvent methanol, which can be miscible with dichloromethane or water.
6) Air enters the packing
This reason is relatively common but also difficult to find. SPE cartridges are composed of sieve plates, fillers, and column tubes. They are not an organic whole. Therefore, during transportation, especially long-distance transportation, there will be slight loosening, leading to air entering the filler is generally not easily detected by the naked eye. The air-containing filler causes the activation solvent to flow slowly under the action of air pressure. Speaking of this, you may easily understand why the same batch of small columns may have a slight flow rate irregularity. You can use the SPE device to add positive pressure to pump out the air, and then add the solvent together.