4) The solvent is added in the wrong order. For reversed-phase cartridges, if the water is accidentally used as the activating solvent, a slow flow rate will occur. Because water has poor wettability to hydrophobic reversed-phase packings, water is accidentally activated as methanol and it can’t flow down. Be careful not to make mistakes.
5) The activation and equilibrium solvents are not miscible, Such as methylene chloride activation, water balance column. We will find that after the water is added, the flow rate is super slow. This is due to the poor mutual solubility of water and dichloromethane, which can be solved by adding the transition solvent methanol, which can be miscible with methylene chloride and water.
6) Air enters the packing. This reason is more common but also difficult to find. SPE columns are composed of sieve plates, packings, and column tubes, and are not organic. Therefore, during transportation, especially long-distance transportation, slight looseness will occur, resulting in Air entering the packing is generally not easily detected by the naked eye. Air-containing packings cause the activation solvent to flow down very slowly under the influence of air pressure. Speaking of this, you may easily understand why the small batches of the same batch may have a slight uneven flow rate. You can use the SPE device to add positive pressure to remove the air and add the solvent together.
Due to the complexity of the sample matrix, particles need to be filtered before loading. For example, protein-containing samples need to be treated with acid, salt, organic solvent, and heating. It should be processed by filtration, centrifugation or high-speed centrifugation, and changing the large-pore size packing. Now many third-party testing units have a lot of samples, but the pre-processing steps should not be saved, otherwise, the instrument will be damaged and influence data reliability.
Rinse and elute
Generally, the flow rate seldom appears in the two steps of rinsing and elution, as they are pure solvents without a matrix. Generally, the loading step is not blocked, which will not affect the rinsing and elution. The slow flow rate may occur after the cartridge is rinsed and dried because once the cartridge is dried, air will enter the packing, which returns to the sixth cause of the slow flow rate caused by the activation equilibrium, which can be solved by pressing it slightly.
Of course, when we are doing experiments, not every step is required to be fast. In the small column operation technique, there are two steps that need to control the flow rate, which is loading and elution; these are the process of adsorption and desorption of the target and the column packing by intermolecular forces, and it takes time to act.