How to Solve the Problem of Slow Flow Rate of SPE Cartridges

Solid Phase Extraction (SPE) technology uses the poor ability of analytes to be adsorbed in different packing to purify the target, effectively separating the target from interferences, and greatly enhancing the analysis of analytes, especially trace analysis. The detection ability of the substance improves the recovery rate of the tested sample.

When using SPE pre-treatment, every analysis and testing engineer encounters an annoying and unpleasant problem: slow flow rate, uneven flow rate, blockage, and inability to get down; too small column would cause a collapse. With years of deep accumulation in the field of sample preparation, HAWACH shows you a few tricks to treat the problem of the slow flow of small columns.

Five steps
The basic operation of SPE includes five steps: activation, equilibration, loading, rinsing, and elution. The problem is nothing more than these steps and we’ll look at it one by one.

Activation and equilibrium
Generally, reversed-phase or ion-exchange cartridges use methanol and water to activate and equilibrate. Normal-phase cartridges use petroleum ether or n-hexane to activate equilibrium.

1) The packing is too tight resulting in too slow flow rate. You can speed up the flow rate by pressurizing, or purchase products from manufacturers that have strict quality control on the flow rate. HAWACH has strict quality control ranges for the flow rate of each batch of cartridges and unqualified ones will be eliminated.

2) The particle size of the packing is too small and the flow rate becomes slow. For example, florisil at 100-200 meshes will be slower than 60-100 meshes at the same activating solvent. You can select the appropriate size of the cartridge according to the type of sample matrix and usage habits.

3) The difference in column volume. These two specifications (500mg 3ml; 500mg 6ml) have different packing heights. Obviously, 500mg 6ml has a large diameter and a low filler height, and the flow rate is relatively fast. It is naturally congested and the passing rate is slow. You can choose the appropriate column according to the type of sample matrix and usage habits.

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.

Sample loading
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.