How do you optimize the selectivity of solid phase extraction (SPE)?

1 Sorbent types

Sorbents are the key components of SPE that determine the selectivity of the technique. Sorbents are classified into two main categories: non-polar and polar. Non-polar sorbents, such as silica, alumina, and carbon, are hydrophobic and interact with non-polar analytes by van der Waals forces. Polar sorbents, such as ion exchange, mixed mode, and affinity, are hydrophilic and interact with polar analytes by electrostatic, hydrogen bonding, or specific binding forces. The choice of sorbent type depends on the polarity and charge of your analytes and the matrix.

2 Sorbent size and shape

The size and shape of the sorbent particles also affect the selectivity of SPE. Smaller particles have larger surface area and more active sites, which can increase the retention and elution efficiency of the analytes. However, smaller particles also create higher back pressure and require more solvent for elution. The shape of the particles can be spherical or irregular, which can influence the packing density and flow characteristics of the sorbent bed. The optimal size and shape of the sorbent depend on the sample volume, flow rate, and elution conditions.

3 Sorbent amount and bed depth

The amount and bed depth of the sorbent are important parameters that affect the selectivity of SPE. The amount of sorbent determines the loading capacity and the breakthrough volume of the analytes. The loading capacity is the maximum amount of analytes that can be retained by the sorbent without compromising the recovery. The breakthrough volume is the minimum volume of sample that can cause the analytes to elute from the sorbent. The bed depth is the height of the sorbent layer in the SPE cartridge or column. The bed depth affects the contact time and the mass transfer of the analytes between the sample and the sorbent. The optimal amount and bed depth of the sorbent depend on the concentration, distribution, and affinity of the analytes.

4 Mode of interaction

The mode of interaction refers to the mechanism by which the analytes are retained and eluted from the sorbent. There are four main modes of interaction in SPE: normal phase, reversed phase, ion exchange, and mixed mode. Normal phase mode uses a non-polar sorbent and a polar solvent, and retains polar analytes by polar interactions. Reversed phase mode uses a polar sorbent and a non-polar solvent, and retains non-polar analytes by hydrophobic interactions. Ion exchange mode uses a charged sorbent and a buffered solvent, and retains charged analytes by electrostatic interactions. Mixed mode mode uses a sorbent that combines two or more modes of interaction, and retains analytes by multiple mechanisms. The choice of mode of interaction depends on the selectivity, specificity, and compatibility of your analytes and sorbent.

5 Sample and solvent pH

The pH of the sample and the solvent can affect the selectivity of SPE by altering the ionization state of the analytes and the sorbent. The ionization state affects the polarity, charge, and solubility of the analytes and the sorbent, which in turn affects their retention and elution behavior. By adjusting the pH of the sample and the solvent, you can enhance or reduce the interaction between the analytes and the sorbent. For example, in ion exchange mode, you can increase the retention of acidic analytes by lowering the pH of the sample, and increase the elution of acidic analytes by raising the pH of the solvent.

6 Sample and solvent composition

The composition of the sample and the solvent can also affect the selectivity of SPE by influencing the competition and solvation of the analytes and the sorbent. The competition refers to the presence of other compounds in the sample or the solvent that can interfere with the retention or elution of the analytes by occupying the active sites or forming complexes with the sorbent. The solvation refers to the ability of the solvent to dissolve or displace the analytes from the sorbent by altering their polarity or charge. By changing the composition of the sample or the solvent, you can modulate the competition or solvation of the analytes and the sorbent. For example, in reversed phase mode, you can increase the elution of non-polar analytes by adding a more polar solvent, such as methanol or acetonitrile.

7 Here’s what else to consider

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