Solid Phase Extraction (SPE) is a sample pretreatment technology based on liquid-solid separation and is developed by column liquid chromatography. Since its inception in the late 1970s, SPE technology has developed rapidly in many fields, such as the environment, due to its high efficiency, reliability, and low solvent consumption. It has gradually replaced traditional liquid-liquid extraction in foreign countries and has become a reliable and effective method for sample pretreatment.
SPE technology is based on the principle of liquid chromatography and can be approximated as a simple chromatographic process. The adsorbent acts as a stationary phase and the mobile phase is a water sample during the extraction process. When the mobile phase is in contact with the stationary phase, some of the traces (targets) remain in the stationary phase. At this time, an enriched and purified target can be obtained by eluting with a small amount of a selective solvent. Solid phase extraction can be divided into online extraction line extraction before the extraction and chromatographic analysis are completed simultaneously; while the latter extraction and chromatographic analysis are completed step by step, the two are consistent in principle.
The general solid phase extraction steps include the selection of a solid phase extraction column (ie, adsorbent), column pretreatment, loading, rinsing, and elution. The factors that need to be specifically considered during the experiment are as follows:
1) Selection of adsorbent
a. traditional adsorbent
The most commonly used reversed-phase adsorbents in environmental analysis are more suitable for the enrichment and purification of non-polar to moderately polar organics in water samples. Among them are representative bonded silica gel C18 and bonded silica gel C8. The adsorbent retains the target mainly by generating a non-polar van der Waals force or dispersive force through the carbon-hydrogen bond of the target and the functional group on the surface of the silica gel.
The normal phase adsorbent includes magnesium silicate, amino group, cyano group, diol-bonded silica gel and alumina, etc., mainly through the polar interaction of the polar functional group of the target with the polar functional group on the surface of the adsorbent (hydrogen bonding) Etc.) to retain polar compounds dissolved in non-polar media. Due to its special principle of action, it is often used in environmental analysis in combination with other types of adsorption columns to remove and remove interferences and achieve sample purification.
The ion exchange adsorbent mainly comprises a strong cation and a strong anion exchange resin. The skeleton of these resins is usually a styrene-divinylbenzene copolymer, mainly through a charged group of the target and a charged group on the bonded silica gel. The groups are electrostatically attracted to each other to achieve adsorption.
b. Antibody binding adsorbent (Immunosorbents-IS)
These new adsorbents make full use of the high sensitivity and high selectivity between bioimmune antigen-antibodies, especially for the enrichment and separation of trace organics in water. The characteristic is that since most organic pollutants are low molecular weight substances and cannot induce an immune reaction in animals, it is necessary to bind the undetermined pollutants to the biological macromolecular carrier of bovine serum albumin to make them have immunological antigen activity. , and then injected into a purebred animal (such as rabbit or sheep) to produce an antibody, and a monoclonal antibody corresponding to the organic pollutant is prepared by a hybridoma technique. The antibody-bonded adsorbent is prepared by binding an antibody to a silica gel surface or a polymer surface (such as a C18 stationary phase) of a reversed-phase adsorbent, and can be used for separating and enriching specific contaminants. The development of monoclonal or polyclonal antibodies that specifically detect a variety of preferential contaminants has become a frontier research area for SPE technology.
When the antibody-bonded adsorbent is eluted, a 20% to 80% methanol-water solution can be generally used, and the adsorbent can be used repeatedly after being stored in a refrigerated manner. When performing SPE operation, the appropriate adsorbent should be selected according to the nature of the target. Tables 1-1 give the types of adsorbents and their associated separation mechanisms, eluent properties, and properties of the components to be tested.
The amount of adsorbent is directly related to the nature of the target (polarity, volatility) and its concentration in the water sample. Generally, increasing the amount of adsorbent can increase the retention of the target, and the amount of adsorbent can be determined by plotting the adsorption curve.
2) Column pretreatment
The purpose of activation is to create an environment that is compatible with the sample solvent and remove impurities from the column. Two solvents are usually required to accomplish the task, the first solvent (primary solvent) is used to purify the stationary phase, and the other solvent (final solvent) is used to establish a suitable stationary phase environment to allow proper retention of the sample analyte. Each activating solvent is used in an amount of about 1 to 2 mL/100 mg of the stationary phase.
The final solvent should not be stronger than the sample solvent, and if too strong a solvent is used, the recovery rate will be reduced. It is usually not a problem to use a solvent that is weaker than the sample solution. It has to be noted that the stationary phase cannot be drained during and at the end of the activation process, as this will result in cracks in the packed bed, resulting in low recovery and re-saturation, and the sample is not properly cleaned. If a crack occurs in the column bed during the activation process, the above activation steps are repeated.
3) Loading
The sample is added to the SPE column and the sample solution is forced through the stationary phase to leave the analyte and some sample interferents on the stationary phase. In order to retain the analyte, the solvent that dissolves the sample must be weak. If it is too strong, the analyte will not be retained, and the recovery rate will be very low. This phenomenon is called breakthrough. Whenever possible, use the weakest sample solvent to give the solute the strongest retention or the narrowest band. As long as there is no leakage, a large volume of loading (0.5 to 1 L) is allowed.
Sometimes the fixed sample must be extracted with a strong solvent. Such an extract cannot be directly loaded. Therefore, the extract should be diluted with a weak solvent to obtain a suitable total strength of the solvent for loading. For example, a soil sample is extracted with 50% methanol to obtain 2 mL of extract, which is diluted with 8 mL of water to obtain a 10% methanol solution, so that the reverse phase solid phase extraction column can be directly applied without a leakage problem.
4) Rinse
After the analyte is retained, it is usually necessary to rinse the stationary phase to wash away the unwanted sample components. The elution solvent has a slightly stronger elution strength than or equal to the loading solvent. The rinsing solvent must be as strong as possible to wash away as much of the interfering components as possible, but not so strong that any analyte can be eluted. The solvent volume can range from 0.5 to 0.8 mL per 100 g of stationary phase.
It is not advisable to use too strong solvent when rinsing, too strong solvent will wash off the strong retained impurities. Using too weak a solvent will increase the volume of the rinse. Can be mixed with strong and weak solvents; but the solvent used in combination or before and after must be mutually soluble.
5) elution
After rinsing, the analyte is eluted from the stationary phase, and the amount of eluting solvent is generally 0.5 to 0.8 mL/100 g of the stationary phase. The solvent must be carefully selected, the solvent is too strong, some of the more important components that are retained will be washed out; if the solvent is too weak, more eluent is needed to wash out the analyte, so the solid phase extraction column The concentration effect will be weakened.
Also pay attention to the mutual solubility of the solvent when selecting the elution solvent. The solvent flowing through the column bed must be miscible with the former solvent, and a solvent which is not miscible with the residual solvent in the column cannot fully function with the stationary phase. Of course, proper liquid-solid partitioning does not occur, resulting in poor recovery and Unsatisfactory purification effect. If it is difficult to use a solvent that is miscible, the column bed must be dried; the method of drying is to let nitrogen or air pass through the bed for 10 to 15 minutes; or by centrifugation, the drying effect is better.
In summary, the solid phase extraction technique is simple and easy, and can significantly improve chromatographic separation, prolong the life of the column, and reduce the detection limit of the method. Compared with traditional liquid-liquid extraction methods, SPE solid phase extraction has significant advantages in improving sample throughput; greatly reducing solvent consumption and waste generation; high recovery and good reproducibility; and extremely low impurities. Interference; no emulsification; multiple separation mode selection; easy to automate. However, the consumables of the solid phase extraction column used in the experiment are expensive, and the cost of the experiment cannot be ignored.
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