About the Features and Different Types of SPE Cartridges

Solid phase extraction

Solid phase extraction (SPE) is a sample pretreatment technology composed of cartridges chromatography separation process, separation mechanism, stationary phase, and solvent selection, etc. It is developed by combining liquid-solid extraction and liquid chromatography technology. There are many similarities with high performance liquid chromatography. However, the particle size of SPE cartridges packing material (>40μm) is larger than that of HPLC packing material (3~10μm). The difference between SPE cartridges chromatography and HPLC chromatography is that the cartridges are low, the number of plates is small, the separation efficiency is low, and it is used for one time, which can separate and retain compounds with very different properties.

Because SPE realizes the trinity process of selective extraction, separation, and concentration, with short operation time, small sample size, and few interfering substances, it can be used for the analysis of volatile and non-volatile substances and has good reproducibility. With the help of SPE cartridges, it can be achieved: remove substances that interfere with subsequent analysis from the sample; enrich trace components to improve analysis sensitivity; change the sample solvent to match the analysis method; in-situ derivatization; sample desalination; storage and transportation of samples. As the preferred method for preparing liquid samples, SPE replaces the traditional liquid-liquid extraction method. Therefore, since the emergence of SPE, it has been expanding its application at an annual growth rate of 10%.

Features of SPE cartridge

Compared with LLE (liquid-liquid extraction), the SPE cartridges, an effective method for sample purification and enrichment before analysis and determination, has the characteristics of less organic solvent consumption, convenience, safety, high efficiency, and low cost. The advantages of relatively high analyte recovery rate and good reproducibility of analysis results are widely used in the fields of food, medicine, environmental protection, disease control, and hygiene, commodity inspection, and pesticide residue analysis. In the chromatographic analysis, sample purification can also extend the service life of the chromatographic cartridges; The enrichment and concentration function of SPE sample preparation for the target analyte can improve the analytical method’s detection sensitivity greatly.

SPE is essentially a liquid chromatographic separation, and the adsorbent used is also the same as the liquid chromatographic stationary phase but differs in particle size. The general practice of SPE solid phase extraction column is: using the principle of selective adsorption and selective elution chromatography, the liquid sample is passed through the adsorbent, the analyte is retained, the impurities are washed out with a solvent of appropriate strength, and then washed with a small amount of solvent. The analyte is removed to achieve the purpose of rapid separation, purification, and concentration. There is also a practice of selectively adsorbing interfering impurities and allowing the analyte to be washed away, or adsorbing both the impurities and the analyte, and selectively eluting the analyte with a suitable solvent.

Type of SPE cartridges

(1) SPE cartridges
SPE columns are commonly used. From the outside of the solid phase extraction cartridges, the classic solid phase extraction column is similar to the syringe barrel and is straight. Another type of solid phase extraction column has a funnel-shaped upper end, which is convenient for loading more samples at one time during manual operation.

A simple SPE column is a small glass column with a diameter of a few millimeters or a column made of plastics such as polyintraene, polyethylene, polytetrafluoroethylene, or stainless steel. There is a sintered sieve plate with a pore size of 20μm at the lower end of the column to support the adsorbent. You can also use glass wool to replace the sieve plate, which can support the solid adsorbent and allow the liquid to flow through. Fill the sieve plate with an amount of adsorbent, and then add another sieve plate on the adsorbent to prevent the column bed from being destroyed when the sample is added.

Based on the consideration of purity, medical polypropylene without additives and containing trace impurities is generally used as the column material to avoid contamination of the sample during the extraction process. In order to reduce the impurities in the SPE blank, glass or pure PTFE can be selected as the column material. Sieve plate material is another possible source of impurities, the materials for making sieve plate are polypropylene, pure PTFE, stainless steel, and titanium. The metal sieve plate does not contain organic impurities but is susceptible to acid corrosion. As impurities may be introduced into the sample from the column, sieve plate, and packing, a blank extraction experiment should be done when establishing and verifying the SPE method.

(2) Non-sieve plate SPE cartridges
It is a new type of integrated SPE column. This product carries out special treatment on bonded silica fillers such as C18 and C8 to make it have higher mechanical strength and toughness, without adding polyethylene porous baffle as support in the column. It is the first integrated SPE column in the market.

(3) SPE disk
Another form of SPE cartridge is the SPE disc. The surface is similar to the membrane filter. The disc extractor is a pure polytetrafluoroethylene disc containing filler or a solid glass fiber sheet that does not need to support the filler. The filler accounts for about 60%~90% of the total amount of the SPE disk, and the thickness of the disk is about 1mm. The main difference between the SPE column and disk extractor is the ratio of bed thickness to diameter. For the packing of equal weight, the load area of disk extraction is higher than that of column type. The extraction is 10 times larger, thus allowing the liquid sample to flow through at a higher flow rate.

(4) Solid phase microextraction
In 1990, Pawliszyn, University of Waterloo, Canada, developed a new solid phase microextraction (SPME) separation technology based on solid phase extraction. The American Supelco company launched a commercial SPME device in 1993, which aroused repercussions in the field of analytical chemistry. The SPME device is shaped like a micro-injector, consisting of a handle and an extraction head or fiber head. The handle is used to install or fix the extraction head. It can be used forever: the extraction head is 1cm long, coated with different adsorbents, and connected to stainless steel. The molten fiber on the silk, the thin stainless steel tube is jacketed (to protect the quartz fiber from being broken), the fiber head can be expanded and contracted in the steel tube, and the thin stainless steel tube can penetrate rubber or plastic gaskets for sampling or injection. The extraction head is usually stored in the extraction head sheath.

When in use, the extraction head is immersed in the liquid or on the liquid to extract certain compounds in the concentrated sample, and then directly enter the vaporization chamber of the gas chromatograph without any solvent elution. After the extract is desorbed in the vaporization chamber, it is removed by the mobile phase. Import the chromatographic cartridges to complete the entire process of extraction, separation, and concentration.

The key to SPME is to choose the coating (adsorbent) on the quartz fiber so that the target can be adsorbed on the coating without interfering compounds and solvents. Generally speaking, non-polar coatings should be selected for non-polar targets. Layer, polar coating should be selected for the polar target.

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