Basic principles of solid phase extraction technology

Solid phase extraction (SPE) is a technique that combines liquid-solid extraction and liquid chromatography.
It is mainly used for sample separation, purification and enrichment
.

Solid-phase extraction technology is based on the theory of liquid-solid chromatography.
It uses selective adsorption and selective elution to enrich, separate, and purify samples.
It is a physical extraction process that includes liquid and solid phases, which can also be approximated Seen as a simple chromatographic process
.

The more commonly used solid-phase extraction method is to pass the liquid sample solution through the adsorbent to retain the tested substance, and then use a solvent of appropriate strength to rinse the impurities; then, use a small amount of solvent to quickly elute the tested substance to achieve rapid separation and purification With the purpose of enrichment

Solid phase extraction is also suitable for the enrichment of compounds in water or soil samples, and there are also dedicated solid phase extraction disks
.

When processing large-volume water samples, solid phase extraction has the characteristics of large cross-sectional area, not easy to block, high flow rate, and short processing time

Compared with traditional liquid-liquid partition extraction, solid-phase extraction has the characteristics of Table 2-4
.

Table 2-4 Comparison of advantages and disadvantages of liquid-liquid partition extraction and solid phase extraction

(2) Classification of solid phase extraction

1.

Reversed phase solid phase extraction Reversed phase solid phase extraction consists of a non-polar stationary phase, which is suitable for polar or medium polar sample matrix

(1) Carbon-containing adsorption material
.

The ENVI-carb material comprising graphite, nonporous carbons; multi-walled carbon nanotube

(2) Polymeric adsorbents
.

For example, the ENVI-Chrom P material is composed of styrene-divinylbenzene, which retains some hydrophobic substances containing hydrophilic functional groups, especially aromatic compounds, such as phenol , which are more effective than C-18 bonded silica gel

Due to the adsorption of the carbon-hydrogen bonds in the analyte with the functional groups on the surface of the silica gel, the organic matter in the polar solution (such as the aqueous solution) can be retained on the solid-phase extraction material
.

The adsorption force between these non-polar molecules and non-polar molecules is the dispersion force in the van der Waals force

2.
Normal phase solid phase extraction Normal phase solid phase extraction consists of a polar stationary phase and is suitable for polar analysis of substances
.

Can be used for polar, moderately polar or non-polar sample matrices

Under normal phase conditions, how the analyte is retained depends on the interaction between the polar functional groups of the analyte and the polar functional groups on the surface of the adsorbent, including hydrogen bonds, π-π interactions, dipole-dipole interactions, Dipole-induced dipole mutual

Role and others
.

Use a more polar solvent for elution (solvent intensity factor greater than 0.

3.
Ion exchange solid phase extraction Ion exchange solid phase extraction is divided into anion exchange (such as LC-SAX, LC-NH ₂ ) and cation exchange (such as LC-SCX, LC-WCX)
.
Ion exchange solid phase extraction is suitable for charged compounds
.
The basic principle is electrostatic attraction, that is, the attraction between the charged groups on the compound and the charged groups on the bonded silica gel
.
Ion exchange solid phase extraction is used to remove metal ions in samples, and is more commonly used to extract dissociable compounds in samples
.
In order to attract the compound to the ion exchange resin from the aqueous solution, the pH of the sample must ensure that the functional groups of the separated substance and the functional groups on the bonded silica are both charged
.
If an ion has the same charge as the analyte, it will interfere with the adsorption of the analyte
.
The elution solution generally has a pH that can neutralize the charge on the functional group of the separated substance, or neutralize the charge on the functional group on the bonded silica gel
.
When the charge on the functional group is neutralized, the electrostatic attraction is gone, and the analyte is eluted
.
In addition, the elution solution may also have a strong ionic strength or contain another ion that can replace the adsorbed compound, so that the adsorbed compound will also be eluted
.

4.
Secondary interactions All bonded silica gels have a certain amount of unreacted silanol groups, which makes reverse solid phase extraction, in addition to non-polar interactions, there are also some polar secondary interactions
.
If the non-polar solvent cannot effectively elute the compound from the packing, a part of the polar solvent (such as methanol) can be added to destroy the compound retained by the polar interaction
.
In this case, methanol forms a hydrogen bond with the hydroxyl group on the silica gel, breaking the hydrogen bond between the analyte and the hydroxyl group on the silica gel
.

The silyl hydroxyl group is on the surface of silica gel.
When the pH is greater than 4, it exists as Si-O-
.
At this time, the secondary interaction of cation exchange may also occur on the silica gel matrix, which can adsorb cations or basic compounds
.
At this time, it is necessary to add acid or base to the eluent to adjust the pH
.

New carbon adsorbents such as graphitized carbon have the advantages of uniform structure and wide applicable pH range, and are mainly used for the removal of plant pigments
.
Bonded solid phase extraction agents are mainly bonded silica gels of various types, which are divided into two types: polar and non-polar
.
Polar bonded silica gels include -NH ₂ , CN, etc.
; non-polar bonded silica gels mainly include -C ₈ and -C _18, etc.
, which are suitable for extracting and purifying non-polar and weakly polar components in samples.
Organic Solvent elution
.
The ion exchange type bonding phase mainly includes -COOH, -SO ₃ H and quaternary amino group
.
Macroporous macroporous resin styrene-divinylbenzene (ST-DVB) non-ionic macroporous resin can adsorb non-polar compounds through hydrophobic interaction, and as the relative molecular weight increases, the retention of components in the resin is enhanced
.
Adsorption resins containing polar functional groups such as amide groups, cyano groups, and phenolic hydroxyl groups, which adsorb polar substances through electrostatic interaction
.

In addition to the general-purpose solid-phase extraction agents mentioned above, some highly selective fillers have appeared in recent years, such as phenylboronic acid-bonded silica gel that retains o-hydroxy compounds, which can be used for reversed-phase adsorption on the inner surface of plasma sample protein removal Filler (permeation limited solid phase extraction agent), as well as affinity solid phase extraction agent that fixes a certain specific antibody on agarose or silica gel, and highly specific molecularly imprinted polymer (MIP) solid phase extraction agent, etc.

.

(3) Operation steps and method establishment of solid phase extraction

There are two ways to achieve solid-phase extraction separation and purification of the sample: one is to keep the impurities on the adsorbent, and the component to be tested is not retained or eluted; the other is to retain the component to be tested and the impurities are natural.
Elute or be eluted first, and then the component to be tested is eluted by an appropriate eluent
.
For the pretreatment of large-volume samples such as environmental samples, the latter also has a component enrichment effect
.

The general operation steps of solid phase extraction mainly include: ①Use a strong elution solvent to activate and clean the packing; ②Flush the equilibrium packing with a weak elution solvent; ③Dissolve the sample with a weak solvent and load the sample; ④Use a similar or slightly stronger solvent Wash the solid-phase extraction column with the eluent to remove the interfering components; ⑤Use a strong eluent to elute the solid-phase extraction column, collect the eluent of the target volume, and then concentrate the eluent for chromatographic analysis
.

The key to the establishment of the solid-phase extraction sample pretreatment method is to select the appropriate type of solid-phase extraction agent according to the physical and chemical properties of the sample, and then optimize the equilibrium solvent, elution solvent, and elution solvent according to the recovery rate, and determine the receiving period and time of the elution solvent.
Volume
.

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