disadvantage is the continuous boiling of the solvent, in which the analytes are
dissolved. This thermal stress avoids a successful enrichment of thermolabile substances. Further on, also for this technique the aforementioned restrictions regarding
the selection of solvents are valid. Additionally, only extractants with a lower
density as compared to water can be used, since they need to accumulate above
the water sample surface.
As alternatives, various techniques of liquid/solid extraction techniques are used.
Most common is the solid phase extraction (SPE) technique as illustrated schematically in Fig. 3.6. Here, solid adsorption material is used for the separation of organic
analytes from water or related samples. The material is usually fitted in glass or
plastic microcolumns (fixed with frits) and the liquid sample is passed through the
solid material. The corresponding flow velocity as well as the chemical properties of
the adsorption material are the most important parameters that need to be considered
for a successful extraction. Adsorption materials cover a wide range of polarity from
nonpolar reversed phases (e.g. C 18 coated silica gel) to polar silica gels with various
chemical modifications. Commonly, the SPE follows a four-step procedure starting
with an activation or cleaning cycle, followed by the actual extraction step. Thereafter, the SPE columns need to be dried and finally, the adsorbed substances are
discharged by elution with appropriate organic solvents. The last step can be
modified by a sequential usage of solvents with different polarity (increasing or
decreasing depending on the adsorption phase) for either a fractionation effect (for
fractionation procedures see Sect. 3.3) or as precleaning step.
Since there is automatically a phase interface (solid–liquid) there are no restrictions for adsorption material polarity or elution solvents. Hence, a much wider range
SPE (Solid Phase Extraction)
Analyte
Interfering components
Frit
Frit
Adsorbent
Sample
Solvent
Eluent
1. Conditioning
with solvent
2. Sample
application
3. Precleaning
with solvent
4. Elution
Solvent
Fig. 3.6 Typical procedure of a SPE extraction
3.2 Extraction
23
dissolved. This thermal stress avoids a successful enrichment of thermolabile substances. Further on, also for this technique the aforementioned restrictions regarding
the selection of solvents are valid. Additionally, only extractants with a lower
density as compared to water can be used, since they need to accumulate above
the water sample surface.
As alternatives, various techniques of liquid/solid extraction techniques are used.
Most common is the solid phase extraction (SPE) technique as illustrated schematically in Fig. 3.6. Here, solid adsorption material is used for the separation of organic
analytes from water or related samples. The material is usually fitted in glass or
plastic microcolumns (fixed with frits) and the liquid sample is passed through the
solid material. The corresponding flow velocity as well as the chemical properties of
the adsorption material are the most important parameters that need to be considered
for a successful extraction. Adsorption materials cover a wide range of polarity from
nonpolar reversed phases (e.g. C 18 coated silica gel) to polar silica gels with various
chemical modifications. Commonly, the SPE follows a four-step procedure starting
with an activation or cleaning cycle, followed by the actual extraction step. Thereafter, the SPE columns need to be dried and finally, the adsorbed substances are
discharged by elution with appropriate organic solvents. The last step can be
modified by a sequential usage of solvents with different polarity (increasing or
decreasing depending on the adsorption phase) for either a fractionation effect (for
fractionation procedures see Sect. 3.3) or as precleaning step.
Since there is automatically a phase interface (solid–liquid) there are no restrictions for adsorption material polarity or elution solvents. Hence, a much wider range
SPE (Solid Phase Extraction)
Analyte
Interfering components
Frit
Frit
Adsorbent
Sample
Solvent
Eluent
1. Conditioning
with solvent
2. Sample
application
3. Precleaning
with solvent
4. Elution
Solvent
Fig. 3.6 Typical procedure of a SPE extraction
3.2 Extraction
23