waters and soils, polycyclic aromatic hydrocarbons (PAHs) in drinking water,
polluted industrial and urban atmospheres, and drugs in biological fluids.
Sorbents are either packed into disposable cartridgesthe size of a syringe
barrel, fabricated into disksor incorporated into plastic pipette tips or well
plates. Most SPE is carried out using a small packed bed of sorbent (25–500 mg)
contained in a cartridge made from a polypropylene syringe barrel, the sorbent
being retained in position by polyethylene fritted disks. The sorbent generally
occupies only the lower half of the cartridge, leaving space above to accommo-
date several millilitres of the sample solution or washing and eluting solvents. A
typical cartridge procedure is illustrated in Figure 2 and consists of four distinct
steps:● Sorbent conditioning. The cartridge is flushed through with the sample
solvent to wet the surface of the sorbent and to create the same pH and
solvent composition as those of the sample, thus avoiding undesirable
chemical changes when the sample is applied.
● Sample loading or retention. The sample solution is passed through the
cartridge with the object of eitherretaining the analytes of interest whilst the
matrix components pass through orretaining the matrix components whilst
the analytes pass through. In some procedures, the analyte(s) and one or
more of the matrix components are retained whilst the remainder of the
matrix components pass through.
● Rinsing. This is necessary to remove all those components not retained by
the sorbent during the retention step and which may remain trapped in the
interstitial solvent.
● Elution. This final step is to recover retained analytes, otherwise the matrix-
free solution and rinsings from the second and third steps are combined for
quantitative recovery of the analytes before completion of the analysis.116 Section D – Separation techniques
Table 2. Typical SPE sorbents and interaction mechanisms
Sorbent Polarity Interaction mechanisms
Silica SiO 2 Polar Adsorption;
H-bonding
Florisil, MgSiO 3 Polar H-bonding
alumina Al 2 O 3 Polar H-bonding
Bonded phases (modified silica)
-C 18 H 37 (C18 or ODS) Nonpolar Van der Waals interactions
-C 8 H 17 (C8 or octyl) Nonpolar Van der Waals interactions
-C 6 H 5 (phenyl) Nonpolar Van der Waals interactions
and p–p
interactions
-(CH 2 ) 3 CN (cyanopropyl) Polar Polar interactions;
H-bonding
-(CH 2 ) 3 NH 2 (aminopropyl) Polar H-bonding
-(CH 2 ) 3 C 6 H 4 SO 3 H Ionic Cation exchange
-(CH 2 ) 3 N(CH 3 ) 3 Cl Ionic Anion exchange
Chiral (cyclodextrin) Polar Adsorption;
H-bonding
dipolar interactions
steric effects
Styrene/divinyl benzene co-polymer Nonpolar Size exclusion