space above to accommodate several millilitres of the sample solution or washing and eluting solvents.
Depth filters can also be positioned immediately above the sorbent bed to remove particulate matter
from sample solutions. A typical cartridge procedure is illustrated in Figure 4.9 and consists of four
distinct steps, i.e.
- Sorbent conditioning The cartridge is flushed through with sample solvent to remove impurities and to
ensure that the sample solution will properly wet the surface of the sorbent particles. It also creates a
solvent composition and pH environment compatible with that of the sample solution thus avoiding
undesirable chemical changes when the sample is applied. Furthermore, conditioning each cartridge
before use improves the reproducibility of an analytical method. - Sample loading or retention The sample solution is passed through the cartridge with the object of
either retaining the analytes of interest whilst the matrix components pass through or retaining the
matrix components whilst the analytes pass through. The former option should be chosen if the analyte
(s) is/are present at relatively low or trace levels otherwise the latter can be used, provided the cartridge
capacity to retain matrix components is not exceeded. The sample solvent should have a weak power of
elution compared to the affinity of the sorbent for the components to be retained. 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. The sample size must be selected so as to be well within the capacity of the
sorbent bed for whichever components are to be retained, and the flow rate of the solvent should not be
excessive (normally no more than 10 cm^3 min–^1 ) otherwise retention efficiency will be impaired or
separations incomplete. - Rinsing This is necessary to remove all of those components not retained by the sorbent during the
retention step and which may remain trapped in the interstitial solvent. These could either be analyte(s)
or matrix components, depending on which are not retained by the sorbent. - 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 analyte before completion
of the analysis.
Disks and Pipette-tips
Alternative forms of sorbent bed have been developed. These include thin porous glass-fibre or PTFE
disks in which sorbent particles are embedded, and disposable plastic pipette-tips fitted with small
sorbent beds.
Disks have relatively large cross-sectional areas compared to packed-bed cartridges and thin sorbent
layers (0.5–1 mm thick) containing about 15 mg of material. This reduced bed-mass results in low void
volumes (as little as