Agriculture Reference
In-Depth Information
achieved by addition of various buffers and/or acids into the extraction mixture in
order to adjust the pH and support the transfer of analytes into the organic layer.
Regarding the type and optimal amount of salts, magnesium sulfate (MgSO
4
) and
sodium chloride (NaCl) at a ratio of 1:4 (w/w) have been used in most of applications.
8.2.4 Puri
cation of Sample Extracts
The main goals of the cleanup step are to remove undesirable sample coextracts that
may interfere with the analytes and to preconcentrate target analytes to allow
acceptable sensitivity and selectivity to be achieved [5,17,18,27]. The most frequently
employed sample puri
cation approaches in the analysis of mycotoxins are using
either solid-phase extraction (SPE) or immunoaf
nity column (IAC) cleanup. Addi-
tionally, molecularly imprinted polymers (MIP) cleanup, matrix solid-phase disper-
sion (MSPD), or liquid
liquid partitioning of extract with
n
-hexane, acetone, or ethyl
acetate have been used in this
-
field, but to a limited extent [14].
The SPE cleanup technique is based on the partitioning of analytes and the sample
matrix between mobile and stationary phases. SPE is performed by passing the sample
extract through a disposable cartridge containing sorbents with bound phases (with
C
18
being the most used) and various adsorbents, such as charcoal, Florisil, or Celite.
SPE in three different modes can be generally applied in food analysis: (i) selective
extraction, (ii) selective washing, and (iii) selective elution [4,46]. In the selective
extraction mode, the SPE cartridge retains mycotoxins and allows impurities to pass
through the cartridge. In a subsequent step, target analytes are released from the SPE
stationary phase using a suitable solvent. In the sample washing mode, both analytes
and impurities are
first retained on the SPE sorbent bed, the interfering matrix
components are further rinsed out using strong enough solvent, and analytes are
finely
eluted with other but stronger solvents. In the selective elution mode, interfering
impurities are retained by the stationary phase of the SPE cartridge and the target
mycotoxins are allowed to pass through the column. No washing and elution steps are
further required. Retention and elution of analytes and impurities are strongly
dependent on properties of the stationary bed and elution/wash solvents, which
are commonly designed for speci
solvent combinations.
The description of sorbent and solvent selectivity is thoroughly described in a review
by Lucci et al. [46]. The respective SPE modes are illustrated in Figure 8.2. It is
apparent that for achieving the highest possible sample throughput, the matrix
removal SPE mode is the most desirable as it enables the sample extract cleanup
to be performed in a single step. It is worth noting that the MycoSep SPE cartridges,
which are currently the most frequently used in mycotoxin analysis, are operated in
the matrix removal mode [18].
While SPE offers cleanup and preconcentration for a broad range of analytes, IAC
provides a higher selectivity and speci
c usage of analyte
-
matrix
-
city to target analyte(s). After application of
the sample extract to the IAC, mycotoxins are selectively bound to antibodies (either
monoclonal or polyclonal) that are immobilized in the cyanobromide-activated
sepharose gel [47] present in the cartridge. IAC combines the sample cleanup and
sample concentration modes. Components of the sample matrix that do not interact
with antibodies are gradually eluted from the column during application of the extract.
