Agriculture Reference
In-Depth Information
This method provides a low-cost, sensitive and fast screening alternative
for the detection of samples containing trace amounts of low-molecular weight
analytes such as nitrofurans [124-126]. The chromatographic methods for the
screening of residues consist essentially of three types, high performance thin-
layer chromatography (HPTLC), gas-chromatography (GC) and high
performance liquid chromatography (HPLC) coupled to different detection
systems.
HPTLC has been applied successfully for the qualitative and quantitative
detection of multi-residues in food samples even though its use has rapidly
decreased. Visualisation of the components can be performed either by
spraying an appropriate chromogenic reagent or under UV light. Quantitative
determination is possible through the relative intensity of the spot in the plate,
which is measured against that of the internal standard by scanning
densitometry. Recent developments allow for automation in a similar way to
HPLC with the appropriate equipment [121].
For the analysis of chemical food contaminants and residues, gas
chromatography (GC) is one of main analytical methodologies employed. As
compared to HPLC, GC provides better separation efficiency by the use of
hight efficiency capillary columns and specific detectors like electron-capture
detector (ECD) or nitrogen-phosphorus detector (NPD). After the 80's decade
GC has been traditionally combined with MS, and the availability of relative
affordable benchtop GC-MS instruments, gave preference to GC analysis of
multicomponent contaminant and residue analysis. Thermally labile and/or
large analytes that cannot be easily volatilized like some veterinary forbidden
drugs like anabolic steroids or beta-agonists must be derivatized to shown a
good sensitivity at sub-nanogram level or must be analyzed by HPLC like
mycotoxins or some polar pesticides.
The use of HPLC expanded during the 1990s and the availability of
automation somehow facilitated its use as a screening technique. HPLC is a
separative technique and its ability to detect compounds depends on the type
of detector used. The choice of the detection system is crucial for selectivity
and sensitivity. Some analytes not detected by absorbance, refractive index or
fluorescence may require chemical modifications to render chromophore,
fluorescent or UV-absorbing compounds. Usually, the detection of multi-
residues is based on a solid-phase extraction cleanup followed by filtration and
injection into a reverse phase HPLC with UV-diode array detection
[121].HPLC is being used in control laboratories due to the possibility to
analyse simultaneously multiple residues in a sample in relatively short time.
Recent developments of high speed HPLC, named ultra performance liquid
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