Agriculture Reference
In-Depth Information
1.2 ADVANCED SAMPLE PREPARATION TECHNIQUES
Food safety analysis is a dif
cult task because of the complexity of food matrices and
the low concentrations at which target compounds are usually present. Thus, despite
the advances in the development of highly ef
cient analytical instrumentation for
their
final determination, sample pretreatment remains a bottleneck and an important
part of obtaining accurate quantitative results. A past survey has shown that an
average chromatography separation accounts for about 15% of the total analysis time,
sample preparation for about 60%, and data analysis and reporting for 25% [18,19].
However, some new technologies and automation have signi
cantly accelerated the
sample preparation process.
Sample preparation can involve a number of steps, including collection, drying,
grinding,
filtration, centrifugation, precipitation, dilution, and various forms of
extraction. The most conventional sample preparation methods are protein precipi-
tation (PPT), liquid
liquid extraction (LLE), and solid-phase extraction (SPE). In
addition to these traditional methods, many advanced approaches have been
proposed for pretreatment and/or extraction of food samples. These approaches
include salting out LLE (SALLE) such as QuEChERS (quick, easy, cheap,
effective, rugged, and safe) and SweEt (Swedish extraction technique), supercritical
-
fluid extraction (SFE), pressurized liquid extraction (PLE), microwave-assisted
extraction (MAE), matrix solid-phase dispersion (MSPD), solid-phase microex-
traction (SPME), stir bar sorptive extraction (SBSE), turbulent
ow chromatogra-
phy (TFC), and others [8,20
23]. To avoid overlap with other chapters, only
automation of weighing and preparing standard solutions, QuEChERS, SWEET,
TFC, PLE, automated 96- and 384-well formatted sample preparation, headspace,
SPME, MEPS, and liquid extraction surface analysis (LESA
TM
)arediscussedinthe
following sections.
-
1.2.1 Automation of Weighing and Preparing Standard Solutions
The
first step of an analysis is to weigh standards for calibration solutions. With an
automatic dosing balance, a tablet, paste, or powder sample can be easily weighed into
a volume
ed target concentration can be
obtained by adding the exact amount of solvent automatically.
Many routine sample preparations, such as calibration curve generation, sample
dilution, aliquoting, reconstitution, internal standard addition, or sample derivatiza-
tion are often time consuming. The technology development of liquid handlers has
provided full automation or semiautomation solutions. Basically, there are two
approaches: one is the multiple pipette liquid handler; another is the multifunction
autosampler. For example, a sample preparation workbench was applied to determine
eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in marine oils found
in today's supplement market [24]. The workbench was programmed to methylate the
analytes (derivatization) for each analytical run, to avoid sample exposure to oxygen
in a closed system, and to transfer the top layer of sample to a
flask. Combined with liquid dosing, a speci
final GC vial for
injection. The workbench not only gave results comparable to three widely applied
