Agriculture Reference
In-Depth Information
In practice, a GC-based method consists of the following steps: (i) isolating
analytes from a representative sample (extraction); (ii) separating coextracted matrix
components (cleanup); (iii) identifying and quantifying target analytes (determinative
step), and if suf
rming results by an additional analysis.
In some cases, cleanup steps can be omitted, which is the case with the use of
headspace techniques or the application of injection techniques such as direct sample
introduction/dif
ciently important (iv) con
cult matrix introduction (DSI/DMI). In the latter case, the sample
extract is placed in a microvial that is placed in an adapted GC liner. The solvent is
evaporated and vented at a relatively low temperature. The injector is rapidly heated to
volatilize the GC-amenable compounds, which are then focused at the front of a
relatively cold GC column. The column then undergoes normal temperature pro-
gramming to separate the analytes, followed by cooling to initial conditions. The
microvial is removed and discarded along with the nonvolatile matrix components
that it contains. Thus, only those compounds with the volatility range of the analytes
enter the column [1,2].
The
“
dilute-and-shoot
”
approach frequently used inLC
-
MS is not fully applicable in
GC
MS because the extracts usually contain many nonvolatile matrix coextracts that
can negatively affect method performance [3,4]. In addition, repeated injections of
nonvolatiles lead to their gradual deposition in the GC inlet and/or front part of the GC
column. As a consequence, new active sites can give rise, which may be responsible for
matrix induced signal diminishment. The observed phenomena include (i) gradual
decrease in analyte responses, (ii) distorted peak shapes (broadening and tailing), and
(iii) shifting retention times toward higher values [5]. However, despite these limita-
tions, GC
-
MS remains an essential technique for fast and comprehensive screening of
various food contaminants and naturally occurring organic compounds. In fact, for
compounds with low ionization ef
-
ciency observed in LC
-
ESI-MS (e.g., organo-
chlorine pesticides), GC
MS is a valuable and necessary alternative. Given the
possibility of matrix interference, it is clear that sample preparation plays a crucial
role in the reproducibility, sensitivity, and robustness of GC
-
-
MS methods [6].
2.2.1.1 Fast Gas Chromatography
-
Mass Spectrometry
Fast GC separation is generally desirable because the decreased time of analysis can
increase sample throughput, and consequently, thus decreasing the laboratory oper-
ating costs per sample. Changing either the column geometry or the operational
parameters is the strategy that may enable fast runs. In practice, a combination of both
tactics is commonly employed [7
-
9].
Reduction of column length is a simple, and the most frequently used approach,
in fast GC
MS. In practice, a conventional GC column (usually 30 m) is
replaced by a short column (usually 10m), which in combination with other
approaches signi
-
cantly decreases GC analysis times [7].
Use of a column with a small internal diameter (e.g., 0.10-0.18 mm) is another
way to achieve faster GC analyses. Unfortunately, dif
culties with introducing
