Agriculture Reference
In-Depth Information
larger sample volumes plus lower sample capacity limit their application in real-
world analyses [7].
Decreases in analyte retention factors, and thus faster GC analyses, can be also
achieved by using a column with a thin
m).
However, reduced ruggedness and sample capacity are the trade-offs for
increased speed [7].
film of stationary phase (e.g., 0.1
μ
The most popular approach to fast GC in food analysis represents fast tempera-
ture programming. When using convection heating facilitated by a conventional
GC oven at faster programming rates, heat losses from the oven to the
surrounding environment may cause a poor oven temperature pro
le, and
hence, lower retention time reproducibility. Reducing the effective size of
the oven helps to improve reproducibility. Use of resistive heating is preferred
because of very good retention time repeatability as well as very rapid cool-
down rate, which results in higher sample throughput [7].
Operating the column outlet at low pressures (low-pressure gas chromatography
(LP
MS alternative. The analyses are conducted on a
megabore column (typically 10 m length
-
GC)) is another fast GC
-
×
×
0.53 mm internal diameter
0.25
-
1
μ
m phase) connected through a connector to a short, narrow restriction column
(2
gura-
tion, the entire analytical column is kept under vacuum conditions while the inlet
remains at usual column head pressures in GC. Because optimum carrier gas
linear velocity is attained at a higher value because of increased diffusivity of the
solute in the gas phase, faster GC separations can be achieved with a dis-
proportionately smaller loss of separation power. The advantages of LP
-
5m
×
0.1
-
0.18 mm internal diameter) at the inlet. Using this GC con
GC
involve: (i) reduced peak tailing and width, (ii) increased sample capacity of
megabore columns, and (iii) reduced thermal degradation of thermally labile
analytes [7].
-
Replacing helium by hydrogen carrier gas results in increasing the speed of
analysis as well as lower inlet pressure requirements. This results from the higher
diffusivity of analytes in hydrogen, which allows higher operating linear
velocities without increasing peak broadening. In practice, however, helium
is usually used due to concerns about safety and inertness [8].
In most cases, an increase in separation speed leads to lower chromatographic
resolution and/or sample capacity. The lower chromatographic resolution is not
necessarily the limiting factor in speed because MS has the ability to distinguish
between analytes that have differences in their MS spectra [10]. In addition, MS
systems acquiring full mass spectra can bene
t from automated spectral deconvo-
lution of partially overlapped peaks on the basis of increasing/decreasing ion
intensities in collected spectra [11]. With the exception of certain applications
such as the separation of isomeric compounds, MS can resolve coeluting peaks
spectrometrically. However, the detector must be able to record the narrower peaks
with an acceptable precision,
thereby providing reproducible quantitation and
identi
cation.
