CO2_2016 - page 11

9
Chimica Oggi - Chemistry Today
- vol. 34(2) March/April 2016
Peaks 1 (C
16:2
ω
4
), 2 (7-methyl-C
16:1
ω
10
) and C
17:0
are well
separated from the column bleed, and the other non-
identified compounds. The FAME C
16:2
ω
4
presents an
intermediate number of double bonds (compared to
others present in the fish oil), and is characterized by a
base width (6
σ
) of about 700 ms. Such a value, which can
be considered as an acceptable indication of average
peak width, would lead to a peak capacity of 7.1 per
second-dimension analysis. If the analyte elution window is
considered (80 min or 960 modulations), then the total
peak capacity would reach a value of nearly 6,800,
providing a rough estimation of the separation potential.
The simple FM concept was applied later in
petrochemical-based applications involving QqQ, high-
resolution time-of-flight MS and a helium ionization
detector (13-15), and in perfume analysis using qMS
detection (16).
FM GC×GC applications using gas flows of ≈ 4 mL min
-1
As described in the previous section, efficient FM can be
performed using gas flows in the range 6-8 mL min
-1
.
However, it is obvious that such a gas flow can still be
considered rather high for MS analysis (even though
compatible with most commercial MS devices), with a
further reduction desirable (17). On the basis of the
information reported previously it could be concluded
that a further reduction of the
2
D gas flow could be
achieved just by extending even more the length of the
re-injection period. However, if the re-injection step is too
long, then the
1
D stop-flow period (caused by the
generation of a pressure pulse) will terminate before the
end of the re-injection step. For instance, if a re-injection
step of 900 ms is applied, with an arbitrary stop-flow period
of 600 ms, then first-dimension effluent will enter the
accumulation loop for the remaining 300 ms. Such a factor
will cause a baseline rise before the main modulated
peak. Such a theoretical consideration was evaluated
experimentally: C
9
and C
10
n
-alkanes were subjected to
FM GC×GC-qMS analyses under isothermal conditions
(120°C), and (this time) using a loop of dimensions 20 cm ×
0.51 mm ID. It is worthy of note that, initially, two restrictors
of the same dimensions (10 cm × 0.25 mm ID) were used to
link each metal branch (deriving from the solenoid valve)
to the FM (Figure 1). Such a configuration is defined as
“symmetrical configuration” (SC). The slight pressure drops
generated by the two restrictors are here considered for
calculation of the gas flow/LV conditions. It must be kept in
mind that the gas flow and average LV values, even
though helpful for method optimization, are to be
considered as approximations.
An FM GC×GC-qMS experiment (SC) was carried out at a
2
D flow of ≈ 5 mL min
-1
. The pressures applied generated
an LV during accumulation (4.5 s) of 2.2 cm s
-1
; the LV
during the 700-ms re-injection period increased to 32 cm
s
-1
. As can be observed in the chromatogram expansion
shown in Figure 7A, there is a baseline rise prior to the
modulated peaks (C
9
), due to the excessively long
re-injection step.
The possibility to extend the stop-flow period was
investigated by using two restrictors, of different
dimensions (asymmetrical configuration – AC). A 10 cm ×
0.25 mm ID restrictor was connected to the upstream
union, while a 19 cm × 0.18 mm ID uncoated column was
linked to the 10 cm × 0.25 mm ID uncoated capillary,
Figure 5.
Untransformed FM GC×GC-qMS traces relative to the two-
stage (a) and four-stage (b) modulation process of the C
10
alkane.
Reproduced with permission from Elsevier [Tranchida P.Q., Franchina
F.A., et al., J. Chromatogr. A 2014, 1372, 236-244].
Figure 6.
Single untransformed modulation illustrating peaks 1
(C
16:2
ω
4
), 2 (7-methyl-C
16:1
ω
10
) and C
17:0
FAME. Reproduced with
permission from Elsevier [Tranchida P.Q., Franchina F.A., et al., J.
Chromatogr. A 2014, 1372, 236-244].
1...,2,3,4,5,6,7,8,9,10 12,13,14,15,16,17,18,19,20,21,...68
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