CO2_2016 - page 9

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Chimica Oggi - Chemistry Today
- vol. 34(2) March/April 2016
An apolar 20 m × 0.18 mm ID column was used as first
dimension, while a medium-polarity 2.5 m × 0.10 mm ID
capillary was used as second dimension. The modulation
period was 5 s (re-injection step: 200 ms). The
2
D gas flow
was ≈ 2.5 mL min
-1
, with about 96% of the effluent
directed to waste. A brief calculation gives a total flow
value of just over 60 mL min
-1
! Even though the
chromatography was satisfactory (as can be observed in
Figure 3), the clear disadvantage was a reduced
sensitivity.
In a further FM experiment (focused on the analysis of fatty
acid methyl esters in fish and plasma samples), a single
quadrupole MS (qMS) was used for qualitative purposes; the
2
D gas flow was ≈ 24 mL min
-1
, with about 80% of the effluent
directed to the waste line (6). A high-speed triple
quadrupole (QqQ) MS instrument has also been subjected
to evaluation in FM GC×GC untargeted and targeted
experiments (7). In an application on mandarin essential oil,
approx. 70% of the entire flow (≈ 23 mL min
-1
) exiting the
modulator was vented. In both research works, the sensitivity
problem remained evident.
After the initial three studies (5-7), performed with three
different detectors, a question arose spontaneous: where
does the sensitivity attained in FM GC×GC applications
stand compared to one-dimensional GC experiments (the
latter with no flow diversion)? To answer such a question, a
measurement (and comparison) of sensitivity was made
between FM GC×GC-QqQ MS and GC-QqQ MS analyses
(8). FM GC×GC applications were performed by diverting
about 80% of the ≈ 28 mL min
-1
flow exiting the accumulation
loop to waste. In GC experiments, the column employed
was the same as that used in the first GC×GC dimension
(apolar, 20 m × 0.18 mm ID). It was found that sensitivity,
measured in terms of signal-to-noise (
s/n
) values, was on
average 3-4 times higher in the single-column application.
Such results highlighted the problems that can arise when FM
and MS encounter one another.
FM GC×GC applications using reduced gas flows
(≈ 6-8 mL min
-1
)
Carefully-tuned optimization is very important when using
an FM equipped with an accumulation loop. It is highly
advisable to devote attention to the linear velocity (LV) of
the intra-loop chromatography band, during the
accumulation period. If the chromatography band reaches
the downstream union before the end of the accumulation
period, then breakthrough will occur.
Another fundamental and simple concept is that a
re-injection period of 100 ms, with a flow of 28 mL min
-1
, will
be equivalent to that of a re-injection period of 400 ms,
with a flow of 7 mL min
-1
. So, in principle, it is not necessary
to use gas flows in excess of 20 mL min
-1
to perform
satisfactory flow modulation. However, the duration of the
re-injection process must not be too long, otherwise the
1
D
stop-flow conditions will not be maintained. The main
consequence of such an occurrence will be that
1
D
effluent will start filling the loop before the end of the
re-injection period. Following such considerations, the
seventh port was closed because its function as waste line
was of no use (9). A modulation loop of dimensions 20 cm ×
0.51 mm ID, corresponding to a volume of 40.9 μL, was
used. A case of efficient flow modulation will now be
described and illustrated (Figure 4), using linear C
9
alkane
as test analyte.
generated at the upstream union. The high flow exiting the
modulator, which was necessary for efficient re-injection, was
split between two columns ((each connected to a flame
ionization detector (FID)).
FM GC×GC applications using high gas flows (> 20 mL min
-1
)
On the basis of the FM model proposed by Seeley and
co-authors (4), a fully-integrated 7-port device was
developed and subjected to evaluation (5). A schematic of
the integrated FM, positioned within a dual-oven GC×GC
instrument, is illustrated in Figure 2.
The output ports of the solenoid valve were connected to
positions 2 and 5; the primary column was connected to port
1, while the secondary one was linked to port 6. An uncoated
column was linked to position 7 on one side, to a manually-
regulated split valve on the other, and served as a waste line.
The latter was obviously exploited to reduce the gas flow
entering the second column. Obviously, if desired a second
detector could be used instead of the split valve. The FM was
characterized by internal channels connecting ports 1-2-3
and 4-5-6/7. An external 40-μL accumulation loop (stainless
steel) bridged positions 3 and 4. An FM FID-based experiment
was performed on a typical GC×GC sample, namely diesel.
Figure 2.
Schematic of the integrated FM, positioned within a
dual-oven GC×GC instrument. Reproduced with permission from
Elsevier [Tranchida P.Q., Purcaro G., et al., J. Chromatogr. A 2011,
1218, 3140-3145].
Figure 3.
FM GC×GC-FID chromatogram of a sample of diesel.
Reproduced with permission from Elsevier [Tranchida P.Q.,
Purcaro G., et al., J. Chromatogr. A 2011, 1218, 3140-3145].
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