21
used. Certain peaks were cryogenically focussed to increase signal intensity and directed to a HRMS for ultra sensitive analysis, without the potential risk of peak slicing incumbent with modulation.
Experimental
All experiments were carried out on a Thermo Scientific®
DFS™ High Resolution
GC-MS coupled to a Thermo Scientific Trace™ GCxGC unit equipped with a Thermo Scientific TriPlus™ auto-sampler. The GCxGC was equipped with a cryogenic GC
modulator device using liquid CO2 as a cryogen (Figure 2). A standard split/splitless injector was employed in splitless mode for all injections.
Time controlled CZC experiments were carried out using standard single GC columns; TR-Dioxin 5MS 30 m and 60 m, both I.D. 0.25 mm and 0.1 µm film (Thermo Scientific). The modulator was placed between 1 - 4 m from the MS end of the analytical column.
The cryogenic gas operation was timed controlled and fully integrated within the GC method control software, via time event settings (Figure 3). Only one of the two jets of the modulator was used for the experiments (Figure 2). Complete and fully automated timed CZC methods could be created and sequenced using the Xcalibur® data system.
Magnetic sector GC-HRMS is constrained by acquisition rate [10]. In order to achieve the fast sampling frequencies required for this type of analysis, only a reduced number of selected target masses can be used with magnetic sector GC-HRMS detection. Sampling cycle times of 80 – 100 ms were used corresponding to 10 – 12 Hz (Figure 4). Observed typical baseline GC peak widths with CZC activated were ca. 600-700 ms and 6 – 8 MS data points could be sampled over the peak with the settings and columns used.
HRMS settings were equivalent to standard settings typical in routine dioxin analysis: 45 eV, Resolution 10,000 (10 % valley definition), etc.
Certified standards from Wellington Laboratories, Ontario, Canada were employed. The DFS instrument performance specification standard (20 fg/µL 2378-TCDD; 5 pg/µL 2378- 13
C TCDD) was diluted down to 2
and 0.5 fg/µL for CZC sensitivity experiments. For multi target CZC experiments a certified 1/10 EPA 1613 CSL standard (Wellington) was employed together with a pooled human serum sample (Figure 6-9).
Figure 5c: Cryogenic jet off: step 2 – re-inject peak of interest on 2nd dimension column Figure 5a: Cryogenic jet off results in normal peak width for non-target analytes
Figure 5b: Cryogenic jet on: step 1 – cryo-traps whole 1st
dimension peak of interest
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