Backflush. One of the advantages of PFM GCxGC over thermal modulation GCxGC is that the PFM-GCXGC device can also serve as a built-in backflusher device since it includes a gas supply junction behind the flow impedance of the second dimension GCxGC column. Thus, as shown in Figure 1, after 53 min the pressure program was switched into backflush mode with 30 ml/min which is about 3000 mBar and in addition, the Agilent 7890 GC injector pressure was reduced to 1 PSIG at that time. Backflush is a known effective technique to keep the first dimension column clean from low volatility compounds such as potential oil impurities in jet fuel.
|Figure 2. Total ion mass chromatogram of PFM-GCxGC-MS with Cold EI of jet fuel JP8 (upper trace) and a representative Cold EI mass spectrum of n-C14H30 (bottom trace, 29.374 min elution time). Click to Enlarge.|
- The second dimension GC-MS trace exhibits a mass chromatogram with several peaks. Three peaks are clearly separated while two "shoulders" of peaks hint on additional peaks for at list total of five peaks.
- The second GCxGC cycle has a different structure and also with five compounds peaks.
- The five mass spectra that are plotted on the A-E traces where obtained at five different elution times and are each unique and with abundant molecular ions. A was identified as a branched isomer of C12H26, B was identified as a co-elution combination of C12H24 and C12H22 which are unsaturated and doubly unsaturated alkenes, C was identified as trans-decaline 2-methyl or one of its isomers, D was identified as one of the pentamethyl benzene isomers and E was identified as tetramethyl benzene (one of its isomers).
- A very interesting observation is that a clear correlation exists between the second dimension elution time and molecular weight of that sample compound. The greater is the second dimension elution time the smaller is the mass of the eluting compounds. In other words, in a given cycle, the greater the sample polarity the smaller is its mass. This correlation simplifies and helps in sample identification and is related to the emergence of a novel concept of GCxGCxMS.
GCxGCxMS is a new and interesting concept for having three dimensions of separation. For the original reference to the concept of GCxMS please read F. C. Y. Wang, K. N. Wan and L. A. Green "GCxMS of diesel: A two dimensional separation approach" Anal. Chem. 77, 2777-2785 (2005). According to Wang and co-workers, if the mass spectrum is obtained with a soft ionization method such as field ionization and as a result only molecular ions are generated, the separation of the GC and MS are orthogonal and can yield two independent dimensions of separation of the GC and MS hence named GCxMS. If only molecular ions are produced the mass spectrometer possesses a separation space of the mass divided by the few isotopes of the molecular ions. While unlike standard EI Cold EI provides trustworthy molecular ions, in contrast to field ionization it also produces the usual EI fragments. We note that in GCxGC-MS the mass range of the molecular ions is limited and in our case in a given one GCxGC cycle as shown in Figure 3 it is between 170 and 134 amu. Thus, in GCxGC-MS with Cold EI, upon the exploration of Cold EI mass spectra in a given second GC dimension cycle, any new high mass peak in this 134-170 amu mass range must belong to a new compound. Consequently, the high mass spectral range provides an additional dimension of separation unlike in standard EI in which the molecular ion can be fully missing for branched hydrocarbons or weak and cannot be trusted. Thus, as demonstrated in Figure 3, PFM-GCxGC-MS with Cold EI demonstrates the emergence of novel concept of GCxGCxMS in which the Cold EI mass spectrum provides solid information on the molecular ions hence another dimension of separation.
|Figure 4. Reconstructed mass chromatograms (RSIMs) of the PFM-GCxGC-MS with Cold EI of jet fuel JP8 in the elution time window of 11.90–12.04 minutes using m/z values of 170.2. 168.2, 166.2. 152.1, 148.1 and 134.1. Click to Enlarge.|
In Figure 5 we show at the upper trace another section of the PFM-GCxGC-MS of jet fuel, and this time in a higher elution time window of 28.70-28.77 min. As expected, the number of co-eluting compounds is now bigger and their second dimension GCxGC separation becomes harder. Furthermore, we deliberately selected an elution time window which is of low intensity (<1% of the full chromatogram scale for most compounds) to include a demonstration of the PFM-GCxGC-MS with Cold EI sensitivity. The lower traces are of reconstructed mass chromatograms (RSIM) of compounds with identified molecular ions, and as shown eleven such compounds could be extracted with m/z values according to order of elution times of 212.2, 210.2, 196.2, 208.2, 194.2, 192.2, 190.2, 188.1, 174.1, 160.1 and 156.1.
In Figure 8 below we show a two dimensional plot of the PFM-GCxGC-MS data. As usual, we can see the group type information data as well as a full qualitative assessment of the degree of orthogonality of the two dimensions of separation. We note that the second dimension elution time of aliphatic compounds is controlled by the pressure program used, which also served to reduce and/or eliminate wrap around. Aviv Analytical data presentation software was used.
|Figure 8. A two dimensional plot of PFM-GCxGC-MS with Cold EI data of JP8 jet fuel using the Aviv Analytical qualitative data presentation software. Click to Enlarge.|
- M. Poliak, M. Kochman and A. Amirav "Pulsed Flow Modulation Comprehensive Two Dimensional Gas Chromatography" J. Chromatogr. A. 1186, 189-195 (2008).
- M. Poliak, A. B. Fialkov and A. Amirav "Pulsed Flow Modulation Two-Dimensional Comprehensive Gas Chromatography Tandem Mass Spectrometry with Supersonic Molecular Beams" J. Chromatogr. A. 1210, 108-114 (2008).
- Aviv Amirav, "Pulsed Flow Modulation Gas Chromatography Mass Spectrometry with Supersonic Molecular Beams Method and Apparatus" Israel patent number 176724 (submitted in July 6, 2006) and USA patent number 7518103.