Fast GC’s rapidity gives greater productivity

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  • Published: Aug 15, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: Gas Chromatography
thumbnail image: Fast GC’s rapidity gives greater productivity

Fast GC gives short run times

Traditional GC-MS typically requires a fair amount of time for a sample to be diluted, possibly filtered, and then run on a GC-MS. There are various modern techniques of rapidly introducing samples into a mass spectrometer, such as DESI (desorption electrospray ionisation), DART (direct analysis in real time) and DSA (direct sample analysis), but these do not give the separation possible with a GC system. However, fast GC techniques have now been developed, using LTM (low thermal mass) systems, in which the run time need only be of the order of a minute or two.

The Tel Aviv scientists combined fast GC with an open probe inlet, where samples are directly inserted into a GC system. A high helium flow, acting as a gas purge of the inlet, minimises air contamination of the system. The fast GC was used with a standard mass spectrometer, giving a relatively inexpensive setup that can be used for rapid analysis.

Wide variety of analyses carried out by open probe fast GC-MS

The new system consisted of an open probe sample inlet, purged with helium, connected to a fast GC column (Agilent DB-1HT, 1.5 m length, 0.25 mm internal diameter, 0.1 μm HT1 100% polydimethylsiloxane film). It was found that columns with a smaller bore internal diameter gave poorer performance with ‘real life’ samples. The capillary was set up inside a low thermal mass metal tube, which was resistively heated. A micropump, a frit flow constrictor and a solenoid valve at the inlet were used to reduce the helium flow on the column to 1–2 ml/min. Samples were typically inserted into the probe on the end of a glass melting point capillary, although inert swabs or miniature ChromatoProbe vials could also be used. The GC was connected to a standard GC-MS mass spectrometer, an Agilent 5977A instrument, with an EI (electron ionisation) source.

Cannabis flowers were used as an analyte on the system. The active component of cannabis, THC (tetrahydrocannabinol), was detected in 30 s, with 50 s required for the complete GC cycle, including returning to the starting temperature of 60 °C. The possible use of the system for anti-terrorism was demonstrated by having a researcher touch some TNT and then touch a clean glass surface. A melting point capillary was used to sample the fingerprint and inject it onto the system. TNT was detected with a full analysis cycle time of only 55 seconds.

The system was successfully used to separate fatty acids, their esters, sterols and vitamin E in rapeseed oil, and also to separate the main components in olive oil. Additionally, polybrominated flame retardants in plastics were identified with a 1.5 min cycle time; such plastics need to be removed from recycling streams.

The active pharmaceutical ingredient, alprazolam, along with two fatty acid excipients, was detected in Xanax pills. Fake, dangerously adulterated versions of these pills have caused a number of deaths. In another test, free cholesterol and various fatty acids were detected in a blood sample. In these two examples, cold EI mass spectrometry was used since it gave better results than the standard EI for these substrates. Finally, the system was used to detect pyrene down to femtogram levels; this polycyclic hydrocarbon is not easily detected at low levels by alternative mass spectrometry systems, such as DART, DSA and DESI.

GC now can give near ‘real time’ results

The paper shows that many different substrates can be successfully analysed in a minute or two using open probe fast GC-MS, greatly increasing laboratory productivity. It is clear that GC is now approaching ‘real time’ results, in contrast to the relatively lengthy cycle times for traditional GC-MS. Such techniques could doubtless be applied to the monitoring of industrial chemical reactions, giving improved process control by generating rapid results.

Related Links

Journal of Mass Spectrometry, 2017, 52, 417-426. Keshet et al. Open probe fast GC–MS — combining ambient sampling ultra-fast separation and in-vacuum ionization for real-time analysis.

International Journal of Mass Spectrometry, 2014, 371, 47-53. Amirav et al. Open probe fast GC–MS — real time analysis with separation.

Analytical Chemistry, 2010, 82, 5777-5782. Poliak et al. Open Probe: A device for ultra fast electron ionization mass spectrometry analysis.

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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