Vacuum-UV Lone Star versus MS
Recently, a vacuum ultraviolet (VUV) detector for gas chromatography (GC) was introduced into the market, taking a top five spot in The Analytical Scientist Innovation Awards 2014 in the process. From the stand point of current detector technologies available to the end user, I believe VUV technology is the only worthy alternative to mass spectrometry (MS) when one wants to obtain both qualitative and quantitative information on analytes eluting from a GC column. VUV detection has already been demonstrated for use in analyzing petrochemicals, drugs, pesticides, fatty acids, permanent gases, and other various analytes that are GC-amenable (1, 2, 3). It is a universal detector. All chemical compounds absorb light in the 115 – 240 nm wavelength range probed by the detector, and to this point, all tested analytes have unique gas phase absorption features. Of course, the degree of absorption varies depending on the nature of chromophores on a given compound. The VUV detector is also very fast; it can record full wavelength-range spectra up to 100 Hz, making it quite amenable for use with GC×GC and fast GC applications.
So, the big question here: can VUV realistically compete with MS as the premier comprehensive GC detector? I honestly think it can. I believe it will take some time to understand the potential for de novo compound identification or classification, in the way that it is considered well accepted and commonplace by MS. Yet, the major difference between the two is that MS has a well-developed library; for the VUV detector, researchers are in the early stages of assembling that resource.
For a complete unknown, electron ionization – MS (EI-MS) can provide some significant hints regarding the functionality of the compound. But MS has never been considered a structural identification technique – it can only help aid and confirm assignments. Interpretation of EI-MS mass spectra is a practiced art, much like interpreting an NMR spectrum. It is arguable that VUV spectra provide similar fingerprinting capabilities, where various classes of compounds are accompanied by specific features in the measured absorption range. In fact, for a proposed structure, VUV spectra can be readily predicted using theoretical computations. Mainstream tools to calculate the EI-MS spectrum based on an inputted structure are hardly popularized and well known, to my knowledge.
Still, significantly more research will be needed to understand class- or substituent- specific features in VUV absorption spectra, and to assess whether these can interpreted for practical or routine use. Until this is established, the growth of a VUV spectral library will provide equally convenient use of GC for qualitative analysis as enjoyed by GC-MS users who rely heavily on their MS library.
“So, the big question here: can VUV realistically compete with MS as the premier comprehensive GC detector?”
Finally, VUV detection has the potential to address analytical problems where MS detection fails. MS can have notable problems for the analysis of some isomeric, highly labile, and very low molecular weight analytes. As we have continued to expand the application base of VUV, it has been proven to fill those gaps. Both VUV and MS detectors have excellent sensitivity. MS sensitivity can depend on operation mode (for example, selected ion monitoring in quadrupole) and the distribution of fragment ion intensities for a given analyte. VUV sensitivity depends on chromophores; aromatic compounds have the highest absorption in the ~160–180 nm range. Our ability to apply spectral filters (projection of response based on a selected wavelength range) either pre- or post-run using VUV detection is largely akin to producing extracted ion chromatograms for MS. However, deconvolution of signals from co-eluting compounds is arguably more easily addressed by VUV detection; the general additivity of distinct high-resolution gas phase absorption spectra is a problem that is easily solved to determine contributions from individual components to an observed peak.
Overall, MS has many more years of development and application than VUV detector technology. I do believe that objectively comparing the two provides a nice perspective on some of the similarities associated with the information they provide. A closer look shows the potential where VUV can accommodate some applications that MS cannot. Perhaps the discussion comparing the two will ultimately devolve to price... There are arguments for both sides on that aspect, but I’ll allow you to research that for yourself.
The new sheriff in town by Daniel Armstrong
Wrangling orthogonality in multidimensional chromatography by Michelle Camenzuli
Reigning in multidimensional data by James Harynuk
Vacuum-uv lone star versus ms by Kevin Schug
Rodeo champs by Mark Schure
Young guns: ionic liquids for gcxgc by Jared Anderson
- K. A. Schug et al., “A Vacuum Ultraviolet Detector for Gas Chromatography”, Anal. Chem. 86, 8329-8335 (2014).
- L. Bai et al., “Permanent Gas Analysis using Gas Chromatography with Vacuum Ultraviolet Detection”, J. Chromatogr. A 1388, 244-250 (2015).
- H. Fan et al., “Gas Chromatography - Vacuum Ultraviolet Spectroscopy for Multiclass Pesticide Identification”, J. Chromatogr. A 1389, 120-127 (2015).
Kevin Schug is Shimadzu Distinguished Professor of Analytical Chemistry, University of Texas Arlington, USA.
