The Complementarity of Vacuum Ultraviolet Spectroscopy and Mass Spectrometry for Gas Chromatography Detection

Kevin A. Schug

Vacuum ultraviolet absorption spectroscopy detection for gas chromatography (GC–VUV) was introduced in 2014 (1,2). The first generation of GC–VUV features a detector that simultaneously measures a full wavelength range of absorption for eluted compounds from 120 to 240 nm, where all chemical compounds absorb and have a unique absorption signature. This provides new qualitative analysis capabilities for GC, a tool that has been lacking multiple choices in that regard. Quantitative analysis is grounded in the simple concept of the Beer-Lambert law, where amounts of measured species are directly proportional to the magnitude of absorption. It is worth noting that the absorption intensity at 180 nm for benzene is about 10,000 times higher than its absorption at 254 nm. Because absorption is captured in the gas phase, spectra are much more highly featured than they are in solution phase, where interactions with solvent cause significant spectral broadening. Further, given classes of molecules have similar absorption features and the absorption intensity within a class of molecules is quite predictable. These are some of the aspects that make GC–VUV highly complementary to GC–mass spectrometry (GC–MS). 

GC–MS has been the gold standard for combined qualitative and quantitative analysis of volatile and semivolatile compounds for many years. While some other GC detectors are highly selective, very few provide additional information beyond a retention time for compound identification. In that regard, GC–MS is not foolproof. While it has had many years to advance, and there are a wide variety of detector configurations available to choose from, GC–MS still has issues with differentiating isomeric compounds that have similar fragmentation patterns, especially if they are difficult to separate chromatographically. Additionally, the resolution of coeluted compounds requires sophisticated algorithms and software to tease apart temporal changes in complex fragment ion spectra. Many manufacturers and researchers have devised means for deconvolving overlapping peaks using GC–MS. The key is that there has to be some minimal temporal resolution of the species; perfectly coeluted components cannot be differentiated by these means.

GC–VUV excels in isomer differentiation and peak deconvolution. Electronic absorption of light in the 120–240 nm wavelength range probes the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a molecule. The energy of this light is sufficient to excite electrons that reside in sigma-bonded, pi-bonded, and nonbonded (n) molecular orbitals. The high-resolution (±0.5 nm) gas-phase measurement enables underlying vibrational excitation features to be observed, superimposed on the electronic absorption features in the spectra. The difference in energies between the HOMO and LUMO are highly dependent on atom connectivity and molecular structure and thus, such a measurement is ideal for differentiating various kinds of isomers. While some consideration has to be given to the spectral similarity of coeluted species and their relative abundances (3), research has shown the technique to have exceptional power for deconvolution of coeluted species (4–6), even if there is no temporal resolution between them. The measured absorption spectra of overlapping species is simply a sum of their absorption features, scaled according to their relative abundances. Thus, the deconvolution algorithm easily accommodates a linear combination of scaled reference spectra, to project the contribution of individual species to any overlapping absorption event.

Isomer differentiation is perhaps the most intriguing complement of GC–VUV to GC–MS. The most recent example of the power of GC–VUV was its use for distinguishing a large set of new designer drugs (7). Automated deconvolution of entire GC-VUV chromatograms from complex mixtures, such as gasoline or Aroclor mixtures, is possible using an advanced technique called time interval deconvolution (TID) (8,9). TID can be used to rapidly speciate and classify components in a mixture based on the fact that classes of compounds have similar absorption features, as mentioned previously. When one searches an unknown signal against the VUV library, even if the compound is not in the library, the closest matches will be from the same compound class as the unknown. For GC–MS, though there can be some diagnostic fragments, only matching elemental formulas are generally returned from a search, so reported library matches could be more misleading in this regard. 

From the quantitative standpoint, GC–VUV is about as sensitive as a standard GC–MS (quadrupole system) in scan mode (low- to mid-picogram amounts on column), but GC–VUV is also capable of calibrationless quantitation (10), by virtue of the highly reproducible absorption cross-section (that is, absorptivity) for molecules. This feature has only recently been explored in the context of real-world samples but could be a real time savings for GC analysis in future applications (11).

Some researchers have explored the operation of both VUV and MS detectors on one instrument. The best convention for this coupling in the future is probably going to be based on splitting column flow post-column, as opposed to placing the detectors in tandem. In tandem, the vacuum of the MS system could make it difficult to maintain sufficient peak residence time in the VUV detector; extracolumn band broadening effects are also a consideration. With a split flow, highly complementary qualitative and quantitative information can be obtained from a single separation. For many applications, the ability to collect information on both detectors would be a major advantage. It would significantly boost the confidence of identifying unknown species or targeting species of high interest or concern. Further, a second-generation VUV instrument is now available that features extended spectral ranges (up to 430 nm), higher temperatures (up to 420 °C), and improved detection limits to address challenging GC applications. In any case, the complementarity of GC–VUV and GC–MS cannot be denied. Each technique has advantages and limitations that balance the other. Certainly, the most desirable GC system in the future will contain both detectors to boost analytical performance and confidence in results.


Disclaimer
K.A. Schug is a member of the Scientific Advisory Board for VUV Analytics, Inc.
 

References

1. K.A. Schug, I. Sawicki, D.D. Carlton Jr., H. Fan, H.M. McNair, J.P. Nimmo, P. Kroll, J. Smuts, P. Walsh, and D. Harrison, Anal. Chem.86, 8329–8335 (2014).
2. I.C. Santos and K.A. Schug, J. Sep. Sci. 40, 138–151 (2017).
3. J. Schenk, X. Mao, J. Smuts, P. Walsh, P. Kroll, and K.A. Schug, Anal. Chim. Acta945, 1–8 (2016).
4. L. Bai, J. Smuts, P. Walsh, H. Fan, Z.L. Hildenbrand, D. Wong, D. Wetz, and K.A. Schug, J. Chromatogr. A1388, 244–250 (2015).
5. H. Fan, J. Smuts, P. Walsh, and K.A. Schug, J. Chromatogr. A1389, 120–127 (2015).
6. H. Fan, J. Smuts, L. Bai, P. Walsh, D.W. Armstrong, and K.A. Schug, Food Chem.194, 265–271 (2016).
7. L. Skultety, P. Frycak, C. Qiu, J. Smuts, L. Shear-Laude, K. Lemr, J.X. Mao, P. Kroll, K.A. Schug, A. Szewczak, C. Vaught, I. Lurie, and V. Havlicek, Anal. Chim. Acta971, 55–67 (2017).
8. P. Walsh, M. Garbalena, and K.A. Schug, Anal. Chem.88, 11130–11138 (2016).
9. C. Qiu, J. Cochran, J. Smuts, P. Walsh, and K.A. Schug, J. Chromatogr. A 1490, 191–200 (2017).
10. L. Bai, J. Smuts, C. Qiu, P. Walsh, H.M. McNair, and K.A. Schug, Anal. Chim. Acta953, 10–22 (2017).
11. M. Zoccali, K.A. Schug, P. Walsh, J. Smuts, and L. Mondello, J. Chromatogr. A1497, 135–143 (2017).

Kevin A. Schug is a Full Professor and Shimadzu Distinguished Professor of Analytical Chemistry in the Department of Chemistry & Biochemistry at The University of Texas (UT) at Arlington. He joined the faculty at UT Arlington in 2005 after completing a Ph.D. in Chemistry at Virginia Tech under the direction of Prof. Harold M. McNair and a post-doctoral fellowship at the University of Vienna under Prof. Wolfgang Lindner. Research in the Schug group spans fundamental and applied areas of separation science and mass spectrometry. Schug was named the LCGCEmerging Leader in Chromatography in 2009 and the 2012 American Chemical Society Division of Analytical Chemistry Young Investigator in Separation Science. He is a fellow of both the U.T. Arlington and U.T. System-Wide Academies of Distinguished Teachers.