We Don’t Need No Separation, We Don’t Need No Peaks at All

I concede that the claim I make in the title of this article might be hyperbolic (and the pun would definitely make Roger Waters recoil). However, I believe that the time has come to re-evaluate the role of chromatographic separations in some applications. It’s not 1960, when mass spec was still in its infancy and people were relying on non-selective detectors, like TCD and FID, as their sole sources of data. The data are no longer based on a simple two-dimensional relationship: time versus response. That “response” now has a life of its own: in mass spec it’s the collective intensity of individual and identifiable mass fragments entering the detector; for VUV it’s the sum of the absorbances at each wavelength for molecules passing through the flow cell. In both mass spec and VUV, the spectrum collected at each time point gives information that can be used to identify a compound – a spectral fingerprint. This third dimension of information allows for substantial selectivity. If we have all this extra data, why is there still so much emphasis on improving chromatographic separation?

Mass spectrometry’s selectivity is well documented and a significant factor in its dominance of the analytical market. However, this selectivity must happen on the front end of the run, selecting specific mass fragments in SIM (selected ion monitoring) or ion transitions in SRM/MRM (selected/multiple reaction monitoring), so you better be sure you know what you’re looking for. If you don’t, you’re fishing for analytes in the deep seas of a full scan (and some mass spec deconvolution software is the equivalent of a piece of string attached to a stick...). When those coeluting compounds have overlapping major ion fragments, achieving an accurate deconvolution can be tricky. And if they are isomers? Forget it!

Chromatographic compression is somewhat limited in mass spec because the vacuum state required for the detector restricts higher column flows. Conversely, the VUV detector is at ambient pressure, allowing for significantly higher flow rates. And because all non-enantiomeric compounds have unique VUV absorbance spectra (except carrier gases, conveniently), we can deliberately compress our chromatography. Peaks can then be linearly deconvolved (following Beer’s Law) post-run with a high degree of accuracy using these unique absorbance spectra.

VUV has even removed the need for GC altogether for some applications. A permanent gas mixture from a process line can be streamed through the flow cell, and any fluctuations in relative concentrations can be evaluated in real time. Less emphasis is placed on having symmetric, baseline-separated peaks… or any peaks at all!

VUV certainly isn’t replacing mass spec, at least not any time soon. As my mother says, “Not everything is a competition.” In fact, the two technologies are complementary, working in tandem and using different strengths to provide much more powerful data than either one alone could.

My ultimate point? The qualities that VUV spectroscopy brings to analytical science may change our perspective on the importance of “good chromatography” in general.