Exploiting the Intensity Stability of Broadband Rotational Spectroscopy to Analyze Chemical Mixtures

The introduction of chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy improved the ability to analyze complex chemical mixtures by making it possible to acquire a broadband spectrum in each measurement acquisition. The availability of a broadband spectrum makes it possible to identify components of the mixture through their rotational spectroscopy signature. The ability to measure the full spectrum on each data acquisition combats the challenges caused by drift in the sample sources that can cause time variability in the sample composition. Early applications of broadband rotational spectroscopy to complex mixtures included analyzing reaction products in electric discharge reactions related to astrochemistry and identifying the isomers of water clusters in the size range 2-15. Over time, an appreciation grew for the long-term stability in the relative intensities in spectra from CP-FTMW spectrometers. It is common to observe 0.1% reproducibility in the relative intensities across the full frequency range of the spectrometer. This feature of CP-FTMW instruments has enabled a new set of experimental approaches to the analysis of complex mixtures. The common measurement concept is the variation of the sample composition through control of an external parameter. For traditional chemical mixtures, like volatiles in essential plant oils, the composition of the vapor entrained by the inert carrier gas can be achieved by varying the temperature of the liquid. Analysis of the time variation in the intensity of each transition makes it possible to cluster the peaks associated with each chemically distinct species. In a more difficult analysis challenge, the relative composition of two gases can be varied to help identify the set of mixed clusters that can form in a pulsed jet expansion. Isotope dilution can also be used in the analysis of clusters to provide information about the number of monomers in each cluster – an approach recently used to identify a family of water-14 isomers. Finally, variation of the enantiomer distribution of a sample can be used to quantitatively measure the enantiomeric excess of chiral sample without the need for any spectroscopic analysis.