Molecular rotational resonance (MRR) spectroscopy gives spectral signatures with high chemical selectivity. At room temperature, the peak intensity of the MRR spectrum occurs in the 100 GHz – 1 THz frequency range for volatile species with mass ≤ 100 amu. Advances in high-power sub-mm-wave light sources has made it possible to implement time-domain Fourier transform (FT) spectroscopy techniques that are similar to FT nuclear magnetic resonance (FT-NMR) measurements. In these measurements, the gas sample is excited by a short (200 ns) excitation pulse that creates a macroscopic sample polarization. The electric field of the subsequent transient molecular emission is detected using a heterodyne receiver and a high-speed digitizer. FT-MRR spectroscopy offers speed and sensitivity improvements over absorption spectroscopy. For chemical analysis, FT-MRR spectrometers combine the benefits of broad chemical coverage typical of gas chromatography – mass spectrometry (GC-MS) instruments and the direct measurement capabilities of infrared gas sensors all in a reprogrammable platform. Pulse sequence measurements can be implemented for advanced spectroscopic analysis. Trace level quantitation of volatile species at ppbv concentration can be performed on the time scale of a minute. In cases where the sample is a complex mixture, a double-resonance pulse sequence can be used to achieve chemical selectivity even in cases where spectral overlap occurs. These measurement capabilities are illustrated using the application of FT-MRR spectroscopy to residual solvent analysis of pharmaceutical products.
Pure rotational spectroscopy in the centimeter, millimeter, and THz regions of the electromagnetic spectrum is a
powerful technique for the characterization of polar molecules in the gas phase. Although this technology has a long
history in the research sector for structural characterization, recent advances in digital electronics have only recently
made commercial instruments competitive with established chemical analysis techniques. BrightSpec is introducing a
platform of pure rotational spectrometers in response to critical unmet needs in chemical analysis. These instruments aim
to deliver the operational simplicity of Fourier transform infrared spectrometers in conjunction with the chemical
analysis capabilities of mass spectrometers. In particular, the BrightSpec ONE instrument a broadband gas mixture
analyzer with full capabilities for chemical analysis. This instrument implements Fourier transform millimeter-wave
emission spectroscopy, wherein a brief excitation pulse is applied to the sample, followed by the measurement of the
coherent free induction decay responses of all molecular transitions within the excitation bandwidth. After sample
injection and characterization, the spectrometer returns a list of all known species detected in the sample, along with
their concentrations in the mixture. No prior knowledge about the sample composition is required. The instrument can
then perform double-resonance measurements (analogous to 2-D COSY NMR), direct mass determination through
analysis of the time profile of the molecular signal, and automated isotopic identification as part of a suite of tools that
can return the structural identity of the unknowns in the sample.
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