Fourier transform molecular rotational resonance spectroscopy for reprogrammable chemical sensing

Abstract

Molecular rotational resonance (MRR) spectroscopy gives spectral signatures with high chemical selectivity. At roomtemperature, 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.

Direct Measurements of Small Polar Impurities in Gasoline Mixtures Using Molecular Rotational Resonance Spectroscopy

“…A BrightSpec millimeter wave MRR spectrometer, which is used in this study and operates in the 260–290 GHz range, has been described elsewhere. 15,16 The present spectrometer is integrated with an automatic sampler capable of sample injection, equilibration, and static or dynamics headspace transfer. Briefly, a broadband millimeter wave light source (35 GHz bandwidth) consists of a fast-pulsed arbitrary waveform generator and multiple active multiplier chains.…”

“…Ten to hundred times faster analysis and/or better signal-to-noise (S/N) ratios could be achieved by using a narrowband targeted spectral measurements around specific rotational transitions of known species, as described elsewhere. 16

Figure 2.A broadband rotational spectrum of gasoline resulting from a signal averaging of 100 000 shots in the 265–290 GHz frequency range. (a) Observed rotational spectra of gasoline(red) matched to that of known references for ethanol (blue), toluene (green), acetaldehyde (purple), and ethyl cyanide (black).

…”

“…The static headspace sampling module, which is employed in this work, has been previously reported elsewhere. 15,16 A schematic diagram illustrating the procedure is shown in Fig. 3.…”

“…As an example, the 260–290 GHz MRR technique has previously proven capable of acquiring a broadband rotational spectrum of several polar organic volatiles in a mixture. 6,15,16 The fast speed, high-resolution, direct chemical specificity, and broadband capabilities inherited by the MRR instrument have allowed direct detection of organic volatiles in complex chemical mixtures. In this study, the MRR is employed to accurately determine and characterize small polar impurities in gasoline mixtures.…”