About FT-MRR Spectroscopy

Background on BrightSpec’s MRR Technology

Molecular rotational spectroscopy has been used for decades in predominantly two fields, molecular structure research and astronomy. Physical chemists use the rotational spectrum as a means to study molecular structure of gas phase molecules and weakly bound complexes. The structures can be used to deduce bonding characteristics. Astronomers use the rotational spectrum as a means to study the composition of the interstellar medium (ISM) through high frequency radio telescopes. The rotational spectrum is the same for a gas in the ISM as it is in a terrestrial lab. Astrochemistry research has driven much of the analytical and technological advancement of millimeter/submillimeter rotational spectroscopy.

What Kind of Molecules?

Small: < 150 amu
FT-MRR spectral signatures are influenced by molecular mass. As a molecule gets heavier, the rotational energy splittings get closer together to produce more accessible states over which to distribute the population of an ensemble (causing a reduction in signal strength). The rotational frequencies also generally fall to conserve angular momentum with the increase in inertia. This effect shifts the Boltzman peak of the spectrum to lower frequencies. At room temperature, most small volatiles emit strongest in the millimeter-wave region of the spectrum. Furthermore, as a molecule gets more complex, molecular symmetry gets broken (more principle moments of inertia and more energy states), conformational changes must be considered, and low lying vibrational modes can contribute to the spectrum. Molecules > 150 amu are more amenable for BrightSpec custom microwave spectrometers.

Polar: the more the better
The more polar a molecule is, the stronger the FT-MRR signal will be. The non-zero dipole moment is important because it is the rotating dipole (oscillating electric field) that interacts with the electric field of the spectrometer excitation pulse.

Volatile: gases, vaporous liquids, sublimated solids
Free rotation requires a gas phase, however only small amounts of vapor are necessary since the measurement takes place at approximately 10 milliTorr (or 10-5 atm).

MRR Compared to Other Technologies

  • It’s like Nuclear Magnetic Resonance (NMR) Spectroscopy:
    MRR is high-resolution, structurally specific spectroscopy like NMR, and MRR is more simple instrumentation.
  • It’s like Fourier Transform Infrared (FT-IR) Spectroscopy:
    MRR is simple to use like FT-IR, and MRR is more selective allowing for high dynamic range gas mixture library analysis.
  • It’s like Gas Chromatography Mass Spectrometry (GC-MS):
    MRR can detect hundreds of species with one instrument like GC-MS, yet MRR requires no column separation or lengthy method development.
  • It’s like Cavity Ring Down Spectroscopy (CRDS):
    MRR is very sensitive like CRDS, and MRR can detect dozens of species with one spectrometer.
SelectivityMixtures can be analyzed directly without chemical separation and with confident, automated results.MRR is a high-resolution molecular spectroscopy technique. Analysis does not require complex deconvolution or chemometric techniques.
Structural SpecificityIsomers, conformers, isotopologues, and enantiomers are all distinguishable.The rotational spectrum of a molecule is unique to the moment of inertia of a rotating molecule. Any redistribution in mass shifts the spectrum.
Sensitivity In minutes: femto mol detection limits, 100 ppb quantitation limits, dynamic range of 105.BrightSpec patented chirped-pulse Fourier transform technology maximizes the measurement duty cycle.
PrecisionQuantification and detection of small changes in composition enable on line monitoring and method transfer.The millimeter wave light source is very stable. Consecutive spectra are reproducible to < 0.5%.
SimplicityDirect, automated sampling and no special training is required to operate or interpret results.There are no lasers or complicated optics. The highly stable light field is digitally generated, detected, and analyzed.
Multi-Species SensingOne instrument, many molecules. Spectrometers are reprogrammable for new sample matrixes.The sample excitation and detection is digitally controlled for full-band and targeted monitoring of multiple species in the same instrument.
Breadth of ApplicationHeadspace analysis (solids/liquids), residual solvent analysis, ambient air monitoring, gas mixture/pipeline impurity analysis.MRR is a non-destructive, low pressure gas measurement requiring only a few millitorr of total pressure.