Stephen L. Coy

Fourier transform spectra of overtone bands of HCN from 5400 to 15100 cm−1☆

Abstract The absolute intensities and vibration-rotation constants of 26 overtone and combination bands of HCN are reported. The dynamic range for the absolute intensity measurements is nearly one million to one. Absorption spectra of HCN from 5400 to 15100 cm −1 were obtained using the Fourier transform spectrometer at the Kitt Peak National Solar Observatory with optical path lengths up to 432 m. The frequencies of 1346 assigned HCN lines were used to derive vibration-rotation constants for 23 bands (14 Σ-Σ, 4 Π-Π, 4 Π-Σ, and 1 Σ-Π) of H 12 C 14 N, 3 bands (Σ-Σ) of H 13 C 14 N, and 2 bands (Σ-Σ) of H 12 C 15 N. These new band origin and rotational constant data have been combined with existing data given in the literature to derive an improved set of vibrational (ω, x , and y ) constants and rovibrational (α and γ) constants for HCN. The vibrational dependence of the centrifugal distortion constants has also been examined. One thousand thirtysix derived line areas were used to determine absolute intensities for all 28 bands. Four weak stretch-only bands were observed for the first time: the (300)-(000), the (201)-(000), the (301)-(000) and the (202)-(000). Such data should be an important aid in accurately determining the CN contribution to the potential and dipole moment functions. Finally, we present a comparison of 13 of the measured absolute overtone intensities (stretching states only) with recent ab initio results.

Chemical effects in the separation process of a differential mobility/mass spectrometer system.

In differential mobility spectrometry (also referred to as high-field asymmetric waveform ion mobility spectrometry), ions are separated on the basis of the difference in their mobility under high and low electric fields. The addition of polar modifiers to the gas transporting the ions through a differential mobility spectrometer enhances the formation of clusters in a field-dependent way and thus amplifies the high- and low-field mobility difference, resulting in increased peak capacity and separation power. Observations of the increase in mobility field dependence are consistent with a cluster formation model, also referred to as the dynamic cluster-decluster model. The uniqueness of chemical interactions that occur between an ion and cluster-forming neutrals increases the selectivity of the separation, and the depression of low-field mobility relative to high-field mobility increases the compensation voltage and peak capacity. The effect of a polar modifier on the peak capacity across a broad range of chemicals has been investigated. We discuss the theoretical underpinnings which explain the observed effects. In contrast to the result with a polar modifier, we find that using mixtures of inert gases as the transport gas improves the resolution by reducing the peak width but has very little effect on the peak capacity or selectivity. The inert gas helium does not cluster and thus does not reduce low-field mobility relative to high-field mobility. The observed changes in the differential mobility alpha parameter exhibited by different classes of compounds when the transport gas contains a polar modifier or has a significant fraction of inert gas can be explained on the basis of the physical mechanisms involved in the separation processes.

Control of Chemical Effects in the Separation Process of a Differential Mobility Mass Spectrometer System

Differential mobility spectrometry (DMS) separates ions on the basis of the difference in their migration rates under high versus low electric fields. Several models describing the physical nature of this field mobility dependence have been proposed but emerging as a dominant effect is the clusterization model sometimes referred to as the dynamic cluster–decluster model. DMS resolution and peak capacity is strongly influenced by the addition of modifiers which results in the formation and dissociation of clusters. This process increases selectivity due to the unique chemical interactions that occur between an ion and neutral gas-phase molecules. It is thus imperative to bring the parameters influencing the chemical interactions under control and find ways to exploit them in order to improve the analytical utility of the device. In this paper, we describe three important areas that need consideration in order to stabilize and capitalize on the chemical processes that dominate a DMS separation. The first involves means of controlling the dynamic equilibrium of the clustering reactions with high concentrations of specific reagents. The second area involves a means to deal with the unwanted heterogeneous cluster ion populations emitted from the electrospray ionization process that degrade resolution and sensitivity. The third involves fine control of parameters that affect the fundamental collision processes, temperature and pressure.

Radiation metabolomics and its potential in biodosimetry

Purpose: Radiation exposure triggers a complex network of molecular and cellular responses that impacts metabolic processes and alters the levels of metabolites. Such metabolites have potential as biomarkers for radiation dosimetry. This review provides an overview of radiation signalling and metabolism, of metabolomic approaches used in the discovery phase, and of instrumentation with the potential to assess radiation injury in the field. Approach: Recent developments in fast, high-resolution chromatography and mass spectrometry and new data analysis methods allow the quantitative assessment of thousands of metabolites based on biofluids obtained non-invasively. This complex analysis leads to the discovery-phase identification of groups of metabolites useful for screening and biodosimetry by targeted quantitative measurement. Instrumentation for target analysis can be simpler than that used for discovery, so we examine current technologies based on ion mobility. Conclusions: Recent published results and ongoing studies examine the complex changes in the levels of many metabolites caused by radiation exposure, and identify groups of small-molecule biomarkers for radiation biodosimetry. Based on results showing separation orthogonal to mass, chemical noise suppression, and high sensitivity, differential mobility mass spectrometry (DMS-MS) ion mobility spectrometry appears highly promising for the development of deployable instrumentation.

Microwave detected, microwave–optical double resonance spectra of NO2: A test of Hardwick’s ergodicity conjecture

In a recent paper, Hardwick predicted that the optical spectrum of NO2 should display ‘‘quantum ergodicity’’, meaning that almost all eigenstates allowed by total symmetry should be seen in absorption. We have performed a test of this prediction by using the technique of microwave detected, microwave–optical double resonance. We have determined the spectrum over the range 16 810–17 100 cm−1 for the two ground vibrational states 91,9(F1) and 100,10(F1) of NO2. We observed 324 transitions from the 100,10 state and 364 transitions from the 91,9 state. The number of observed transitions is a factor of 8 greater than that expected if only the allowed rotational transitions to any B2 symmetry vibronic level were observed. The number of observed lines is a factor of 3 less than the number predicted if all selection rules were broken as predicted by Hardwick. Thus we see that Hardwick’s ‘‘ergodicity’’ conjecture is a useful starting point for understanding the spectrum of NO2, but the spectrum is intermediate bet...

Modeling the rotational and vibrational structure of the i.r. optical spectrum of NH3

Abstract Current progress on the rotational assignment of NH 3 vibrational bands from 4800 cm −1 to 18 000 cm −1 is reported. We discuss a vibrational Hamiltonian for regions containing overtones or combinations of the three normal modes, excluding the symmetric inversion motion. The Hamiltonian is capable of modeling the normal mode to local mode transition observed in the spectrum as well as the observed band origins, rotational constants and intensifies. It includes the Darling—Dennison (local mode—normal mode) coupling, the (ν 1 ν 3 )-2ν 4 Fermi resonance, and one significant (ν 1 ν 3 )-ν 2 interaction. We illustrate the intramolecular vibrational dynamics predicted by the model Hamiltonian.

Rotational structure of ammonia N–H stretch overtones: Five and six quanta bands

The five and six quanta bands of the N–H overtone bands of NH3, with origins near 15 450 and 18 110 cm−1, respectively, have been rotationally analyzed. Assignments of 318 and 105 lines in the two regions have been made on the basis of microwave‐detected microwave‐optical double resonance spectra and of photoacoustic absorption spectra. Both regions contain hybrid bands with parallel and perpendicular components which show Coriolis interactions between the optically bright components, and frequent, generally patternless, interactions with background dark vibrational states. These interactions cause almost all lines to be broken into several components. The five quanta region also shows a strong Fermi interaction, suggested due to 4νN–H+3νa2, which causes a negative inversion splitting in the upper states. The relative strengths of the parallel and perpendicular bands are (0.3:0.7) and (0.35:0.65) for the five and six quanta regions, in contrast to lower energies where parallel bands are weak or absent. Th...

Selection and generation of waveforms for differential mobility spectrometry

Devices based on differential mobility spectrometry (DMS) are used in a number of ways, including applications as ion prefilters for API-MS systems, as detectors or selectors in hybrid instruments (GC-DMS, DMS-IMS), and in standalone systems for chemical detection and identification. DMS ion separation is based on the relative difference between high field and low field ion mobility known as the alpha dependence, and requires the application of an intense asymmetric electric field known as the DMS separation field, typically in the megahertz frequency range. DMS performance depends on the waveform and on the magnitude of this separation field. In this paper, we analyze the relationship between separation waveform and DMS resolution and consider feasible separation field generators. We examine ideal and practical DMS separation field waveforms and discuss separation field generator circuit types and their implementations. To facilitate optimization of the generator designs, we present a set of relations that connect ion alpha dependence to DMS separation fields. Using these relationships we evaluate the DMS separation power of common generator types as a function of their waveform parameters. Optimal waveforms for the major types of DMS separation generators are determined for ions with various alpha dependences. These calculations are validated by comparison with experimental data.

Numerical pattern recognition analysis of acetylene dispersed fluorescence spectra

Polyad quantum numbers have been assigned to 134 vibrational levels of the X1Σg+ state of acetylene with internal energies from 3,000 to 15,000 cm−1. These polyad assignments have been made possible by two advances: (1) the recording of new, rigorously calibrated acetylene A1Au→X1Σg+ dispersed fluorescence spectra, and (2) the development of a numerical pattern recognition technique which identifies groups of transitions in the spectra that terminate on eigenstates with the same polyad quantum numbers. This pattern recognition technique is based on the Extended Cross-Correlation, which has been reported previously in this Journal [J. Chem. Phys. 107, 8349, 8357 (1997)], and requires neither a priori knowledge of the number of polyads in the spectra nor the pattern of spectral lines that is associated with each polyad. No evidence for the breakdown of the polyad quantum numbers is found, at the 7 cm−1 resolution of our spectra, at internal energies up to at least 15,000 cm−1. The ability to assign polyad...

Speed dependence of microwave rotational relaxation rates

For most molecular systems, rotational relaxation rates depend strongly on absorber speed. The influence of this speed dependence on transient decays in the microwave region and on the measurement of rotational relaxation rates, line positions, and line shifts is described. Measurement of the speed dependence of relaxation rates may be made from the nonexponential behavior of transient decays which are more sensitive to the speed dependence than line shapes. We describe how these transient experiments may be performed using either amplitude modulation or Stark switching, how the data may be analyzed using calculations of nonexponentiality from pressure‐dependent sources, and give experimental results for these transitions: OCS J=0−1 and J1–2, H2CO 212−211, NH3 (3,2), and ethylene oxide 431−422. Previous transient experiments need to be re‐evaluated in terms of a speed dependent relaxation rate. The speed dependence may make the measured rate an ill‐defined population average which cannot be compared to st...