Kermit K. Murray

Methylene: A study of the X̃ 3B1 and ã 1A1 states by photoelectron spectroscopy of C2H− and CD2−

Photoelectron spectra are reported for the CH2(X 3B1)+e−←CH−2 (X 2B1) and CH2(a 1A1)+e−←CH−2 (X 2B1) transitions of the methylene and perdeuterated methylene anions, using a new flowing afterglow photoelectron spectrometer with improved energy resolution (11 meV). Rotational relaxation of the ions to ∼300 K and partial vibrational relaxation to <1000 K in the flowing afterglow negative ion source reveal richly structured photoelectron spectra. Detailed rotational band contour analyses yield an electron affinity of 0.652±0.006 eV and a singlet–triplet splitting of 9.00±0.09 kcal/mol for CH2. (See also the following paper by Bunker and Sears.) For CD2, results give an electron affinity of 0.645±0.006 eV and a singlet–triplet splitting of 8.98±0.09 kcal/mol. Deuterium shifts suggest a zero point vibrational contribution of 0.27±0.40 kcal/mol to the observed singlet–triplet splitting, implying a Te value of 8.7±0.5 kcal/mol. Vibrational and partially resolved rotational structure is observed up to ∼9000 c...

Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)

This document contains recommendations for terminology in mass spectrometry. Development of standard terms dates back to 1974 when the IUPAC Commission on Analytical Nomenclature issued recommendations on mass spectrometry terms and definitions. In 1978, the IUPAC Commission on Molecular Structure and Spectroscopy updated and extended the recommendations and made further recommendations regarding symbols, acronyms, and abbreviations. The IUPAC Physical Chemistry Division Commission on Molecular Structure and Spectroscopy’s Subcommittee on Mass Spectroscopy revised the recommended terms in 1991 and appended terms relating to vacuum technology. Some additional terms related to tandem mass spectrometry were added in 1993 and accelerator mass spectrometry in 1994. Owing to the rapid expansion of the field in the intervening years, particularly in mass spectrometry of biomolecules, a further revision of the recommendations has become necessary. This document contains a comprehensive revision of mass spectrometry terminology that represents the current consensus of the mass spectrometry community.

Photoelectron spectroscopy of the halocarbene anions HCF−, HCCl−, HCBr−, HCI−, CF−2, and CCl−2

The 488 nm photoelectron spectra are reported for the HCX(X1A’)+e−←HCX−(X2A‘) and HCX(a3A‘)+e−←HCX−(X2A‘) transitions in HCF−, DCF−, HCCl−, HCBr−, and HCI− and for the CX2(X1A1)+e−←CX−2(X2B1) transitions in CF−2 and CCl−2 . Adiabatic electron affinities are found to be 0.557±0.005 eV (HCF), 0.552±0.005 eV (DCF), 1.213±0.005 eV (HCCl), 1.556±0.008 eV (HCBr), 1.683±0.012 eV (HCI), 0.179±0.005 eV (CF2), and 1.603 ± 0.008 eV (CCl2). Bounds for the triplet excitation energies are determined for all the halocarbenes for which photoelectron spectra were observed, with the exception of CCl2. For the HCX halocarbenes, upper bounds for the triplet excitation energies are 14.7±0.2 kcal/mol (HCF, DCF), 11.4±0.3 kcal/mol (HCCl), and 9±2 kcal/mol (HCBr). A more detailed analysis of HCF suggests the actual triplet excitation energy to be 11.4±0.3 kcal/mol, 14.7±0.2 kcal/mol, or 8.1±0.4 kcal/mol, with the first value the most likely. Since the multiplicity of the ground state of HCl is not known, we report the ener...

DNA sequencing by mass spectrometry.

Mass spectrometry has the potential to replace gel electrophoresis for fast DNA sequencing. In this tutorial paper, it is discussed how far mass spectrometry of DNA has come since the advent of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) and how much remains to be done before mass spectrometry will be a useful tool for DNA sequencing. A brief description of MALDI and ESI mass spectrometric analysis of DNA is presented along with the different strategies for DNA sequencing using mass spectrometry. These include mass spectrometric analysis to replace gel separation in Sanger sequencing, enzymatic ladder sequencing and sequencing by gas-phase fragmentation. The future role of mass spectrometry in DNA sequencing in the Human Genome Project and beyond is also discussed.

Microfluidic chips for mass spectrometry-based proteomics

Microfluidic devices coupled to mass spectrometers have emerged as excellent tools for solving the complex analytical challenges associated with the field of proteomics. Current proteome identification procedures are accomplished through a series of steps that require many hours of labor-intensive work. Microfluidics can play an important role in proteomic sample preparation steps prior to mass spectral identification such as sample cleanup, digestion, and separations due to its ability to handle small sample quantities with the potential for high-throughput parallel analysis. To utilize microfluidic devices for proteomic analysis, an efficient interface between the microchip and the mass spectrometer is required. This tutorial provides an overview of the technologies and applications of microfluidic chips coupled to mass spectrometry for proteome analysis. Various approaches for combining microfluidic devices with electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) are summarized and applications of chip-based separations and digestion technologies to proteomic analysis are presented.

Coupling matrix-assisted laser desorption/ionization to liquid separations

This review discusses the off-line and on-line coupling of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to liquid chromatography (LC) and capillary electrophoresis (CE). Off-line methods involve collecting fractions or depositing a sample on a prepared target prior to MALDI analysis. Because LC fraction collection is straightforward, off-line MALDI LC-MS is relatively easy to implement and widely used. Collecting CE fractions poses a greater challenge due to the small sample volume and the high voltage electrical connection that must be maintained during separation. Methods for off-line MALDI CE-MS employing a porous glass joint, liquid sheath flow, low-volume buffer, or direct deposition are discussed. On-line coupling of MALDI to liquid separations involves introducing the sample directly into the mass spectrometer, using either a continuous-flow (CF) probe or an aerosol spray. With CF MALDI, liquid samples are delivered to the probe through a silica capillary. Aerosol MALDI relies on a pneumatic nebulizer to deliver samples to the mass spectrometer, where ions are formed through irradiation of the particles. CF MALDI and aerosol MALDI have both been coupled to LC separations, although further refinements in both techniques are needed. Future prospects for liquid separations and both off-line and on-line MALDI are discussed.

Laser photoelectron spectroscopy of the formyl anion

The 488 nm photoelectron spectra of HCO− and DCO− show vibrational structure in the X 2A’ state of neutral formyl radical up to 10 000 cm−1 above the vibrational ground state. Electron affinities are found to be 0.313±0.005 eV for HCO and 0.301±0.005 eV for DCO. The CH bond strength and heat of formation of HCO− and the gas phase acidity of formaldehyde are derived from these data. A Franck–Condon analysis of the photoelectron spectra provides an estimate of the equilibrium geometry of the anion. Transitions to excited vibrational states of HCO enable the determination of a complete set of quadratic anharmonicities.

The visible photoabsorption spectrum of Ar+3

The photodissociation cross section of Ar+3 was measured at a number of wavelengths between 1064 and 320 nm. A single broad and featureless band was observed peaking near 520 nm with a width of ≈2600 cm−1 and a peak cross section of ≈10−16 cm2. Consideration of the electronic structure of Ar+3 indicates that the measured spectrum is equivalent to the photoabsorption spectrum. Two ionic products, Ar+ and Ar+2, were observed in the photodissociation of Ar+3, indicative of at least two exit pathways and suggestive of two electronic transitions.

Small Molecule Ambient Mass Spectrometry Imaging by Infrared Laser Ablation Metastable-Induced Chemical Ionization

Presented here is a novel ambient ion source termed infrared laser ablation metastable-induced chemical ionization (IR-LAMICI). IR-LAMICI integrates IR laser ablation and direct analysis in real time (DART)-type metastable-induced chemical ionization for open air mass spectrometry (MS) ionization. The ion generation in the IR-LAMICI source is a two step process. First, IR laser pulses impinge the sample surface ablating surface material. Second, a portion of ablated material reacts with the metastable reactive plume facilitating gas-phase chemical ionization of analyte molecules generating protonated or deprotonated species in positive and negative ion modes, respectively. The successful coupling of IR-laser ablation with metastable-induced chemical ionization resulted in an ambient plasma-based spatially resolved small molecule imaging platform for mass spectrometry (MS). The analytical capabilities of IR-LAMICI are explored by imaging pharmaceutical tablets, screening counterfeit drugs, and probing algal tissue surfaces for natural products. The resolution of a chemical image is determined by the crater size produced with each laser pulse but not by the size of the metastable gas jet. The detection limits for an active pharmaceutical ingredient (acetaminophen) using the IR-LAMICI source is calculated to be low picograms. Furthermore, three-dimensional computational fluid dynamic simulations showed improvements in the IR-LAMICI ion source are possible.

Intact and top-down characterization of biomolecules and direct analysis using infrared matrix-assisted laser desorption electrospray ionization coupled to FT-ICR mass spectrometry

We report the implementation of an infrared laser onto our previously reported matrix-assisted laser desorption electrospray ionization (MALDESI) source with ESI post-ionization yielding multiply charged peptides and proteins. Infrared (IR)-MALDESI is demonstrated for atmospheric pressure desorption and ionization of biological molecules ranging in molecular weight from 1.2 to 17 kDa. High resolving power, high mass accuracy single-acquisition Fourier transform ion cyclotron resonance (FT-ICR) mass spectra were generated from liquid- and solid-state peptide and protein samples by desorption with an infrared laser (2.94 µm) followed by ESI post-ionization. Intact and top-down analysis of equine myoglobin (17 kDa) desorbed from the solid state with ESI post-ionization demonstrates the sequencing capabilities using IR-MALDESI coupled to FT-ICR mass spectrometry. Carbohydrates and lipids were detected through direct analysis of milk and egg yolk using both UV- and IR-MALDESI with minimal sample preparation. Three of the four classes of biological macromolecules (proteins, carbohydrates, and lipids) have been ionized and detected using MALDESI with minimal sample preparation. Sequencing of O-linked glycans, cleaved from mucin using reductive β-elimination chemistry, is also demonstrated.