Temperature dependence of field desorption mass spectra. Investigation of peptide synthesis intermediates

A pulse mode programmer has been developed for field desorption mass spectrometry that provides setting of the emitter heating current during the scans, switching off the emitter heating current during the intervals between consecutive scans and increasing it by 0.5 mA at the start of each scan. Using this heating device, the temperature dependence of field desorption mass spectra of some peptide synthesis products has been studied.     

Liquid chromatography-mass spectrometry of trace compounds with a moving-belt interface and multi-dimensional chromatography

A high-performance liquid chromatographic method with on-line mass spectrometric detection is described for the structural analysis of a number of synthetic impurities, present at trace levels in almitrine. To obtain mass spectra with various ionization methods and high-resolution mass measurements, a moving-belt liquid chromatograph-mass spectrometer interface is used. A two-column switching system allows the injection of large amounts of almitrine, from which the trace compounds are trapped on a second column, while discarding the major component. This permits the introduction of the impurities into the mass spectrometer by elution of the second column, without the risk of introducing too large an amount of the major compound into the mass spectrometer. The mass spectra thus obtained are of sufficient quality to permit a correct structural assignment of the impurities.     

Temperature dependence of solid uranyl-tris-acetate fluorescence spectra

Fluorescence spectra of Na [UO₂ acetate₃] measured at liquid nitrogen temperature are evaluated as the sum of vibronic transition bands, 8 base frequencies were found which build long progressions with the total-symmetric uranyl vibration. IR, FIR and Raman spectra were used to assign the vibrational bands.     

Temperature dependence of the surface free energy of a crystal-liquid interface

A dimensionless complex containing the surface free energy of a crystal-liquid interface γ, and the entropy jump, temperature, and density of a crystal phase is described using the phenomenology of thermodynamic similarity; this complex remains constant at the melting line. It is demonstrated that the complex refines the result obtained by Skripov and Faizullin in [6] and enables us to estimate the temperature dependence of γ. Our calculations show that the surface free energy of the crystal-liquid interface of normally melting compounds is a monotonically increasing function of temperature.     

Temperature dependence of resonance Raman spectra of carotenoids

To understand the mechanism of the photoprotective and antioxidative functions of carotenoids, it is essential to have a profound knowledge of their excited electronic and vibronic states. In the present study we investigate the most powerful antioxidants: β-carotene and lutein by means of resonance Raman spectroscopy. The aim was to study in detail their Raman spectra in solution at room temperature and their changes as a function of temperature. To measure the spectra in their natural environment pyridine has been used as a solvent. It has been chosen because of its polarizability (n=1.5092) which is close to that of membrane lipids and proteins. The temperature dependence of the most intensive ν(1) band in the range from 77 K to 295 K at 514.5 nm excitation has been obtained. It was found that in pyridine the CC stretching frequency, its intensity, line shape, and line width are very sensitive to the temperature (the sensitivity being different for the two studied carotenoids). The observed linear temperature dependence of the CC stretching frequency is explained by a mechanism involving changes of the vibronic coupling and the extent of π-electron delocalization. The different behavior of the temperature-induced broadening of the ν(1) band and its intensity for the two studied carotenoids can be associated with the different nature of their solid matrices: glassy for β-carotene and crystalline-like for lutein, owing to their different chemical structures.     

Program for elemental analysis using low or medium resolution mass spectra

A program, called ELANAL, has been written in basic which computes all possible empirical formulae for a molecular ion or a fragment ion containing up to eleven common elements. The program then calculates the expected intensities of the peaks following the molecular or fragment ion due to heavier isotopes in the formulae. These calculated intensities are compared with the intensities in an experimentally determined mass spectrum.     

Temperature dependence of the Raman spectra of individual carbon nanotubes

Resonant Raman scattering (RRS) spectra of individual carbon nanotubes on a SiO2 substrate have been investigated first in the temperature range of 100-600 K (Phys. Rev. B 2002, 66, 115411). It was revealed by the intensity abnormality of the radial breathing mode (RBM) that the carbon nanotubes have a temperature-dependent density of electronic states. This means that the previously reported temperature coefficients of RBM of carbon nanotubes are smaller than their "real" ones for the bulk samples of single- or double-walled carbon nanotubes. Comparatively, the G line of individual nanotubes shows no observable difference relative to the bulk samples.     

Fast-atom bombardment-induced condensation of glycerol with ammonium surfactants II: Time dependence of mass spectra and tandem mass spectra

Studies of fast-atom bombardment (FAB)-induced condensation between trimethyltetradecylammonium cations and glycerol have been extended to consider spectral time dependence. To enhance reproducibility of time dependence, a modified FAB target was used. FAB mass spectrometry of deuterium-labeled surfactants and FAB/collision-induced dissociation (CID) of nonlabeled material demonstrate that products of condensation at the surfactant “head group” predominate early in the analysis, while tail adducts become prominent later. This time dependence correlates with the expected surface activity of the products. It is incompatible with gas-phase reaction, but consistent with reaction in the condensed phase. Subtle variations in the surface activities of various condensation products (derived from changes in the number of hydroxyls from the reactive glycerol radical or in the position of attack along the surfactant chain) are reflected in the time dependence of FAB and CID spectra. CID spectra of deuterium-labeled cations provide evidence for intramolecular hydrogen transfer from the surfactant tail to the head within a surfactant radical. This transfer shows no significant kinetic isotope effect.     

Atmospheric pressure thermospray ionization using a heated microchip nebulizer

When a standard atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI) ion source is used without applying the corona discharge or photoirradiation, atmospheric pressure thermospray ionization (APTSI) of various compounds can be achieved. Although largely ignored, this phenomenon has recently gained interest as an alternative ionization technique. In this study, this technique is performed for the first time on a miniaturized scale using a microchip nebulizer. Sample ionization with the presented microchip‐APTSI (µAPTSI) is achieved by applying only heat and gas flow to a nebulizer chip, without any other methods to promote gas‐phase ionization. To evaluate the performance of the described µAPTSI setup, ionization efficiency for a set of test compounds was monitored as the microchip positioning, temperature, nebulizer gas flow rate, sample solution composition, and solvent flow rate were varied. The µAPTSI mass spectra of the test compounds were also compared to those obtained with ESI and APCI. The µAPTSI produces ESI‐like spectra with low background noise, favoring the formation of protonated or deprotonated molecules of compounds that are ionizable in solution. Multiple charging of peptides without in‐source fragmentation was also observed. Unlike ESI, however, the µAPTSI source can tolerate the presence of mobile phase additives like trifluoroacetic acid (TFA) without significant ion suppression. The µAPTSI source can be used with standard mass spectrometer ion source hardware, being a unique alternative to the present interfacing techniques. Copyright © 2009 John Wiley & Sons, Ltd.     

Studies on the complexation of polyols and carbohydrates with excess borate using thermospray mass spectrometry

Complex formation between different polyols, carbohydrates and borate was studied by thermospray mass spectrometry. The analytes were dissolved in 0.1 M ammonium borate buffer and introduced through a standard thermospray interface into a quadrupole mass spectrometer. The formation of various complexes which gave rise to relatively abundant ions in the thermospray spectra was observed. A correlation has been found between the relative intensities of these ions and the orientation of the hydroxyl groups involved in the complexation.     

Analysis of vitamin D and its metabolites using thermospray liquid chromatography/mass spectrometry.

A new method is described for the analysis of vitamin D and its metabolites utilizing thermospray (TSP) mass spectrometry as an on-line detector for high performance liquid chromatography. Ionization conditions were optimized for use with isocratic reversed phase chromatography. TSP mass spectrometry was employed in series with a UV absorbance detector to facilitate comparisons between the two methods of detection. Positive ion TSP mass spectra were recorded for vitamin D2, vitamin D3, 25-hydroxyvitamin D3 (25(OH)D3), 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 24,25-dihydroxyvitamin D3 (24,25(OH)2D3). The spectra contained protonated molecular ions, ammonium adduct ions and fragment ions due to the loss of one or more molecules of water. A comparison of quantitative precision was made by determining UV absorbance and TSP standard curves for vitamin D3 using two different methods: (1) External standard method with post-column (post UV detector) addition of ammonium acetate. (2) As (1) but using the method of internal standards with a closely eluting internal standard (vitamin D2). In each case the quantitative precision (correlation coefficient) for UV absorbance detection was superior owing to intrinsic instability of the TSP ion beam. A stable isotopically labelled internal standard was employed in the development of an assay for 1,25(OH)2D3. The assay was used to quantify in vitro enzymic conversion of 25(OH)D3 to 1,25(OH)2D3 in guinea pig and sheep renal mitochondrial incubations. TSP LC/MS was also applied to analysis of an extract of human blood plasma in which D3 and each of its principal metabolites were identified in a single analysis.     

Improvements in instrumentation for thermospray operation on a magnetic sector mass spectrometer

Changes in the design of a thermospray liquid chromatography-mass spectrometry ion source can be made to improve the sensitivity towards solute related ions and therefore extend the practical utility of the complete system. The addition of a discharge ionization facility provides much greater scope for gradient elution analyses and forms the basis of a method which offers increased structural information. All of these changes are illustrated by practical examples.     

Repeller-Induced dissociation of alkyl alcohols in thermospray mass spectrometry

Six alkyl alcohols were studied using thermospray mass Spectrometry. Whereas the dominant ion in the spectrum up to a repeller potential of 120 V was [M + NH₄]⁺, above that potential [M + H]⁺ and fragment ions appeared. The fragments observed were largely due to hydrogen release from alkyl ions ([C_nH_2n+1]⁺ – H2 → [C_nH_2n-1]⁺) and loss of water or some other stable molecule from the same species. The results are compared with those from ionization of the same alcohols under electron impact and photoionization conditions and with results obtained for methanol under thermospray conditions.