Andrew Proctor

Curve Fitting Analysis of ESCA Ni 2p Spectra of Nickel-Oxygen Compounds and Ni/Al2O3 Catalysts:

Nonlinear least-squares curve fitting is used to analyze Ni 2p spectra obtained from various nickel-oxygen compounds and Ni/Al2O3 catalysts. Apart from NiO, which shows a characteristic discrete satellite peak 2 eV from the main 2p3/2 peak, all other spectra exhibit variable peak asymmetry assigned to multiplet splitting effects. By the use of a single extra peak to describe this asymmetry, the relative satellite/main peak intensity ratio can be used as an indication of the chemical state of nickel. Results for Ni/Al2O3 catalysts agree with reduction studies from previous work about the nature of nickel species on the surfaces of these catalysts at various nickel loadings.

Important Aspects Concerning the Quantification of Biomolecules by Time-of-Flight Secondary-Ion Mass Spectrometry

Fundamental aspects regarding the use of time-of-flight secondary-ion mass spectrometry (TOF-SIMS) as a quantitative tool for the analysis of organic compounds are reported. The following factors are discussed: (1) the use of Poissons law to correct for dead-time in single-ion data collection; (2) practical considerations concerning the analysis of “real world” samples; and (3) the effect of the etching process on the reproducibility of the intensity ratio (analyte/internal standard) of Ag-cationized species. To evaluate the importance of these factors, we used cocaine and cyclosporin A (CsA) as analytes because they show protonated and Ag-cationized species, respectively, in their SIMS spectra. Correction for detector dead-time using Poissons law of single-ion counting expanded the dynamic range for cocaine by ∼2 orders of magnitude. For analyses requiring only a small dynamic range (i.e., CsA), the correction improved the % RSD of the slope from 2.43 to 0.87%. The maximum secondary-ion (SI) yield of CsA (Ag-cationized species) occurs at a CsA concentration ∼3 orders of magnitude higher than the therapeutic levels in blood (25–2000 ng/mL). It is discussed how this problem should be addressed. Analysis of variance (ANOVA) indicates that Ag substrates must be etched under identical conditions to obtain quantitative results when species requiring cationization are being analyzed.

Inelastic Background and Peak Area Determination in X-Ray Photoelectron Spectroscopy (ESCA)

Analysis of the Au 4f ESCA spectra of thin gold films deposited over a graphite base shows that as the separation from the main photoelectron peak, ΔE, increases the background decreases. Conversely, the effect of a contaminant overlayer on solid gold gives rise to a background which increases as ΔE increases. There is, therefore, a considerable variety of background profiles which can be encountered in real samples, which depends markedly on the element, its concentration, and its distribution in the sample. Over the region of the photoelectron line signal itself, the simple integral description which assumes that the background is proportional to the number of electrons of higher kinetic energy is reasonably valid and is suggested for peak area measurement when a nonlinear profile is needed. Direct integration and curve fitting techniques constitute the two main methods of peak area measurement. Simulated Ni 2p spectra are used to compare the effectiveness of the simple integral and linear background profiles for producing the correct peak areas. Although the simple integral profile shows more consistency and is to be preferred in general, certain fortuitous cancellation effects, particularly with non-monochromatized spectra, make areas using the linear background fairly accurate in some instances. Curve fitting can overcome many of the problems of direct integration resulting from severe peak overlap. Two types of Gaussian/Lorentzian functions with a suitable exponential tailing function, which has a finite area over reasonable integration limits, are discussed with regard to their use in non-linear least-squares curve fitting methods. Also discussed is the usefulness of composite and difference spectra as simpler methods of determining the relative areas of overlapping peaks.

The adsorption of metal oxyanions on alumina

The adsorption of perrhenate, permanganate, molybdate, tungstate, and chromate on alumina was studied at pH 4.5. The effect of nitrate in the impregnation solution on the adsorption of these ions was examined as well as the effect of washing the freshly prepared catalysts. The uptake of perrhenate and permanganate decreased with increasing nitrate in the impregnation solution; the uptake of molybdate, tungstate, and chromate was unaffected. Washing the catalysts almost completely removed perrhenate, permanganate, and chromate, but allowed a significant amount of molybdate and tungstate to remain on the alumina. It is concluded that tungstate and molybdate adsorb on two types of surface sites, producing loosely and tightly bound surface species. From the amounts of molybdate and tungstate adsorbed, and from EXAFS results from washed and unwashed Mo/Al2O3 samples, both loosely and tightly bound molybdate and tungstate are polymeric in nature. The results are discussed in terms of electrostatic and surface reaction models.

Determination of the Mo surface environment of Mo/TiO2 catalysts by EXAFS, XANES and PCA

The local structure of oxidic Mo/TiO2 catalysts (0.5 to 13.5 wt.% Mo) has been studied using EXAFS and XANES. Both EXAFS and XANES results suggest that the Mo surface phase is octahedrally coordinated for all Mo loadings. The EXAFS results were also examined using principal component analysis (PCA) to determine the number of Mo species present on the Mo/TiO2 catalysts. Results from PCA of the Mo EXAFS spectra suggested the presence of three Mo species: two surface species and bulk MoO3.

Quantitative Analysis of Peptides by Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight Mass Spectrometry

Quantitative analysis of peptides using internal standards was carried out with the use of matrix-assisted laser desorption/ionization (MALDI) on a time-of-flight mass spectrometer in a ferulic acid matrix. Spectra were collected and averaged over several areas of the sample surface at constant laser energy, and a defocused beam was used to improve reproducibility. A calibration curve was plotted, showing a correlation coefficient of 0.992 and a relative standard deviation of 1.83%. The accuracy and precision of this technique were sensitive to the crystal structure of the matrix and saturation effects of the instruments detector.

Determination of sulfur forms on coal surfaces by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS or ESCA) was used to monitor changes in the amount of organic and pyritic sulfur, and inorganic sulfur present on coal surfaces following hydrochloric and nitric acid treatments. The results show that after treatment with HCl the oxidized sulfur was totally removed from the surface. Subsequent leaching with HNO 3 decreased the atomic abundance of sulfide forms on the coal surface.

Application of sequential paired covariance to liquid chromatography-mass spectrometry data enhancements in both the signal-to-noise ratio and the resolution of analyte peaks in the chromatogram

Abstract The algorithm of sequential paired covariance (SPC) has been previously reported to dramatically enhance the signal-to-noise (S/N) ratio for on-line separations combined with mass spectrometry. That initial study focused on a limited number of data sets derived from the combination of capillary electrophoresis (CE) with time-of-flight mass spectrometry using an electrospray interface. Results from the initial study clearly demonstrated that a significant enhancement (almost two orders of magnitude) in the S/N ratio of the eluting peaks in the electropherogram could be obtained, facilitating identification of the analytes. In this report, the algorithm has been applied to liquid chromatography-mass spectrometry data obtained on a triple quadrupole instrument and we have evaluated the general applicability of the SPC approach to several types of microcolumn separations with mass spectrometric detection, including CE coupled with Fourier transform ion cyclotron resonance mass spectrometry. In all the cases we tested, we found the algorithm enhanced the S/N ratios of the resulting chromatograms or electropherograms to a similar extent. This report further demonstrates the SPC approach to enhance the resolution as well as the S/N ratio of the eluting peaks of a complex peptide mixture. While many variations of the algorithm are possible, we have also found higher order covariance (e.g., 3rd order) is useful for eliminating coincidental noise in sequential mass spectra, giving the potential to extract broad, low intensity analyte peaks. We also demonstrate the sequential covariance approach for enhancing the S/N ratio of mass spectra.

A quantitative study of in vitro hepatic metabolism of tacrolimus (FK506) using secondary ion and matrix-assisted laser desorption/ionization mass spectrometry.

The identification and simultaneous quantification of Tacrolimus and its hepatic metabolites in baboons has been achieved using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry and static secondary-ion mass spectrometry (TOF-SIMS). Little fragmentation, high sensitivity and tolerance to contamination are the major advantages of these methods, allowing facile identification and quantification of metabolites produced in vitro with minor analyte isolation. Based on the MALDI and TOF-SIMS results, seven metabolites have been identified: de-methylated, di de-methylated, hydroxylated, di hydroxylated, de-methylated hydroxylated, dihydrodiol, and di de-methylated hydroxylated. The concentrations of the parent drug and its major metabolites (e.g. de-methylated, di de-methylated) were measured using Rapamycin as an internal standard. The time course of Tacrolimus and its major metabolites as a function of incubation time was calculated. Good correlation between SIMS and MALDI results was obtained.

EXAFS Smoothline Determination Using Splines: Finding the Optimal Smoothline and Fast Computation

The feasibility of the PC IMSL cubic spline smoothing routine CSSMH (ICSSCU in the mainframe edition) for numerical determination of the EXAFS smoothline (μo) is discussed. Examination of the cubic spline algorithm employed suggests that a relatively simple coding modification results in a considerably more efficient algorithm for use in EXAFS. The rudimentatary code already exists in the literature. This allows incorporation of the modified code into current programs without the large memory overhead necessary when IMSL is being used. The degree of flexibility is determined by a parameter which can have values in the range 0 to 1. The EXAFS smoothline requires that the algorithm be used at one extreme end of this range of flexibility. At the other end of this range the routine can be used for interpolation. A suggested path for finding the optimal smoothline, based on the use of coarse and fine smoothing parameters, is discussed.