David H. Powell

A Dual Platform for Selective Analyte Enrichment and Ionization in Mass Spectrometry Using Aptamer-Conjugated Graphene Oxide

This study demonstrates the use of aptamer-conjugated graphene oxide as an affinity extraction and detection platform for analytes from complex biological media. We have shown that cocaine and adenosine can be selectively enriched from plasma samples and that direct mass spectrometric readouts can be obtained without a matrix and with greatly improved signal-to-noise ratios. Aptamer-conjugated graphene oxide has clear advantages in target enrichment and in generating highly efficient ionization of target molecules for mass spectrometry. These results demonstrate the utility of the approach for analysis of small molecules in real biological samples.

Proton Affinities of Eight Matrices Used for Matrix‐assisted Laser Desorption/Ionization

Protonated molecules of analytes in matrix-assisted laser desorption/ionization (MALDI) are frequently the most intense ions observed, especially when the concentration of alkali metal cations is low. Examination of the laser desorption mass spectra of MALDI matrices usually shows the presence of both molecular radical ions M+• and [M + H]+ ions. With some matrices, the intensity of the [M + H]+ ion is greater than that of the molecular radical ion, e.g. with 2,5-dihydroxybenzoic acid. A logical source for the ions of protonated analyte in MALDI is proton donation from the [M + H]+ ions of the matrix, but donation could also occur from the radical molecular ions. A knowledge of the proton affinities of the common MALDI matrices might be helpful in understanding why some matrices are ‘hotter’ than others and lead to more post-source as well as prompt decay. The ground-state proton affinity of eight common MALDI matrices were determined. For each matrix, the [M + H]+ ion was generated by methane chemical ionization, trapped and isolated in a Fourier transform ion cyclotron resonance mass spectrometer, allowed to cool for 5 s and reacted with reference compounds of known proton affinities. In some cases, the matrix proton affinities are low enough that proton transfer can occur from the ground state [M + H]+ ion to MALDI analytes; in other cases, the matrix proton affinities are so high that some other mechanism for proton transfer is required.

Aptamer Conjugated Multifunctional Nanoflowers as a Platform for Targeting, Capture and Detection in Laser Desorption Ionization Mass Spectrometry

Although many different nanomaterials have been tested as substrates for laser desorption and ionization mass spectrometry (LDI-MS), this emerging field still requires more efficient multifuncional nanomaterials for targeting, enrichment, and detection. Here, we report the use of gold manganese oxide (Au@MnO) hybrid nanoflowers as an efficient matrix for LDI-MS. The nanoflowers were also functionalized with two different aptamers to target cancer cells and capture adenosine triphosphate (ATP). These nanoflowers were successfully used for metabolite extraction from cancer cell lysates. Thus, in one system, our multifunctional nanoflowers can (1) act as an ionization substrate for mass spectrometry, (2) target cancer cells, and (3) detect and analyze metabolites from cancer cells.

Species Differentiation of a Diverse Suite of Bacillus Spores by Mass Spectrometry-Based Protein Profiling

ABSTRACT In this study, we demonstrate the versatility of matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOFMS) protein profiling for the species differentiation of a diverse suite of Bacillus spores. MALDI-TOFMS protein profiles of 11 different strains of Bacillus spores, encompassing nine different species, were evaluated. Bacillus species selected for MALDI-TOFMS analysis represented the spore-forming bacterial diversity of typical class 100K clean room spacecraft assembly facilities. A one-step sample treatment and MALDI-TOFMS preparation were used to minimize the sample preparation time. A library of MALDI-TOFMS spectra was created from these nine Bacillus species, the most diverse protein profiling study of the genus reported to date. Linear correlation analysis was used to successfully differentiate the MALDI-TOFMS protein profiles from all strains evaluated in this study. The MALDI-TOFMS protein profiles were compared with 16S rDNA sequences for their bacterial systematics and molecular phylogenetic affiliations. The MALDI-TOFMS profiles were found to be complementary to the 16S rDNA analysis. Proteomic studies of Bacillus subtilis 168 were pursued to identify proteins represented by the biomarker peaks in the MALDI-TOFMS spectrum. Four small, acid-soluble proteins (A, B, C, and D), one DNA binding protein, hypothetical protein ymf J, and four proteins associated with the spore coat and spore coat formation (coat JB, coat F, coat T, and spoIVA) were identified. The ability to visualize higher-molecular-mass coat proteins (10 to 25 kDa) as well as smaller proteins (<10 kDa) with MALDI-TOFMS profiling is critical for the complete and effective species differentiation of the Bacillus genus.

The coupling of direct analysis in real time ionization to Fourier transform ion cyclotron resonance mass spectrometry for ultrahigh-resolution mass analysis

Direct Analysis in Real Time (DART) is an ambient ionization technique for mass spectrometry that provides rapid and sensitive analyses with little or no sample preparation. DART has been reported primarily for mass analyzers of low to moderate resolving power such as quadrupole ion traps and time-of-flight (TOF) mass spectrometers. In the current work, a custom-built DART source has been successfully coupled to two different Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers for the first time. Comparison of spectra of the isobaric compounds, diisopropyl methylphosphonate and theophylline, acquired by 4.7 T FT-ICR MS and TOF MS, demonstrates that the TOF resolving power can be insufficient for compositionally complex samples. 9.4 T FT-ICR MS yielded the highest mass resolving power yet reported with DART ionization for 1,2-benzanthracene and 9,10-diphenylanthracene. Polycyclic aromatic hydrocarbons exhibit a spatial dependence in ionization mechanisms between the DART source and the mass spectrometer. The feasibility of analyzing a variety of samples was established with the introduction and analysis of food products and crude oil samples. DART FT-ICR MS provides complex sample analysis that is rapid, highly selective and information-rich, but limited to relatively low-mass analytes.

MOCVD of tungsten nitride (WNx) thin films from the imido complex Cl4(CH3CN)W(NiPr)

Thin films of tungsten nitride (WNx) were deposited by MOCVD from the single-source precursor Cl4(CH3CN)W(N i Pr). Films were analyzed by X-ray diffraction (XRD), Auger electron spectroscopy (AES) and cross-section scanning electron microscopy (X-SEM), while the film resistivity was determined by four-point probe. Film growth rates ranged from 10 to 27 ( A/min within a temperature range of 450–7001C. The apparent activation energy for film growth in the kinetically controlled regime was 0.84 eV. Films grown at temperatures below 5001C were amorphous, with minimum film resistivity and sheet resistance of 750mO cm and 47O/&, respectively, occurring for deposition at 4501C. r 2002 Elsevier Science B.V. All rights reserved.

Non-regulated organic compounds in Florida sediments

Abstract Sediment sample extracts initially analyzed by gas chromatography/mass spectrometry during a state-wide study of organic priority pollutants, were re-evaluated using a broad spectrum approach. Mass spectra for most of the non-regulated organic compounds present in each sediment sample extract were screened. Mass spectral data for these non-regulated organic compounds were organized into a spreadsheet database. The non-regulated organic compounds in the database were prioritized and fifteeen of the most frequently occurring compounds or compound groups were chosen for further study and potential identification. Four compounds, calamenene, dehydroabietine, retene and benzo(b)naphtho(2,1-d)thiophene were positively identified. Tetrahydroretene was tentatively identified while three compounds remain unidentified. Seven compound groups, including alkylbenzenes, alkylphenols and polycyclic aromatic hydrocarbons (PAHs) contained many isomers so complete identification of the individual isomers within each group was not attempted. Information on possible sources and environmental or toxicological impact of these chemicals was obtained from the literature.

Exposed Loop Domains of Complexed 14-3-3 Proteins Contribute to Structural Diversity and Functional Specificity

The 14-3-3 family of proteins functions through protein:phosphoprotein interactions, the nature of which has been elucidated using x-ray crystallography. However, some key structural features in nonconserved regions have yet to be fully resolved, leaving open questions regarding the functional selectivity of 14-3-3 family members for diverse clients. In an effort to study surface accessible structural features in 14-3-3 containing macromolecular complexes and to illuminate important structure/function variations among the 14-3-3 isoforms, we determined the epitopes for three unique monoclonal antibodies (mAbs) developed against the Arabidopsis (Arabidopsis thaliana) G-box DNA:protein complex. The epitopes mapped to different loops in a phylogenetically important subset of the 13 14-3-3 family members. All three epitopes were on a common exposed face of complexed 14-3-3s. Two of the mAbs recognized linear sequences within loops 5 and 6, while the third mAb recognized 14-3-3 residues surrounding the pivotal medial Gly in the divalent cation-binding domain of loop 8, together with distal residue(s) in the putative dynamic 10th helix that has yet to be determined by crystallography. Gly at this loop 8 position is unique to nonepsilon 14-3-3 isoforms of the plant kingdom, suggesting that this region constitutes a plant-specific key functional 14-3-3 feature and highlighting that the loop 8 region is functionally significant. Mutagenesis of the medial amino acid in the loop 8 domain changed the flexibility of the C terminus and altered client peptide-binding selectivity, demonstrating the functional significance of the surface accessible, evolutionarily distinct loop 8 domain.

Reactive desorption electrospray ionization for rapid screening of guests for supramolecular inclusion complexes

Reactive desorption electrospray ionization (DESI), an ambient technique, has been explored as a tool for the development of a fast screening approach for supramolecular complexes capitalizing on the specificity of mass spectrometric detection. A library of twelve potential guests for inclusion by a beta-cyclodextrin host was initially screened via DESI using a spray solution incorporating the host directed toward an array of deposited guests. The steroid nortestosterone was used to verify the applicability of reactive DESI for complexation experiments with beta-cyclodextrin. Results from the DESI experiment and results from an analogous electrospray ionization (ESI) mass spectral screen were compared with solution-phase data obtained by nuclear magnetic resonance (NMR) spectroscopy. The complexes detected using DESI were identical to those determined using NMR, validating the applicability of the technique to supramolecular applications, but the ESI data exhibited significant disparities, predominantly due to the interference of nonspecific artifacts.

Mechanism of forming ohmic contacts to GaAs

The distribution of Si dopant at the Au/GaAs interface after heat treatment has been studied using spatially resolved secondary‐ion mass spectrometry. Previously it has been shown that heat treatment changes as deposited Au thin film from Schottky contacts to ohmic contacts. The present study shows that the transition to an ohmic contact results from segregation of dopants in areas where GaAs is decomposed by reacting with the Au overlayer. Thus the ohmic contact is spatially very inhomogeneous at the metal/semiconductor interface. The mechanism leading to concentrations of the Si and nonuniform ohmic contacts is discussed, and segregation to the solid during decomposition of the GaAs is the most likely mechanism.