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J H Callahan

Publications and source records attributed to J H Callahan.

6 recordsLinked to original sources
PubMed2002

Scanning ultraviolet two-step laser mass spectroscopy of polycyclic aromatic hydrocarbon distributions on creosote-contaminated soil particles.

The distribution of polycyclic aromatic hydrocarbons (PAHs) on creosote-contaminated soil has been examined with scanning ultraviolet two-step laser desorption/laser ionization mass spectroscopy (UV-L2MS). The instrument has been constructed in-house by modifying a reflectron time-of-flight mass spectrometer. Two-dimensional chemical maps were accurately generated from model patterned PAH distributions. From examination of three-dimensional substrates, the depth of field of the experiment allows surfaces with roughness of up to 120 microm to be treated as a two-dimensional system and still achieve an accurate representation of the surface deposits. Soil was obtained from a former wood treatment facility. Individual particles of 100-1000 microm were mounted on indexed sample plates and examined by reflectance infrared microscopy, optical microscopy, and imaging UV-L2MS. The most intense PAH signals were associated with regions on the particles where clay/organic carbon deposits were found.

Creosote↗
PubMed2001

Application of quantitative chemometric analysis techniques to direct sampling mass spectrometry.

This paper explores the use of direct sampling mass spectrometry coupled with multivariate chemometric analysis techniques for the analysis of sample mixtures containing analytes with similar mass spectra. Water samples containing varying mixtures of toluene, ethyl benzene, and cumene were analyzed by purge-and-trap/direct sampling mass spectrometry. Multivariate calibration models were built using partial least-squares regression (PLS), trilinear partial least-squares regression (tri-PLS), and parallel factor analysis (PARAFAC), with the latter two methods taking advantage of the differences in the temporal profiles of the analytes. The prediction errors for each model were compared to those obtained with simple univariate regression. Multivariate quantitative methods were found to be superior to univariate regression when a unique ion for quantitation could not be found. For prediction samples that contained unmodeled, interfering compounds, PARAFAC outperformed the other analysis methods. The uniqueness of the PARAFAC model allows for estimation of the mass spectra of the interfering compounds, which can be subsequently identified via visual inspection or a library search.

Journal Article↗
PubMed1997

A spray management valve for hand-compression sprayers.

The commercially available spray management valve provided consistent flow rates when used with hand-compression sprayer systems. The 15-psi spray management valve maintained a constant flow rate of 180, 150, and 155 ml/min in combination with a fine 45 degrees flat fan nozzle and Hudson. B&G, and Chapin hand-compression sprayers, respectively. The 30-psi spray management valve maintained a constant flow rate and adequate spray-on time when combined with the coarse flat fan nozzle for each of the 3 hand-compression sprayers tested.

Mosquito Control↗
PubMed1995

Noncovalent protein--oligonucleotide interactions monitored by matrix-assisted laser desorption/ionization mass spectrometry.

Positive ion mode matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was used to explore nonspecific interactions between proteins and oligonucleotides. The formation of noncovalent complexes showed correlation with the type of oligonucleotide bases and with the amino acid composition of the proteins. Among the four DNA homooligomers, abundant protein-nucleic acid complexes were detected for pd(T)n, whereas negligible attachment was evident for pd(A)n, pd(C)n, and pd(G)n. Mixed base sequence nucleic acids (pd(AGCTCAGCTT) and d(TTAGCAGCTT) also showed affinity to Arg-Lys. The protein affinity of pd(T)n turned out to be nonspecific and produced a larger variety of complexes when the number of basic residues in the protein was increased. Complexation of pd(T)n with small basic dipeptides (Arg-Lys or His-His) led to significant improvement in the mass resolution for positive ions. For example, the mass resolution of the pd(T)20/Arg-Lys complex exhibited about 4 times improvement over pd(T)20 alone. The protein--oligonucleotide interactions were also pH and matrix dependent. Lowering the pH from its original value (pH = 1.7) led to diminishing complex related signal, whereas increasing the pH resulted in the appearance of a larger variety of complexes. 2,5-Dihydroxybenzoic acid matrix demonstrated much greater tendency to produce complex ions than did the three other matrix materials we tested. A possible explanation of the observed phenomena was based on pH-controlled ion pair formation between oligonucleotides and proteins.

Acid-Base Equilibrium↗
PubMedDate not supplied

Mass spectrometric analysis of anatoxin-a.

Secondary ion mass spectrometry (SIMS), gas chromatography/mass spectrometry (GC/MS), desorption chemical ionization/mass spectrometry (DCI/MS), and thermospray/mass spectrometry (TSP/MS) were evaluated for the detection of a low-molecular weight biological toxin, anatoxin-a. The lowest detection limit was obtained using isobutane or ammonia DCI/MS. Tandem mass spectrometry (MS/MS) was used to enhance the selectivity and sensitivity of the analysis. The detection limit for the pure material is 10 pg, and 100 pg of the toxin can be analyzed directly from urine using DCI/MS/MS.

Bacterial Toxins↗