PubMed Health⌕ Search

Biomedical subjects

Martin J Snowden

Publications and source records attributed to Martin J Snowden.

6 recordsLinked to original sources
PubMed2006

Accuracy vs time dilemma on the prediction of NMR chemical shifts: a case study (chloropyrimidines).

The nuclear magnetic shieldings of two chloropyrimidine species have been predicted and analyzed by means of ab initio and DFT methods. The results have been compared with the experimental values and with those from other database-related approaches. These dataset-based techniques are found to be particularly valuable because of the accurate and instantaneous prediction of the 13C chemical shifts. On the other hand, only a few quantum chemistry based approaches were showed to be the most precise to predict 1H chemical shifts and to elucidate unequivocally the 1H NMR spectra of the regioisomeric mixture under study. Special emphasis was put on incorporating the solvent effect, implicitly, or explicitly. The influence of the level of theory and basis set in the predicted values has also been discussed.

Chlorine↗
PubMed2005

Theoretical prediction of the enantiomeric excess in asymmetric catalysis. An alignment-independent molecular interaction field based approach.

[reaction: see text] The enantiomeric excess of three different asymmetric catalyses has been predicted in excellent agreement with the experiments using a 3D-QSPR approach. In particular, GRid INdependent Descriptors generated from molecular interaction fields together with a simple partial least-squares method were found to be adequate to describe the enantioselectivity induced by these metal-ligand complexes.

Catalysis↗
PubMed2005

Raman line mapping as a fast method for analyzing pharmaceutical bead formulations.

This paper describes the use of principal component analysis (PCA) to de-noise Raman spectra and considerably shorten data acquisition time in Raman mapping experiments. A solid dosage pharmaceutical material (bead) is mapped by a Raman line-mapping system. The mapping acquisition time was varied from 30 s (usually employed in practice) to only 3 s. Apparently excessive noise in the maps measured for 3 s is removed by PCA and the maps of all three components of the bead are then binarized and compared. It is found that spatial difference is negligible despite the remarkably different acquisition times employed. The spectra acquired for 3 s and reconstructed via PCA are found to largely overlap with the spectra acquired for 30 s. The signal to noise ratio of the Raman mapping spectra does not obey the expected root t dependence, thereby preventing straightforward estimation of the shortest usable acquisition time (which is to a lesser extent also a function of the binarization threshold). The results reveal that Raman microscopy can be considered a fast method for mapping some materials, in contrast to the established opinion that it is an inherently slow technique.

Chemistry, Pharmaceutical↗
PubMed2005

Rapid, accurate and precise quantitative drug analysis: comparing liquid chromatography tandem mass spectrometry and chip-based nanoelectrospray ionisation mass spectrometry.

We have developed a liquid chromatography tandem mass spectrometry (LC-MS/MS) system capable of achieving better than 2% accuracy, routinely over a wide concentration range of 1800 ng mL-1. We demonstrate that the necessary high precision, high accuracy and rapid analysis can be achieved using LC-MS/MS technology. Automated nanoelectrospray ionisation tandem mass spectrometry (nanoESI-MS/MS) technology can be employed to eliminate the chromatographic step completely. In this paper, nanoESI-MS/MS is evaluated and compared directly with LC-MS/MS for the quantitative analysis of two-test analytes, amitriptyline (ATT) and 5-methoxytryptamine (5-MTT), in aqueous/organic mixture. Calibration curves were found to be linear over a wide concentration range of 1800 ng mL-1 for both analytes using LC-MS/MS. Using nanoESI-MS/MS ATT gave a linear response while 5-MTT gave a non-linear response using nanoESI-MS/MS over the same concentration range as in LC-MS/MS. Accuracy and precision values of quality control samples (QCs) at four concentration levels were analysed in replicates of six at each level using 5-MTT and ATT as test analytes for both techniques. The LC-MS/MS system was capable of achieving accuracy levels of 99.50101.96% for ATT and 100.17100.40% for 5-MTT. Accuracy levels using nanoESI-MS/MS were not comparable to LC-MS/MS, they ranged from 90.09100.18% for ATT and 95.95113.55% for 5-MTT. The precision values obtained for nanoESI-MS/MS were in good agreement with those obtained by LC-MS/MS.

5-Methoxytryptamine↗
PubMed2005

Analyzing Raman maps of pharmaceutical products by sample-sample two-dimensional correlation.

Sample-sample (SS) two-dimensional (2D) correlation spectroscopy is applied in this study as a spectral selection tool to produce chemical images of real-world pharmaceutical samples consisting of two, three, and four components. The most unique spectra in a Raman mapping spectral matrix are found after analysis of the covariance matrix. (This is obtained by multiplying the original mapping data matrix by itself.) These spectra are identified by analyzing the slices of the covariance matrix at the positions where covariance values are at maxima. Chemical images are subsequently produced in a univariate fashion by visually selecting the wavenumbers in the extracted spectra that are least overlapped. The performance of SS 2D correlation is compared with principal component analysis in terms of highlighting the most prominent spectral differences across the whole data set (which typically comprises several thousand spectra) and determining the total number of species present. In addition, the selection of the unique spectra by SS 2D correlation is compared with the selection obtained by the orthogonal projection approach (OPA). Both comparisons are found to be satisfactory and demonstrate that a quite simple SS 2D correlation routine can be used for producing reliable images of unknown samples. The main benefit of using SS 2D correlation is that it is based on a few data processing commands that can be executed separately and produce results that are closely related to the chemical features of the system.

Algorithms↗