Michael W. Blades

Investigation of Selected Baseline Removal Techniques as Candidates for Automated Implementation

Observed spectra normally contain spurious features along with those of interest and it is common practice to employ one of several available algorithms to remove the unwanted components. Low frequency spurious components are often referred to as ‘baseline’, ‘background’, and/or ‘background noise’. Here we examine a cross-section of non-instrumental methods designed to remove background features from spectra; the particular methods considered here represent approaches with different theoretical underpinnings. We compare and evaluate their relative performance based on synthetic data sets designed to exemplify vibrational spectroscopic signals in realistic contexts and thereby assess their suitability for computer automation. Each method is presented in a modular format with a concise review of the underlying theory, along with a comparison and discussion of their strengths, weaknesses, and amenability to automation, in order to facilitate the selection of methods best suited to particular applications.

Excitation temperature and electron density in the inductively coupled plasma—aqueous vs organic solvent introduction

Abstract Using a photodiode array spectrometer, spatially resolved Fe I, excitation temperatures and electron densities have been measured for an ICP with both water and xylene solution introduction. The excitation temperature is lower in the ICP with organic aerosol than in the ICP with aqueous aerosol at a fixed power and height. For an ICP with organic aerosol the input power must be increased by approx. 0.5 kW to simulate the temperatures reached in an aqueous ICP.

Assessing differentiation status of human embryonic stem cells noninvasively using Raman microspectroscopy.

Raman microspectroscopy is an attractive approach for chemical imaging of biological specimens, including live cells, without the need for chemi-selective stains. Using a microspectrometer, near-infrared Raman spectra throughout the range 663 cm(-1) to 1220 cm(-1) were obtained from colonies of CA1 human embryonic stem cells (hESCs) and CA1 cells that had been stimulated to differentiate for 3 weeks by 10% fetal bovine serum on gelatin. Distributions and intensities of spectral bands attributed to proteins varied significantly between undifferentiated and differentiated cells. Importantly, compared to proteins and lipids, the band intensities of nucleic acids were dominant in undifferentiated cells with a dominance-reversal in differentiated cells. Thus, we could identify intensity ratios of particular protein-related bands (e.g., 757 cm(-1) tryptophan) to nucleic acid bands (784 cm(-1) DNA/RNA composite) that were effective in discriminating between spectra of undifferentiated and differentiated cells. We observed no discernible negative effects due to the laser exposure in terms of morphology, proliferation, or pluripotency of the stem cells. We conclude that Raman microscopy and complementary data processing procedures provide a rapid, noninvasive approach that can distinguish hESCs from differentiated cells. This is the first report to identify specific Raman markers for the differentiation status of hESCs.

Hollow-core photonic crystal fiber-optic probes for Raman spectroscopy

We have implemented a new Raman fiber-optic probe design based on a hollow-core photonic-crystal excitation fiber surrounded by silica-core collection fibers. The photonic-crystal fiber offers low attenuation at the pump radiation wavelength, mechanical flexibility, high radiation stability, and low background noise. Because the excitation beam is transmitted through air inside the hollow-core fiber, silica Raman scattering is much reduced, improving the quality of the spectra obtained using probes of this design. Preliminary results show that the new probe design decreases the Raman background from the silica by approximately an order of magnitude compared to solid-core silica Raman probes.

An evaluation of ion-atom emission intensity ratios and local thermodynamic equilibrium in an argon inductively coupled plasma

Abstract Spatially resolved, radial electron density ( n e ) profiles have been measured at rf power settings of 0.75, 1.25 and 1.75 kW, and vertical heights of 4,8,12,16,20 and 24 mm above the load coil. These measured values of n e have been used to construct a theoretical local thermal equilibrium (LTE) framework. Ion-atom emission intensity ratios for Mg and Cd calculated from this framework have been compared to experimentally measured values. The measured ion-atom emission intensity ratios are found to be (ess than calculated LTE ratios. This suggests that the aerosol channel of the inductively coupled plasma can be characterized as an ionizing plasma.

Accuracy and Precision of Manual Baseline Determination

Vibrational spectra often require baseline removal before further data analysis can be performed. Manual (i.e., user) baseline determination and removal is a common technique used to perform this operation. Currently, little data exists that details the accuracy and precision that can be expected with manual baseline removal techniques. This study addresses this current lack of data. One hundred spectra of varying signal-to-noise ratio (SNR), signal-to-baseline ratio (SBR), baseline slope, and spectral congestion were constructed and baselines were subtracted by 16 volunteers who were categorized as being either experienced or inexperienced in baseline determination. In total, 285 baseline determinations were performed. The general level of accuracy and precision that can be expected for manually determined baselines from spectra of varying SNR, SBR, baseline slope, and spectral congestion is established. Furthermore, the effects of user experience on the accuracy and precision of baseline determination is estimated. The interactions between the above factors in affecting the accuracy and precision of baseline determination is highlighted. Where possible, the functional relationships between accuracy, precision, and the given spectral characteristic are detailed. The results provide users of manual baseline determination useful guidelines in establishing limits of accuracy and precision when performing manual baseline determination, as well as highlighting conditions that confound the accuracy and precision of manual baseline determination.

Comparison of dopants for charge exchange ionization of nonpolar polycyclic aromatic hydrocarbons with reversed-phase LC-APPI-MS.

Atmospheric pressure photoionization (APPI) is capable of ionizing nonpolar compounds in LC/MS, through charge exchange reactions following photoionization of a dopant. Recently, several novel dopants—chlorobenzene, bromobenzene, 2,4-difluoroanisole, and 3-(trifluoromethyl)anisole—have been identified as having properties making them well-suited to serve as dopants for charge exchange ionization under reversed-phase LC conditions. Here, we report the results of experiments comparing their effectiveness to that of established dopants—toluene, anisole, and a toluene/anisole mixture, for the charge exchange ionization of model nonpolar compounds—the 16 polycyclic aromatic hydrocarbons (PAHs) identified by the US EPA as priority pollutants—when using a conventional reversed-phase LC method. Chloro- and bromobenzene were found to be much more effective than toluene for all the PAHs, due to the relatively low reactivity of their photoions with the solvent. Their overall performance was also better than that of anisole, due to anisole’s ineffectiveness toward higher-IE compounds. Further, the experiments revealed that anisole’s performance for higher-IE compounds can be dramatically improved by introducing it as a dilute solution in toluene, rather than neat. The two fluoroanisoles provided the highest overall sensitivity, by a slim margin, when introduced as dilute solutions in either chloro- or bromobenzene.

Asymmetric Abel Inversions on Inductively Coupled Plasma Spatial Emission Profiles Collected from a Photodiode Array

The utility of a photodiode array based spatial emission profiling system has been enhanced by using a method for performing an Abel inversion on asymmetric lateral data which preserves the asymmetry in the radial emission profiles. This procedure has been implemented with an APPLE II plus computer, coupled with a Reticon RL-1024S photodiode array that has been masked with a narrow exit slit. The method has been tested with a theoretical set of lateral data and with actual measured spatial emission profiles.

Spatial profiles of electron density in the inductively coupled plasma

Abstract Spatially resolved, radial electron number density (ne) profiles have been measured at rf power settings of 1.00, 1.25, 1.50, 1.75 and 2.00 kW, and vertical heights of 4, 8, 12, 16 and 20 mm above the load coil. These profiles have been condensed and presented as electron density contour plots for each input power. The precision of the method was evaluated by doing ten replicate measurements of electron density. The relative standard deviation varied between 2 and 10 % with the maximum at the plasma centre. Electron density was measured with and without the presence of the easily ionizable element—Cs, and no significant difference was observed.

Some considerations regarding temperature, electron density, and ionization in the argon inductively coupled plasma

Abstract The measured density of electrons in the ICP cannot be explained on the basis of a pure LTE calculation. A mechanism which involves radiation trapping and the transfer of excitation energy from the annular regions of the ICP to the aerosol channel is offered. This mechanism called “assisted ionization” leads to a more accurate prediction of electron density at a particular temperature. Assisted ionization is the result of the coupling of high energy resonance radiation from Ar(I) in the annular regions of the ICP into the analyte channel. The response of analyte atoms and ions to temperature and electron density in the channel can be estimated by inclusion of the analyte ionization equilibrium in an overall equilibrium which includes argon atoms and excited state argon species.