Jack X. Zhou

A REVIEW OF RECENT APPLICATIONS OF NEAR INFRARED SPECTROSCOPY, AND OF THE CHARACTERISTICS OF A NOVEL PbS CCD ARRAY-BASED NEAR-INFRARED SPECTROMETER

ABSTRACT Near-infrared spectroscopy (NIRS) has been gaining popularity as an analytical tool due to advances in the power of personal computers, which allow the extraction of chemical and physical information about a sample, and the utilization of cost efficient and robust CCD array-based spectrometers. In this review, applications of NIRS are explored, within various fields of analytical chemistry, which take advantage of the inherent rapid analysis time and minimal sample preparation that is possible. For the purposes of illustration, the usefulness and limitations of a novel, PbS array-based spectrometer are described in the context of its application to the analysis of a variety of samples.

Resonant Laser Ablation of Metals Detected by Atomic Emission in a Microwave Plasma and by Inductively Coupled Plasma Mass Spectrometry

It has been shown that an increase in sensitivity and selectivity of detection of an analyte can be achieved by tuning the ablation laser wavelength to match that of a resonant gas-phase transition of that analyte. This has been termed resonant laser ablation (RLA). For a pulsed tunable nanosecond laser, the data presented here illustrate the resonant enhancement effect in pure copper and aluminum samples, chromium oxide thin films, and for trace molybdenum in stainless steel samples, and indicate two main characteristics of the RLA phenomenon. The first is that there is an increase in the number of atoms ablated from the surface. The second is that the bandwidth of the wavelength dependence of the ablation is on the order of 1 nm. The effect was found to be virtually identical whether the atoms were detected by use of a microwave-induced plasma with atomic emission detection, by an inductively coupled plasma with mass spectrometric detection, or by observation of the number of laser pulses required to penetrate through thin films. The data indicate that a distinct ablation laser wavelength dependence exists, probably initiated via resonant radiation trapping, and accompanied by collisional broadening. Desorption contributions through radiation trapping are substantiated by changes in crater morphology as a function of wavelength and by the relatively broad linewidth of the ablation laser wavelength scans, compared to gas-phase excitation spectra. Also, other experiments with thin films demonstrate the existence of a distinct laser–material interaction and suggest that a combination of desorption induced by electronic transition (DIET) with resonant radiation trapping could assist in the enhancement of desorption yields. These results were obtained by a detailed inspection of the effect of the wavelength of the ablation laser over a narrow range of energy densities that lie between the threshold of laser-induced desorption of species and the usual analytical ablation regime. Normal ablation employs high-power lasers in an attempt to create a vapor plume without selective vaporization, and with a stoichiometry that accurately represents the stoichiometry of species in the solid sample. RLA, as a method of selective vaporization, appears to provide an opportunity to exploit selective vaporization in new ways.

Laser-excited atomic fluorescence spectrometry in a graphite furnace with an optical parametric oscillator laser for sequential multi-element determination of cadmium, cobalt, lead, manganese and thallium in Buffalo river sediment

It is demonstrated, for the first time, that solid-state lasers based on optical parametric oscillation (OPO) allow relatively rapid sequential multi-element analysis of samples by laser-excited atomic fluorescence spectrometry (LEAFS) in a graphite furnace. These lasers are tunable by facile computer keyboard control over the wavelength region 220–2000 nm. A method is described for the sequential multi-element determination of cadmium, cobalt, lead, manganese and thallium in a river sediment standard reference material (NIST SRM 2704) by graphite furnace LEAFS. With a slew rate of 0.125 nm s–1, the OPO laser could be tuned to cover the wavelength range needed for these elements, from 228 to 304 nm, in 15 min. This allowed each element to be determined sequentially with the analysis time determined primarily by the slow heating cycle of the furnace rather than the laser wavelength tuning. Detection limits in the multi-element mode were 545, 111, 28, 445 and 24 fg for cadmium, cobalt, lead, manganese and thallium, respectively, limited primarily by the low repetition rate of the laser (10 Hz). The multi-element detection limits were within a factor of 2–4 of those in the single element mode. Higher excitation energies, by a factor of 2–5, were required to optically saturate the transitions of the analytes in the sediment sample solution compared with aqueous standards. By use of several aliquots of one sample solution, and simple aqueous calibration, it was possible to analyze the sample, accurately, for the five elements over a concentration range between 1 ng ml–1 for thallium and 460 ng ml–1for manganese. Different dilutions were not necessary owing to the long calibration range of the technique. The high sensitivity of LEAFS allowed sufficient initial dilution to remove an interference on thallium that is normally irresolvable by atomic absorption measurements of the same sample in the same graphite furnace.

Teaching Raman Spectroscopy in Both the Undergraduate Classroom and the Laboratory with a Portable Raman Instrument

Abstract We have evaluated a small portable Raman instrument on loan from B&W Tek, Inc., and have determined that it can successfully be used in the classroom both as a visual aid for teaching the fundamentals of Raman spectroscopy and for a variety of undergraduate experiments as a normal component of an instrumental analysis class. Having portable Raman instrumentation would allow the instructor to demonstrate the principles of Raman spectroscopy, as well as the concepts of calibration curves, blank subtraction, detection limits, and regression analysis. Both qualitative and quantitative types of experiments were done for solid Tylenol tablets, aqueous solutions of isopropyl alcohol, dimethyl sulfoxide, methanol, and ethanol, and gaseous CO2 and N2O4. Additionally, surface‐enhanced resonance Raman spectra of Rhodamine 6G were obtained using a chloride ion–activated silver colloid. Spectra from the B&W Tek, Inc., instrument were comparable to literature Raman spectra.

Reverse Intensity Correction for Raman Spectral Library Search

A reverse intensity correction method was developed for spectral library searches to correct for instrument response without the side effect of magnifying the noise in the low responsivity region of test spectra. Instead of applying relative intensity correction to the sample test spectra to match the standardized library spectra, a reverse intensity correction is applied to the standardized library spectra to match the uncorrected sample spectrum. This simple procedural change improves library search performance, especially for dispersive charge-coupled device Raman analyzers using near-infrared excitations, where the instrument response often varies greatly across the spectral range, and signal-to-noise ratio in the low responsivity regions is typically poor.