Thomas M. Niemczyk

Temperature determinations in the inductively coupled plasma using a Fourier transform spectrometer

Abstract A vertical profile of iron I excitation temperature in the inductively coupled plasma was determined using spectroscopic techniques based on the Einstein-Boltzmann expression for spectral line intensity and the powerful information gathering ability of a high resolution Fourier transform spectrometer. Calculated temperature values were found to be critically dependent on the oscillator strength values chosen from the literature; however, a qualitative picture of the vertical excitation temperature gradient in the plasma was well-defined and was found to reach a maximum value in the normal analytical zone of the plasma. Excitation temperatures for iron I, nickel I, cobalt I, and vanadium I and II in the normal analytical zone of the plasma were also determined by the same method.

Chemical sensors based on hydrophobic porous sol-gel films and ATR-FTIR spectroscopy

A new chemical sensor based on attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy has been fabricated by coating an ATR crystal with a hydrophobic mesoporous silica film. The highly porous coating can extract hydrophobic analytes, such as benzene, from aqueous solutions and concentrate them inside the film. The enrichment of the analytes inside the film provides the sensor with enhanced sensitivity. The hydrophobic porous film can efficiently exclude water from the region probed by the evanescent wave, thus eliminating the spectral interference due to water absorption bands. Measurement of nitrogen adsorption and desorption isotherms of the films using surface acoustic wave techniques has been used to characterize the pore structure of the films. Relationships between the structure of the porous film and sensor performance are discussed.

Intra-Alkali Matrix Effects in the Inductively Coupled Plasma

Spatial profile studies are used to determine the interference effects of alkali matrix elements on the observed emission of alkali analyte elements in the inductively coupled plasma. These interferences are referred to as intra-alkali matrix effects. It is found that significant enhancement of the analyte emission occurs low in the plasma in the initial radiation zone where atomic emission is most intense and that a slight downward shift in the vertical position of the emission peak is caused by the presence of the alkali matrix. A “same value” point occurs in the normal analytical zone where no net effect of the matrix on analyte emission is apparent. In the tail plume of the plasma, some lesser enhancement is again observed. The magnitude of the enhancement effect increases as the ionization potential of the matrix element decreases, and the effect appears linear with matrix concentration. The data suggest that the interference mechanism is related to an increase in electron density in the lower region of the plasma, and analytical correction for the interference is recommended for alkali determinations in the inductively coupled plasma.

Chemical Sensors Based on Surface-Modified Sol-Gel-Coated Infrared Waveguides *

Attenuated total reflectance sampling, or waveguide sampling, is commonly used to obtain infrared spectra of aqueous solutions. The detection limits achieved for the determination of organic analytes in aqueous solutions using waveguide sampling are limited by the strong water absorption features and relatively small effective sample thickness due to the short penetration depth of the evanescent wave into the sample. Sol-gel processing has been used to produce a porous silica film on the surface of an infrared waveguide. The surface of the sol-gel film was modified with a silanizing reagent to produce a hydrophobic surface. The resulting waveguide was used to obtain infrared spectra from samples of benzonitrile in water. The benzonitrile spectral features obtained with the modified waveguide are enhanced by more than a factor of 103 when compared with those obtained with an unmodified waveguide.

“Top-Down” versus “Side-On” Viewing of the Inductively Coupled Plasma

The inductively coupled plasma is viewed by a “top-down” optical configuration, and the analytical performance is compared to conventional “side-on” viewing in terms of sensitivity, detection limits, linear dynamic range, self-reversal effects, and multielement performance. This comparison is made for a selection of eleven atom and ion lines of eight elements. The results of this study indicate distinct advantages in “top-down” viewing including improved sensitivity, lower detection limits, better signal-to-background ratios, and better compromise viewing position for multielement analysis. An exception to these advantages is increased self-absorption effects observed for the alkali elements.

Quantitative determination of bucindolol concentration in intact gel capsules using Raman spectroscopy.

The ideal quality control method for pharmaceutical products should be capable of rapid nondestructive testing of intact tablets or capsules. Raman spectroscopy using near-infrared excitation is shown to be capable of obtaining useful spectral data directly from drug formulations in gel capsules and from the gel capsules inside blister packs. The Raman data collected from the capsules inside blister packs containing 0-100 mg of the active ingredient (bucindolol), when coupled with multivariate calibration, resulted in a calibration SEP of 3.36 mg. The largest source of error was found to be due to sample inhomogeneity. Even so, the method is shown to have significant potential as a rapid nondestructive quality control method for pharmaceutical samples.

Reaction rate studies of atomic germanium (3P0,1) and silicon (3PJ) with various oxidizers

Abstract The gas phase reactions of Ge(3P0,1) and Si(3PJ) with O2, NO and N2O have been studied in a flow tube system at 350 K. Atomic Ge and Si were produced by flowing GeH4 and SiH4 through a hollow cathode discharge. The subsequent disappearance of the Ge and Si atoms was followed with atomic absorption spectroscopy. Rate constants were determined for the reactions at 4 and 5 torr pressures.

Excitation Temperatures in the Hollow Cathode Discharge

Excitation or spectroscopic temperatures have been determined for various operating conditions of a hollow cathode discharge. It was found that the temperatures measured do not significantly change over the range of conditions studied. These results are compared to those obtained in other low-pressure discharges. The temperature values as well as trends are found to be very similar.

Use of CLS to Understand PLS IR Calibration for Trace Detection of Organic Molecules in Water

Partial least-squares (PLS) and principal component regression (PCR) methods applied to spectral data can generally provide excellent quantitative analysis precision, but extraction of qualitative spectral interpretation from the models can be more difficult. For example, we have achieved sensitivity in the parts-per-million range for polar organic compounds in aqueous solutions using infrared (IR) spectroscopy and modified sol-gel-coated ATR sensors. The interpretation of PLS or PCR loading vectors obtained from calibrations involving orthogonally designed solutions of acetone and isopropanol in water yields a misleading understanding of the mechanism for the IR detection of isopropanol on these sensors. Examination of the loading vectors from PLS or PCR or the first weight-loading vector from the PLS model would suggest that the spectral calibration is based largely on the interaction of the isopropanol with the surface modifier of the sol-gel coating. However, a classical least-squares (CLS) analysis of the data shows clearly that this interaction is not a significant source of the spectral calibration, but rather the calibration is primarily due to the spectroscopic signal of the isopropanol analyte. In this case, the misleading qualitative interpretation of the PLS and PCR models is the result of the spectral variation being dominated by the effects of spectrometer drift. CLS can overcome this problem if a parameter is included in the CLS calibration that adequately represents the drift. In the example presented here, time of spectral data collection is an appropriate drift-related parameter that can be added to the CLS calibration concentration model in order to provide the qualitative information needed to correctly interpret the spectral data. Other methods to include the effects of spectrometer drift in the CLS model are also presented.

Enhancing IR Detection Limits for Trace Polar Organics in Aqueous Solutions with Surface-Modified Sol-Gel-Coated ATR Sensors

An experimental investigation into the use of surface-modified sol-gel-coated Si attenuated total reflectance (ATR) mid-infrared sensors was completed to determine the detection limits for trace amounts of polar organic molecules in aqueous solutions. The surfaces of porous sol-gel films were modified to be mostly hydrophobic in order to concentrate the polar organic analytes in the film while largely excluding the water solvent. This modification of the Si ATR sensor coupled with multivariate partial least-squares calibration methods allowed approximately three orders of magnitude improvement in detection limits for acetone when compared to detection limits from univariate analysis of spectra obtained on uncoated Si ATR sensors. The comparable improvement for isopropanol in aqueous solution was over two orders of magnitude. Studies performed on mixtures of acetone and isopropanol in water confirmed that quantitative analyses could be performed on these mixture systems with cross-validated standard errors of prediction (CVSEP) of 0.5 and 8.7 ppm, respectively. Using three times the CVSEP as an estimate of the detection limit translates to detection limits of 1.5 and 26 ppm for acetone and isopropanol, respectively.