Scott R. Goode

Correction for Light Absorption in Fluorescence Studies of Protein-Ligand Interactions

It is shown that absorption of the excitation light can lead to substantial systematic errors in fluorescence measurements of equilibrium constants for formation of protein-ligand complexes. The assumptions about the optical arrangement of the fluorescence spectrometer involved in the calculation of the correction of this absorption are discussed. A general semiempirical correction procedure which can be used for (calculated) absorbance values as high as 5 is described. The importance of choosing the excitation wavelength so as to minimize the necessity for these corrections is emphasized.

Dual-pulse laser-induced breakdown spectroscopy with combinations of femtosecond and nanosecond laser pulses

Nanosecond and femtosecond laser pulses were combined in an orthogonal preablation spark dual-pulse laser-induced breakdown spectroscopy (LIBS) configuration. Even without full optimization of interpulse alignment, ablation focus, large signal, signal-to-noise ratio, and signal-to-background ratio enhancements were observed for both copper and aluminum targets. Despite the preliminary nature of this study, these results have significant implications in the attempt to explain the sources of dual-pulse LIBS enhancements.

Energy Dependence of Emission Intensity and Temperature in a LIBS Plasma Using Femtosecond Excitation

In this paper, we investigate the effect of laser energy on laser-induced breakdown emission intensity and average temperature in a short-pulse plasma generated by using 140 fs laser excitation. Both line emission and continuum background intensity and plasma temperature decrease very rapidly after excitation compared to the more conventional nanosecond pulse excitation. Both emission intensity and plasma temperature increase with increasing laser energy. However, the intensity increase appears to be mostly related to the amount of material ablated. Also, nongated laser-induced breakdown spectroscopy (LIBS) is demonstrated using a high-pulse (1 kHz) pulse repetition rate.

Some Comparisons of LIBS Measurements using Nanosecond and Picosecond Laser Pulses

Laser-induced breakdown spectra were measured by using a 1.3 ps laser pulse on glass, steel, and copper. Material ablation with the use of picosecond excitation is very precise with well-formed sharp-edged craters. The spectra obtained with 570 nm, 1.3 ps excitation decay more quickly and show significantly lower background emission than those that use 1064 nm, ∼ 7 ns excitation. The background was low enough that excellent laser-induced spectroscopy (LIBS) spectra were obtained on the three samples by using a single 1.3 ps laser pulse and a nongated detector. Similar results were obtained by using nanosecond excitation but with higher relative background signals. The radiance was similar with the use of pico- or nanosecond excitation; however, the radiant intensity was larger with nanosecond excitation because of the larger plasma.

Analysis of Aqueous Solutions by Laser-Induced Breakdown Spectroscopy of Ion Exchange Membranes

Elemental analysis of solutions can be achieved by concentrating and immobilizing the metal ions into a commercially available ion exchange polymer membrane followed by laser-induced breakdown spectroscopy. Two methods of sample preparation were investigated: filtering the solution through the ion exchange membrane with suction, and placing the membrane in the solution and allowing the ions to equilibrate with the membrane. The membrane was then ablated with the focused energy of a Nd:YAG laser at 1064 nm. The emitted light was collected by an echelle spectrometer through a fiber-optic cable and detected with an intensified charge-coupled device (CCD). Ten different metals, most covered by the Resource Conservation and Recovery Act (RCRA), were studied. The concentrations of barium, cadmium, chromium, cobalt, copper, silver, lead, mercury, nickel, and zinc can be measured simultaneously with limits of detection ranging from 2 μg/mL to 4 ng/mL. The linear range is 2–6 orders of magnitude depending upon the element and sampling method. The major advantages of the technique are the multielement capability and the ease of sample preparation.

Identifying alloys by laser-induced breakdown spectroscopy with a time-resolved high resolution echelle spectrometer

Laser-induced breakdown spectroscopy (LIBS) produces line spectra from solid samples without any pre-treatment. The spectra were reduced to a matrix of intensities for the most intense lines and a number of different methods of identification were applied to determine if metal alloys could be distinguished by their spectra. The methods included principal component analysis, cluster analysis, multiple discriminant analysis, and spectral matching with a similarity index. The unsupervised methods, principal component analysis and cluster analysis, showed that the samples can be divided into groups based on their LIBS spectra. Discriminant analysis achieved an overall 97% correct classification into metal groups. Finally, a spectral matching approach was used to compare 234 individual spectra to a library of 39 average spectra. The accuracy of the matches varied from 97.4% correct prediction of the class of material to 79.9% correct identification of the specific alloy.

Speciation of Chromium via Laser-Induced Breakdown Spectroscopy of Ion Exchange Polymer Membranes

A new method for the speciation of ng/mL concentrations of Cr(III) and Cr(VI) solutions with analysis by laser-induced breakdown spectroscopy (LIBS) is reported. Speciation is achieved by pre-concentration of the chromium onto commercially available cation exchange polymer membranes. Chromium(III) is removed directly by cation exchange; chromium(VI) in the filtrate is reduced to Cr(III) and concentrated onto a second cation exchange membrane, affording independent measurement of both species. Large volumes of waters containing Cr(III) and Cr(VI) can be concentrated onto the membranes and directly analyzed by laser-induced breakdown spectroscopy. The estimated limit of detection corresponds to 500 ng of Cr on the membrane: if a solution volume of 1 L is used, then the detection limit corresponds to a solution concentration of 0.5 ng/mL. Excellent separation of the chromium species is attained. Results show that overall method efficiencies range from 94–116% and are independent of the matrix. The influence of pH has been measured, and although Cr(VI) converts to Cr(III) in acidic solutions, the total Cr recoveries are not appreciably influenced by pH over the range of natural waters (4 to 9). In addition, speciation was performed in the presence of a number of different cations and showed that the method is robust in many different and complex matrices.

Laser-induced breakdown spectroscopy for the detection of gunshot residues on the hands of a shooter

Laser-induced breakdown spectroscopy (LIBS) has been used to determine whether the hands of a suspected gun user contain traces of gunshot residue. Samples are obtained by pressing adhesive tape against the skin of the suspect and analyzing the tape directly. When the suspect has fired multiple shots, or if the gun has not been cleaned, the gunshot residue provides a spectral signature that is readily apparent, but a person who has fired a single shot from a clean gun is not so easy to identify. The error rates associated with the LIBS identification of a subject who fired one shot from a clean gun have been evaluated by Monte Carlo simulation techniques, and criteria are proposed for defining a positive or a negative test result.

A Critical Evaluation of the Tangential Flow Torch Microwave-induced Plasma Detector for Gas Chromatography

The tangential-flow torch was evaluated as part of an atomic emission microwave plasma detector for gas chromatography. Emission intensities for the elements carbon, sulfur, and bromine in a variety of organic molecules were studied. Calibration plots showed good linearity, indicating that the chemical matrix has little effect on the relative degree of atomization.

Influence of pressure on the properties of a microwave-induced plasma

Abstract The argon excitation temperature, OH rotational temperature, electron density, and relative line radiances have been measured as the pressure is varied from 10 to 760 torr in a unique, variable-pressure microwave-induced plasma system. The influence of power and flow rate on low (50 torr), medium (220 torr), and atmospheric-pressure plasmas is presented. Changes in behavior of the plasma at pressures below 100 torr are observed which are consistent with a relative decrease in nonradiative processes and thus a proportional increase in radiative processes at reduced pressures.