Roger P. Sperline

Anomalous Dispersion Effects in the IR-ATR Spectroscopy of Water

The distortion of band shapes seen in infrared attenuated total reflection (IR-ATR) spectroscopy of strongly absorbing materials such as water, relative to transmission sampling, is shown here to be due to the anomalous dispersion (AD) of water. This distortion occurs in addition to the normal 1/v dependence, and, contrary to previous reports, is shown to not be due to chemical changes of water at the interface between the optical element and bulk solution. IR-ATR spectra of water were modeled with approximation-free calculations. The modeled spectra are compared with experimental ATR spectra for different internal reflection elements, and the results are discussed in terms of the AD optical effect.

The development of a micro-Faraday array for ion detection

Abstract A micro-Faraday array detector was evaluated for use as an ion detector for mass spectrometry. This charge-integrating detector was based upon the merging of technologies from the fields of CCDs and infrared (IR) multiplexers. Measurements were performed by exposing the detector to an Ar+ ion beam of low flux. The array detector responds to both positive and negative charges and preliminary results indicate a detection limit of 100 ions. Current data indicate that the linear dynamic range of the device is over five orders of magnitude. The capability of the device to perform specialized charge read out modes could theoretically both lower the detection limit by a factor of seven and increase the linear dynamic range to nine orders of magnitude using non-destructive read outs.

Quantitative Analysis of Adsorbed Serum Albumin on Segmented Polyurethane Using FT-IR/ATR Spectroscopy

FT-IR attenuated total reflection (ATR) spectroscopy was applied to quantitatively determine the extent of bovine serum albumin adsorbed onto a biomedical-grade poly(ether)urethane film deposited on a ZnSe internal reflection element (IRE). The method of adsorption density determination was based on the optical principles for a stratified medium consisting of three layers. The spectral peak area due to bulk solution was less than 1% of total peak area. The measured adsorption density of albumin in a flowing system was 3.9 μg/cm2 at a solution concentration of 4.5 g/100 mL.

Performance evaluation of a miniature ion mobility spectrometer drift cell for application in hand-held explosives detection ion mobility spectrometers

AbstractThe implementation of hand-held ion mobility spectrometers (IMS) requires the development and evaluation of miniature drift cells providing high sensitivity while maintaining reasonable resolution. This manuscript describes the construction of a miniature IMS designed for such an application and its characterization by evaluation of the detection limits and resolution of the system with seven explosive compounds including trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), pentaerythritol tetranitrate (PETN), 2,4,6-trinitrophenyl-N-methylnitramine (Tetryl), nitroglycerin (NG), 2,4-dinitrotoluene (2,4 DNT), and 2,6-dinitrotoluene (2,6 DNT). Miniature IMS Assembly

Evaluation of a 512-channel Faraday-strip array detector coupled to an inductively coupled plasma Mattauch-Herzog mass spectrograph.

A 512-channel Faraday-strip array detector has been developed and fitted to a Mattauch-Herzog geometry mass spectrograph for the simultaneous acquisition of multiple mass-to-charge values. Several advantages are realized by using simultaneous detection methods, including higher duty cycles, removal of correlated noise, and multianalyte transient analyses independent of spectral skew. The new 512-channel version offers narrower, more closely spaced pixels, providing improved spectral peak sampling and resolution. In addition, the electronics in the amplification stage of the new detector array incorporate a sample-and-hold feature that enables truly simultaneous interrogation of all 512 channels. While sensitivity and linear dynamic range remain impressive for this Faraday-based detector system, limits of detection and isotope ratio data have suffered slightly from leaky p-n junctions produced during the manufacture of the semiconductor-based amplification and readout stages. This paper describes the new 512-channel detector array, the current dominant noise sources, and the figures of merit for the device as pertaining to inductively coupled plasma ionization.

Mechanism of formation of humus coatings on mineral surfaces 2. Attenuated total reflectance spectra of hydrophobic and hydrophilic fractions of organic acids from compost leachate on alumina

Abstract Hydrophobic and hydrophilic fractions were isolated from a compost leachate. The adsorption isotherms of both fractions on alumina were measured by attenuated total reflectance infrared spectroscopy. The shapes of the adsorption isotherms of the two fractions were different. The isotherms for the hydrophilic fraction showed little change in surface excess with increasing solution concentration above 4 mg L −1 . The isotherms for the hydrophobic fraction, on the other hand, displayed a marked increase in surface excess with increasing solution concentration. This increase is evidence for the formation of aggregates (admicelles or hemimicelles) on the alumina surface. Linear dichroism calculations indicated that more of the carboxylate groups in the adsorbed hydrophobic molecules than in the absorbed hydrophilic fraction were free to rotate. The hindered rotation of the carboxylate groups in the adsorbed hydrophilic-fraction molecules probably indicates that these groups are bound to surface aluminum ions by a bidentate mechanism in which the two oxygen atoms of a single carboxylate group bind to separate aluminum ions.

New Quantitative Technique for Attenuated Total Reflection (ATR) Spectrophotometry; Calibration of the “CIRCLE” ATR Device in the Infrared

A general method for spectroscopic determination of the incidence angle and number of reflections in ATR devices is described. This technique extends the inherent advantages of ATR by simplifying general quantitative spectroscopic measurements. Effects of polarization, beam focusing, errors of alignment, indices of refraction, handling of the optical elements, and surface activity of internal standard are discussed. Measured ATR absorbances are fitted mathematically with the use of incidence angle as a parameter. A standardizing solution, molar absorptivities for that solution, and knowledge of the dimensions of the internal reflection optical element are required. Application of the method to a cylindrical internal reflection (CIR) device gives the averaged incidence angle in the CIR with greater accuracy than is otherwise obtainable. With the use of a side-focusing Fourier transform spectrophotometer, the average incidence angle in the Spectra-Tech, Inc. “Micro-cell” “CIRCLE” CIR was 49.68 degrees ± 0.78%, and the number of solution-sensing internal reflections was 7.52 ± 1.95%.

Evaluation of a fourth-generation focal plane camera for use in plasma-source mass spectrometry

A fourth-generation focal plane camera containing 1696 Faraday-strip detectors was fitted to a Mattauch-Herzog mass spectrograph and characterized for its performance with inductively coupled plasma ionization. The camera provides limits of detection in the single to tens of ng L−1 range for most elements and has a linear dynamic range of at least nine orders of magnitude. Isotope-ratio precision better than 0.02% has also been achieved with this device, and this fourth-generation system features the broadest simultaneous mass range obtainable to date with this family of focal plane camera detectors.

Mechanism of formation of humus coatings on mineral surfaces 1. Evidence for multidentate binding of organic acids from compost leachate on alumina

Abstract Measurements of the infrared linear dichroism of carboxylate groups of organic acids from compost leachate adsorbed to an alumina surface and the enthalpy of adsorption of this reaction have been made. The linear dichroism measurements indicated that the carboxylate groups are not free to rotate. This limited rotation probably results from bidentate binding of the carboxylate groups. The molar enthalpy of adsorption of the acids is approximately −100 kJ mol−1. This high value for enthalpy of adsorption may best be explained by assuming that two or more carboxylate groups on a single dissolved organic carbon (DOC) molecule coordinate to the surficial aluminium ions.

Use of an ambient ionization flowing atmospheric-pressure afterglow source for elemental analysis through hydride generation

An ambient mass spectrometry ionization source based on an atmospheric-pressure flowing afterglow has been coupled to a Mattauch-Herzog mass spectrograph capable of simultaneous acquisition of a range of mass-to-charge values by means of a Faraday-strip array detector. The flowing afterglow was used as the ionization pathway for species produced by hydride generation. This ionization strategy circumvents problems, such as discharge instabilities or memory effects, induced by introducing the gaseous sample into the discharge. The generated spectra show both atomic and molecular peaks; calibration curves were calculated for both peak types with limits of detection for arsenic below 10 ppb. This study demonstrates the ability to use an ambient mass spectrometry source, commonly used for molecular analyses, for the detection of gas phase elemental species with the possibilty of performing speciation by coupling with a separation technique.