Fourier Transform Infrared Instrument Research Articles

Discrimination of 1990s Original Automotive Paint Systems: A Collaborative Study of Black Nonmetallic Base Coat/Clear Coat Finishes Using Infrared Spectroscopy

The 1990s saw the introduction of significantly new types of paint binder chemistries into the automotive finish coat market. Considering the pronounced changes in the binders that can now be found in automotive paints and their potential use in a wide variety of finishes worldwide, the Paint Subgroup of the Scientific Working Group for Materials (SWGMAT) initiated a validation study to investigate the ability of commonly accepted methods of forensic paint examination to differentiate between these newer types of paints. Nine automotive paint systems typical of original equipment applications were acquired from General Motors Corporation in 1992. They consisted of steel panels coated with typical electrocoat primers and/or primer surfacers followed by a black nonmetallic base coat and clear coat. The primary purpose of this study was to evaluate the discrimination power of common forensic techniques when applied to the newer generation original automotive finishes. The second purpose was to evaluate interlaboratory reproducibility of automotive paint spectra collected on a variety of Fourier transform infrared (FT-IR) spectrometers and accessories normally used for forensic paint examinations. The results demonstrate that infrared spectroscopy is an effective tool for discriminating between the major automotive paint manufacturers' formulation types which are currently used in original finishes. Furthermore, and equally important, the results illustrate that the mid-infrared spectra of these finishes are generally quite reproducible even when comparing data from different laboratories, commercial FT-IR instruments, and accessories in a "real world," mostly uncontrolled, environment.

Comparison of the FT-IR Mapping and Imaging Techniques Applied to Polymeric Systems

Two Fourier transform infrared (FT-IR) spectrometers with microspectroscopic capabilities—one equipped with a focal plane array (FPA) mercury-cadmium-telluride (MCT) detector (imaging) and the other a single-element MCT detector (mapping)—are compared. Two samples, a one-dimensional diffusion system and a two-dimensional phase-separated system, were studied with both analytical tools. Diffusion regions and concentration gradients were determined in the diffusion sample. Phase-specific chemical composition data were obtained in the phase-separated sample. The utility of each instrument for specific cases and their respective limitations are illustrated. It is shown that the FPA system has the ability to sample a large sample area with high spatial resolution in a short time without adverse diffraction effects. Because of this very rapid sampling, many dynamic processes can be studied which are otherwise poorly monitored by the point-by-point mapping technique. The spatial resolution achievable, the quality of data obtained [signal-to-noise ratio (SNR)], and temporal resolution between mapped areas are intimately connected in the mapping system. However, the imaging systems have a fixed spatial resolution (dependent on the optics and detector) and the SNR considerations determine temporal resolution. However, it was also shown that by aperturing to a localized region, the single-element detector system is able to arrive at phase concentration information more rapidly, with less cumbersome processing and with higher SNRs than is possible with the current FPA technology. The chemical information obtained from both instruments is, within a small experimental error, identical. This work demonstrates what can be accomplished with each analytical FT-IR instrument, and serves as a comparative study to demonstrate the value of each analytical tool in specific situations.

Effect of Spectral Resolution on Pattern Recognition Analysis Using Passive Fourier Transform Infrared Sensor Data

The Fourier transform infrared (FT-IR) spectral data of two nerve agent simulants, diisopropyl methyl phosphonate (DIMP) and dimethyl methyl phosphonate (DMMP), are used as test cases to determine the spectral resolution that gives optimal pattern recognition performance. DIMP is used as the target analyte for detection, while DMMP is used to test the ability of the automated pattern recognition methodology to detect the analyte selectively. Interferogram data are collected by using a Midac passive FT-IR instrument. The methodology is based on the application of pattern recognition techniques to short segments of single-beam spectra obtained by Fourier processing the collected interferogram data. The work described in this article evaluates the effect of varying spectral resolution on the pattern recognition results. The objective is to determine the optimal spectral resolution to be used for data collection. The results of this study indicate that the data with a nominal spectral resolution of 16 cm−1 provide sufficient selectivity to give pattern recognition results comparable to that obtained by using higher resolution data. We found that, while higher resolution does not increase selectivity sufficiently to provide better pattern recognition results, lower resolution decreases selectivity and degrades the pattern recognition results. These results can be used as guidelines to maximize detection sensitivity, to minimize the time needed for data collection, and to reduce data storage requirements.

Calibration of an FT-IR Spectrometer for Ambient Air Monitoring Using PLS

The objective of this work has been to develop a robust calibration method for simultaneous multigas detection with a Fourier transform infrared (FT-IR) system. Calibration models for the identification and quantification of 23 gases in the presence of high concentrations of background gases such as water vapor, carbon dioxide, and methane have been obtained for an FT-IR instrument with 0.7-cm−1 resolution. The calibration models have been tested on a breadboard instrument for trace gas measurement in manned space missions. The results show that FT-IR combined with multivariate methods such as partial least-squares (PLS) and proper pretreatment of the infrared spectra used in calibration is well suited for this purpose. A procedure for baseline drift compensation has been introduced to make the system insensitive to baseline drift and variations in transmittance. This baseline drift compensation also reduces the need for background measurements. Further, a procedure for incorporating a priori information about the instrument signal-to-noise ratio (SNR) and the absorption strength of interfering absorption lines has been developed. Indoor air monitoring and industrial process monitoring are other possible application areas for these techniques. Parts of this work have been performed in a project for the European Space Agency (ESA) in cooperation with Kayser-Threde GmbH and Daimler-Benz Aerospace, Dornier GmbH.

Determination of Fat, Protein, and Lactose in Raw Milk by Fourier Transform Infrared Spectroscopy and by Analysis with a Conventional Filter-Based Milk Analyzer

The accuracy of fat, crude protein (CP), true protein (TP), and lactose determinations of raw milk by Fourier transform infrared (FTIR) spectroscopy and by analysis with a conventional filter-based milk analyzer was assessed in 6 trials performed over a 10-month period. At each trial, 30 bulk milk samples collected from 15 European countries and 11 reconstituted milks made from raw milk components were analyzed. When calibrations were performed with reconstituted milks at each trial, accuracy standard deviations for fat, CP, TP, and lactose were, respectively, 0.050, 0.048, 0.035, and 0.076 g/100 g for the filter instrument and 0.047, 0.046, 0.042, and 0.065 g/100 g for the FTIR instrument. When a single calibration was made instead of calibrations at each trial, accuracy standard deviations increased for the filter instrument to 0.130, 0.119, 0.121, and 0.083 for fat, CP, TP, and lactose, respectively, and for the FTIR instrument to 0.082, 0.053, 0.044, and 0.084 g/100 g. Because the FTIR instrument provides more spectral information related to milk composition than does the filter instrument, single-calibration FTIR analysis of milk samples collected in different seasons is more accurate. Using reconstituted milks, prepared such that there is no correlation between fat, CP, and lactose, provides a more robust calibration than using genuine bulk milk, especially when milks with unusual composition are analyzed.

An interlaboratory comparison of sample presentation methods for the analysis of aqueous solutions using fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy is becoming much more widely used as an analytical technique. It appears to be particularly useful for quantitative analysis, especially of aqueous solutions. However, there is a considerable variation in the cost and quality of instrumentation as well as a large number of accessories for sample presentation employing attenuated total reflectance (ATR). The objectives of the work reported in this paper were to investigate the effect of the choice of instrumentation on the quality of quantitative analyses as well as the effect of variable acquisition parameters. A series of experiments was designed in which samples were distributed to a number of workers using a range of FTIR instruments from different manufacturers and of differing quality. The workers were allowed to use the accessory of their own choice and to carry out a quantitative analysis of glucose solutions using their normal operating parameters. In following experiments the effects of instrumental and purge were considered and quantitative analysis was performed using a set of prescribed acquisition conditions. The results showed that there were significant differences in the quality of the spectra produced on the different combinations. However, although each worker produced quite different linear calibrations (in terms of slope and intercept), they all predicted the concentration of an unknown with a similar accuracy. However, later experiments showed that the low-cost bench-top instruments were not always so successful when the peaks used in the analysis were affected by residual water vapour interferences. Instrumental drift was noticeably worse for the low-cost instruments but this was an effect that could easily be eliminated by suitable baseline correction.

New Design for Fourier Transform Infrared Vibrational Circular Dichroism Spectrometers

Fourier transform infrared (FT-IR) spectrometers are commonly designed with small-area detectors and tight focusing mirrors. Vibrational circular dichroism (VCD) measurements made with such FT-IR instruments contain polarization artifacts, and VCD measurements on both enantiomers (or one enantiomer and racemic mixture) are required in order to reduce these artifacts. This restriction limits the VCD measurements to only those samples for which both enantiomers (or one enantiomer and racemic mixture) are available. Recently a modified design was reported in the literature where the mirrors between sample and detector were replaced with a BaF2 lens, and a larger-area detector was substituted for the smaller-area counterpart. These modifications successfully alleviated some of the artifact problems. This design, however, is not suitable for polarizing interferometers, where polarizations exiting the interferometer are to be preserved to a high degree of purity. In addition, it is not clear whether the throughput enhancement advantage realized with a larger-area detector completely offsets the disadvantage from increased noise with detector area. Furthermore, BaF2 lenses reduce the broad range routinely available on an FT-IR instrument. Here we report a new design that replaces all the mirrors at the exit port of the interferometer with two KBr lenses and retains the full spectral range (4000–400 cm−1) of mid-infrared FT-IR spectrometers. VCD measurements obtained with small- (1 × 1 mm) and large- (4 × 4 mm) area detectors are found to have similar signal quality.

Advantages and Disadvantages in the Use of Fourier Transform Infrared (FTIR) and Filter Infrared (FIR) Spectrometers for Monitoring Airborne Gases and Vapors of Industrial Hygiene Concern

Abstract The important differences between Fourier transform infrared (FTIR) and filter infrared (FIR) systems for air monitoring are explored, and the strengths and weaknesses of these technologies when applied to industrial hygiene problems are defined and illustrated with actual workplace air monitoring examples. The discussion of the differences in FTIR and FIR instrumentation is structured around the principal components of these spectrophotometers: the optical, sample, and data handling and interpretation systems. Workplace air monitoring applications that are discussed as illustrations of the relative utility of these systems are remote sensing at a hazardous waste site, emissions from a chemical vapor deposition furnace, irritating odors from resin regeneration, and trichloroethylene and Freon® samples from a degreasing operation. In addition, cautionary notes are given for the application of FTIR by nonspectroscopists. Finally, the direction of future research and technical advances are discussed.

Diffuse reflectance Fourier transform IR spectroscopy

Fourier transform infrared (FTIR) spectroscopy has greatly enhanced the utility and range of applications of infrared (IR) spectroscopy in chemistry. Until the development of routine FTIR instrumentation, applications of IR were essentially limited to direct transmission measurements. Newer sampling methods in FTIR spectroscopy can be successful with less than 1% throughput of the infrared radiation and yet yield a spectrum with acceptable signal-to-noise ratio. The decreasing cost of FTIR spectrometers has made a number of such sampling techniques accessible for routine applications without compromises in performance. This article will describe one of these, diffuse reflectance.

Detection of atmospheric pollutants by direct analysis of passive Fourier transform infrared interferograms

Methodology is developed that allows interferograms from a passive Fourier transform infrared (FTIR) spectrometer to be analyzed directly. With a digital filtering approach, a set of generalized procedures is developed for real-time monitoring of atmospheric pollutants. These procedures allow the spectrometer to be mobile, overcoming problems associated with changing spectral backgrounds and with severe data processing requirements. The methodology allows customized digital filters to be developed that remove information from the interferogram except that associated with frequencies characteristic of a target chemical species of interest. To test the methodology, a filter is derived that is specific of for SF/sub 6/. With the use of a passive FTIR instrument mounted in a helicopter, several passes are made by a ground source of SF/sub 6/, thereby simulating the action of a mobile pollution monitoring system. Interferograms collected during this experiment are used to evaluate the computed filter.

Analysis of a Polarizing Michelson Interferometer for Dual Beam Fourier Transform Infrared, Circular Dichroism Infrared, and Reflectance Ellipsometric Infrared Spectroscopies

The properties of a Fourier transform infrared (FT-IR) spectrometer designed around a polarizing Michelson interferometer (PMI) are analyzed with respect to applications in both conventional absorption as well as polarization spectroscopies. The PMI design is that of Martin and Puplett and consists of a metal grid polarizing beam splitter combined with rooftop retromirrors set to rotate the polarization direction in each beam by 90°. If used in conjunction with additional grid polarizers, positioned according to application, the result is an FT-IR instrument that can function both as a dual beam absorption spectrometer and as a polarization spectrometer. Operating in the former mode, the instrument is predicted to achieve speeds comparable to those achievable using a nonpolarizing dual beam Michelson interferometer but to cover a much wider spectral range due to the achromatic nature of metal grid polarizing beam splitters. In the latter mode, it is predicted to give at least double the effective throughput achievable with conventional instruments equipped with external polarization modulating optics, and again to cover a much wider spectral range. It would extend the capability of determining circular dichroism spectra into the far IR (down to ∼3 cm−1) for the first time. With the development of a process for forming a metal grid polarizer of fine spacing (∼0.1 μm) on a KBr substrate, the instrument would be capable of covering the spectral range 25 000 to 3 cm−1 or lower using only two beam splitters, i.e., in two ranges: 25 000 to 450 cm−1 using a KBr-supported metal grid, and 600 to 3 cm−1 or less using an unsupported wire grid.