Infrared trace analysis

Raman measurements [INFRARED TECHNOLOGY AND RAMAN SPECTROSCOPY - RAMAN SPECTROSCOPY] (Vol 14) -trace analysis of [IHACE AND RESIDUE ANALYSIS] (Vol 24)... 

Infrared microscopes can focus the beam down to a 20-pm spot size for microprobing in either the transmission or reflection mode. Trace analysis, microparticle analysis, and spatial profiling can be performed routinely. 

The first (direct reading) method is fairly simple and results are available immediately. However, the instruments have limited sensitivity and must be recalibrated periodically. The second (absorption in a liquid or adsorption on a medium) and third (gas container) methods are generally considered more sensitive and more accurate method for trace analysis by gas chromatographs, infrared... 

The Analysis of Extraterrestrial Materials. By Isidore Adler Chemometiics. By Muhammad A. Sharaf, Deborah L. Illman, and Bruce R. Kowalski Fourier Transform Infrared Spectrometry. By Peter R. Griffiths and James A. de Haseth Trace Analysis Spectroscopic Methods for Molecules. Edited by Gary Christian and James B. Callis... 

The ability to recover monolayers and subject them to meaningful analysis has become practical only in recent years because of the development of new methods of trace analysis. High-performance liquid chromatography and vapor phase chromatography allow separation and identification of such small quantities (54a). Attenuated total reflectance techniques for infrared analysis (56) and field desorption mass spectrometry (68) have been applied to the trans-... 

Trace analysis by IR spectroscopy, involving pre-concentration, separation, and computer techniques has been reported by Hannah et al. (1978). The term trace analysis is used to refer to concentrations in the ppm range up to 1%. In instances where interference is at its minimum, analysis may be performed in a straightforward manner by using difference techniques. However, there are many cases in which analysis may be complicated by the fact that the trace material is structurally similar to the matrix material. Moreover, the presence of other trace compounds is intolerable if their spectra interfere with that of the compound under investigation. In this case it is often necessary to use pre-concentration or separation techniques. This method is illustrated by analyses of aromatic isomers, gasoline additives, drugs, and polymer additives. The different aspects of trace analyses by infrared spectroscopy are discussed by Smith (1986). 

Trace analysis is usually a discontinuous process comprising several procedures in sequence sampling, enrichment, separation, and measurement. In this section a technique is described which intends to integrate all necessary procedures in order to facilitate continuous measurements. It combines reversible extraction of the analyte by membranes with the measurement of the absorption of its infrared bands. 

Smith AL (1979) Applied Infrared Spectroscopy. John Wiley Sons, New York Smith AL (1986) Trace Analysis by Infrared Spectroscopy. In Christian GD, Callig JB (eds) Trace Analysis. John Wiley Sons, New York... 

TLC can provide other information critical in compound identification. Colors from selective chemical detection reactions, behavior in ultraviolet light, absorbance and/or fluorescence spectra obtained directly on the chromatogram by use of a spectrodensitometer or in solution after elution and Rp values of derivatives prepared by reaction before, during, or after development can be combined with the R p values of the sample to increase the degree of probability of correct identification [67,68]. Infrared (IR) and mass spectrometry combined with chromatography can provide unequivocal identification if sufficient sample is available. This is often not the case in trace analysis where nanogram amounts of material may be separated and detected by TLC, whereas microgram quantities are needed for conventional IR spectrometric confirmation of the trace substances. 

Particularly in trace analysis, artifacts are recorded that occur because of the so-called Raman bands of the solvent. These result in incoherent scattering by the solvent at high excitation intensities (- Infrared and Raman Spectroscopy). To avoid this, in good-quality equipment, excitation radiation is produced with the aid of a monochromator rather than a filter, so that these additional Raman lines can be cut off by the second (observation) monochromator. In fluorescence measurements, the spectrum of pure solvent should always be recorded as a blank before the sample is investigated. 

Another important application of infrared gas analysis is for trace analysis, such as for the analysis of a dilute mixture (in the ppm range) or an environmental specimen. In such cases, individual analytes are measured from the high ppb levels to the lO s or lOO s of ppm. For such analyses, extended path lengths are required, and typically multipass gas cells from 1 to 20 m in path length are used. One very specific application is an open-path measurement for ambient air monitoring in manufacturing plants or in toxic waste sites in which no cell is used. Instead, a source and interferometer combination are focused on a remote detection system with the aid of special telescope optics. In such cases, several hundreds of meters of effective path length are used. 

McKelvie, 2008). Detection methods have included UV/Vis spectroscopy (the largest number of applications for its robustness, versatility, simplicity, and low cost), luminescence and chemiluminescence (CL) (which offer low detection limits and high sensitivity, being therefore especially favored for biological, biochemical, and trace analysis), atomic absorption and emission spectroscopy (which benefit enormously from automated sample pretreatment, used for matrix removal and analyte accumulation), electrochemistry (pH, fluoride ion selective electrodes, stripping voltammetry and conductivity), turbidimetry, vibrational spectroscopy (Fourier transform infrared spectroscopy [FTIR] and Raman) and mass spectrometry. 

L. A. L. Smith, Trace Analysis by Infrared Spectroscopy, in G. D. Christian and J. B. CaUis (Eds.), Trace Analysis Spectroscopic Methods for Molecules, Wiley, 1986, pp. 175-284. Excellent. 

An advantage of off-line measurement is tiiat in general different analytical methods can be applied to the same sample, matching the proj rties of different compounds in the system (GC, HPLC, SFC, mass spectrometry (MS), nuclear magnetic resonance (NMR), infrared spectroscopy (IR), and capillary electrophoresis). The analytes become diluted in a solvent, so that higher concentrations of compounds can be examined. However, dilution may also be a disadvantage because trace analysis becomes problematic and the used solvent may disturb analysis. Broad solvent peaks in GC or HPLC may mask underlying substances. 

Scale of Operation Molecular UV/Vis absorption is routinely used for the analysis of trace analytes in macro and meso samples. Major and minor analytes can be determined by diluting samples before analysis, and concentrating a sample may allow for the analysis of ultratrace analytes. The scale of operations for infrared absorption is generally poorer than that for UV/Vis absorption. 

Reference methods for criteria (19) and hazardous (20) poUutants estabHshed by the US EPA include sulfur dioxide [7446-09-5] by the West-Gaeke method carbon monoxide [630-08-0] by nondispersive infrared analysis ozone [10028-15-6] and nitrogen dioxide [10102-44-0] by chemiluminescence (qv) and hydrocarbons by gas chromatography coupled with flame-ionization detection. Gas chromatography coupled with a suitable detector can also be used to measure ambient concentrations of vinyl chloride monomer [75-01-4], halogenated hydrocarbons and aromatics, and polyacrylonitrile [25014-41-9] (21-22) (see Chromatography Trace and residue analysis). 

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

The preferred quantitative deterrnination of traces of acetylene is gas chromatography, which permits an accurate analysis of quantities much less than 1 ppm. This procedure has been highly developed for air poUution studies (88) (see Airpollution control methods). Other physical methods, such as infrared and mass spectroscopy, have been widely used to determine acetylene in various mixtures. 

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