Sampling dispersion instrumentation

Fig. 1. Comparison of amide V VCD for an identical sample of poly-L-lysine in D20 as measured on the UIC dispersive instrument (top) and on the ChirallRFT-VCD instrument (at Vanderbilt University, kindly made available by Prof. Prasad Polavarapu). Sample spectra were run at the same resolution for the same total time ( 1 h) in each case. The FTIR absorbance spectrum of the sample is shown below. VCD spectra are offset for sake of comparison. Each ideal baseline is indicated by a thin line, the scale providing a measure of amplitude. Noise can be estimated as the fluctuation in the baseline before and after the amide V, which indicates the S/N advantage of the single band dispersive measurement.

All of the usual sampling techniques used in infrared spectroscopy can be used with FT-IR instrumentation. The optics of the sampling chamber of commercial FT-IR instruments are the same as the traditional dispersive instruments so the accessories can be used without modification for the most part. To make full use of the larger aperature of the FT-IR instrument, some accessories should be modified to accomodate the larger beam. The instrumental advantages of FT-IR allow one to use a number of sampling techniques which are not effective using dispersive instrumentation. Transmission, diffuse reflectance and internal reflectance techniques are most often used in the study of epoxy resins. 

In the absence of sample in the optical path, the detector perceives the full intensity I0 of light emitted by the source for the wavelength interval selected by the entrance slit of the dispersive instrument. When a sample is present, the detector perceives a reduced intensity / (Fig. 14.5a). If the source emits a continuum of light, the ratio I0/I will always be close to 1 because the absorption lines of elements are very fine (1 x 10-3 nm). On the other hand, if the source emits only at the wavelengths that... 

Limited sample clean-up could overload the analytical column, and residual matrix components can accumulate on the column after multiple injections. The residual matrix components can also solidify and deposit over a period of time in the LC-MS ionization source or vacuum interface, resulting in a decrease in ion transfer efficiency. The decrease in instrumentation performance (i. e., signal intensity) can be monitored by the signals of system-suitability samples dispersed within an analytical batch. The practice of replacing the pre-column in every run and scrubbing the analytical column periodically with a cleaning mobile phase will help to maintain instrument performance. 

The fundamental difference between a dispersive and a FTIR instrument consists of the way of scanning the sample. In a dispersive instrument, the polychromatic source is monochromatized by a prism or a grating. These separated frequencies are measured independently. 

C. The Basic Elements of the Experimental Setup. The basic elements of TRRR experiments are a photolysis source a laser probe source (whose scattered radiation by the photolabile sample contains the vibrational spectra of the photodecomposed sample and its transients) a dispersing instrument (e.g., a spectrometer) and an optical multichannel analyzer (OMA) system used as a detector. 

In the past, VCD has been observed via one of two experimental approaches dispersive and Fourier transform (FT) instrumentation. For biological samples, where there are normally no enantiomeric samples available to check for the accuracy of the measurement, dispersive instruments have been used nearly exclusively. These instruments will be described in detail in Section 2.2, and FT-VCD instrumentation in Section 2.3. 

The intensity of a Raman signal is governed by a number of factors, including incident laser power, frequency of the scattered radiation, efficiency of the grating (in the case of dispersive instruments) and detector, absorptivity of the materials involved in the scattering, molar scattering power of the normal mode, and the concentration of the sample. This situation is further complicated by the fact that many of these parameters are frequency-dependent, as indicated in the following equation ... 

One of the main advantages of Raman spectroscopy over IR is that water is a weak Raman scatterer. The spectrum of water causes little interference so that spectra of solutes can be measured in aqueous solutions. A good example of the reduced interference from water is shown for two pharmaceuticals in Fig. 7-28. The Raman spectra of damp and dry samples of acetaminophen and ibuprofen are shown in the figure. Bands due to water are not observed in the spectra. Near and mid-IR of these same samples exhibited relatively strong absorbances due to water. These Raman spectra were measured on a dispersive instrument and were excited with an Ar-ion laser emitting at 488 nm. The background for the acetaminophen sample is flat, whereas ibuprofen exhibits a background characteristic of fluorescence. 

In FTIR spectroscopy, an interference wave interacts with the sample in contrast to a dispersive instrument where the interacting energy assumes a well-defined wavelength range. The interference wave is produced in an interferometer (Fig. 4.1.1), the most common of which is the Michelson interferometer. A computer is used to control the interferometer, to collect and store data, and to perform the Fourier transformation. In addition, the computer performs post-spectroscopic operations such as spectral presentation, resolution enhancement, calibration, and calculation of correlation equations. 

For DRIFT studies, a wood wafer, paper sheet, or milled wood sample dispersed in KBr (or KC1) is placed in a cup at the focal point of the concave, ellipsoidal mirror so that the incident light is focused on the sample. The scattered light coming from the sample is collected from the concave mirror and directed by a suitable mirror system to the detector of the FTIR instrument. The pressure used for smoothing the sample has to be adjusted so that reproducible results can be obtained (Yeboah et al. 1984). The contribution of specular reflectance can be diminished by reducing the particle size and by increasing the sample dilution. For powder samples, as indicated above, the diluent is KBr or KCI. Good results are normally obtained with alkali halide powders that contain 1-2% of sample. In certain cases, the sample concentration may be increased up to 10%. 

Analysis schemes developed for identifying clay minerals in the TEM based on EDS spectra (e.g., Murdoch et al.100) are inappropriate for colloidal samples dispersed on polycarbonate filters due to complications associated with the various sample-beam-substrate interactions that differ dramatically from that of ideal samples or standards with smooth polished surfaces.94 96 101 102 Correction procedures that account for the influence of particle size and morphology on x-ray spectra have been widely available for some time,101102 but these techniques have not been applied to the analysis of environmental particulates. To overcome the limitation of quantitative elemental analysis, some research groups have compared the x-ray spectra for sample colloids to the spectra for various minerals of similar size and composition under the same instrumental and sample preparation conditions to calibrate instrumental response.7 24 93 Noting the resolution problems associated with SEM analysis of submicron colloids, several research groups have chosen TEM as the primary discrete particle analysis technique,21 52 103 104 or have combined TEM analysis techniques, such as electron diffraction and x-ray microanalysis, to confirm conclusions drawn from SEM surveys.7,93 105... 

Double-beam or dispersive instruments in which the IR radiation from a single source is split into two identical beams. One beam passes through the sample and the other is used as a reference and passes through air or the pure solvent used to dissolve the sample. The difference in intensity of the two beams is detected and recorded as a peak the principal components of this type of instrument are shown in Fig. 28.2. The important controls on the spectrometer are ... 

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