Infrared analysis sample cells

Infrared spectroscopy is routinely used for the analysis of samples in the gas, liquid, and solid states. Sample cells are made from materials, such as NaCl and KBr, that are transparent to infrared radiation. Gases are analyzed using a cell with a pathlength of approximately 10 cm. Longer pathlengths are obtained by using mirrors to pass the beam of radiation through the sample several times. 

Photometric Moisture Analysis TTis analyzer reqiiires a light source, a filter wheel rotated by a synchronous motor, a sample cell, a detector to measure the light transmitted, and associated electronics. Water has two absorption bands in the near infrared region at 1400 and 1900 nm. This analyzer can measure moisture in liquid or gaseous samples at levels from 5 ppm up to 100 percent, depending on other chemical species in the sample. Response time is less than 1 s, and samples can be run up to 300°C and 400 psig. 

One cm3 of the reactant/product/catalyst mixture was sampled periodically during the reaction for the transmission infrared analysis (Nicolet Magna 550 Series II infrared spectrometer with a MCT detector). The concentrations of reactants and products were obtained by multiplying integrated absorbance of each species by its molar extinction coefficient. The molar extinction coefficient was determined from the slope of a calibration curve, a plot of the peak area versus the number of moles of the reagent in the IR cell. The reaction on each catalyst was repeated and the relative error for the carbamate yield measured by IR is within 5%. 

Infrared analysis is usually used as a qualitative method to identify substances. Liquids are usually analyzed as pure substances in cells with very small optical path lengths of 0.1-1.0 mm. Usable spectra can be obtained by placing a drop of relatively non-volatile sample between two sodium chloride plates, allowing them to be held together by capillary action. 

The most straightforward method for analyzing a solid material by infrared spectrometry is to dissolve it in a suitable solvent and then to measure this solution using a liquid sampling cell such as one of the several described in Section 8.8. Thus it becomes a liquid sampling problem, the experimental details of which have already been discussed (Section 8.8). It is the only method of solid sampling suitable for quantitative analysis because it is the only one that has a defined and reproduced pathlength. 

Quantitative analysis procedures using infrared spectrometry utilize Beer s law. Thus only sampling cells with a constant pathlength can be used. Once the percent transmittance or absorbance measurements are made, the data reduction procedures are identical with those outlined in Chapter 7 (preparation of standard curve, etc.). 

A gas-washing bottle (Figure 4.7B) may also be used for trapping. This technique is especially useful in conjunction with infrared analysis. The sample is simply bubbled through the anhydrous solvent as it exits the chromatographic column. The solution is then placed in a liquid sample infrared cell. A matching cell containing only the solvent is placed in the reference beam. An infrared spectrum of the sample may then be recorded. 

Conventionally, infrared spectroscopy is carried out in the transmission mode, where the light passes through a sample cell with a defined thickness. There are two main disadvantages of this technique for the purpose of reaction analysis. 

In these experiments, nickel hydroxide was mixed in a proportion of 12.4% with finely divided silica (Cabosil), pressed in a die and dehydrated at 200°, under vacuum, in the infrared analysis cell. Composition of the sample was therefore different from the composition of the samples used in the gravimetric or calorimetric work [NiO(200°)] and possible effects of the support cannot be, a priori, completely excluded. Calorimetric experiments with the supported samples have shown, however, that their reactivity toward CO is very similar to the reactivity ofNiO(200°). 

Dispersive Infrared Spectroscopy The dispersive IR spectrometer generally incorporates an IR broadband source, sample cell, a diffraction grating and one or more IR detectors. Dispersive IR instruments may provide simultaneous or sequential measurements. Respectively, the instrument may have a fixed grating and many detectors, or a movable grating and a single detector. In some cases, the grating may be replaced by one or more optical filters to resolve the desired wavelengths. A reference cell and associated optics to perform simultaneous differential analysis are also incorporated to improve sensitivity or reliability of measurement. 

The infrared analysis was performed using samples cured between silver chloride plates. Conditioning of the resin was achieved by immersion in water. High temperature scans were made using a temperature regulated sample cell. Spectra were obtained with a Perkin-Elmer 283 spectrophotometer. 

The experimental technique that has provided the vast majority of results on E V transfer from excited halogens is that introduced in 1974 by Leone and Wodarczyk. In brief, X is produced by a photolysis source, and the concentrations of X and of the vibrationally excited collision partner are monitored by observing their time-dependent infrared fluorescence. In our laboratory the apparatus has taken the form depicted in Fig. 1. It consists basically of a source and sample cell, a detector system, and an electronic system for signal enhancement and analysis. 

Ideally, the intensity (/o) of infrared radiation incident upon a sample cell is reduced (to f) by the absorption of the samples. Actually, some of the incident energy is scattered by the sample and this scattered energy makes the Beer-Lambert law inaccurate, especially at high values of absorbance [4]. The baseline method for quantitative analysis is an empirical method used to establish a calibration curve of log (/q//) versus concentration. Infrared absorption bands may overlap neighboring bands or may appear on a sloping background, so transmittance is measured in practice as shown in Figure 8.13. The absorbance. A, is determined from measurements of / and 7o, then a calibration curve of absorbance versus concentration is plotted. 

By far the most common use of mid-infrared radiation for process analysis is in the non-dispersive infrared analysers that are discussed below. The widespread use of FTIR spectrometers in the mid-lR has yet to be fully realized in process analytical apphcations. The requirements for the optical components and the wavelength sta-bihty of the instraments available have, until recently, detracted from the use of this region of the spectrum in on-line process analysis. Optical fibers that provide such a benefit to the apphcations of NIR (see below) are not available for the mid-IR in robust forms or forms that are capable of transmitting over more than a few tens of metres. Improvements and developments to sample cells, particularly designs of attenuated total reflectance (ATR) cells, for use with mid-lR are being made and will influence the application of the technique. An impressive list of apphcations including both FTIR and the NDIR approaches has been compiled (2, 3]. 

To conclude this article, it is important to state that, in general, commercial oenological laboratories are equipped with automated instrumentation that carry out the above analyses (and others besides) in a single step. The most widely used instrumental technique is based on FTIR analysis. The infrared spectrum of an organic solution such as wine presents complex absorption spectra characteristic of the different wine components. The Michelson interferometer, which is at heart of the FTIR method, is based on the division of a polychromatic band of infrared radiation into two beams which then follow different optical pathways one beam traverses the sample cell directly, while the other is reflected on a mobile mirror before arriving at the sample cell. For each elemental wavelength arriving at the detector cell there will be a phase difference, which is continuously varied... 

Figure 1 Examples of gas cells for mid-infrared transmission measurements (A) photograph of a multiple-pass gas cell ( Long Path Miniceir), with a high path-to-volume (530 ml) ratio. Allows paths from 1.2 m (eight passes) to 7.2 m (48 passes) (B) schematic of a multipass cell with transfer optics for use in a center-focus sample compartment (C) and (D) photographs of Pyrex and stainless steel bodied 10-cm pathlength cells, respectively. ((A and B) Reproduced by kind permission of Infrared Analysis, Inc., Anaheim CA, USA. (C and D) Reproduced by kind permission of Specac Ltd., Orpington, Kent, UK.)...

With modem sampling techniques, good quantitative infrared analysis with virtually every type of sample is practicable however, liquids are ideal for this purpose, being measured in a liquid cell of fixed thickness, either as 100% sample or diluted with solvent. In this connection it should be taken into account that there are no ideal solvents for infrared spectroscopy [35]. In addition, because absorption bands and path length can be influenced by the temperature of the transmission cell, it is advisable to control the temperature. 

Normally, one would not usually consider that the change from an amorphous or glassy state to a crystalline or ordered state is readily discerned by infrared absorption. With some polymers, such as polyethylene terephthalate (PET), this change in structure gives rise to diffefrences in the infrared spectrum that can be used for quantitative analysis of samples for crystalline content. With PET, a band at 10.2 was shown by Cobbs and Burton [ ] to be a function of crystallinity and variously annealed samples were run for infrared absorption and density. The density of amorphous PET is 1.33, while the density for crystalline polymer is 1.47 according to x-ray unit cell measurements. From the density-infrared absorbance samples it was then possible to construct a calibration curve of absorbance versus percent crystallinity. 

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