Sample compartment spectrometer

In the most general temis, an infrared spectrometer consists of a light source, a dispersmg element, a sample compartment and a detector. Of course, there is tremendous variability depending on the application. 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

The technique used to acquire the data in this paper was SNIFTIRS. A schematic diagram of the required apparatus is shown in Figure 5, and has been described in detail elsewhere. The FTIR spectrometer used was a vacuum bench Bruker IBM Model IR/98, modified so that the optical beam was brought upwards through the sample compartment and made to reflect from the bottom of the horizontal mercury surface. The methods used herein are adapted from a configuration that has been used by Bewick and co-workers (21) at Southampton. 

Electrochemical measurements were performed in an electrochemical cell equipped with quartz windows which fit into the sample compartment of a Cary 14 spectrometer. The cell (CHjCN, 0.1N TEAP vs S.C.E.) employed three electrode (Pt auxiliary electrode) potentiostatic control. A Tacussell PRT Potentiostat and PAR model 175 signal generator were used for the measurements. 

For measurements at temperatures other than ambient, cells with double walls, which can be thermostatted, are also available commercially. If measurements are required at temperatures between ca. —5°C and room temperature, the sample compartment of the spectrometer can be flushed with dry air or nitrogen to reduce condensation on the cell windows. Below ca. — 5 °C the windows can be covered with a thin polythene film, but measurements below —25 °C are very troublesome. The problems associated with low temperature spectroscopic measurements were solved by enclosing the cell in an air-tight box fitted with glass windows (Dadley and Evans, 1967). The box was so designed that it fitted into the spectrophotometer and the air inside the box was dried with phosphoric oxide which, it is claimed, stopped condensation even at temperatures as low as — 60 °C glass windows could be used because only absorptions above 380 nm were of interest. 

Spectrometer was used. Normal operating conditions were used normal slit, IX expansion, 12-minute scan speed (4000-200 wave numbers), and normal gain, in accordance with Perkin-Elmer setup instructions. A Beckman 2 cm path length, near infrared silica cell (holds 8 ml sample) was used to hold the desorbing solution in the sample compartment (Figure 1). 

HWGs have successfully been applied to a wide variety of gas-sensing applications [44-52]. Micheels et al. [46] coupled a coiled MIR HWG to a FT-IR spectrometer measuring VOCs in the headspace of water samples. Yang et al. [47,48] partitioned organics from water or the headspace above a soil sample into the coating of a HWG. The waveguide was then inserted into the sample compartment of the FT-IR. 

A shutter and pure solvent in the sample compartment of this pseudo-double beam spectrometer permits acquisition of both a dark current (0% T) and an emmisivity/responsivity (100% T) data array. These consist of digitized responses from each of the pixels in the array under conditions where the integration time and the speed of read-out is identical to that planned for the measurement of the spectrum of a sample. The dark-current data will provide information as to the shot-noise and other inherent... 

The next class of VCD instruments to be developed was centered around a Fourier transform infrared (FT-IR) spectrometer. The idea was to design the sample compartment to be the same as in a dispersive VCD instrument, including a photoelastic modulator. To measure VCD, the detector signal is first sent to a lock-in amplifier to demodulate the high-frequency polarization modulation. The output of the lock-in is a VCD interferogram which is Fourier transformed in much the same way as the ordinary transmission interferogram. 

An airtight sample holder or at least an outer container for the sample should be used when possible. The spectrometer can be placed in a fume cupboard to avoid vapor hazard, but good ventilation of the sample compartment can be enough. It should be remembered that no sample preparation should be carried out in the same fume cupboard to avoid contamination of the instrument. Decontamination of an FTIR spectrometer is very difficult if not impossible. 

The NIR spectrometer used for method development and sample analysis was a Foss NIR Systems Model 6500 Forage Analyzer with a sample transport module and a standard reflectance detector array. The transport module moves the sample compartment up and down during data collection, thereby allowing a more representative spectrum to be obtained from bulky heterogeneous samples. The reflectance array uses two silicon detectors to monitor visible light from 400-850 nm and four lead-sulfide detectors to monitor NIR light from 850-2500 nm. Natural product sample compartment cells in 1/4-cup and 1-cup sizes were used as sample holders in the transport module. This instrument has a maximum resolution of 2 nm. 

Fig. 6.8-5 shows part of a spectrum of C2H2 (6 = 1.177 cm ). A is obtained from this spectrum. The resulting sample temperature is T = 309 K, a value which is to be expected for a sample in a gas cell inside the closed. sample compartment of an IR spectrometer. 

The cell shown in Fig. 7 has been designed to be placed outside the sample compartment of the spectrometer. It has the advantage of requiring only a small volume of electrolyte (c. 5 ml). The solution can be replaced while the working electrode is kept under potential control. This can be very useful in adsorption experiments with organic fuels, as we shall see in the sections devoted to adsorption of alcohols. 

To run a spectrum of a neat liquid (free of water) remove a demountable cell (Fig. 3) from the desiccator and place a drop of the liquid between the salt plates, press the plates together to remove any air bubbles, and add the top rubber gasket and metal top plate. Next, put on all four of the nuts and gently tighten them to apply an even pressure to the top plate. Place the cell in the sample compartment (nearest the front of the spectrometer) and run the spectrum. 

Fig. 1 Accessories for diffuse reflectance spectroscopy (A) Integrating sphere with hemispherical radiation collection (B) Accessory based on ellipsoidal mirrors, used within the sample compartment of the spectrometer (C) Rotational ellipsoidal mirror device with dedicated detector and (D) Bifurcated fiber optic-based accessory (also shown is the random mixture of fibers for illumination and detection compared with devices based on reflection optics the acceptance cone for radiation delivery and collection is limited and depends on the refractive indices of the core and cladding material).

Most reflectance spectroscopy is carried out utilizing an accessory that can be easily inserted into and removed from the sampling compartment of a conventional spectrometer. These accessories are designed for each application and usually consist of mirrors or prisms for reflecting or focusing the radiation. Most of the sampling accessories for the FTIR spectrometers and the FTIR microscopes are available commercially. 

An absorption spectrometer requires a separate radiation source and a sample compartment that holds containers for the sample and blank. With an emission spectrometer, the sample is introduced directly into a hot plasma or flame where excitation and emission occur. 

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