Sample compartment, control

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. 

The core components of a CE instrument are a power supply, a detector, and devices that allow for temperature control of the capillary and sample compartment. A wide variety of commercial CE instruments are available, from simple modular systems to fully integrated automated systems under computer control. 

When assays are conducted with a instrument such as a spectrophotometer, the sample compartment in the instrument should also be equilibrated to the same temperature. Typically, the control of temperature inside the sample compartment is less than that of the water bath or circulator. Many instruments now provide a temperature readout for the sample compartment that allows an investigator the opportunity to directly assess temperature control issues in the instrument. 

The injection temperature can be a signiflcant issne for thermally unstable samples or where samples are stored for hours in an antosampler prior to injection. For this reason, most manufacturers sell autosamplers with optional thermostated sample compartments. This can be done either by placing the sample tray in an air bath oven or by a condnctive temperature control of the sample rack. The need to keep samples cool prior to injection when conpled with elevated temperature separation increases the complexity of the flow system reqnired. For such application, a separate mobile phase pre-heater with a low volnme placed between the injector and the column is a good choice. Alternatively, the injector valve wonld need to be monnted ontside the antosampler or in the column oven to insure preheating of the mobile phase before the colnmn. 

Figure 6.14 Four-compartment cell for controlled-potential electrolysis and coulo-metric titrations. Two central bridge compartments can be emptied to sample compartment by application of nitrogen pressure and refilled by vacuum 1, polyethylene top 2, lock ring 3, combination glass-calomel electrode 4, N2 inlet tube 5, N2 outlet tube 6, Pt gauze electrode 7, cell rinse assembly 8, polyethylene spray shield 9, 0.1 M KC1 in 3% agar gel 10, Ag anode.

Turn on the spectrophotometer and let it warm up for a few minutes. Turn the wavelength control knob to read 340 nm. With no sample tube in the sample compartment, adjust the amplifier control knob so that 0% transmittance or infinite absorbance is read. 

Figure 1. In-situ FTIR catalyst light-off temperature measurement system. 1. compressed air 2. mass flow controller 3. gas lecture bottle 4. MKS pressure gauge 5. exhaust 6. IR cell 7. IR window 8. catalyst sample 9. thermocouple 10. sample compartment of Nicolet FTIR Model 60SX 11. chart recorder 12. digital thermometer display.

Figure 3.4-1 Optical diagram of a commercial Michelson interferometer for infrared and Raman spectroscopy (Bruker IFS 66 with Raman module FRA 106). CE control electronics, D1/D2 IR detectors, BS beamsplitter, MS mirror scanner, IP input port, S IR source, AC aperture changer, XI — X3 external beams, A aperture for Raman spectroscopy, D detector for Raman spectroscopy, FM Rayleigh filter module, SC sample compartment with illumination optics, L Nd.YAG laser, SP sample position.

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. 

If the thermostatic control is obtained with gaseous nitrogen, a metallic cell holder can be used. A cell holder consists of an outer brass cylinder with the thermostatically regulated nitrogen flow circulating inside. Holes can be made in the cylinder in order to allow optical observations. To avoid condensation and ice formation on the walls at low temperatures, the cell holder is isolated in a sample compartment overpressurized with dry nitrogen. This kind of apparatus is used for spectroscopic determinations at subzero temperatures (Maurel et al., 1974). 

All activity measurements were conducted in an in-situ infrared reactor cell placed in the sample compartment of a DIGILAB 15C Fourier Transform Infrared (FTIR) Spectrometer. The reactor, described in detail elsewhere [11], consisted of two aluminum flanges with CaF2 IR transparent windows, a gas inlet and outlet, and two foil fast response thermocouples which were placed in direct contat with the catalyst. The reactor temperature was maintained constant by external heaters controlled by a temperature programmed controller. A Teflon coated recycle pump permitted to maintain near isothermal conditions and improve the mixing in the reactor. The reactor and associated lines were tested for activity at the highest temperature used, and it was found to have negligible activity. 

Fig. 10.3 Scheme of automatic computer-controlled spectrometer sample compartment. (Reproduced from [7] with permission of Pergamon Press Ltd.). 

Part was transferred to a 1cm silica cell and the reaction followed at an appropriate wavelength in the spectrophotometer, of which the sample compartment was maintained at a controlled temperature by the circulation through surrounding coils of water from a thermostat. 

The sample compartment contains a specially constructed cell holder. It can be temperature controlled as well as stirred and suf lies positions for photo diodes which allow continuous mcmitoring of the intensity of the irradiation source. This specific cell holder is demonstrated in Fig. 4.7 in more detail than in Fig. 4.5. 

The combination of an irradiation and measurement device needs some modification of the sample compartment of the spectrometer. In some cases problems with the geometry can arise. For this reason another approach was tried using the advantage of fibre optics. A bundle of quartz fibres (approx. 1 cm in diameter) allows process-controlled irradiation of the cell in a conventional double beam spectrometer (see Fig. 4.9) without any modification of the sample compartment. Only the cell holder has to be modified to allow the output of the fibre to be fixed [53]. 

The similarities of TG and DTA are obviously great, at least instrumentally. As a consequence, many commercial instruments are designed to perform both types of analysis the heating device, temperature-control unit, atmospheric control, and recording device are essentially used in common and are contained in a single control unit, only the thermobalance and DTA sample compartments being separate. 

The major pieces of apparatus used to perform the abbreviated e xposure experiment consist of an FT-IR spectrophotometer (Nlcolet 7199) with a DTGS detector, a controlled environmental chamber (CEEC) with monitoring system and gas supply, and a solar simulator. The polymer-coated mirror sample Is mounted Inside the CEEC, which Is mounted Inside the sample compartment of the FT-IR. Details of the Instrumentation have been presented elsewhere [8]. A schematic of the CEEC Is shown In Fig. 3. 

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