Sample compartment

The sample cells used in fluorescence spectroscopy have all four sides dear, since in most instruments the fluorescence photons are detected at an angle of 90° to the propagation direction of the exdtation beam. The 90° geometry is used in order to minimize any interference in the fluorescence hght detection by the exdtation light beam. In most cases the cuvettes are made from fused silica (quartz) but if the exdtation and fluorescence wavelengths are above 300 nm plastic disposable cuvettes may be used. 

The deactivation processes of Si(0) are often highly temperature dependent 

1 The rhodamine dyes are considered to be toxic. It is important to wear gloves while handling the dyes and to avoid all spills. Weigh 15 mg of rhodamine B into the scintillation vial. 

2 Add 0.5 ml of ethanol to aid the solubility of the rhodamine. Add 4.5 ml of ethylene glycol and cap the vial. Shake the solution and then hold it in a warm sonication bath until there is no evidence of any particulate matter in the sample. The sonicator assists in the solution of the rhodamine. 

3 Totally fill the cuvette with the rhodamine solution using a Pasteur pipette. Stopper the cuvette, and wrap the stopper and top of the cuvette with a small piece of polyfilm. Wipe the outside of the cuvette with a tissue wet with methanol and then a dry tissue. 

After leaving the monochromator the radiation is directed to the sample compartment by a rotating sector mirror, where it is alternately focused on the substance to be examined (which is contained in a cell with quartz windows) and a reference cell (which holds the pure solvent used to dissolve the sample). The system now has two beams, hence the name double-beam spectrophotometer. After passing through the sample where the absorption of radiation may occur, the beams are recombined. 

Following the wavelength selection by the monochromator, the beam passes on to the sample compartment where the sample solution, held in the cuvette, is positioned in its path. The sample compartment is an enclosure with a lid that can be opened and closed in order to insert and remove the cuvette. When the lid is closed, the compartment should be relatively free of stray light, although this is not a requirement if a xenon arc lamp is used as the source. This is because of the high intensity of the xenon arc lamp. The cuvette is held snugly in a spring-loaded holder. 

Some spectrophotometers are single-beam instruments, and some are double-beam instruments. In a double-beam instrument the light beam emerging from the monochromator is split into two beams at some point between the monochromator and the detector. The double-beam design provides certain advantages that we will discuss shortly. 

FIGURE 8.6 An illustration (top view) of the double-beam design utilizing two cuvette holders in the sample compartment. 

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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. 

Because Raman spectroscopy requires one only to guide a laser beam to the sample and extract a scattered beam, the technique is easily adaptable to measurements as a function of temperature and pressure. High temperatures can be achieved by using a small furnace built into the sample compartment. Low temperatures, easily to 78 K (liquid nitrogen) and with some diflSculty to 4.2 K (liquid helium), can be achieved with various commercially available cryostats. Chambers suitable for Raman spectroscopy to pressures of a few hundred MPa can be constructed using sapphire windows for the laser and scattered beams. However, Raman spectroscopy is the characterizadon tool of choice in diamond-anvil high-pressure cells, which produce pressures well in excess of 100 GPa. ... 

Fig. A.3. Light meter used by the author (Model 8020, Pelagic Electronics). For total light measurements, the signals are integrated with capacitors. Milliammeter reading is automatically reset at full-scale position, and the number of resets is digitally indicated below the meter. The box at the right contains a photomultiplier and sample compartment.

At the time of writing, in all papers published on adsorption studies on oxides surfaces, spectra have been reported of samples held at the ambient temperature of the sample compartment. It is obvious that when dealing with very volatile adsorbates, low temperature sample cells may be required to increase adsorption and also to prevent rapid desorption of the adsorbed species. In some instances, it is also desirable to record the spectra of species held at elevated temperatures for better correlation with industrial catalytic systems. It should be noted that there are only a few infrared spectra reported in the literature for high temperature studies of catalytic reactions. Sample emission at elevated temperature is a significant experimental complication in investigations of this type. 

Two identical reaction solutions were prepared, one at T,(= 25.000 °C) in the sample compartment of a double-beam spectrophotometer, the other at T2( = 27.170 °C) in the reference beam. A direct recording of AAbs = Absi - Abs2 was made as a function of time while the difference in reaction temperature was maintained to 0.0001 °C. Evaluate kffk and AW1 for the run shown note this calculation is possible with an arbitrary time axis. 

In those experiments, the solvent is distinguished from the host material by the huge difference in the transverse relaxation times. The technique to be described here monitors interdiffusion between two sample compartments initially filled with deuterated and undeuterated liquids (or gels) of the same chemical species. Bringing the compartments into contact initiates interdiffusion. Mapping of the proton spin density thus permits the evolution of the corresponding concentration profiles to be followed. 

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... 

A diagram of a typical interferometer (Michelson type) is shown in Figure 7.8. It consists of fixed and moving front-surface plane mirrors (A and B) and a beamsplitter. Collimated infrared radiation from the source incident on the beamsplitter is divided into two beams of equal intensity that pass to the fixed and moving mirrors respectively. Each is reflected back on itself, recombining at the beamsplitter from where they are directed through the sample compartment and onto the detector. Small... 

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. 

After extraction, samples to be analyzed are dissolved in a solvent, commonly acetonitrile, heptane, hexadecane, or water. A scan of the pure solvent in the sample cell, typically made of quartz (Figure 14.3 [A]), is first obtained. The dissolved sample is placed in the sample cell, which is placed in the sample compartment of the instrument, and a spectrum is obtained. 

When using a spectrophotometer for a colorimetric analysis, both the 0% and 100% transmittance (oo and 0 absorbance) readings must be set. Once the instrument has warmed up, with the light beam blocked and with nothing in the sample compartment, the readout is set to 0% transmittance (oo abs.). Again, this measurement is done to set / in the absorbance equation shown earlier. A blank, a solution containing all the components used in the analysis except the analyte being measured, is placed in a cuvette, placed in the sample... 

Hamilton Umicon Lumicon chemi- and biolumium assay luminometer This equipment is used in test-tube scale luminescent immunoassays. With its sample compartment (thermostatted by means of Peltier elements, which allow the temperature to be set from 15°C to 40°C with a precision of 0.1°K) this instrument is suitable for the measurement of temperature-sensitive bioluminescence resulting from enzymic reactions and also in phagocyte-mediated luminescence measurements. 

FIGURE 8.7 The double-beam design in which the second beam passes directly to a detector without passing through the sample compartment. 

A photomultiplier tube is a light sensor combined with a signal amplifier. The light emerging from the sample compartment strikes the photosensitive surface and the resulting electrical signal is amplified. 

FIGURE 8.15 An illustration of an FTIR instrument showing the light source, the interferometer, the sample compartment, and detector. 

Why is the sample compartment a box isolated from the rest of the instrument ... 

In summary, the basic fluorometer, and thus the basic fluorescence detector, consists of a light source and a wavelength selector (usually a filter) for creating and isolating a desired wavelength, a sample compartment, and a second wavelength selector (another filter) with a phototube detector for isolating... 

As with the UV absorption detector, the sample compartment consists of a special cell for measuring a flowing, rather than static, solution. The fluorescence detector thus individually measures the fluorescence intensities of the mixture components as they elute from the column (see Figure 13.10). The electronic signal generated at the phototube is recorded on the chromatogram. 

The sample compartment must consist of a light-tight box so that no stray light—only light from the instrument s light source—reaches the light sensor. 

The precalibration of a single-beam spectrophotometer consists of tweaking the readout to read 100% T when the blank is in the sample compartment. 

It is possible to assess the proportion of stray light by measuring the amount of radiation transmitted by samples that are optically opaque at the wavelength to be assessed but that transmit radiation of other wavelengths. The instrument is set to zero and 100% transmittance in the normal way and the opaque substance introduced into the sample compartment. The amount of light transmitted by the sample, measured in percentage transmittance, is... 

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. 

FIAs can be based on steady-state intensity measurements without probe amplification, owing to the sensitivity of detection that is possible with fluorescence instrumentation, which exceeds that of spectrophotometers by two or three orders of magnitude. A sensitive fluorometer has been described for an estradiol assay(36) in which the limit of estradiol detection is 3 x KT11 M. Estradiol antibody labeled with rhodamine B is reacted with estradiol samples. Unreacted labeled antibody is removed with Sepharose-estradiol-casein beads, and the remaining fluorescence is directly proportional to the analyte concentration. The detection limit of rhodamine B on the same fluorometer is 5 x 1(T12 M. This instrument uses a 0.75 mW green helium-neon (HeNe) laser to irradiate the sample from above, at the air-liquid interface, to increase the light path and to decrease surface reflections. The sample compartment has a top-mounted photon trap, and a mirror mounted on the side of the sample compartment opposite the PMT to enhance detection. 

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