Variable pathlength cells

Spectrophotometer cell, n - an apparatus which allows a liquid sample or gas to flow between two optical surfaces which are separated by a fixed distance, referred to as the sample pathlength, while simultaneously allowing light to pass through the liquid. There are variations of this including variable-pathlength cells, and multi-pass cells, and so on. 

When I was a boy, Uncle Wilbur let me watch as he analyzed the iron content of runoff from his banana ranch. A 25.0-mL sample was acidified with nitric acid and treated with excess KSCN to form a red complex. (KSCN itself is colorless.) The solution was then diluted to 100.0 mL and put in a variable-pathlength cell. For comparison, a 10.0-mL reference sample of 6.80 X 10-4 M Fe3+ was treated with HN03 and KSCN and diluted to 50.0 mL. The reference was placed in a cell with a 1.00-cm light path. The runoff sample exhibited the same absorbance as the reference when the pathlength of the runoff cell was 2.48 cm. What was the concentration of iron in Uncle Wilbur s runoff ... 

Sealed and semi-permanent cells are most common, but variable pathlength cells are also available. A simple version of a liquid sample cell is the demountable Nujol cell which is suitable for qualitative examination of pastes or viscous liquids. The use of spacers in these devices may or may not be necessary, depending on the viscosity of the sample under investigation. NaCl, KBr, Csl, and polyethylene window materials are convenient if the sample does not contain water. A drop of liquid or paste is placed on one of the windows, and the two windows are clamped with front and back metal plates. Semi-permanent cells are suitable for quantitative work and can be cleaned easily after use (Fig. 5.2.1). 

Variable-pathlength cells suffer from similar disadvantages and they are difficult to take apart. The calibration therefore suffers and the cells have to be calibrated regularly. 

Schneider et al. have reported a cell for HP IR spectroscopy which combines both transmission and reflectance methods for in situ investigations of multiphase reactions [47]. The upper part of the cell can be monitored by variable pathlength... 

Solvent compensation is slightly more cumbersome than in ordinary work because all absorptions become sharper and increase in intensity with decreasing temperature. Use of a variable-pathlength reference cell alleviates most of the problems. The method of choice, however, is to run solvent spectra at various temperatures and use computer subtraction which is nowadays available even with medium-priced dispersive instruments. 

It is possible to purchase variable-pathlength liquid transmission cells, which commonly have a pathlength that can be varied between 5 pm and 5 mm. A suitable sample must be chosen, preferably a neat solvent that has relatively wide peaks, the resolution of the spectrometer must be set so that the resolution parameter, p, is no greater than 0.5, and a Norton-Beer medium apodization function should be applied. A series of measurements at increasing pathlengths should yield a linear Lambert plot if the instrument is functioning correctly. Some instruments will exhibit linear Lambert plots up to 4 AU however, this is unusual, and an acceptable... 

FIGURE 4.26 A variable pathlength sealed liquid cell. (Photo courtesy International... 

Apparatus Use a suitable variable-wavelength spectrophotometer capable of measuring percent transmittance throughout the visible spectrum and designed to permit the use of sample and reference cells with pathlengths of 2 to 4 cm. The transmittance of all paired cells should agree within 0.5%. 

However, because we have used an ATR cell in this current study we cannot use the Beer-Lambert law directly, as we do not have a pathlength for the calculation. However, as this variable does not change during the experiment we can ignore this factor and exploit the fact that absorbance is directly proportional to the sample concentration. Therefore, we can write the following ... 

The demountable is by far the easiest to maintain as it can be readily dismantled and cleaned. The windows can be repolished, a new spacer supplied and the cell reassembled. The permanent cells are difficult to clean and can become damaged by water. The pathlengths need to calibrated regularly if quantitative work is to be undertaken. Variable path-length cells suffer from similar disadvantages and they are difficult to take apart. The calibration therefore suffers and the cells have to be calibrated regularly. 

An alternative stopped-flow attachment, designed for use with Jasco J-700 series spectropolarimeters, is the HI-TECH Scientific SHU-61CD with pneumatic drive. Its temperature range is 5-80 C, its pathlength is 2 mm, it has a 3.5 mm window size and a cell volume of 19 (i,l. The mixer is integral to the cell. 200 p,l is required per shot with 2 ml to prime the flowlines. Drive syringes may be altered to allow variable mixing. Hi-Tech do not offer a dedicated stopped-flow circular dichroism system. 

A UV-Vis spectrum is subject to a number of variables, among which are solvent, concentration of the solution being examined, and the pathlength of the cell through which the light passes. The amount of light absorbed by a particular solution is quantitatively defined by the Beer-Lambert law (Eq. 8.14). 

Fig. 14.1 Various absorption and emission cells, a Thin pathlength demountable flow cell b 0 cm pathlength cylindrical absorption cell c triangular emission cell for use with highly absorbing solutions d variable path length absorption flow cell with stainless steel sleeve and screw e standard 1 and 5 mm path length absorption cells, and micro cell f 0.1 mm path length cylindrical cell (the 0.1 mm path length is behind the cell face placed down on the table) g 5 cm cylindrical absorption cell h 4 cm absorption cell i standard 1 cm emission cell. The 5p UK coin, included to give some idea of scale, has a diameter of 18 mm...

As with other types of absorption spectroscopy e.g. UVA IS) the basis of quantitative analysis in transmission IR spectroscopy is Beer s law. This requires few components and no peak overlap. Although deviations from Beer s law exist, these can usually easily be dealt with. The challenge in FTIR quantitation for polymers is sample thickness. In infrared, sample concentration and optical pathlength can seldom be controlled as tightly as in UVAHS spectrometry. This is primarily due to the absence of suitable materials (solvents and cuvets) that are transparent over a sufficiently wide frequency range. Use of peak ratios standardises the absorbance signal and eliminates the thickness variable. Alternatively, use can be made of sealed cells with constant path-length. 

Hirt and King have described the use of variable thickness (micrometer Baly cells for short-path measurements in the ultraviolet. Mitzner used thin cells to get the ultraviolet spectra of pure liquids in order to avoid solvent effects. Jones and Keir described a low-temperature cell for the Cary. Hamner et aL described a 50-cm heated gas cell for the ultraviolet. Cylindrical silica gas cells of 100 mm pathlength are commercially available. So are continuous-flow cells, short-path cells (to 0.3 mm), 50-cm... 

---