Path length, spectrophotometry

Several advantages offered by CE, such as a high efficiency, rapid method development, simple instrumentation, and low sample consumption, are the main reasons for its success in a variety of fields. UV-VIS spectrophotometry is probably the most widely used detection technique with CE because of the simplicity of the on-line configuration. However, its sensitivity, directly related to the optical path length afforded by the I.D. of capillaries, which is in the micrometer range, is low, and it remains the major bottleneck of this technique (see... 

The quality of an analytical separation technique is not only characterized by its separation efficiency but also by its detection sensitivity. UV-VIS spectrophotometry and fluorescence are commonly used simple detection methods for CEC. However, not all compounds absorb UV light and even fewer compounds fluoresce. Although high mass sensitivity can be achieved, the concentration sensitivity in CEC is generally poor since short optical path lengths that equal the internal diameter of the capillary are used. Dittmann et al. [55] discussed thoroughly the limits of sensitivity in CEC detection. 

The limit-of-detection for fiber optics is higher than for conventional spectrophotometry because the loss of signal down the fiber decreases the S N ratio. This loss of sensitivity may be overcome with a longer path length cell. 

For absorptive transition, as in atomic absorption spectrophotometry, it can be shown that the measurable integrated inherent intensity of an absorption line, expressed as absorbance A, per unit concentration of absorber c and optical path length d in the flame, is given by... 

A complete description of the flow-through detector, including operating conditions, should be provided. In spectrophotometry, the flow cell is usually distinct from the detector therefore, its description, including light optical fibres (if relevant), is required. It is important to specify the inner volume and optical path length of the flow cell and, for specific applications, e.g., multi-site detection [114], information about its illuminated portion is also relevant. In luminometric procedures, the shape of the flow cell as well as its arrangement in relation to the detector should also be reported. 

Atomic absorption follows an exponential law for the intensity of transmitted light as a function of the path length b, similcir to Lambert s law in molecular spectrophotometry ... 

One good thing about this new method is that it is performance based. The analyst can use an alternative approach to oil and grease isolation and recovery. One approach is to use solid-phase extraction (SPE) to isolate the oil and grease, whereas the other approach is to find a way to eliminate use of the solvent. The recently introduced use of infrared cards by 3M Corporation, whereby the oil and grease remains on a thin film of Teflon, is an example of the latter alternative to the conventional liquid-liquid extraction (LLE). The thin film that now offers a fixed path length, is then inserted into the sample holder of a conventional infrared spectrophotometer and the absorbance of the C — H stretching vibration at 2900 cm is directly related to concentration via the Beer Lambert Law of Spectrophotometry discussed in Chapter 4. 

Under optimum conditions a precision of 0.002 pH can be obtained with glass electrodes, compared with 0.0005 pH or better for spectrophotometry with a 100 mm path length. Systematic errors are currently estimated to be similar for both techniques, of the order of 0.004 pH. The systematic errors for spectrophotometric measurements are likely to decrease in the near future as further indicator pK determinations become available and as more accurate methods of correcting for the added indicator are developed. Spectrophotometry is therefore expected to become the method of choice for the most accurate and precise measurements of seawater pH. 

One of the most common uses of spectrophotometry is to quantify the concentration of solutions from their absorbance spectra. The Beer-Lambert [17] law states that the absorbance of a solution is linearly proportional to its concentration c, and the path length of the light in the solution. The proportionality coefficient is the molar absorptivity K. Thus,... 

UV microspectroscopic applications may find their origin in the fact that analytical problems in synthetics production and moulding are often associated with the determination of additives in polymers while only small sample quantities are available. Examples are the determination of stabiliser distributions or the characterisation of inhomogeneities in foils. Qualitative and quantitative characterisation of such small quantities by in-polymer UV spectrophotometry requires small pinholes (0 0.085 cm). Se-han et al. [401] have described a sample holder provision to allow for distribution analysis of phenolic stabilisers in polymers by direct UV analysis. For purposes of scanning (in mm steps) a specimen holder with x, y adjustments permits determination of the stabiliser concentration in desired increments of an industrially produced foil of relatively constant path length (200 /xm). This is a way to check the distribution of substances in polymers and, if required, to optimise the manufacturing process. 

In spectrophotometry the absorbance per unit length of path through the sample e is defined as... 

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