Ambient light interferes

For Raman analysis, sample preparation is much easier than with IR. In fact, the source light is simply focussed onto the solid or liquid sample directly. If a cuvette is used, quartz or glass windows can be used. If a slide or surface is used, a background spectrum should be taken to remove the possibility of any interfering peaks. Glass tubes are often used and since water is a weak Raman scatterer, aqueous samples can be easily analysed. Reflectance measurements, as distinct from transmissive measurements above, can also be made and are useful for studying Aims on metal surfaces or samples on diamond surfaces. Measurements should also ideally take place in the dark to remove ambient light interferences. 

The polymer can also be used to tailor the selectivity and sensitivity of the optical sensor due to enrichment of the analyte by the polymer material. Furthermore, the polymer may the permeable for gases but not by ions again inducing selectivity for certain analytes. Finally, the polymer can provide optical isolation against ambient light and thus prevent bleaching and light interference. 

Electrochemical sensors have several disadvantages with respect to optical sensors (i) they are based on electrodes and require a reference electrode (ii) the liquid-liquid junction is easily perturbed by external factors (iii) they are sensitive to electrical interferences (iv) miniaturization is not easy and their cost is relatively high. However, optical sensors also have some disadvantages (i) ambient light can interfere (ii) the range over which the concentration of an analyte can be accurately measured is often limited (iii) they have generally limited long-term stability. 

Schmid et al. used the same principle to develop sensors to be incorporated into FI systems for the determination of ascorbic acid in fruit juices [38] and that of lactic acid in dairy products [39]. The membrane used in both applications consisted of decacyclene dissolved in silicone rubber that was treated similarly as the membrane in glucose sensors (Fig. 3.4.B). The oxygen optrode was coated with a sheet of carbon black as optical insulation in order to protect it from ambient light or intrinsic sample fluorescence. Ascorbic acid oxidase or lactic acid oxidase was immobilized by adsorbing it onto carbon black and cross-linking it with glutaraldehyde. The FI system automatically buffered and diluted the food samples, thereby protecting the biosensor from a low pH and interferents. 

Plain fiber sensors are simpler in design and manufacture than are indicator-phase sensors (Section 18.2.2). However, they are mostly less selective and suffer from interferences by any substance that adsorbs at the same analytical wavelength, even small changes in the optical parameters such as refractive index at the sensing tip, and ambient light. They therefore are usually operated at at least two wavelengths and/or with background subtraction. 

In terms of sensor development several of these sources have distinct advantages, notably LEDs, pulsed xenon lamps and lasers. LEDs have the advantage that they are inexpensive and simple, although not very intense. However, they may find application in the development of simple purpose-built equipment. Pulsed xenon sources when coupled to a gated detector are particularly useful because a probe based on this type of system may be used in an open beaker without interference from ambient light. Laser sources have the advantages of high power and excellent collimation. The laser sources... 

Interference from ambient light drift due to aging effects poisoning by SOj, HjS... 

In the case of a single lifetime with no interference from ambient or backscattered light only a single frequency is necessary to determine the lifetime of the luminescence. Determination of the phase and modulation at multiple frequencies is necessary to characterize complex decays in fiberoptic sensors. 

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