Probe-type spectrophotometers

These solid state detectors are multichannel devices. They have large number of photon-detecting registers. The registers are semiconductor capacitors that have been formed on a silicon chip. These detectors, which can detect very low levels of light, are primarily used in probe-type, near-infrared, and visible spectrophotometers and in fluorometers. 

Photometric titrations are ordinarily performed with a spectrophotometer or a photometer that has been modified so that the titration vessel is held stationary in the light path. Alternatively, a probe-type cell, such as that shown in Figure B-18, can be employed. After the... 

Kweon GY, Lund E, Maxton C et al (2009) Soil profile measurement of carbon contents using a probe-type vis-NlR spectrophotometer. J Biosyst Eng 34 382-389... 

In vitro techniques for studying DDI potential are based on the metabolism of known marker substrates. Two assay types are typically used to study DDIs the turnover of drug-like probes monitored by LC-MS/MS methods or the use of spectrophotometer (plate reader) based methods. As each technique has unique advantages and shortcomings, assay use has not been standardized across the industry. Although techniques based on the turnover of radiolabeled substrates have also been developed [94—97], these methods are used infrequently and will not be discussed further. 

The identification of a compound using only its retention time is vulnerable to error. It is essential that a standard compound is injected in order to verify the retention time. As is the case in gas chromatography, more sophisticated detectors can be used. These detectors provide complementary information and can be installed at the end of the column. These can be other types of spectrophotometers or a mass spectrometer and they are used simultaneously as classical detectors (to obtain the chromatogram) or for identification purposes of the analytes (cf. Chapter 16). For example, the coupling of HPLC to NMR, which has long been considered impossible, has now been realised through the miniaturisation of the probes and the increased sensitivity of the NMR instruments (cf. Chapter 9). 

Fortunately, automated fiber-optic probe-based dissolution systems have begun to appear for these solid dosage-form applications. One such system uses dip-type UV transflectance fiber-optic probes, each coupled to a miniature photodiode array (PDA) spectrophotometer to measure drug release in real time. This fiber-optic dissolution system can analyze immediate- and controlled-release formulations. The system is more accurate and precise than conventional dissolution test systems, and it is easier to set up than conventional manual sampling or automated sipper-sampling systems with analysis by spectrophotometry or HPLC. 

Double beam spectrophotometers allow differential measurements to be made between the sample and the analytical blank. They are preferable to the single beam instruments for cloudy solutions. The bandwidth of high performance instruments can be as small as 0.01 nm. For routine measurements such as monitoring a compound on a production line, an immersion probe is employed. Placed in the sample compartment of the apparatus this accessory contains two fibre-optics, one to conduct the light to the sample and another to recover it after absorption in the media studied. Two types exist by transmission for clear solutions and by attenuated total reflection (ATR) for very absorbent solutions (Figure 9.17). 

Applications of the fibre optics transmittance or ATR probe are in quality control, reaction monitoring, skin analysis, goods-in checking, analysis at high and low temperature, radioactive or sterile conditions, and hazardous environments. Applications of the reflectance probe are for turbid liquids, powders, surface coatings, textiles, etc. By using an on-line remote spectrophotometer, real-time information is gathered about a chemical process stream (liquids, films, polymer melts, etc.), as often as necessary and without the need to collect samples. This determines more reliable process control. Remote spectroscopy costs less to maintain and operate than traditional techniques. Fernando et al. [48] have compared different types of optical fibre sensors to monitor the cure of an epoxy resin system. 

Abstract Far-ultraviolet (FUV) absorption spectroscopy provides molecular information about valence electronic transitions a, n, and Jt electron excitation and charge transfer (CT). FUV spectral measurements of liquid water and aqueous solutions had been limited, because the absorptivity of liquid water is very intense (absorptivity 10 cm at 150 nm). We have developed an attenuated total reflection (ATR)-type FUV spectrophotometer in order to measure FUV spectra of liquid water and aqueous solutions. The ATR-FUV spectroscopy reveals the features of the valence electronic transition of liquid water. This chapter introduces a brief overview of the first electronic transition (A. Y) of liquid water (Sect. 4.1) and the FUV spectral analyses (140-300 nm) of various aqueous solutions including how the hydrogen bonding interaction of liquid water affects the A <— X transition of water molecules (Sect. 4.1) how the A Y bands of water molecules in Groups 1, 11, xm, and lanthanoid (Ln +) electrolyte solutions are associated with the hydration states of the metal cations (Sects. 4.2 and 4.3) how the protonation states of amino acids in aqueous solutions affect the electronic transition of the amino acids (Sect. 4.4) and the analysis of O3 pulse-photolytic reaction in aqueous solution using a nanosecond pump-probe transient FUV spectrophotometer (Sect. 4.5). 

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