The spectral sensitivity characteristics of a photomultiplier are mainly determined by the material of the photoelectric surface. For this reason, photomultipliers are used in high-grade instruments. If there is sufficient light intensity, this feature is not particularly relevant, but as the light intensity decreases, this feature becomes increasingly useful. The most important feature of a photomultiplier is that it achieves a significantly high level of sensitivity that cannot be obtained with other optical sensors. The photoelectrons emitted from the photoelectric surface repeatedly cause secondary electron emission in sequentially arranged dynodes, ultimately producing a large output for a relatively small light intensity. Among spectrophotometers of medium or higher grade that use photomultipliers, which will be described later, as detectors, there are models for which large sample compartments are made available in order to allow the analysis of large samples or the attachment of large accessories.įig.10 Spectral Sensitivity Characteristics of a Photomultiplier2)Ī photomultiplier is a detector that uses the fact that photoelectrons are discharged from a photoelectric surface when it is subjected to light (i.e., the external photoelectric effect). The various accessories are attached by replacing these cell holder units or by replacing the entire sample compartment. 9, hold square cells with optical path lengths of 10 mm. In a standard configuration, the sample compartment contains cell holders that, as shown in Fig. An explanation of the difference between single-beam and double-beam spectrophotometers is given in the Q&A of previous issue of UV Talk Letter. A spectrophotometer in which only one beam passes through the sample compartment is called a “single-beam spectrophotometer”. The monochromatic light that leaves the spectrometer is split into two beams before it enters the sample compartment. 9) pass through the compartment, and that this is therefore the sample compartment of a “double-beam spectrophotometer”. You can see that two light beams (indicated by red arrows in Fig. 9 shows an example of a standard sample compartment. In some cases, however, minor alterations in structure result in disproportionate changes in the compactness of the crystal lattice, and this fact makes it difficult to develop broad generalizations applicable to large groups of compounds.Fig. It is postulated that electron-withdrawing and electron-donating groups at the N1-position influence the strength of the hydrogen bonds which form and, hence, the tendency of these compounds to exhibit more than one crystalline form. The p-amino group, the acidic N1-hydrogen atom, and the oxygens of the sulfonamide group have been implicated in the various hydrogen-bonding arrangements which distinguish one polymorphic form from another. The screening procedures used in this research identified polymorphism in eight compounds and solvates in two compounds. Sixteen sulfonamides were selected for study. and the American Pharmacists Association J Pharm Sci 99:3874–3886, 2010Īn evaluation of entropies and enthalpies of transition and fusion and a comparison of X-ray diffraction patterns and IR spectra of polymorphic forms of structurally related compounds were undertaken to obtain information which might prove useful in correlating the frequency of occurrence of polymorphism with certain aspects of chemical structure. The results provided confidence that the current development form was the most stable polymorph, with a low likelihood for the existence of a more-stable anhydrous form. The development of these screens was a critical and alternative approach to circumvent solvation issues associated with more conventional screening methods. Application of these targeted approaches, comprising over 100 experiments, produced only the known anhydrous forms, without appearance of any new forms. With the aid of this analysis, two screening approaches were devised which targeted high-temperature desolvation as a means to increase conformational populations and enhance overall probability of anhydrous form production. After early polymorph screening using common techniques yielded mostly solvates and failed to uncover several key anhydrous forms, it became necessary to devise new approaches based on an advanced understanding of crystal structure and conformational relationships between forms. In all, 5 anhydrous forms and 66 solvated forms have been discovered. The presented case illustrates an extensively polymorphic compound with an additional propensity for forming stable solvates. Polymorph screening for an API with a complex polymorphic profile can present a significant challenge. Elucidation of the most stable form of an active pharmaceutical ingredient (API) is a critical step in the development process.
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