Radiometry, detection method

As the enzyme itself is usually the focus of interest, information on the behavior of that enzyme can be obtained by incubating the enzyme with a suitable substrate under appropriate conditions. A suitable substrate in this context is one which can be quantified by an available detection system (often absorbance or fluorescence spectroscopy, radiometry or electrochemistry), or one which yields a product that is similarly detectable. In addition, if separation of substrate from product is necessary before quantification (for example, in radioisotopic assays), this should be readily achievable. It is preferable, although not always possible, to measure the appearance of product, rather than the disappearance of substrate, because a zero baseline is theoretically possible in the former case, improving sensitivity and resolution. Even if a product (or substrate) is not directly amenable to an available detection method, it maybe possible to derivatize the product with a chemical species to form a detectable adduct, or to subject a product to a second enzymatic step (known as a coupled assay, discussed further later) to yield a detectable product. But, regardless of whether substrate, product, or an adduct of either is measured, the parameter we are interested in determining is the initial rate of change of concentration, which is determined from the initial slope of a concentration versus time plot. 

Detectors that have produced the lowest detection Emits to date are based on fluorescence with precolumn derivatization, mass spectrometry, radiometry, and amperometry. Applications are described below for these detection systems employed with conventional CE instrumentation. For applications of these detection methods to CE analysis using microfabricated devices, the reader is referred to a review article.3... 

Methods for bulk analysis and particle analysis of nuclear materials for detection of undeclared activities were described in a review article (Piksaikin et al. 2006). The bulk detection methods included radiometry (alpha, beta, and gamma spectrometry) based on the natural decay of the radionuclides, including the use of 234Th/23°Th gamma activity ratio for age determination (see detailed discussion later) and ratio that should be about 21 for undisturbed ores higher in mining... 

Emission spectroscopy is the analysis, usually for elemental composition, of the spectmm emitted by a sample at high temperature, or that has been excited by an electric spark or laser. The direct detection and spectroscopic analysis of ambient thermal emission, usually ia the iafrared or microwave regioas, without active excitatioa, is oftea termed radiometry. la emission methods the sigaal iateasity is directiy proportioaal to the amouat of analyte present. 

The natural substrates for lipases are triglycerides but, because of the complexity of these and the fact that they seldom contain a chromophore or other label to enable ready detection of the products, several synthetic substrates have been developed. These enable different detection techniques such as spectrophotometry, fluorimetry, chromatography, or radiometry to be used. It is important to note that, by definition, true lipases are active only on water-insoluble esters while esterases cleave only water-soluble esters (Jaeger et al., 1994). Thus, it is important that methods used for milk and milk products use substrates, which detect true lipase but not esterases as lipases play a major role in the hydrolysis of milk fat, while the role of esterases is considered insignificant (McKay et al., 1995). 

For reflective samples, temperature changes may cause variations in the sample s reflectance giving rise to photothermoreflectance spectrometry. Increased blackbody emission accompanies sample heating by the excitation beam. An infrared detector may be used to measure this blackbody emission signal and to detect temperature changes in the sample caused by optical absorption. This remote method of detection is termed photothermal radiometry. 

Infrared photothermal radiometry can be used to measure thermal anisotropy, by displacing excitation and detection spots on the sample, and measuring the transverse heat flow between these spots. As the direction of the spot displacement is varied, directional variations are sampled. Entire images of the thermal anisotropy may be scanned using this method. Such images may be used to detect defects in aligned materials, where a defect destroys the local alignment of bulk fibers or molecular chains. 

The Fourier-transform infrared system (FTIR) is a well-known spectroscopic technique based on the absorption of infrared photons that excite vibrations of molecular bonds. Molecules such as UsOg, UO2, UO3, Th02, have characteristic absorption bands in the infrared region that can be used like a fingerprint to detect their respective presence. FTIR radiometry has become a relatively mature and reliable method for the identification and measurement of chemicals emitted from stacks and its potential for passive standoff detection of nuclear material is under investigation (Puckrin and Theriault 2006). 

---