Sensitivity shifts

The second procedure is to measure the luminescence intensities at various Ca2+ concentrations and plot log (light intensity) against —log [Ca2+] for each aequorin. Examples of this method are shown in Fig. 4.1.14. This method provides more detailed information on the sensitivity of each aequorin. Generally, an increase in Ca2+ sensitivity shifts the curve to the left. 

The analysis of a sample from an incubation mixture that contains significant differences in substrate and product concentrations presents some problems, since the detector must be set so that the substrate peak is on scale, but also it must be set to detect small amounts of product. Usually different sensitivity settings are required to put both components on scale. For these cases we have adopted a procedure that might be called the sensitivity shift. In this procedure, the injection is made with the sensitivity set at a value that allows for the detection of one of the compounds. If this is the product, the detector is set at its maximum sensitivity. As soon as the product has emerged, the sensitivity of the detector is changed (either manually or electronically by computer) to a value that allows the substrate peak to appear completely on scale. 

It is of interest to note the use of the sensitivity shift procedures, illustrated in Figure 9.55. Arrows on the figure indicate where detector sensitivity was increased to allow for the detection of low levels of the product. 

Trace concentrations below 10" % are below the sensitivity range of many methods. To have a possibility of determining them by spectrophotometric methods a preliminary concentration of trace components usually becomes necessary (see Chapter 1 on methods of preconcentration and separation of elements). Depending on the kind of sample and its weight (e.g., 10 g, 100 g, or more) such an operation can increase the sensitivity (shift the limit of determination) by 1-2 or more orders of magnitude. In such a way, the sensitivity of spectrophotometric methods can be increased to lO -lO" %. 

Fig. 4. Average a-wave amplitude ( SEM) as a function of intensity from n-3-deficient (filled circles, n = 12) and control rats (unfilled circles, n = 12). The lines show the best-fitting Naka-Rushton function, which indicate that n-3-deprived animals have a loss of approx 35% amplitude and a sensitivity shift of 0.3 log units compared to controls. Note that below -2.0 log cd/s/m" the a-wave vanishes into noise.

Among the key technical elements is the establishment of sensitivity baselines, since they form the basis for all monitoring studies on sensitivity shifts. In past cases of rapid development of resistance, the lack of such figures has caused problems. 

As mentioned, Fig. 8.5 shows temperature effects at steady state after all members of the pressure transducer have come to thermal equilibrium with one another. Tests show that several minutes may be required for thermal equilibrium to be established. During this period of thermal shock, temperature effects are a combination of the two steady-state effects shown namely zero shift and sensitivity shift. It is not unusual for a pressure transducer to show a false output of 85% of full scale during this transition period. Accordingly, avoiding thermal shock effects is most important for making a valid measurement. 

The temperature in the living cell has a large influence on the metabolic activity of an organism. Some rare earth complexes have been developed as in vivo thermometers, because both of contact and pseudo-contact shifts vaiy with the local temperature. The Pr + complex with cyclen 93 (scheme 19) is the first example of an in vivo NMR thermometer based on a rare earth complex (Roth et al., 1996). The methoxy signal of the ligand sidearm did not broaden and sensitively shifted in vivo, upon raising the body temperature of a rat by a few degrees. Tm + complexes with cyclens 94 and 95 also exhib-... 

---