Relaxation —continued instruments

The rotational relaxation of DNA from 1 to 150 ns is due mainly to Brownian torsional (twisting) deformations of the elastic filament. Partial relaxation of the FPA on a 30-ns time scale was observed and qualitatively attributed to torsional deformations already in 1970.(15) However, our quantitative understanding of DNA motions in the 0- to 150-ns time range has come from more accurate time-resolved measurements of the FPA in conjunction with new theory and has developed entirely since 1979. In that year, the first theoretical treatments of FPA relaxation by spontaneous torsional deformations appeared. 16 171 and the first commercial synch-pump dye laser systems were delivered. Experimental confirmation of the predicted FPA decay function and determination of the torsional rigidity of DNA were first reported in 1980.(18) Other labs 19 21" subsequently reported similar results, although their anisotropy formulas were not entirely correct, and they did not so rigorously test the predicted decay function or attempt to fit likely alternatives. The development of new instrumentation, new data analysis techniques, and new theory and their application to different DNAs in various circumstances have continued to advance this field up to the present time. 

Quantitative measurements in NMR are based on the area of the signals present in the spectrum. Signal areas can be produced as numerical values proportional to the area or, on less modern instruments, from the integration plots that are superimposed on the spectrum (Fig. 9.1). For the proton, the precision obtained in area measurements does not exceed l % even if continuous wave instruments are used at slow scanning speeds. In l3C NMR, it is preferable to add a relaxation reagent in order to avoid saturation related to relaxation times that alter the intensity of the signal. Using the molar ratios that are easily accessible from the spectrum, it is possible to deduce concentrations. 

Conventional FTIR instruments, in which the interferometer mirror is translated at a constant velocity, are ideally suited to the analysis of steady state infrared emission. However, time resolution of the infrared emission is required in many applications, such as the measurement of absolute rate constants for the formation or subsequent relaxation of a vibrationally excited species. It is then necessary to follow the intensity of the emission (at a particular wavenumber if state-specific rate constants are required) as a function of time. For continuous-wave experiments, crude time resolution... 

It is interesting to consider hardness as an example of how mechanical tests for rubber have, or have not developed. Firstly, despite the very imprecise relationship with modulus and the lack of any fundamental significance, hardness measurements have continued to be used and even now new ones are being introduced. The far more sensible method of measuring force to produce a given deformation, which would also allow stress relaxation to be conveniently measured, has not been adopted. However, the instrumentation has been updated so that the old measure can be made with electrical transducers and fed directly to a computer. On the other hand, perhaps the fact that hardness is a non-destructive method that can be applied to virtually any product is justification that it should thrive. 

The proton lineshape test uses chloroform in deuteroacetone typically at concentrations of 3% at or below 400 MHz, and 1% at or above 500 MHz. Older instruments and/ or probes of lower sensitivity or observations via outer decoupler coils may require 10% at 200 MHz and 3% at 500 MHz to prevent noise interfering with measurements close to the baseline. A single scan is collected and tbe data recorded under conditions of high digital resolution (acquisition time of 16 s ensuring the FID has decayed to zero) and processed without window functions. Do not be tempted to make measurements at the height of the satellites themselves unless these are confirmed by measurement to be 0.55%. Since these arise from protons bound to C, wbicb relax faster than those of tbe parent line, they may be relatively enhanced if full equilibrium is not established after previous pulses. The test result for a 400 MHz instrument is shown in Fig. 3.79. The traditional test for proton resolution, which dates back to the continuous-wave (CW) era (o-dichlorobenzene in deuteroacetone), is now no longer employed and has become part of NMR history. 

Turbidity meters with narrow-band near infrared (NIR) light sources (peak output at 0.86 pm spectral bandwidth <0.06 pm) are recommended by ISO. Such instruments reduce problems of algal build-up on the optical surfaces, are less affected by color, and are more sensitive to the slightly larger particles typical of sediment transport systems. However, some relaxation of the infrared protocol is tolerable for field instruments operating in continuous monitoring mode. 

In stress relaxation measurements an instantaneous strain is imposed on the sample and maintained constant, and the time-dependent stress is measured at constant temperature. Stress relaxation can be measured by TMA, provided the instrument is capable of maintaining a constant strain and continuously monitoring the stress. 

Clearly some simple method must he devised to characterize the mecheuiical response as a function of time (and of course temperature). Creep tests were a possibility hut the continued strain with time wo ild cause changes in structure and thereby make the structure-property relationship difficult to interpret. Stress relaxation was also a possibility but at that point there were instrumentation difficulties in obtaining short time measurements. What was needed was some simple method that wo ild cover a wide range of time scale of loading in which the applied strain magnitude was a controlled variable. This line of thought led to the development of the rotating beam dynamic tester ... 

---