Instrumental limitations high frequencies

With appropriate calibration the complex characteristic impedance at each resonance frequency can be calculated and related to the complex shear modulus, G, of the solution. Extrapolations to zero concentration yield the intrinsic storage and loss moduli [Gr] and [G"], respectively, which are molecular properties. In the viscosity range of 0.5-50 mPa-s, the instrument provides valuable experimental data on dilute solutions of random coil (291), branched (292), and rod-like (293) polymers. The upper limit for shearing frequency for the MLR is 800 Hz. High frequency (20 to 500 K Hz) viscoelastic properties can be measured with another instrument, the high frequency torsional rod apparatus (HFTRA) (294). 

If we know that the instrument response is band limiting with frequency cutoff Q, we may likewise process the estimate. The resulting solution will then also be band limited. The high-frequency spectral structure beyond cutoff Q that we would wish to restore is forever lost to these linear methods. The data contain no information about the high-frequency content. We must wait until Chapter 4 to see how straightforward and seemingly unimportant... 

H (f) = 0 at high frequencies ( f > fo), where fo is the limiting cut-off frequency of the high-frequency filter, which is chosen according to the noise performance of the measuring instrument and the resolving time of the retrieved signal. 

The most obvious solution to problems where both high and low frequency noises are embedded in the data are better instrumental practices. For example, by sufficient coaddition, the high frequency noise component of spectroscopic imaging data can be averaged out. However, when better instrumental practices are unavailable or impractical, other means must be found to enhance the spectroscopic imaging data so that information may be extracted. The method of using second-derivative spectra, shown in this section can be used with success if the high frequency noise in the data is limited. 

The high cost of commercial superconducting spectrometers and of the continuing supply of liquid helium needed has limited the number of installations of these instruments, and, in some countries, such high-frequency p.m.r. spectra are available only from a national service center. However, some savings are realized because of the fact that, once established in the persistent mode, superconducting solenoids require neither a power supply nor cooling water. 

The creep function J(t) is the transient strain per unit stress in a step-stress experiment. The resolution at short times is also limited from instrument response and sensitivity. J(t) at short times may also be derived from the high frequency complex compliance data. 

Broaden the frequency range As described in Section 19.5.3, an insufficient frequency range will reduce the ability to identify system characteristics by regression. Typically, an increase in frequency range is constrained at high frequencies by instrument limitations and at low frequencies by nonstationary behavior. 

In practice, spectral noise plots can be obtained only in a frequency range that is more limited than in FIS. On the high-frequency side, the limit is imposed hy the instrumental noise, whereas in the low-frequency region, the time of acquisition becomes very long [118]. 

In a Curie-point pyrolyzer, an oscillating current is induced into the pyrolysis filament by means of a high-frequency coil. It is essential that this induction coil be powerful enough to permit heating the wire to its specific Curie-point temperature quickly. In such systems, the filament temperature is said to be self-limiting, since the final or pyrolysis temperature is selected by the composition of the wire itself, and not by some selection made in the electronics of the instrument. Properly powered, a Curie-point system can heat a filament to pyrolysis temperature in milliseconds. Providing that wires of the same alloy composition are used each time, the final temperature is well characterized and reproducible. 

In spite of these limitations, continuum source background correction may be used with good accuracy for many analyses. It offers low cost, wide applicability, operation at high frequencies, and little degradation in detection limits or linear dynamic range. It is commonly found in commercial instrumentation alone or with other methods of correction. 

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