The Density Detector

Owing to its nature, the arrangement is inherently very sensitive to mechanical and electrical noise. Mechanical vibrations can be avoided by mounting the balance on a very heavy support. Electrical disturbances must be minimized by careful earthing. Temperature gradients within or around the apparatus can seriously disturb the system and therefore should also be minimized. 

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For polymer systems without UV activity the combination of a RI detector with a density (D) detector can be used. The working principle of the density detector is based on the mechanical oscillator method. Since this detector yields a signal for every polymer, provided that its density is different from the density of the mobile phase, this detector can be regarded as universal [29,30,36]. The separation of mixtures of polystyrene and polybutadiene by SEC with dual den-sity-RI detection is presented in Figs. 7 and 8. In a first set of experiments, the response factors of both polymers in both detectors have to be determined. Then from the intensity of each slice of the elution curves in both detectors, the mass distribution of both polymers across the elution volume axis can be calculated. As can be seen in Fig. 7, a separation into the component peaks is obtained due to the fact that the molar masses of PS and PB are sufficiently different. For both components the individual elution profiles can be determined and using corresponding calibration curves for PS and PB the individual MMDs can be calculated. The same information can be extracted from an experiment where the molar masses of the components are similar and SEC separation does not work (see Fig. 8). Again the individual mass distributions are obtained and the MMDs for PS and PB can be determined. 

An example of the separation of a mixture of tert-Boc-valine and phenylanaline methyl ester from a Sephadex LH-20 column monitored by the density detector is shown in figure 16. Each peak represents 50 mg of solute and thus the sensitivity is extremely low. Although it is a bulk property detector, and thus will detect all substances that have a density that differs from that of the mobile phase, it will obviously not tolerate gradient elution. Density measurement may be a basis for LC detection, and, in fact, this work has proved its validity. Nevertheless,... 

A simple, non-selective pulse starts the experiment. This rotates the equilibrium z magnetization onto the v axis. Note that neither the equilibrium state nor the effect of the pulse depend on the dynamics or the details of the spin Hamiltonian (chemical shifts and coupling constants). The equilibrium density matrix is proportional to F. After the pulse the density matrix is therefore given by and it will evolve as in equation (B2.4.27). If (B2.4.28) is substituted into (B2.4.30), the NMR signal as a fimction of time t, is given by (B2.4.32). In this equation there is a distinction between the sum of the operators weighted by the equilibrium populations, F, from the unweighted sum, 7. The detector sees each spin (but not each coherence ) equally well. 

The composition of PPG—PEG blends has been determined using gpc with coupled density and RI detectors. PEG and PPG have different response factors for the density and RI detectors which were exploited (173). An hplc system with CHROMPAC RP-18C2g column at 298°C and acetonitrile—water or methanol—water as the mobile phase has been used to gather information about the functionaUty of PPO (174). 

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

Radiation-Density Gauges Gamma radiation may be used to measure the density of material inside a pipe or process vessel. The equipment is basically the same as for level measurement, except that here the pipe or vessel must be filled over the effective, irradiated sample volume. The source is mounted on one side of the pipe or vessel and the detector on the other side with appropriate safety radiation shielding surrounding the installation. Cesium 137 is used as the radi-... 

In SXAPS the X-ray photons emitted by the sample are detected, normally by letting them strike a photosensitive surface from which photoelectrons are collected, but also - with the advent of X-ray detectors of increased sensitivity - by direct detection. Above the X-ray emission threshold from a particular core level the excitation probability is a function of the densities of unoccupied electronic states. Because two electrons are involved, incident and the excited, the shape of the spectral structure is proportional to the self convolution of the unoccupied state densities. 

Single-component PDA equipment is similar to LDA, but two detectors (not one) are installed with different detection angles. By means of simultaneous processing of signals supplied by the two detectors, information on the velocity and on the size of the scattering objects can be acquired. Therefore, velocity distribution, size distribution, and number density (local concentration)... 

The density section of the tool, also seen in Figure 4-281, uses a 1.7 curie (Ci) of 137-cesium (Ce) gamma ray source in conjunction with two gain-stabilized scintillation detectors to provide a high-quality, borehole compensated density measurement. 

Findings with PDU. Work with the PDU largely paralleled the bench-scale reactor tests there was one important addition—extensive three-phase fluidization studies. As was mentioned, the PDU is equipped with a traversing gamma-ray density detector that is capable of measuring bed density to within dbO.Ol specific gravity units. Thus, we could measure and correlate fluidized bed expansion as a function of liquid and gas velocities and physical properties, and could also determine the... 

The thickness of a Mossbauer sample affects not only the strength of the Mossbauer signal but also the intensity of the radiation arriving at the detector because the y-rays are inherently attenuated by the sample because of nonresonant mass absorption caused by the photo effect and Compton scattering as mentioned earlier. The counting rate C in the detector decreases exponentially with the density of the absorber,... 

In this expression, n/p] pec is the mass absorption coefficient of X-rays from element A in the specimen, a is the detector take-off angle, p is the density of the specimen... 

Some of the 220 detectors recently recovered have been analysed not only for radon exposure but also to determine the value of F (the equilibrium factor) in the houses. A preliminary set of such F factor results, obtained by analysing the inner and outer LR- 115 track densities of each detector, are presented in Table III for 12 houses with mean indoor radon concentrations greater than 200 Bq/nP. In Table III are also presented radon daughter doses estimated using the individually determined equilibrium factor values F together with the doses estimated on the basis of an assumed mean F factor value of 0.45. 

The s.a.n.s. experiments were carried out using the D17 camera at the I.L.L., Grenoble. Data were collected at two wavelengths, 0.8 and 1.4 nm at a sample to detector distance of 1.8 m. The overlapping spectra were combined to give a sufficiently wide Q range to enable the data to be numerically inverted to obtain the density distributions. The latex dispersions were prepared at a solids concentration of 4% and polymer solution concentrations between 200 and 300 ppm. 

Fig. 4.10. Portion of the red spectrum of the H II galaxy Tololo 0633-415 with a redshift of 0.016, showing diagnostic features for helium (Ha and X 6678), electron density ([S n]) and ionization ([S hi]). The features marked cosmic ray are due to impacts of charged particles on the CCD detector. After Pagel et al. (1992).

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