Sampling interface

The use of air-bome ultrasound for the excitation and reception of surface or bulk waves introduces a number of problems. The acoustic impedance mismatch which exists at the transducer/air and the air/sample interfaces is the dominant factor to be overcome in this system. Typical values for these three media are about 35 MRayls for a piezo-ceramic (PZT) element and 45 MRayls for steel, compared with just 0.0004 MRayls for air. The transmission coefficient T for energy from a medium 1 into a medium 2 is given by... 

Figure 4.9. Shock pressure versus particle velocity for engineering materials, geological material, and explosive detonation products. Intersection of detonation product curves with nonreactive media predicts shock pressure and particle velocity at an explosive sample interface. (After Jones (1972).)...

As long as 9 > 9, and the sample is not absorbing, the reflectivity of the prism/sample interface will be complete. However, at any wavelength where the sample is absorbing, the reflectivity will be attenuated or less than complete. Thus, an absorption spectrum that is similar to that obtained in transmission can be produced in ATR. 

FIGURE 5-11 The recognition process occurring at the TDMAC/PVC membrane/sample interface used for measurements of heparin. (Reproduced with permission from reference 26.)... 

Figure 7. Families of cellular interfaces computed for System I with k = 0.865 as a function of increasing P in a A /2 sample size. The cells are represented by the dimensionless arc length. The letters refer to sample interface shapes shown in Figure 8.

Figure 8. Sample interface shapes for System I with k = 0.865 for the parameter values marked on Figure 7.

Figure 13. Sample interface shapes from each of the families shown in Fig. 12.

Figure 17. Sample interface shapes for System III for increasing P and A = 1.0 as computed using the mixed cylindrical/cartesian representation.

Fig. 2.4.3 (a) Image of a flat oil-glass interface centered along the sensitive volume. The length of the pulses was set to 5 ps to excite a slice thicker than the PSF width. The dashed line displays the step sample interface, (b) PSF obtained as the derivative of the step image shown in (a). It is symmetrical and has a linewidth of about 2.3 pm. 

The simplest IR sensor would consist of a source, a sample interface and a detector. Although quite sensitive, such an arrangement would have no selectivity as any IR absorbing substance would cause an attenuation of the detected radiation. To get the selectivity that is a main driving force behind the application of IR systems, the radiation has to be spectrally analysed. This can be accomplished either by measurement at discrete wavelengths or, for multi-component sensors or samples containing (potentially) interfering substances, by full spectral analysis of the collected radiation. 

The only potential that varies significantly is the phase boundary potential at the membrane/sample interface EPB-. This potential arises from an unequal equilibrium distribution of ions between the aqueous sample and organic membrane phases. The phase transfer equilibrium reaction at the interface is very rapid relative to the diffusion of ions across the aqueous sample and organic membrane phases. A separation of charge occurs at the interface where the ions partition between the two phases, which results in a buildup of potential at the sample/mem-brane interface that can be described thermodynamically in terms of the electrochemical potential. At interfacial equilibrium, the electrochemical potentials in the two phases are equal. The phase boundary potential is a result of an equilibrium distribution of ions between phases. The phase boundary potentials can be described by the following equation ... 

Recently, comparatively inexpensive, very reliable, and stable single quadrupole mass spectrometers have entered the market. These spectrometers can be coupled to GC, LC, and CE separation methods simply by modifying the sampling interfaces. Although these detectors are more expensive than most conventional detectors including the versatile electron capture and diode array absorbance detectors used for GC and LG respectively, the reduction in sample preparation effort and their increased specificity can often rapidly... 

Fiber-optic-coupled spectrophotometers (single beam and double beam) are the best choice for on-line analyses. The advent of nonsolarizing optical fiber has made possible on-line analyses in which the spectrophotometer may be located remotely from the process and light is carried to/from the process by the optical fiber. A rugged probe or flow cell provides the sample interface. 

Design and selection of the sample interface is vital to provide the best-quahty data for an analysis. The sample interface may be located in the sample cavity of a spectrophotometer, as in the cases of laboratory cuvettes, vials, and flow cells. The sample interface may also be fiber-coupled and located closer to the process. Fiber-optic sample interfaces include flow cells, insertion probes, and reflectance probes. 

Section 5.3 described a number of alternative design and implementation strategies for near-infrared analyzers, suitable for operation in a process analytical environment. However, none of these analyzers can operate without a robust, maintainable and repeatable sampling interface with the process sample under consideration. In addition to this question of the optical interface to the sample, there is a whole wider area of concern, which is how far the particular sample interface is representative of the sample in the process as a whole. This complex issue is not addressed here, and is dealt with separately in Chapter 3. 

Are there multiple process streams to be sampled If so, can they be physically stream switched, or does each stream require a dedicated sampling interface ... 

What are the quantitative spectroscopic demands of the calibration How much impact will variation in the sampling interface (as opposed to analyzer instability) have on the likely calibration snccess and maintainability What issues concerning transport of laboratory-developed calibration models to the process analyzer need to be addressed ... 

Depending on the answers to these qnestions, varions sampling interface strategies may be developed which... 

Of the analytical techniques available for process analytical measmements, IR is one of the most versatile, where all physical forms of a sample may be considered - gases, liquids, solids and even mixed phase materials. A wide range of sample interfaces (sampling accessories) have been developed for infrared spectroscopy over the past 20 to 30 years and many of these can be adapted for either near-lme/at-lme production control or on-line process monitoring applications. For continuous on-line measurements applications may be limited to liquids and gases. However, for applications that have human interaction, such as near-line measurements, then all material types can be considered. For continuous measurements sample condition, as it exists within the process, may be an issue and factors such as temperature, pressure, chemical interfer-ants (such as solvents), and particulate matter may need to be addressed. In off-line applications this may be addressed by the way that the sample is handled, but for continuous on-line process applications this has to be accommodated by a sampling system. 

---