The sampling interface

Section 3.3 described a number of alternative design and implementation strategies for NIR analysers, suitable for operation in a process analytical environment. However, none of these analysers can operate without a robust, maintainable and repeatable sampling interface with the process sample under consideration. In terms of practical issues this is certainly the most demanding aspect of all. It requires a number of issues to be addressed and clearly understood for each specific application to be attempted. These include  

Depending on the answers to these questions, various sampling interface strategies may be developed which will include  

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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. 

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 ... 

Optics Issues relative to hygroscopic optics and the need to pay attention to mirror mounts relative to vibration and/or thermal effects have already been addressed. Zinc selenide is an important alternative material, especially when antireflection (AR) coated. If potassium bromide absolutely has to be used for its lower transmission range (down to 400 cm ) then a protective coating such as Parylene must be used. Most process analyzers use protective windows between the spectrometer and the sample interface. If used, back reflections from the window surfaces into the interferometer must be avoided because these will cause photometric errors. Such reflection can be eliminated by wedging or tilting the optical windows, relative to the beam path. 

Note that the examples above are over-simplifications to some extent, and other critical elements, such as optics, the associated electronics and the sampling interfaces are also important for a final implementation. The latter is particularly the case for laser instruments, where attention to optical interfacing is essential and often features specialized optical fiber-based optical coupling. 

Li etal. discuss the use of on-line Raman spectroscopy to dynamically model the synthesis of aspirin, one of the most documented and well-understood reactions in organic chemistry. That makes it an excellent choice for building confidence in the sampling interface, Raman instrumentation, and analysis procedures. The researchers used wavelets during analysis to remove fluorescent backgrounds in the spectra and modeled the concentrations with multiple linear regression.53... 

Motion Artifacts and Skin Heterogeneity A key component to obtaining accurate and robust calibrations is the sample interface. The sample interface should ideally limit motion while maintaining a constant pressure and temperature. One approach to combat inadvertent motion artifacts is to intentionally build motion into the calibration model. This can be achieved by scanning the laser spot within a larger area. 

Consideration needs to be given to the sample interface and the measurement technique used. Some techniques relate to the whole sample whereas others are very much surface measurements. For example, microwave spectroscopy and infrared transmission measurements provide values on the bulk sample whereas X-ray fluorescence and Raman spectroscopy are very much surface techniques, only penetrating the sample to a limited degree. 

Effective process control needs reliable, robust, often rapid inputs from measurement techniques. Spectroscopy can provide such inputs if the right technique is selected, the sample interface is weU designed and the data analysis is carried out effectively. 

Instruments vary widely in their design depending upon the purpose for which they are built. Common features include a source of radiation, a means of bringing the radiation and the sample of interest together in a ceU or probe, and a detector. In applications to process measurement perhaps the most distinctive feature is the sample interface. The source of radiation used and the detectors are similar and often identical to laboratory-based instmmentation. Almost all of today s instra-ments include data acquisition and control electronics together with a user interface in a computerized form. To obtain the optimum performance from analytical and control systems, links to distributed control systems for feed-back and feed-forward control are vital. 

The remainder of this article examines the main subsystems of a Raman instrument the laser source, the sample interface, the spectrometer, and the... 

The sample interface brings together the laser illumination and the spectrometer field of view at the desired location on the sample. The three most common sample interfaces are a sample compartment, a fiber optic probe, and a Raman microscope. Common to all three interfaces are the needs to condition the laser beam that illuminates the sample and to collect light from the sample. 

An important aspect of plasma mass spectrometry is the sampling interface. Ions are physically extracted from the plasma into a mass spectrometer which is required to be at extremely low pressure, so the sampling interface must be in direct contact with the plasma (usually an ICP). The problem of extracting ions from an extremely hot plasma at atmospheric pressure into a mass spectrometer at 10 Pa (10 atm) is overcome by making use of a series of differentially pumped vacuum chambers held at consecutively lower pressures. A schematic diagram of the ICP-MS... 

Depth profiles of one or both sides of the failure may also be helpful, or even necessary, depending on the situation. Figure 16 shows an example of an interfacial or near-interfacial failure between the oxide and the titanium adherend. Because the freshly exposed metal surface oxidized immediately, the initial surface spectra could not distinguish between the interfacial failure and a failure entirely within the oxide.<86,87) Depth profiles may also be needed in the analysis of complex structures, such as those comprising many layers. In these cases, the sample interface chemistry might occur at several different points. However, a profile through several layers should allow the specific locus of failure to be identified.<87)... 

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