Sample interface probes

There have been substantial changes in Raman sample interfaces recently. The approaches now can be divided broadly into two categories based on the sampling volume. The probes intended to sample small volnmes include the more traditional noncontact or standoff probes, immersion probes, and optical microscopes. Large volume sampling approaches are newer and include WAl and SORS probes and transmission confignrations. 

Raman spectroscopy s flexible and minimally intrusive sampling configurations reduce sample pretreatment demands. Sampling interfaces can be engineered for almost any environment, but these requirements must be clearly defined before probes are built. No probe can be made to survive all environments so there may be some design trade-offs. In a noncontact system, the laser usually comes to a focus several inches 

Though the use of transmission geometry is common for many other spectroscopic techniques, it has not been widely nsed for Raman spectroscopy [39] In this case, illumination and collection optics are on opposite sides of the sample. The actual generation and travel of Raman photons through the sample is convoluted, but it is safe to conclude that the bulk of the sample is probed [40,41]. The large sample volume probed results in reduced subsampling errors. In one example, the use of the transmission mode enabled at least 25% reduction in prediction error compared to a small sampling area probe [42]. The approach is insensitive 

In order for the data generated by the analyzer to be useful, it must be transferred to the operation s centralized control or host computer and made available to process control algorithms. Vendor packages manage instrument control and can do spectral interpretation and prediction or pass the data to another software package that will make predictions. Most vendors support a variety of the most common communications 

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The mass spectrometer should provide structural information that should be reproducible, interpretable and amenable to library matching. Ideally, an electron ionization (El) (see Chapter 3) spectrum should be generated. An interface that fulfils both this requirement and/or the production of molecular weight information, immediately lends itself to use as a more convenient alternative to the conventional solid-sample insertion probe of the mass spectrometer and some of the interfaces which have been developed have been used in this way. 

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. 

A quantitative bioassay for erythromycin 2 -ethylsuccinate (EM-ES, M, 861 Da), a prodrug of the macrolide antibiotic erythromycin, using Cf-FAB LC-MS was described by Kokkonen et al. [53-54]. Reversed-phase LC of extracted plasma samples was performed at a flow-rate of 1 ml/min. In order to meet the flow-rate requirements of the Cf-FAB interface, i.e., 15 pl/min, without splitting, the phase-system switching approach [53] was used. After post-column dilution of the column effluent with water, the eluent fraction of interest was enriched on a short precolumn, from which the compound of interest was desoibed and transferred to the Cf-FAB interface probe. A [ Hj]-analogue was used as internal standard. Good linearity was observed in the range of 0.1 to 10 pg/ml EM-ES in plasma. The within-ran precision was ca. 6%. The accuracy and inter-day precision, determined at 1.05 pg/ml in plasma, were 0.93 0.11 pg/ml and 12%, respectively (n=6). The determination limit was 0.1 pg/ml [54]. 

Figure 12.4. Effect of the focusing lens at the laser-fiber interface on the size of the light exit cone at the sample or probe head. (Angles not drawn to scale.)...

Figure 2. Schematic depiction of physical processes affecting the laser probe beam for an opaque homogeneous sample including thermoelastic deformation of the air-sample interface and thermal lens effects in the air above the sample.

The interface between the sample and the spectrometer is vital wherever a spectrometer is sited. The sample can be piped into the spectrometer or, in some cases the radiation used by the spectrometer can be transmitted to a convenient sample or probe location point using optical fibres or other light-pipe devices. The sample presented to the spectrometer must be representative of the material from which the measurement is required and the interaction between the radiation of the spectrometer and the sample must be suitable for the measurement to be made (sufficient power and suitably clean interface). 

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. 

In the case of oversized samples (e.g., works of art), substrates with unusual shapes or difficult to get at samples such as board components or recessed sample areas, coatings can be remotely probed using a fiber-optic interface that directs the light from the source into a sampling probe head and thereafter collects the transmitted light into the detector of an IR spectrometer. The probe head (a mini-turized ATR element or a reflectance probe with actual sample interface areas as... 

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. 

Jin, D.-Q., Zhu, Y, Fang, Q. (2014) Swan Probe A Nanoliter-scale and High-throughput Sampling Interface for Coupling Electrospray Ionization Mass Spectrometry with Microfluidic Droplet Array and Multiwell Plate. Anal. Chem. 86 10796-10803. 

Sampling interface Insertion probe (transmission) Insertion probe (backscatter) Insertion probe (transmission)... 

In some process examples, the reaction environment are relatively mild. The majority of process environments, however, are more challenging with both elevated temperatures and pressures. When faced with a new on-line trial, one of the first questions that needs to be addressed is how to interface to the process of interest. In this subsection, examples will be provided in order to facilitate an appropriate selection of probe to sample interfaces. 

In voltage probes the total current vanishes. However, the partial ionic and electronic current there need not vanish, it is only their sum that vanishes. Thus matter and charge may flow across the interface probe/sample, when the sample is a MIEC. 

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