Flexible sample interfaces

For smaller samples, Raman spectra can be collected through a microscope. Process microspectroscopy systems, such as might be used for semiconductor chip manufacturing or pharmaceutical high throughput 

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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... 

Fig. 9.7.23 A most recent variation on the theme of interfacing a CE chip to a flexible sample line for continuous monitoring. See text for details.

Table 7.87 shows the main features of on-line micro LC-GC (see also Table 7.86). The technique allows the high sample capacity and wide flexibility of LC to be coupled with the high separation efficiency and the many selective detection techniques available in GC. Detection by MS somewhat improves the reliability of the analysis, but FID is certainly preferable for routine analysis whenever applicable. Some restrictions concern the type of GC columns and eluent choice, especially using LC columns of conventional dimensions. Most LC-GC methods are normal-phase methods. This is partly because organic solvents used as eluents in NPLC are compatible with GC, making coupling simpler. RPLC-GC coupling is demanding water is not a suitable solvent for GC, because it hydrolyses the siloxane bonds in GC columns. On-line RPLC-GC has not yet become routine. LC-GC technology is only applicable to compounds that can be analysed by GC, i.e. volatile, thermally stable solutes. LC-GC is appropriate for complex samples which are difficult or even impossible to analyse by a single chromatographic technique. Present LC-GC methods almost exclusively apply on-column, loop-type or vaporiser interfaces (PTV).

To optimize the applicability of the electrothermal vaporization technique, the most critical requirement is the design of the sample transport mechanism. The sample must be fully vaporized without any decomposition, after desolvation and matrix degradation, and transferred into the plasma. Condensation on the vessel walls or tubing must be avoided and the flow must be slow enough for elements to be atomized efficiently in the plasma itself. A commercial electrothermal vaporizer should provide flexibility and allow the necessary sample pretreatment to introduce a clean sample into the plasma. Several commercial systems are now available, primarily for the newer technique of inductively coupled plasma mass spectroscopy. These are often extremely expensive, so home built or cheaper systems may initially seem attractive. However, the cost of any software and hardware interfacing to couple to the existing instrument should not be underestimated. 

Many manufacturers now offer other sample injection systems compatible with the vacuum lock used for the solids probe. These include small (e.g., 75-ml) heatable batch inlet systems, usually accessible via syringe (gas syringe or GC microliter syringe for liquids), which can be particularly useful as inlets for mass reference compounds. Other probes are designed as flexible, easily removed connections to a gas chromatograph via some form of interface. 

There are of course many other similarities and differences, and some of them are listed in Table 5.1 without further explanations. In general, STM is very versatile and flexible. Especially with the development of the atomic force microscope (AFM), materials of poor electrical conductivity can also be imaged. There is the potential of many important applications. A critically important factor in STM and AFM is the characterization of the probing tip, which can of course be done with the FIM. FIM, with its ability to field evaporate surface atoms and surface layers one by one, and the capability of single atom chemical analysis with the atom-probe FIM (APFIM), also finds many applications, especially in chemical analysis of materials on a sub-nanometer scale. It should be possible to develop an STM-FIM-APFIM system where the sample to be scanned in STM is itself an FIM tip so that the sample can either be thermally treated or be field evaporated to reach into the bulk or to reach to an interface inside the sample. After the emitter surface is scanned for its atomic structure, it can be mass analyzed in the atom-probe for one atomic layer,... 

Many variations of the design of the cells shown in Figure 9.3 have been reported. Reasonable cost, simple construction, flexibility, and ease of correct use have led to widespread acceptance of these general designs. These cell designs fit most aqueous and nonaqueous sample requirements where the presence of oxygen or water is not critical. When the removal and exclusion of these contaminants is required, special care must be taken to work in an inert atmosphere. A dry box (Chap. 19) or a cell that can be interfaced to a vacuum line (Chap. 18) may be required. 

The data reduction hardware is based on a Hewlett-Packard 9825A desk top computer. It is supported by one megabyte of flexible disk storage, a printer/plotter and the necessary interface equipment for on-line LSC data collection. Reliability was a prime factor when the hardware was chosen. The LSC(s) and data system run virtually unattended, 24 hours a day, 365 days a year. Samples are typically counted for two minutes each plus one minute for the external standard. Therefore, data from the four counters are received by the HP9825A at an average interval of 45 seconds. With such a demand on the system, computers require good service support, more so than other instruments. Since installation in May, 1977, there have been less than two work days of cumulative downtime. 

The detected current is influenced considerably by any incomplete contact between sample and electrode, bubbles, grain boundary, and so on. So it is necessary to be careful in preparing the contact conditions for the sample and electrode interface in polymer systems. When the polymers are flexible films or soft paste, good contact is obtained by simply pressing. For hard or brittle polymer systems, it is difficult to get good contact by the pressing pressure. To improve the contact, it is necessary to melt the polymer material or to sputter electrode material on the... 

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