Laboratory methods instrument configurations

Lacking assay accuracy may also stem from the fact, that most LC-MS/MS methods used in clinical laboratories are still locally designed laboratory-developed tests operating on very heterogeneous instrument configurations. Consequently,... 

Reversed phase chiral separations are desired simply for efficiency in generating results from laboratories whose instrumentation is routinely configured to run in reversed and not normal phase modes.Normal phase conditions are less attractive to the analytical chemist for this reason and deter laboratory efficiency. Typical commercial chiral LC columns found on pharmaceutical reversed phase LC chiral method development screens are listed in Table 8. Table 11 shows suggested chromatographic conditions employed in reversed phase chiral screening. 

Before considering the special requirements for automated on-line determination of metals from industrial effluents, it is worthwhile examining the features of standard laboratory procedures associated with the off-line determination of copper as a dithiocarbamate complex by liquid chromatography with electrochemical detection. The off-line determination of copper as its diethyldithiocarbamate complex in aqueous samples, zinc plant electrol3d e, and urine have been described [3, 7, 10] using reverse phase liquid chromatography with amperometric detection. A standard instrumental configuration for the conventional laboratory off-line method as used in these studies is depicted in Fig. 7.2. 

The early experiments reported by Knowles s laboratory (Abbott et al., 1979) utilized a method of configurational analysis that was based on metastable-ion mass spectroscopy and required instrumentation that is not routinely available to most investigators in addition, this approach involved the chromatographic separation of hydrolytically labile species, which also discouraged routine adoption of the very elegant mass spectral technique. 

The precise configuration can be chosen according to the laboratory workload. Table 2.2 clearly illustrates the capacities of the various models. These instruments will support a wide range of analytical methods including end-point assays at one or two wavelengths and with one or two steps as well as rate analyses. 

All of the laboratory instrumental methods have been practically adapted for on-site analytical procedures, from pH measurement to NMR analyses. The instruments are generally less versatile than their laboratory counterparts. Dedicated to particular measurements and suitable for harsh environments or hazardous areas, they must be robust, so their design is different. They are configured for industrial process control. Many of them are process analysers for concentration measurements. A continuous sample is introduced via a diaphragm pump into a small tank. At intervals the sample or calibration standards are pumped into the chemistry stream where they are treated depending of the method of analysis (UV, IR). 

Thermal diffusion of petroleum samples is carried out in the annular space defined by two coaxial cylinders whose surfaces are separated by distances of approximately 0.2 mm. These surfaces are maintained at different temperatures. Figure 3.15 demonstrates a laboratory setup34 used to practice this technique. Separation is performed by filling the annular space with the sample, then allowing the system to equilibrate for a period up to several days. In one configuration, the cylinder diameters are about 0.5 in., the annular spacing is about 0.0115 in., and the vertical length is 6 ft. The sample is injected at the center position and the product samples are taken off at a number of sample points, frequently ten of these, on the vertical axis after an appropriate time (3 to 10 days in practice) for the diffusional separation to take place. The samples, a few milliliters or less in size, are then analyzed by modem instrumental methods. 

Atmospheric pressure chemical ionization (APCI) is the ionization source that provides lower chemical noise and, subsequently, lower quantification limit than electrospray ionization (ESI) which is more robust. The use of mass spectrometric methods can be expected to increase, particularly as they become easier to use and the costs of instrumentation continue to fall. Despite the enormous progress in analytical technologies, methods based on HPLC with fluorescence detection are the most used today for aflatoxins instrumental analysis, because of the large diffusion of this configuration in routine laboratories. 

One of the many remarkable features of modern GC instruments is their ability to perform fast GC without special modifications or expensive accessories. High-speed GC can be achieved using short, narrow-bore colmnns, resulting in analyses that are 5 times faster than with traditional methods run on conventional laboratory GCs. These GCs offer the capability to carry out fast GC without the need for cryofocusing or thermal desorption devices which may limit the flexibility or performance of the instrmnent. Properly configured for fast GC, the system can perform all types of analyses using existing detectors, injectors, and flow controllers. 

Recently, different approaches to the problem of determining absolute configuration have emerged, based on NMR spectroscopy. These techniques are very appealing, because of the undoubted advantages, which include the following (a) the instrument is available in most laboratories (b) an in-depth understanding of the fundamentals of the method is not necessary to apply this method (c) a small amount of sample is needed, and this can be recovered and (d) because the analysis is conducted in solution, it is applicable to both solid and liquid samples. 

---