Injection systems preparative techniques

A flow-injection system with electrochemical hydride generation and atomic absorption detection for the determination of arsenic is described. This technique has been developed in order to avoid the use sodium tetrahydroborate, which is capable of introducing contamination. The sodium tetrahydroborate (NaBH ) - acid reduction technique has been widely used for hydride generation (HG) in atomic spectrometric analyses. However, this technique has certain disadvantages. The NaBH is capable of introducing contamination, is expensive and the aqueous solution is unstable and has to be prepared freshly each working day. In addition, the process is sensitive to interferences from coexisting ions. 

User experience of monitoring techniques in oil and gas production has been reviewed and indicates success and failure for the same methods by different operators. A survey of current monitoring practice in UK offshore fields has been published and other experience related to oil/gas production has be reported . A draft document has been prepared by CEA Task Group E2-5 providing guidelines for monitoring sea-water injection systems. 

Several sample preparation techniques are performed inside the inlet system. Large-volume injection can be carried out by a number of methods including programmed temperature vaporisation (PTV). Automated SPE may be interfaced to GC using a PTV injector for large volume injection. SPE-PTV-GC with on-column injection is suited to analysis of thermola-bile compounds. 

Solid-phase microextraction (SPME) is effectively a miniamrised version of SPE. Instead of using a packed cartridge, a rod is typically used, which is coated with the stationary phase. This is dipped into a solution of the analyte and allowed to extract for a pre-determined period of time. After this incubation period, the rod is removed from the solution and may be inserted directly into the injection system of the GC or HPLC. All of these operations can be automated on an autosampler. Clearly, the success of this technique depends intimately on the affinity of the analyte for the stationary phase. Frost, Hussain and Raghani [34] used SPME with GC-FID to measure benzyl chloride and chloroethylmethyl ether (amongst other process impurities) in pharmaceutical preparations. 

Apart from thermodesorption, all other sample preparation techniques are normally directed toward production of a liquid extract that is subsequently injected into a GC/MS system. For the simplest of the samples, neat organic liquids or concentrated solutions, the sample preparation method will consist of diluting the sample with a suitable, clean solvent (most often dichloromethane). 

Dialysis can also be used as an on-line sample preparation technique for the deproteinization of biological samples prior to HPLC. Selecting the appropriate semipermeable membrane for the dialyzer can prevent interference from large macromolecules. Samples are introduced into the feed (or donor) chamber, and solvent is pumped through the lower acceptor (or recipient) chamber. The smaller molecules diffuse through the membrane to the acceptor chamber and are directed to an HPLC valve for injection. In case of low concentrations of compounds of interest, a trace enrichment step may be required this is accomplished with a column placed downstream of the dialyzer that will retain the analyte until the concentration is sufficient for detection. After this step, the analyte can be backflushed into the HPLC system. The technique is useful for blood studies as sampling can be achieved continuously without blood withdrawal. A commercial on-line system, such as Asted from Gilson, used for both cleanup and enrichment by a combination of dialysis with SPE, is shown in Fig. 7. 

There are many sample preparation techniques listed in texts, from a simple filtration or centrifugation to many other kinds of extraction procedures, including both liquid-liquid and solid-phase extraction. When any type of sample preparation is used, it often is done manually if only a few samples are involved. If a large number of samples are to be analyzed, the entire procedure should lend itself to automation. Regardless of the number of samples, most sample preparation is done off-line that is, the samples are prepared first with one of the methods listed, then placed into an automated sample injection system for sequential analysis of all samples. 

The purpose of sample preparation is to create a processed sample that leads to better analytical results compared with the initial sample. The prepared sample should be an aliquot relatively free of interferences that is compatible with the HPLC method and that will not damage the column. The whole advanced analytical process can be wasted if an unsuitable preparation method has been employed before the sample reaches the chromatograph. Specifically, analytical work with samples from fermentation processes require a sample pre-treatment that eliminates the fermentation broth before the analytes can be injected into the chromatographic columns. This is primarily to remove macromolecular sample constituents, which easily clog the columns. Complex matrices often require a more selective sample preparation than for instance pharmaceutical solutions. In practice the choice of sample-preparation procedure is dependent on both the nature and size of the sample and on the selectivity of the separation and detection systems employed. Sample pre-treatment may includes a large number of methodologies. Ideally, sample preparation techniques should be fast, easy to use and inexpensive. In papers I and II careful sample pre-treatment was performed before all injections. 

Filtration is used as a sample-preparation method to remove particulates and debris that can potentially foul the LC lines, column frits, or mass spectrometer interface. Also, it is generally accepted that all workstations and pipetting systems can beneht from sample hltration because of the universal issues related to plasma clot formation that introduce pipetting challenges. Applications that use hltration include the removal of a mass of precipitated protein or of debris from raw plasma before use with any of the traditional sample-preparation techniques, as well as direct injection techniques (turbulent how... 

Another current trend that is well underway is the use of more specific analytical instrumentation that allows less extensive sample preparation. The development of mass spectrometric techniques, particularly tandem MS linked to a HPLC or flow injection system, has allowed the specific and sensitive analysis of simple extracts of biological samples (68,70-72). A similar HPLC with UV detection would require significantly more extensive sample preparation effort and, importantly, more method development time. Currently, the bulk of the HPLC-MS efforts have been applied to the analysis of drugs and metabolites in biological samples. Kristiansen et al. (73) have also applied flow-injection tandem mass spectrometry to measure sulfonamide antibiotics in meat and blood using a very simple ethyl acetate extraction step. This important technique will surely find many more applications in the future. 

Restricted-access materials (RAM) are biocompatible sample preparation supports that enable the direct injection of biological fluid into a chromatographic system. The technique was introduced in 1991 by Desilets et al., who also established the acronym RAM. Sorbents used in RAM represent a special class of materials that are able to fractionate a biological sample into a protein matrix and an analyte fraction, based on molecular weight cutoff. Macromolecules are excluded and interact only with the outer surface of the particle support, which is coated with hydrophilic groups. This minimizes the adsorption of matrix proteins. Applications of RAMs have been reviewed by several research groups. 

The injection process introduces the prepared sample or reagent into the flowing carrier stream within the manifold. Ideally, the injector system should be designed so as to provide a high sample flow rate. Injection systems typically employ electrokinetic mobility or hydrodynamic pressure techniques. In the former systems, the sample flow into the microchannel is controlled by the application of an external electric field to the reservoir, while in the latter systems, a pressure difference is created in the reservoir using either a positive pressure (pistmi-type) technique or a suction pressure (vacuum) technique. 

Hankemeier et al., studied large-volume injection combined with GC/DD-FTIR. A loop-type injection interface was chosen because of its rather simple optimization. Large-volume injection by means of a loop-type interface can be carried out successfully in conjunction with GC/DD-FTIR. The hyphenation permits enhanced detectability of analytes by about two orders of magnitude when compared with conventional split/splitless ones. As demonstrated, the determination and identification of PAHs in river water is possible down to a level of 0.5 p-g/L, even when using simple micro hquid-liquid extraction as a sample preparation technique. The present system may, therefore, be considered a viable approach to trace-level environmental analysis. 

The preparation technique of the disposable sensing elements used in this flow-injection amperometric immunofiltration system demonstrated high reproducibility of the analytical parameters. The variation between the responses to a standard concentration of mortalized cells (100 cell/ml) obtained with membranes prepared from one and the same batch was 16 %. It should be noted that the data points in Figures 4, 5 and 6 (curves a) are obtained as an average of 4 independent measurements, eadi performed by an individual sensing element. The error bars in Figures 4,5 6 represent the standard deviation obtained from these 4 independent measurements. 

Solid-phase microextraction (SPME) is a solvent-free sample preparation technique. The volume of the extraction phase is very small compared to the sample volume. The extraction is not exhaustive, but is based on equilibrium between the sample and the extraction phase, which is located on a fiber. SPME involves an adsorption step of the analyte, from a gas headspace or in a liquid sample (direct immersion), and a desorption step, which often is coupled directly with injection in the analytical system. Although SPME is mainly used in combination with GC, it has also been automated for HPLC. Eigure 9.10 shows a schematic representation of an SPME device. 

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