Sample introduction, continuous

Lan and Mottola [14] have presented two continuous-flow-sensing strategies for the determination of C02 in gas mixtures using a special reaction cell. Both approaches are based on the effect of the complex of Co(II) with phthalocyanine as a rate modifier of the CL emission generated by luminol in the absence of an added oxidant agent, which is enhanced by the presence of C02 in the system. This enhancement allows the fast and simple determination of carbon dioxide at ppm levels (v/v) in atmospheric air and in human breath. In the first case, a continuous monitoring system was applied however, because the flow of expired gas is not constant, a discrete sample introduction approach was used in the analysis of C02 in breath. 

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

The sample introduction unit was constructed from inert materials, which minimizes the introduction of metal contamination into the system. The samples or digestion acids make contact only with PTFE, Kel-F, glass, acid-resistant rubber and platinum-iridium (9 + 1) alloy. In addition, the construction materials were Hmited to acid-grade Arborite (ureaformaldehyde laminate). Perspex and stainless-steel. The unit was constructed in three continuous sections a heated sample compartment, a turntable mechanism and heat-exchanger compartment, and a pump compartment. 

Flow-injection sample introduction has been successfully applied in the analysis of standard reference materials and in the measurement of accurate and precise isotope ratios, and, hence, isotope dilution analysis. The rapid sample throughput possible with FI should allow a four-fold increase in the sampling rate compared with conventional nebufization techniques. Also, the amount of sample consumed per analytical measurement by FI is considerably less than continuous nebufization. TTiese considerations are of particular importance for the cost-effective operation of ICP-MS. 

Elements such as As, Se and Te can be determined by AFS with hydride sample introduction into a flame or heated cell followed by atomization of the hydride. Mercury has been determined by cold-vapour AFS. A non-dispersive system for the determination of Hg in liquid and gas samples using AFS has been developed commercially (Fig. 6.4). Mercury ions in an aqueous solution are reduced to mercury using tin(II) chloride solution. The mercury vapour is continuously swept out of the solution by a carrier gas and fed to the fluorescence detector, where the fluorescence radiation is measured at 253.7 nm after excitation of the mercury vapour with a high-intensity mercury lamp (detection limit 0.9 ng I l). Gaseous mercury in gas samples (e.g. air) can be measured directly or after preconcentration on an absorber consisting of, for example, gold-coated sand. By heating the absorber, mercury is desorbed and transferred to the fluorescence detector. 

The chromatographic process begins by injecting the solute onto the top of the column. The solvent need not be the mobile phase, but frequently it is appropriately chosen to avoid detector interference, column/analyte interference, loss in efficiency, or all of these. Sample introduction can be accomplished in various ways. The simplest method is to use an injection valve. In more sophisticated LC systems, automatic sampling devices are incorporated where sample introduction is done with tire help of autosamplers and microprocessors. It is always best to remove particles from the sample by filtering, or centrifuging since continuous injections of particulate material will eventually cause blockage of injection devices or columns. 

Although new types of columns will undoubtedly continue to be introduced, at present much research is being performed for the purpose of improving the existing ones. For packed columns the primary goal is the reduction in the number of active sites, while for capillaries it is the reproducible and uniform coating of capillaries with inner diameters closer to the theoretical optimum for mass transfer, the latter will unfortunately require further improvements in sample introduction and detection before the predicted improvements in resolution can be fully realized. 

Schaub,T. M., Linden, H. B., Hendrickson, C. L., and Marshall, A. G. (2004). Continuous flow sample introduction for field desorption/ionization mass spectrometry. Rapid Commun. Mass Spectrom. 18,1641-1644. 

Atomization sources can also be classified according to the technique of introducing the sample into the atomizer. Sample introduction is continuous when the sample is aspirated during fixed flow and noncontinuous when a discrete volume of sample is introduced into the atomizer. The first system (flame- and plasma-based atomizers) supplies a constant atomic signal in the second one (electrothermal atomizers) the atomic signal reaches a maximum value and then drops to zero. 

For continuous sample introduction, gated injection was adopted. With EK flow, the analyte continually flowed in parallel with a separation buffer to the analyte waste reservoir (see Figure 4.15). Injection of the sample analyte was achieved by interrupting the flow of the buffer for a short time (known as the injection time) so that the analyte stream was injected. This scheme was achieved by four reservoirs (without considering the reagent reservoir) and two power supplies [317], Gated injection has also been achieved using one power supply and three solution reservoirs [564]. 

Pure HDF injection can be achieved for sample introduction by using vacuum suction. The sample was sucked through an inserted capillary into a short section of microchannels between three ports. Different amounts could be selected by filling different sections of the microchip [813]. Pressure injection of a DNA sample was also achieved via a transfer capillary sequentially to a five-channel microchip. The use of one capillary allowed for automated sampling necessary for continuous monitoring of an enzymatic DNA restriction digestion experiment [320],... 

Fang, Q., Xu, G.-M., Fang, Z.-L., High throughput continuous sample introduction interfacing for microfluidic chip-based capillary electrophoresis systems. Micro Total Analysis Systems, Proceedings 5th XTAS Symposium, Monterey, CA, Oct. 21-25, 2001, 373-374. 

Roddy, E.S., Price, M., Ewing, A.G., Continuous monitoring of a restriction enzyme digest of DNA on a microchip with automated capillary sample introduction. Anal. Chem. 2003, 75, 3704-3711. 

Figure 6.20 Flow cell with horizontal DME for continuous monitoring (a) sample introduction tube (b) solution outlet (c) DME capillary (d) O-ring seals (e) waste mercury (/) agar salt bridge to counter electrode (g) SCE reference electrode with agar plug and fine glass frit (h) ground-glass joints.

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