Flow injection systems coupled with detectors

A highly sensitive fluorescence quenching method for the determination of silicate based on the formation of an ion associate between molybdosilicate and Rhodamin B (RB) in nitric acid medium was developed. A flow-injection system coupled with a fluorescence detector was used for the measmement of fluorescence intensity at 560 and 580 ran as excitation and emission wavelengths, respectively. The calibration graph for Si showed a linear range of 0.1-5 ng/cm with correlation coefficient of 0.9999, and the detection limit of 0.06 ng/cm. The proposed method was successfully applied to the determination of silicate in ultrapurified water with satisfactory results. 

Solvent extraction offers unique advantages among separation techniques. A system based on extraction into a polymer [poly(vinyl chloride)] as solvent was examined here because of possible advantages in speedy simplicity, sample size, solvent handlingy etc.f especially when coupled with flow injection and an amperometric detector. Solutes examined included salicylic acid and 8-hydroxy quinoline. The apparatus typically consisted of 0.8-mm i.d. X 170-cm coiled tubing that could be connected directly to the injection loop of a flow-injection amperometric detector system containing a nickel oxide electrode. 

Principles and Characteristics As mentioned already (Section 3.5.2) solid-phase microextraction involves the use of a micro-fibre which is exposed to the analyte(s) for a prespecified time. GC-MS is an ideal detector after SPME extraction/injection for both qualitative and quantitative analysis. For SPME-GC analysis, the fibre is forced into the chromatography capillary injector, where the entire extraction is desorbed. A high linear flow-rate of the carrier gas along the fibre is essential to ensure complete desorption of the analytes. Because no solvent is injected, and the analytes are rapidly desorbed on to the column, minimum detection limits are improved and resolution is maintained. Online coupling of conventional fibre-based SPME coupled with GC is now becoming routine. Automated SPME takes the sample directly from bottle to gas chromatograph. Split/splitless, on-column and PTV injection are compatible with SPME. SPME can also be used very effectively for sample introduction to fast GC systems, provided that a dedicated injector is used for this purpose [69,70],... 

The traditional HPLC instrument is composed of two different parts the first part separates the components of the sample and the other part accomplishes the detection of the components separated. The part of the HPLC carrying out the separation contains a column, an injection device and the eluent delivery system (pump with filters, degasser and transfer tubing, eventually a mixer for gradient elution). One or more detectors, a signal output device coupled with appropriate software, are responsible for detection and primary data evaluation. Pumps deliver the eluent or the different components of the eluent into the column with a precise, constant and reproducible flow rate. 

Compared with the ICP, other atomic spectrometric detectors are not widely coupled to HPLC. Several interfaces have been described for AAS detector. Methods include a rotating platinum spiral collection system (Ebdon et al., 1987) and a flow injection thermospray sample introduction system (Robinson and Choi, 1987). Post-column hydride generation is also popular with AAS detection as will be described later. Pedersen and Larsen (1997) used an anion-exchange column to separate selenomethionine, selenocysteine, selenite and selenate with both FAAS and ICP-MS. The detection limits for the FAAS system were lmg H1 compared with 1 fig l-1 for ICP-MS. HPLC-MIP systems have been described to an even lesser extent. These either use elaborate interfaces to overcome the problems of quenching the low-power plasma (Zhang and Carnahan, 1989) or use a modified argon/oxygen mixed gas plasma (Kollotzek et al., 1984). 

Nonchromatographic separation coupled to atomic detectors Flow injection analysis (FIA) strategies may provide an efficient, continuous, and automatic method for preconcentration and/or separation of the sought species from a complex matrix, prior to its final determination. Furthermore, FIA systems can be interfaced easily with atomic detectors making them very convenient in speciation analysis. A typical application is selective preconcentration on... 

Low-dispersion flow injection techniques (dispersion values of 1-3) have been used for high-speed sample introduction to such detector systems as inductively coupled plasma atomic emission, flame atomic absorption, and specific-ion electrodes. The justification for using flow injection methods for electrodes such as pH and pCa is the small sample size required ( 25 pL) and the short measurement time ( 10 s). That is, measurements are made well before steady-state equilibria are established, which for many specific-ion electrodes may require a minute or more. With flow injection measurements, transient signals tor sample and standards provide excellent accuracy and precision. For example, it has been reported that pH measurements on blood serum can be accomplished at a rate of 240/h with a precision of 0.002 pH. 

Electrodes modified only with porphyrins or porphyrins associated with other species were explored as potentiometric detectors for different applications. A manganese(lll) porphyrin was utilized as the main component of recovered glassy carbon electrodes used as potentiometric detector of thiocyanate in urine. This sensor was coupled to a FIA system to optimize parameters such as membrane composition, pH, and flow injection parameters. Under the best conditions, a Nemstian response with 58.0 mV per decade of thiocyanate concentration was recorded. The new sensor presented a linear response in the 4.2 x to... 

For hot injectors capillary columns with an internal diameter of 0.32 mm are more suitable, as the transfer of the sample from the inlet liner to the column takes place more rapidly with less diffusion. However, on coupling with MS detectors, the higher column flow rate can exceed the maximum compatible flow rate in some quadrupole MS systems, and need to be checked. Columns with an internal diameter of 0.25 mm are the popular alternative and should be used in hot injectors with 2 mm internal diameter inserts and low injection volumes to obtain optimal results. If inserts with a wide internal diameter of4 mm are used for larger injection volumes, the solvent effect must be particularly exploited for focusing (Figure 2.67). 

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