Sampling systems continuous mode

It is used in IC systems when the amperometric process confers selectivity to the determination of the analytes. The operative modes employed in the amperometric techniques for detection in flow systems include those at (1) constant potential, where the current is measured in continuous mode, (2) at pulsed potential with sampling of the current at dehned periods of time (pulsed amperometry, PAD), or (3) at pulsed potential with integration of the current at defined periods of time (integrated pulsed amperometry, IPAD). Amperometric techniques are successfully employed for the determination of carbohydrates, catecholamines, phenols, cyanide, iodide, amines, etc., even if, for optimal detection, it is often required to change the mobile-phase conditions. This is the case of the detection of biogenic amines separated by cation-exchange in acidic eluent and detected by IPAD at the Au electrode after the post-column addition of a pH modiher (NaOH) [262]. 

The on-line interface of flow manifolds to continuous atomic spectrometric detectors for direct analysis of samples in liquid form typically requires a nebuliser and a spray chamber to produce a well-defined reproducible aerosol, whose small droplets are sent to the atomisation/ionisation system. A variety of nebulisers have been described for FAAS or ICP experiments, including conventional cross-flow, microconcentric or Babington-type pneumatic nebulisers, direct injection nebuliser and ultrasonic nebulisers. As expected, limits of detection have been reported to be generally poorer for the FIA mode than for the continuous mode. 

There are several reasons for this exceptional situation. First, production of transactinides at accelerators implies a thermalization of the primary products in a gas, usually helium. It is rather straightforward to connect such a recoil chamber to a gas chromatographic system. Second, gas phase separation procedures are fast and may be performed in a continuous mode. Third, at the exit of the chromatographic column separated volatile species can be easily condensed as nearly weightless samples on thin foils. This enables detection of a decay and spontaneous fission (SF) of the separated products with supreme energy resolution. 

Ultrasonic nebulizers have also been employed in continuous flow systems as interfaces between sample preparation steps in the analytical process and detection by virtue of their suitability for operating in a continuous mode. Thus, preconcentration devices have commonly been coupled to atomic spectrometers in order to increase the sensitivity of some analytical methods. An enhancement factor of 100 (10 due to USNn and 10 due to preconcentration) was obtained in the determination of platinum in water using a column packed with polyurethane foam loaded with thiocyanate to form a platinum-thiocyanate complex [51]. An enhancement factor of 216 (12 with USNn and 18 with preconcentration) was obtained in the determination of low cadmium concentrations in wine by sorption of metallic complexes with pyridylazo reagents on the inner walls of a PTFE knotted reactor [52]. One special example is the sequential determination of As(lll) and As(V) in water by coupling a preconcentration system to an ICP-AES instrument equipped with a USN. For this purpose, two columns packed with two different resins selective for each arsenic species were connected via a 16-port valve in order to concentrate them for their subsequent sequential elution to the spectrometer [53]. 

A first report appeared in 1992 from Flentge [153] who developed an enzyme reactor for the amperometric detection of both metabolites, but only the detection system for choline was coupled with microdialysis. The enzymes (one or both) were physically entrapped between two cellulose nitrate filters, and the sandwich reactor was put on-line after an HPLC column for the separation step, or else placed directly after the microdialysis probe for the online measurement of choline. The sensitivity of this system was sufficient to measure acetylcholine and chohne in rat striatum when used after the HPLC column, while the dilution due to the microdialysis sampling only permitted determination of choline, even if in a continuous mode. 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

The principle behind the FIA technique (reversed mode) has been exploited to develop a completely automated instrument [44] (Fig. 15.15) in which the sample is continuously pumped along the system and in which different injection valves [45] or a single injection valve aided by a selecting valve [46] are used to introduce the reagents required for the determination of each analyte in the appropriate sequence and at the required time intervals. The reacting plug is driven to the fiow-ceii of a photometric detector, and the... 

An Sl E system can be operated in one of two ways. In the dynamic extraction mode, the valve between the extraction cell and the restrictor remains open so that the sample is continually supplied with fresh supercritical fluid and the extracted material flows into the collection vessel where depressuriz-ation occurs. In the static extraction mode, the valve between the extraction cell and the restrictor is closed and the extraction cell is pressurized under static conditions. After a suitable period, (he exit valve is opened and the cell contents arc iransferred through the rcsiricior by a dynamic flow of fluid from the pump. The dynamic mode is more widciv used than the static mode. 

In this type of system a resistively heated platinum or nichrome wire coil or ribbon is used to rapidly heat the sample. The wire is continuously swept with carrier gas, whereupon the pyrolysis vapors are transported into the chromatographic column. Heating times are relatively large (up to 20 s) for this system, which may lead to nonrepeatable pyrograms and secondary reactions. The pyrolysis conditions, sample size, and location must be carefully controlled to obtain repeatable data. Two possible heating modes are available for this system pulse mode or programmed mode. For most forensic applications the pulse mode has been used. 

Most of the commercially available porosimeters include certain common features. First, the sample is evacuated and then the penetrometer is backfilled with Hg in the low-pressure port. The seeond step of the low-pressure analysis is the eolleetion of the data at pressures up to the last low pressure point specified. When the low-pressure analysis is complete, the high-pressure measurement is carried out up to the maximum pressure. Pore volume data are calculated by determining the volume of Hg remaining in the penetrometer stem. When the maximum pressure is aehieved, the extrusion curve starts by reducing slowly the applied pressure. Commercial instruments ean work in one or both modes ineremental and continuous. In the former the pressure, or amount of Hg introdueed, is inereased step by step and the system is allowed to stabilize before the next step. In the continuous mode the pressure is increased continually at a predetermined rate [106]. 

Some of the well known advantages of FI over continuous systems are (i) the sample and/or reagent consumed volumes are reduced by several orders of magnitude (ii) automation is easier (iii) the precision of the method is better (iv) the sample throughput is higher (v) matrix effects can be eliminated, although some studies have indicated that the interferences caused by nitric acid can be more severe in FI than in continuous mode and (vi) the sensitivity and limits of detection are in some cases better. FI-based methods are also interesting for the analysis of hazardous samples and the minimization of waste. 

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