Preconcentration system flow rates

More recently, Simon and Dasgupta (110) used a parallel-plate wet denuder in which the scrubber liquid is made to flow down two closely spaced silica-coated parallel plates facing each other with the air sample flowing upward in the gap. Excellent collection efficiency (90% + ) was established for S02 even at flow rates of 15 L/min. The denuder effluent was preconcentrated and chromatographed as in reference 108. On the basis of signal-to-noise data observed at S02 test concentrations of 19 pptrv, the calculated LOD of the system is on the order of 0.5 pptrv. 

Off-line dicarbamate solvent extraction and ICP-MS analysis [317] provided part-per-trillion detection limits Cd (0.2 ppt), Co (0.3 ppt), Cu (3 ppt), Fe (21 ppt), Ni (2 ppt), Pb (0.5 ppt), and Zn (2 ppt). Off-line matrix removal and preconcentration using cellulose-immobilized ethylenediaminetetraacetic acid (EDTA) have also been reported [318]. Transition metals and rare earth elements were preconcentrated and separated from the matrix using on-line ion chromatography with a NTA chelating resin [319]. Isotope-dilution-based concentration measurement has also been used after matrix separation with a Chelex ion-exchange resin [320]. The pH, flow rate, resin volume, elution volume, and time required for isotope equilibration were optimized. A controlled-pore glass immobilized iminodiacetate based automated on-line matrix separation system has also been described [321]. Recoveries for most metals were between 62% and 113%. 

The elution flow-rate is an important parameter in column preconcentration which is usually optimized for maximum sensitivity. However, the speed of elution is also a crucial factor for the efficiency of on-line preconcentration systems, because in most cases the eluent flow is connected directly (some after merging with reagent streams) with the detector. This is particularly important for detection systems which require a certain sample delivery rate for optimum response, e.g., the flame AA or ICP spectrometer, and will be discussed in more detail in section 4.6.3. 

In ICP spectrometry, optimum sample introduction rates are much lower than those for flame AAS. In a FI column preconcentration system the optimum conditions for elution are closer to the required sample introduction rates. Therefore, on-line elution flow-rates are quite similar to the normal sample introduction rates of 1-2 ml min for ICP spectrometric systems. 

FI column preconcentration manifolds used for flame AAS can be easily adapted to work with ICP spectrometry, usually adjusting the elution flow-rate to lower values. Thus the manifold in Fig. 4.7. which was originally designed for flame AA has been readily adapted to an ICP system with minor modifications [41,42]. 

A low-temperature flame is used to prevent the excitation of most other metals Wide dynamic range with online dilution and/ or preconcentration steps Flow rate in the flow system should be carefully selected and compared with the aspiration rate of the nebulizer Sample spends short time in contact with nebulizer and burner, so that the measuring system is readily rinsed between consecutive injections High sample throughput High tolerance to saline matrices Better adapted to SIA technique Improved sensitivity with regard to FAAS due to the rapid atomization of the entire sample... 

Figure 5.17 shows an SIA system for the AAS determination of iron by preconcentration of the metal on a Chelex 100 column and elution with nitric acid [5]. The system uses a Crison Compact titrator to operate both burettes in such a way that the nebulizer will continuously receive liquid at a constant flow-rate. Another interesting example of coupling ASS detection and flow techniques is a multisyringe flow injection analysis (MSFIA) system with cold-vapor AAS developed to determine mercury [6]. 

The sample passes through the GFF filter (300 mm ID, 0.5 i,m pore size), PUFP (100 mm ID, 100 mm height), and back-up PUFP (100 mm ID, 100 mm height). The system design makes it possible to sample up to several cubic meters of water at the flow rate of 1-2 L/min ( = 60-120 L/h = 1440-2880 L/day). The in situ preconcentration system was applied to several water samples 100-200 L and 1500-2000 L of river and tap water samples were collected, respectively. 

Variables associated with the preconcentration system performance, such as pH, sampling flow rate and eluent concentration were optimised using a basic simplex as multivariate technique. 

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