Flow preconcentration systems

Experimental designs were used to set up a held flow preconcentration system (six important variables were considered) coupled to a minicolumn concentration system filled with Chelite P... 

M. C. Yebra and A. Moreno-Cid, Optimisation of a field flow preconcentration system by experimental design for the determination of copper in sea water by flow-injection-atomic absorption spectrometry, Spectrochim. Acta, Part B, 57(1), 2002, 85-93. 

M.C. Yebra, A. Garcia, N. Cairo, A. Moreno-Cid, and L. Puig. Design of a field flow preconcentration system for cadmium determination in seawater by flow-injection-atomic absorption spectrometry. Talanta 56 777-785,2002. 

Olsen et al. [660] used a simple flow injection system, the FIAstar unit, to inject samples of seawater into a flame atomic absorption instrument, allowing the determination of cadmium, lead, copper, and zinc at the parts per million level at a rate of 180-250 samples per hour. Further, online flow injection analysis preconcentration methods were developed using a microcolumn of Chelex 100 resin, allowing the determination of lead at concentrations as low as 10 pg/1, and of cadmium and zinc at 1 pg/1. The sampling rate was between 30 and 60 samples per hour, and the readout was available within 60-100 seconds after sample injection. The sampling frequency depended on the preconcentration required. 

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

Beck et al. [61] used flow injection magnetic sector ICP-MS to determine cadmium, copper, nickel, zinc, and manganese in estuarine waters. The online preconcentration system used Toyopearl A-T Chelate 650 H as chelating resin, and was validated for an alkaline water standard reference material (SLEW-2). 

Fig. 5.11 Flow diagram of preconcentration system described by Knapp et al. [41].

For method development and display of the run status, a flow-chart of the TraceCon preconcentration system is used to visuahze the current system state. During the execution of a method the flow chart displays the current activity of the TraceCon apparatus in the run mode. Run errors can also be easily identified, so that runs can be aborted if parts of the system fail. 

Figure 4.14 — (A) Flow injection system for the preconcentration and determination of copper P peristaltic pumps A 0.5 M HNOj B sample q = 2.5 mL/min) C water (jq = 0.5 mL/min) E 1 M NaNOj/O.l M NaAcO, pH 5.4 q = 0.5 mL/min F 1 M NaAcO/2 x 10 M Cu pH 5.0 (9 = 1.0 mL/min) 3-5 valves ISE copper ion-selective electrode W waste I and II 2 and 3 mL of chelating ion exchanger for purification III 100 fil of chelating ion exchanger for metal ion preconcentration. (B) Scheme of the flow system for the determination of halides A 4 M HAcO/1 M NaCl/0.57 ppm F B 1 M NaOH/0.5 M NaCl C, mixing coil (1 m x 0.5 mm ID PTFE tube) Cj stainless-steel tube (5 cm x 0.5 mm ID) ISE ion-selective electrode R recorder. (Reproduced from [128] and [129] with permission of Elsevier Science Publishers and the Royal Society of Chemistry, respectively).

A solid -phase preconcentration system was implemented. Several chemical and flow-related parameters potentially influencing the enrichment factor were studied, of which only three were revealed to be significant... 

C. R. Teixeira-Tarley, A. F. Barbosa, M. Gava-Segatelli, E. Costa-Figueiredo and P. Orival-Luccas, Highly improved sensitivity of TS-FF-AAS for Cd(II) determination at ng L levels using a simple flow injection minicolumn preconcentration system with multiwall carbon nanotubes, J. Anal. At. Spectrom., 21(11), 2006, 1305-1313. 

S. Rio Segade and J. F. Tyson, Determination of methylmercury and inorganic mercury in water samples by slurry sampling cold vapor atomic absorption spectrometry in a flow injection system after preconcentration on silica C18 modified, Talanta, 71(4), 2007, 1696-1702. 

Determination of inorganic anions by capillary electrophoresis is critically compared with ion chromatographic determinations on the basis of recent literature in the field. After a very brief summary of the theoretical background, the selection and optimization of the running electrolyte system are discussed, especially in connection with modification of the electroosmotic flow. Preconcentration techniques are surveyed, as are the approaches to the sample introduction and analyte detection. The principal analytical parameters of the determinations are evaluated and illustrated on selected applications described in the literature. 1997 Elsevier Science B.V. 

Various workers [32-34] have discussed mass spectrometric and other methods for the determination of plutonium in soils. Plutonium in soils has been quantified using 238plutonium as a yield tracer. Hollenbach et al. [36] used flow injection preconcentration for the determination of 230Th, 234 U, 239Pu and 240Pu in soils. Detection limits were improved by a factor of about 20, and greater freedom from interference was observed with the flow injection system compared to direct aspiration. 

R. M. Cespon Romero, M. C. Yebra-Biurrun, M. P. Bermejo-Barrera, Preconcentration and speciation of chromium by the determination of total chromium(III) in natural waters by flame atomic absorption spectrometry with a chelating ion-exchange flow injection system, Anal. Chim. Acta, 327 (1996), 37-45. 

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]. 

Tungsten-coil atomizers have to some extent been used in connection with atomic techniques in general and ETA-AAS in particular, and their popularity continues to rise. Their high simplicity and low cost make them attractive alternatives to graphite furnaces for many applications. However, they are much more interference-prone than their graphite counterparts. The interferences experienced by W-coil atomizers have been overcome in various ways, however. Thus, Barbosa et al. [25] avoid the interference from a salt matrix and implement a preconcentration step by electrochemically reducing Pb onto the coil surface. They use a flow injection system to deliver the sample through an anode inserted in the tip of the autosampler and the W-coil itself as cathode. The use of a W-coil as a platform inside a Massmann-type furnace provides no appreciable improvement over wall atomization [26]. 

In a flow injection system the sample flow and a reagent flow are continuously brought together so as to allow a chemical reaction to take place. This reaction produces a gaseous compound, which has to be separated off as in hydride generation, or forms a complex, which can be adsorbed onto a solid phase to be isolated and preconcentrated. In the latter case, elution with a suitable solvent is carried out and the analytes are led on-line into the AAS system. 

Z.-L. Fang, M. Sperling, B. Welz, Flame atomic-absorption spectrometric determination of lead in biological samples using a flow-injection system with online preconcentration by coprecipitation without filtration, J. Anal. At. Spectrom. 6 (1991) 301. 

Chen, H., Jin, J. and Wang, Y., 1997, Flow- injection on-line Coprecipitation-preconcentration System Using Copper (II) diethyldithiocarbamate as Carrier for Flame Atomic Absorption Spectrometric Determination of Cadmium, Lead and Nickel in Environmental Samples, Analytica Chimica Acta, 353, 181-188. 

Mussel, TORT-1 CRM Cd Microwave digestion in closed vessel with HNO3 WDM] Preconcentrate on chelating resin, A/A flame [WDM-SEP/ via flow injection system [SEP/CONC-FAAS] CONC-FAAS] Enriquez-Domi-nguez et al. (1998)... 

Enriquez-Dominguez me, Yebra-Biurrun MC and Bermejo-Barrera MP (1998) Determination of cadmium in mussels by flame atomic absorption spectrometry with preconcentration on a chelating resin in a flow injection system. Analyst (London) 123 105-108. 

Liu X and Fang Z (1995) Flame atomic absorption spectrometric determination of cobalt in biological materials using a flow-injection system with on-line preconcentration by ion-pair adsorption. Anal Chim Acta 316 329-335. 

Z. Fang, S. Xu, and S. Zhang, The Determination of Trace Amounts of Heavy Metals in Waters by a Flow-Injection System Including Ion-Exchange Preconcentration and Flame Atomic Absorption Spectrometric Detection. Anal. Chim. Acta, 164 (1984) 41. 

P. Hernandez, L. Hernandez, and J. Losada, Determination of Aluminium in Hemodialysis Fluids by a Flow Injection System with Preconcentration on a Synthetic Chelate-Forming Resin and Flame Atomic Absorption Spectrophotometry. Fresenius Z. Anal. Chem., 325 (1986) 300. 

Flow injection is a method using on-line discrimination chemical reactions. Direct selective preconcentration is performed on microcolumns, the analytes being concentrated up to three to four orders of magnitude and injected into the detector. Usually as separation techniques ion-exchange, liquid-liquid extraction, gas diffusion are used. Flow injection systems are connected either to ETA AS or ICP-AES detection systems. These methods have been used for determinations of redox species such as Crflll)/ Cr(IV), Fe(II)/Fe(III), As(III)/As(V), Se(IV)/Se(VI) in soil extracts and water samples. 

Here the model is subjected to a more general treatment to include FI preconcentration systems using other flow-through detectors. 

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. 

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