Preconcentration detectors

Frequently, preconcentration of an analyte is necessary because the detector used for quantitation may not have the necessary detectabiUty, selectivity, or freedom from matrix interferences (32). Significant sample losses can occur during this step because of very small volume losses to glass walls of the recovery containers, pipets, and other glassware. 

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

Analysis of environmental samples is similar to that of biological samples. The most common methods of analyses are GC coupled to MS, ECD, a Hall s electrolytic conductivity detector (HECD), or a flame-ionization detector (FID). Preconcentration of samples is usually done by sorption on a solid sorbent for air and by the purge-and-trap method for liquid and solid matrices. Alternatively, headspace above liquid and... 

Aniline, methyl aniline, 1-naphthylamine, and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin-layer, dual glassy-carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behavior of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nmol/l were achieved using colourimetric and amperometric cells, respectively, when using an in-line preconcentration step. 

A typical extraction manifold is shown in Figure 13.2. The sample is introduced by aspiration or injection into an aqueous carrier that is segmented with an organic solvent and is then transported into a mixing coil where extraction takes place. Phase separation occurs in a membrane phase separator where the organic phase permeates through the Teflon membrane. A portion of one of the phases is led through a flow cell and an on-line detector is used to monitor the analyte content. The back-extraction mode in which the analyte is returned to a suitable aqueous phase is also sometimes used. The fundamentals of liquid liquid extraction for FIA [169,172] and applications of the technique [174 179] have been discussed. Preconcentration factors achieved in FIA (usually 2-5) are considerably smaller than in batch extraction, so FI extraction is used more commonly for the removal of matrix interferences. 

Basically, these involve the addition of three electronically activated Teflon valves to direct the flow of gas from the gas-hquid separator and over the Galahad instrument. One valve is controlled directly from the vapour generator and this only diverts the argon containing mercury stream over the gold trap for a preset time. The additional two valves are controlled by the Galahad cycle so that the revaporized mercury can be directed to the Merlin detector for measurement. In this manner the flows for preconcentration and the flows for measurement can be optimized individually. The operation of these valves is shown schematically in Fig. 7. IS. 

Fig. 7.17 Atomic fluorescence detector response from samples preconcentrated on gold traps prior to revaporization.

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

Simple alkyl nitrates are also commonly measured using GC-ECD, usually with preconcentration either by cryotrapping or using a solid sorbent (e.g., Atlas and Schauffler, 1991 Ridley et al., 1997). Another approach is GC with an NO detector as described earlier (e.g., Flocke et al., 1998). In this approach, the compounds are converted to NO over a catalyst as they emerge from the GC column, and the NO measured by its chemiluminescence reaction with 03. 

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. 

Analytical separation of several organics from water by PVC polymer is feasible. A solvent extraction model describes the separation dynamics and pH dependence. Selectivity via pH control of the extraction step and preconcentration of analyte can be accomplished. These results suggest that other polymer solvent extraction schemes can be developed by using this approach. The flow-through amperometric technique provides a well-suited detector component for the technique. 

What is required from the analysis Is it qualitative identification of components in a mixture Will you require high-resolution separation of everything or do you just need good resolution in a portion of the chromatogram Can you sacrifice resolution to shorten the analysis time Do you need quantitative analysis of one or many components Do you need high precision Will analytes be present in adequate concentration or do you need preconcentration or a very sensitive detector for ultratrace analysis How much can the analysis cost Each of these factors creates trade-offs in selecting techniques. 

J. K. Robinson, M. J. Bollinger, and J. W. Birks, Luminol/H202 Chemiluminescence Detector for the Analysis of NO in Exhaled Breath, Anal. Chem. 1999, 71, 5131. Many substances can be analyzed by coupling their chemistry to luminol oxidation. See, for example, O. V. Zui and J. W. Birks, Trace Analysis of Phosphorus in Water by Sorption Preconcentration and Luminol Chemiluminescence, Anal. Chem. 2000, 72, 1699. 

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