Samples preconcentration

M. E. Swaitz and M. Merion, On-line sample preconcentration on a packed-inlet capillary for improving the sensitivity of capillary electi ophoretic analysis of pharmaceuticals , 7. Chromatogr. 632 209-213 (1993). 

The procedure described earlier for sample preconcentration can be easily extended for the online extraction of solid samples, e.g., powdered plant materials. Horizontal conbguration of the chromatographic plate in the chamber facihtates this procedure, because the sample to be extracted is then placed on a carrier plate at the begiiming part of the adsorbent layer (or in the scrapped channel of the adsorbent layer), which should be directed upward [15,26]. The chamber is covered with a narrow plate, and the development is started with a snitable extracting solvent. In some cases, it is advantageous to put the narrow plate directly on the adsorbent layer to press the sample to be extracted. Extracted components are preconcentrated on the adsorbent layer at the end of the narrow plate, as shown in Fignre 6.26 [15]. 

Many process mixtures, notably fermentations, require sample preconcentration, microdialysis, microfiltration, or ultrafiltration prior to analysis. A capillary mixer has been used as a sample preparation and enrichment technique in microchromatography of polycyclic aromatic hydrocarbons in water.8 Microdialysis to remove protein has been coupled to reversed phase chromatography to follow the pharmacokinetics of the metabolism of acetaminophen into acetaminophen-4-O-sulfate and acetaminophen-4-O-glucu-ronide.9 On-line ultrafiltration was used in a process monitor for Aspergillus niger fermentation.10... 

Speciation involves a number of discrete analytical steps comprising the extraction (isolation) of the analytes from a solid sample, preconcentration (to gain sensitivity), and eventually derivatisation (e.g. for ionic compounds), separation and detection. Various problems can occur in any of these steps. The entire analytical procedure should be carefully controlled in such a way that decay of unstable species does not occur. For speciation analysis, there is the risk that the chemical species can convert so that a false distribution is determined. In general, the accuracy of the determinations and the trace-ability of the overall analytical process are insufficiently ensured [539]. 

Similar comments apply in the case of organics in seawater, because it has now become possible to resolve the complex mixtures of organics in seawater and achieve the required very low detection limits. Only since the advent of sample preconcentration and mass spectrometry coupled with gas chromatography and high-performance liquid chromatography, and possibly derivatisa-tion of the original sample constituents to convert them into a form suitable for chromatography, has this become possible. 

Winge et al. [730] have investigated the determination of twenty or more trace elements in saline waters by the inductively coupled plasma technique. They give details of experimental procedures, detection limits, and precision and accuracy data. The technique when applied directly to the sample is not sufficiently sensitive for the determination of many of the elements at the low concentrations at which they occur in seawater, and for these samples preconcentration techniques are required. However, it has the advantages of being amenable to automation and capable of analyzing several elements simultaneously. 

Abollino et al. [690] compared cathodic stripping voltammetry and graphite furnace AAS in determination of cadmium, copper, iron, manganese, nickel, and zinc in seawater. The effects of UV irradiation, acidification, and online sample preconcentration were studied. 

Sample preconcentration was performed by means of an automated on-line SPE sample processor Prospekt-2 (Spark Holland, Emmen, The Netherlands). Oasis HLB cartridges (Waters, Barcelona, Spain) were used to preconcentrate cannabi-noids present in the water samples whereas isolation of the rest of the compounds was done in PLRPs cartridges (Spark Holland). Before extraction, influent samples were diluted with HPLC water (1 9, v/v) to reduce matrix interferences and to fit some analyte concentrations, e.g., cocaine (CO) and benzoylecgonine (BE), within the linear calibration range. A sample volume of 5 mL was spiked with the internal standard mixture (at 20 ng/L) in order to correct for potential losses during the analytical procedure, as well as for matrix effects. Elution of the analytes to the LC system was done with the chromatographic mobile phase. 

As a consequence, for unequivocal identification of the constituents of complex mixtures found in surfactant blends and also in the analyses of surfactants and their metabolites in environmental samples, MS and tandem mass spectrometry (MS-MS) have proved to be more advantageous and are discussed thoroughly in Chapter 2. To optimise the output of reliable results and to save manpower and time certain procedures in sample preconcentration, clean-up and separation prior to MS examinations are inevitable. These are discussed in the present book in more detail in Chapter 3. 

Analytical methods for detecting phenol in environmental samples are summarized in Table 6-2. The accuracy and sensitivity of phenol determination in environmental samples depends on sample preconcentration and pretreatment and the analytical method employed. The recovery of phenol from air and water by the various preconcentration methods is usually low for samples containing low levels of phenol. The two preconcentration methods commonly used for phenols in water are adsorption on XAD resin and adsorption on carbon. Both can give low recoveries, as shown by Van Rossum and Webb (1978). Solvent extraction at acidic pH with subsequent solvent concentration also gives unsatisfactory recovery for phenol. Even during carefully controlled conditions, phenol losses of up to 60% may occur during solvent evaporation (Handson and Hanrahan 1983). The in situ acetylation with subsequent solvent extraction as developed by Sithole et al. (1986) is probably one of the most promising methods. 

Sample preconcentration techniques are used with two purposes (1) to increase concentration in order to achieve detection and (2) to eliminate disturbances of the electrophoretic system during hydraulic or electrokinetic sample introduction when the conductivity of the sample is significantly different from that of the analysis buffer. It is important to keep sample manipulations and modifications to a minimum, and a rule of thumb is to prepare the sample so that its composition is at the same pH as the analysis buffer. It is also advantageous... 

Trippel-Schulte P, Zeiske J, Kettrup A. 1986. Traee analysis of seleeted benzidine and diaminodiphenylmethane derivatives in mine by means of liquid ehromatography using preeolumn sample preconcentration, UV and electrochemical detection. Chromatographia 22(1-6) 138-148. 

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

An advantage of CIEF is that the total length of the capillary is filled with sample and ampholyte mixture thus, larger quantities of proteins can be separated. It therefore can be advantageously used as a sample preconcentration step prior to capillary gel electrophoresis. Furthermore, CIEF is a suitable and very precise method for measuring the isoelectric point of biomolecules. 

Frommberger, M., Schmitt-Kopplin, R, Ping, G., Frisch, H., Schmid, M., Zhang, Y., Hartmann, A., and Kettrup, A., A simple and robust set-up for on-column sample preconcentration-nano-hquid chromatography-electrospray ionization mass spectrometry for the analysis of N-acylhomoserine lactones, Analytical and Bioanalytical Chemistry 378(4), 1014—1020, 2004. 

Because most organics are present at ppb-ppt concentrations in air, sample preconcentration is required. This is typically accomplished by direct cryotrapping of air, trapping on a solid sorbent, or sampling into an evacuated canister followed by cryotrapping the canister contents prior to injection onto the GC column. Combinations of these, such as cryotrapping followed by transfer to a solid sorbent, have also been used (e.g., Shepson et al., 1987). 

J. R. Hutchinson, P. Zakaria, A. R. Bowie, M. Macka, N. Avdalovic, and P. R. Haddad, Anion-Exchange Capillary Electrochromatography and In-Line Sample Preconcentration in Capillary Electrophoresis, Anal. Chem. 2005, 77, 407. 

Packed column technology has been used in airborne gas chromatographs for the separation and quantitation of sulfur species (46, 47) and peroxyacetic nitric anhydride (48). The combination of sample preconcentration and sensitive detectors has yielded detection limits that are superior to corresponding continuous sensors. For S02, a detection limit of 25 pptrv was claimed, and for peroxyacetic nitric anhydride the detection limit was roughly 60 pptrv for an 50-cm3 air sample. Analysis times for samples were on the order of 10 min. 

Pushing detection limits of nitroaromatic explosives into the parts per trillion (ppt) level requires sample preconcentration. Collins and coworkers used solid-phase extraction (SPE) of explosives from sea water which was followed by rapid on-chip separation and detection [18]. Explosives were eluted from SPE column by acetonitrile and were injected in the microchip separation channel. Lab-on-a-chip analysis was carried out in nonaqueous medium. The mixed acetonitrile/methanol separation buffer was used to produce the ionized red-colored products of TNT, TNB and tetryl [27,28]. The chemical reaction of the bases (hydroxide and methoxide anions) with trinitroaromatic explosives resulted in negatively charged products, which were readily separated by microchip... 

As techniques for chemical analysis are used in continually smaller domains, experimental challenges for inherently insensitive methods such as NMR spectroscopy become increasingly severe. Among the various schemes to boost the intrinsic sensitivity of an NMR experiment, the development of small-volume RF probes has experienced a renaissance during the past decade. Commercial NMR probes now allow analyses of nanomole quantities in microliter volumes from natural product extracts and combinatorial chemical syntheses. Figure 7.3.1.9 illustrates the range of volumes that can be examined by NMR probes and accessories such as microsample tubes and inserts. With recently reported advances in sample preconcentration for microcoil NMR analysis [51], dilute microliter-volume samples can now be concentrated into nanoliter-volume... 

Supercritical fluid extraction [153,154], accelerated solvent extraction [68] and subcritical fluid extraction [107,155] have been studied. To reduce the equipment cost and the analysis time in the extraction process and sample preconcentration, a solid-phase microextraction method was proposed by Pawliszyn and coworkers [156-158]. 

TABLE 5.2 Sample Preconcentration Using Different Stacking Methods... 

The ESI-LC/MS-based approaches that feature ion trap (Gatlin et al, 1998 Washburn et ah, 2001) and quadrupole time-of-flight (QTOF) (Blackburn and Moseley, 1999) mass spectrometers are routinely used for the identification and characterization of proteins. Nanoelectrospray LC/MS formats (Figure 6.3) are used to provide lower limits of detection and fully automated sample preconcentration and desalting. On-line LC/MS approaches for protein expression profiling are also used with ESI-TOF (Banks and Gulcicek, 1997 Chong et al., 2001) and ESI-Fourier transform (FT) (Kelleher... 

Blanco, R.M., M.T. Villanueva, J.E.S. Uria, and A. Sanz-Medel. 2000. Field sampling, preconcentration and determination of mercury species in river waters. Anal. Chim. Acta 419 137-144. 

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