Packed microcolumns

Microcolumns use less solvent and, because the sample is diluted to a lesser extent, produce larger signals at the detector. These columns are made from fused silica capillaries with internal diameters of 44—200 pm and lengths of up to several meters. Microcolumns packed with 3-5-pm particles have been prepared with column efficiencies of up to 250,000 theoretical plates. 

Monolithic columns are comparatively easy to prepare. This is particularly true for capillary columns, which are known to be tedious to pack with particles. Furthermore, the reproducibility of microcolumn packing is low. 

Xan et al. [124] described a highly selective method for the determination of traces of lead in soils and sediments. It is based on the preconcentration of lead on a microcolumn packed with a macrocycle immobilised on silica gel. 

Both Cr111 and Cr concentrations in natural water samples were measured by flame AAS after pre-concentrations of the chromium species on microcolumns packed with activated alumina (acidic form) (Sperling et al., 1992). An FI manifold was used in this work to obtain conditions for species-selective sorption and subsequent elution of the chromium species directly to the nebuliser of the spectrometer. In this procedure, water samples were maintained at a safe pH of 4 prior to analysis. Analytical conditions of pH 2 and 7 were attained by adding buffers on-line only fractions of a second before the corresponding chromium species was sorbed into the column. In this manner, any risk of losses of analytes and/or shifts in equilibria between the species at pH 2 and 7 were minimised. The detection limits were 1.0 and O.Smgdm 3 for Cr111 and Cr, respectively. 

A field method of pre-concentration and preservation of organo-mercury species extracted from water was described (Jian and McLeod, 1992). Acidified water samples were passed through microcolumns packed with sulfhydryl cotton. The packing retained organo-mercury species, but not inorganic mercury. The contents were analysed later in a laboratory. 

Chromatographic method The determination of silicate based upon the adsorption of preconcentrated phosphomolybdic and silicomo-lybdic heteropoly acid (HPA) in the dynamic mode on a microcolumn packed with an Amberlite XAD-8 polyacrylate adsorbent is reported [49]. The method is based on the adsorption of HPAs followed by desorption with acetonitrile and determination by reversed-phase HPLC. Another highly sensitive HPLC method for the simultaneous determination of soluble silicate and phosphate in environmental waters is used in ion-pair liquid... 

Pervaporators are amenable to coupling to any type of detector, whether molecular or atomic, via an appropriate interface such as a transport tube, a microcolumn packed with... 

In direct headspace analysis, the sample e g. serum or urine, is equilibrated with the headspace in a suitable container. A protion of headspace gas is then injected for analysis. More elaborate headspace trapping devices combine separation of the volatiles from the sample matrix with subsequent enrichment of the constituents. Such a system, suitable for small volumes of body fluids, is known as the transevaporator sampling technique. It contains a microcolumn packed with Porasil E (pore silica gel), into which the sample is injected. In one mode of use, helium is passed through the column to remove the volatiles which are then collected in a trap (Tenax-GC, a porous polymer, 2,6-diphenyl-p-phenylene oxide). 

A microcolumn packed with 3 g of activated silica gel is used for sample cleanup and fractionation. An aliquot of the extract (containing about 20 mg oil or TSEM) is then transferred to the silica gel cleanup column to remove polar components and other interferences. The column is eluted first with hexane, which recovers samrated hydrocarbons as Fraction 1 (FI). The mixture of hexane/DCM or hexane/benzene (1 1, v v) is used to elute the aromatic compounds as Fraction 2 (F2). Half of FI and half of F2 are combined into the Fraction 3 (F3). These three fractions are concentrated to appropriate volume (0.5 to 1.0 ml) by nitrogen purge. The quantitation internal standards are 5-a-androstane, di4-terphyl, and C30 17(3(H), 21(3(H)-hopane. The Fraction 3 is analyzed for quantitation of the TPH and the UCM by GC-FID. The Fraction 1 is analyzed for determination of n-alkanes by GC-FID and biomarker terpanes and steranes by GC-MS. The Fraction 2 is analyzed for the determination of alkylated PAH homologues and other EPA priority unsubstantiated PAHs by GC-MS. 

Cappiello s research group applied micro-HPLC to the screening of water samples for pesticide contamination. With electron ionization mass spectrometric detection and the use of two microcolumns packed with a Cig silica-based stationary phase, they performed the preconcentration of water samples and successfully detected trace levels of pollutants in similar samples. They also showed that ion-interaction micro-HPLC on a Cig stationary phase with hexylamine as the ion-pair reagent and coupling to particle beam mass spectrometry could be successfully used for the analysis of herbicides ranging from acidic species such as... 

Pervaporators are amenable to coupling to any type of detector via an appropriate interface such as a transport tube, a microcolumn packed with adsorptive or ion-exchange material, or a gas liquid separator. The acceptor stream can be either liquid or gaseous depending on the characteristics of the detector. The detectors most frequently used are the spectroscopic - atomic or molecular, electroanalyti-cal (potentiometric, voltammetric), electron capture, and flame ionization types. The low selectivity of some of these detection techniques is overcome by that of the pervaporation step, endowing the overall analytical process with the selectivity required for the analysis of complex matrices. The potential use of the pervaporation technique for sample insertion into water-unfriendly detectors such as mass spectrometers or devices such as those based on microwave-induced plasma remains unexplored. 

A flow-injection online preconcentration and separation system with microwave-assisted desorption is proposed for the efficient determination of trace amounts of Pt by FA AS. A microcolumn packed with thiourea modified silica, placed in a microwave oven, was used to collect Pt from the sample solutions. The detection limit was 0.060 p,g/ml. The results obtained by the proposed method for three metalliu gical samples, a secondary nickel alloy, anode slim, and a C0CI2 solution, were in good agreement with the recommended... 

Different procedures based on that chemistry and using FIA [183], SIA [166,184], MSFIA [185,186], and MCFIA [187] have been described. Most of them make use of online sample preparation, including micellar-assisted preconcentration (cloud point extraction (CPE)) or solid-phase extraction (SPE) [185,186,188]. To this end, either a microcolumn packed with Amberlite XAD-4 resin [185] or C18-modified silica material may be used. 

Online sorptive preconcentration exploiting renewable solid surfaces, the so-called bead injection (Bl), in the miniaturized LOV platform has been hyphenated to GC separation with electron-capture detection for automated determination of trace levels of PCBs in solid-waste leachates. To this end, the MSFIA system presented in Chapter 3, Figure 3.19, has been implemented. It comprises a mul-tisyringe burette, an extra solenoid valve, an LOV, and an injection valve (IV) as well as reversed-phase copolymeric beads with hydroxylated surface (Bond Elut Plexa) included into the channels of a poly(ether imide) LOV microconduit, thus serving as a transient microcolumn-packed reactor for preconcentration of organic species [194]. 

Su and coworkers used silica gel with immobilized polymelamine, poly(amidoamine), or poly(propylenimine) dendrons as the microcolumn packing for the flow injection preconcentration and separation of traces of Pt(IV) ions for flame atomic absorption spectroscopy determination.Polyamine dendron-functionalized polystyrene was... 

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