Preconcentration stationary phase

H. Irth, R. Tocklu, K. Welten, G. J. de Jong, U. A. Th Brinkman and R. W. Frei (1989), Trace-level determination of 3 -azido-3 -deoxythymidine in human plasma by preconcentration on a silver (I)-thiol stationary phase with on-line reversed-phase high-performance liquid chromatography , J. Chromatogr. 491 321 -330 (1989). 

A programmed temperature-vaporization (PTV) injector (with a sorbent-packed liner) was used to preconcentrate and inject the sample. Thermal desorption was performed and the analytes were passed to a primary column (16 m X 0.32 mm i.d., film thickness 5 p.m, 100% methyl polysiloxane) and separated according to analyte vapour pressure. Selected heart-cuts were transferred to a second column (15 m X 0.53 mm i.d., Al203/Na2S04 layer, open tubular column with 10 (im stationary phase) where final separation was performed according to chemical functionality. 

The sensitivity of the technique depends mainly on the value of the partition coefficient of the analytes partitioned between the sample and the liber stationary phase. The efficiency of preconcentration depends on both the type of liber used and its thickness (amount). This type of liber affects the amount and character of the sorbed species.51 The general rule like dissolves like applies here, that is, polar compounds are sorbed on polar libers, and nonpolar compounds on nonpolar libers. A broad range of standard libers is commercially available. 

Solid-phase microextraction (SPME) — is a procedure originally developed for sample preconcentration in gas chromatography (GC). In this procedure a small-diameter fused silica optical fiber, coated with a liquid polymer phase such as poly(dimethylsiloxane), is immersed in an aqueous sample solution. The -> analytes partition into the polymer phase and are then thermally desorbed in the GC injector on the column. The same polymer coating is used as a stationary phase of capillary GC columns. The extraction is a non-exhaustive liquid-liquid extraction with the convenience that the organic phase is attached to the fiber. This fiber is contained in a syringe, which protects it and simplifies introduction of the fiber into a GC injector. Both uncoated and coated fibers with films of different GC stationary phases can be used. SPME can be successfully applied to the analysis of volatile chlorinated organic compounds, such as chlorinated organic solvents and substituted benzenes as well as nonvolatile chlorinated biphenyls. 

The term solid-phase extraction was introduced by personnel of the J. T. Baker Company in 1982. The method consists of retention of the analytes from a liquid or gaseous sample to a solid stationary phase and subsequent removal of analytes using an appropriate eluent. The main purpose of SPE is isolation and preconcentration of compounds of interest or sample clean-up and simplification of the matrix. Application of this sample preparation technique also allows extract fractionation. As a result of significant reduction in the volume of organic solvents used, high recovery, and the possibility of process automation, SPE is a good alternative for conventional liquid-liquid extraction. According to their affinity for the compound of interest, stationary phases are classified as follows ... 

Countercurrent chromatography has been mainly developed and used for preparative and analytical separations of organic and bio-organic substances [1], The studies of the last several years have shown that the technique can be apphed to analytical and radiochemical separation, preconcentration, and purification of inorganic substances in solutions on a laboratory scale by the use of various two-phase liquid systems [2], Success in CCC separation depends on choosing a two-phase solvent system that provides the proper partition coefficient values for the compounds to be separated and satisfactory retention of the stationary phase. The number of potentially suitable CCC solvent systems can be so great that it may be difficult to select the most proper one. 

It was demonstrated that systems with DEHPA can extract and preconcentrate Zr(IV), Hf(IV), and Nb(V) into the organic stationary phase and can separate them from the majority of other elements [I]. The preconcentration of Zr(IV) and Hf(IV) and subsequent back-extraction, as well as the selective extraction of Zr(IV), Hf(IV), Nb(V), and Ta(V) into a 4-mL volume of the organic phase can be performed with a mixture of DEHPA and BPHA. 

On the other hand, the capability of sample preconcentration for instruments such as AAS, ICP-AES, ICP-MS, and so forth was studied [3]. After metal ions were enriched, they were eluted almost simultaneously by inorganic acid at low pH, because of their diffusion in the column is at a disadvantage for improvement of the detection limits. It has been demonstrated that metal ions such as Ca, Cd, Mg, Mn, Pb, and Zn were enriched with a good recovery at a concentration of 10 ppb each in 500 mL of the sample solution. However, the final enriched sample volume eluted from the CCC column was as large as several milliliters, due to longitudinal diffusion of the sample band in the retained stationary phase [1,3]. Additional band spreading occurred in the flow tube when the concentrated solution was eluted with an acid solution for subsequent analysis. 

Naturally-occurring humic-metal complexes have been isolated from estuarine systems and seawater using solid phase extraction (SPE) onto a Cig HPLC column to preconcentrate the sample (JO-12). Samples were subsequently eluted from the SPE colunm at a much higher concentration and injected onto another HPLC column and detected by UV absorbance and a metal-sensitive detector, such as atomic fluorescence spectroscopy. The concentration of metal-humic complexes in natural aquatic environments was then calculated. However, there was some evidence of competitive binding of the metal ion between the organic matter and free silanol groups in the stationary phase resulting in a loss of metal in the column and erroneously low metal values (10). 

The introduction of microprocessor technology, in connection with modem stationary phases of high chromatographic efficiencies, makes it a routine task to detect ions in the medium and lower ppb concentration range without pre-concentration. The detection limit for simple inorganic anions and cations is about 10 ppb based on an injection volume of 50 pL. The total amount of injected sample lies in the lower ng range. Even ultrapure water, required for the operation of power plants or for the production of semiconductors, may be analyzed for its anion and cation content after preconcentration with respective concentrator columns. With these pre-concentration techniques, the detection limit could be lowered to the ppt range. However, it should be emphasized that... 

The efficiency and selectivity of fuUerenes as adsorbents from aqueous solutions has resulted in a number of analytical applications of C g and C g as chromatography stationary phases, as chemical sensors and, especiaUy, as sorbents for the preconcentration of analytes. In the latter case, the adsorption properties of fuUerenes are more useful for inorganic and organometaUic compounds than for organic compounds. On the other hand, the fuUerenes exhibit a selectivity for aromatic compounds and planar molecules that makes them very attractive as stationary phases for liquid chromatography. 

When large volumes of water are sampled, the extraction of analytes is directly performed in the field water is passed through a cartridge containing a suitable stationary phase, for instance XAD-2 resin. Generally, before the preconcentration cartridge, a filtering system, with a pore size lower than 1 /rm, is positioned, and the particulate matter recovered is stored and analyzed separately. 

Several techniques have been used to overcome the problem of low column loadings on capillary columns. Capillary columns have been used after preconcentration of the alkylderivatives on a wide-bore fused-silica column or by solid-phase microextraction (SPME). " Large volume injection techniques have been applied on capillary columns coated with 0.25 /rm DB-5. Multicapillary GC (MCGC) (919 capillaries, 1 m X 40/rm i.d. coated with 0.2/rm SE 30 stationary phase (Alltech)) coupled to allows column loadings and carrier gas flow... 

Flame Photometric Detector (PFPD) or an ICP-MS. The various sample treatment steps, liberation of the compounds, their derivatization and preconcentration via headspace on to a SPME phase, all occur in the same vial, hmiting contamination and loss risks. SPME is a solvent-free sample preparation method in which a fused-sUica fiber coated with a polymeric organic stationary phase is used to extract organic compounds directly from aqueous or gaseous samples.Further GC separation of the compounds and MS, FPD, or ICP-MS detection allows very sensitive determinations. This method will be further referred to as the SPME method. 

A 100-ml intake was used for analysis. Extraction was performed with 50 ml NaOH and 2 ml sodium acetate/acetic acid (2 mol 1 ) in 10 ml hexane. Cleanup was carried out using silica gel, followed by preconcentration over a N2 stream to a volume of 1 ml. Derivatization was performed with 10% NaBE4 in acetic acid at pH 4. Separation was by CGC (column of 30 m length, 0.32 mm internal diameter, DB-5 as stationary phase, 0.25-/rm film thickness He as carrier gas air/H2 as make-up gases injector temperature at 80°C). Detection was by QFAAS at 283.3 nm (detector temperature at 750°C). Calibration was by standard additions, using M3PbCl provided by SM T. 

SPME was hrst used by Pawliszyn et al. in 1990. It is a two-step process conductive to the simultaneous extraction and preconcentration of analytes form sample matrices. In the first step, a fused-silica fiber coated with a polymeric stationary phase is exposed to the sample matrix where the analyte partitions between the matrix and the polymeric stationary phase. In the second step, the fiber/analyte is transferred to the analytical instrument for desorption, separation, and quantification. SPME has a number of advantages over traditional extraction techniques for pesticides. In fact, it is fast, simple, solvent-free, and easily automated for both GC and HPLC instruments. It exhibits good linearity and sensitivity. Thus, carbamate and organophosphorus pesticides in golf course samples were successfully extracted by SPME and analyzed by HPLC by Jinno et al. ... 

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