Solid direct sample injection

There are basically three methods of liquid sampling in GC direct sampling, solid-phase extraction and liquid extraction. The traditional method of treating liquid samples prior to GC injection is liquid-liquid extraction (LLE), but several alternative methods, which reduce or eliminate the use of solvents, are preferred nowadays, such as static and dynamic headspace (DHS) for volatile compounds and supercritical fluid extraction (SFE) and solid-phase extraction (SPE) for semivolatiles. The method chosen depends on concentration and nature of the substances of interest that are present in the liquid. Direct sampling is used when the substances to be assayed are major components of the liquid. The other two extraction procedures are used when the pertinent solutes are present in very low concentration. Modem automated on-line SPE-GC-MS is configured either for at-column conditions or rapid large-volume injection (RLVI). 

Section II covers the latest trends in reducing sample preparation time, including direct sample infusion/injection and on-line solid phase extraction (SPE). In Section III, we focus on newer trends in stationary phases and how these phases hope to offer different selectivities compared to current CIS-based phases. Section IV briefly provides a few observations on how new detectors are increasing the versatility of HPLC. Finally, in Section V we examine monolithic columns, small particles packed in short columns, high-temperature LC, ultra high-pressure LC, and parallel injection techniques. 

The sample preparation techniques described in the Basic and Alternate Protocols require 1 to 3 hr prior to sample injection into the HPLC. No additional time for sample cleanup is necessary if the sample is directly injected into the HPLC column without solid-phase extraction. 

Soils, sediments, or solid wastes mixed with water and subjected to purge and trap concentration aqueous extract may directly be injected onto GC sample/extract analyzed as above. 

While it is wonderful to be able to inject neat samples directly, sample preparation can often improve selectivity and sensitivity. If the resolution is poor, the salt content of the sample too high, or the capillary fouls, consider a sample cleanup. This can include liquid-liquid extraction, solid-phase extraction, supercritical fluid extraction, protein precipitation, or dialysis, depending on the solutes and application [38]. The final sample diluent should be a solution that is CE-Mendly. That usually means low ionic strength compared to the BGE. 

Calibration is carried out using standard calibration curves. The simplicity, repeatability, and low cost of the method have allowed its use for routine determination of trihalomethanes in tap water. SOME has also been compared with solid phase microextraction (SPME), purge and trap (P T), and direct aqueous injection (DAI) [10]. This technique offers accuracy comparable with that obtained using P T and DAI. With respect to conventional LEE, the SDME method is more accurate. In contrast to DAI and P T, it requires no special equipment. SDME has been used for extraction of chlorophenols [II], pesticides [12, 13], warfare agents [14], and butanone derivatives [15], and for control of food products [16]. The low costs of the SDME method (typical GC syringe and 2-3 pL of solvent), simplicity, and short extraction time (approximately 15 min) make it particularly suitable for preliminary analyses of organic pollutants in water samples. It can also be an effective alternative to SPME, as it does not require the use of expensive instrumentation. 

For many analytical methods using HPLC as the end step, the samples presented to the analyst cannot be injected directly into the instrument. For example, the sample may be a large volume of water where the analytes are present at low concentrations, a solid such as soil or foodstuffs, or a biological specimen where numerous other compounds are present. Sample preparation is therefore needed to isolate the analytes of interest, to pre-concentrate them in order to lower detection levels and also to protect the analytical column from substances which may potentially damage the bed of packing material. Despite many advances in instrumentation for HPLC, only rarely are samples (particularly complex samples) injected without some form of sample pre-treatment. 

The characterization of water-soluble components in slurries is one use of SPME with mixed solid-liquid samples. In one application, dried homogenized solid samples (10 mg of sewage sludge or sediment) were slurried in 4 ml of H,0 saturated with NaCl and adjusted to pH 2 with HCl for extraction for 1-15 h, which was followed by desorption into 4 1 methanol/ethanol over 2 min. The extracted compounds were either injected into a liquid chromatograph or fed directly via an electrospray ionization interface to a mass spectrometer with 1 s miz scans from 50-700 or selected-ion monitoring. The major components extracted included phthalates, fatty acids, non-ionic surfactants, chlorinated phenols and carbohydrate derivatives [235]. 

Because no separation is used, only crude information about the purity of the compounds can be obtained. For example, if unreacted, synthetic starting materials (Fig. 10b) are present in a sample of a combinatorial product (Fig. 10a), these can show up in the mass spectrum of the crude product (Fig. 10c). Because the starting materials are often structurally different from the finished product, a simple extraction can be used after the synthesis to remove much of the unused reactants and obtain a cleaner mass spectrum (Fig. lOd) for a solution-phase product. Conversely, washing the resin after solid-phase synthesis or using a scavenger resin in a solution-phase synthesis can also yield improved purity by removing these excess reactants. When direct flow injection is used to characterize combinatorial libraries, it is best to avoid dimethyl sulfoxide (DMSO) as a solvent, because it interferes with reliable ionization of the analytes. 

When using GC organic acids are typically transferred into an organic solvent followed by derivatization, which makes organic acids suitable for GC measurement. Exceptions are direct aqueous injections of water samples (e.g., wastewater) onto specialty GC columns (Section II.C.l). Transfer into organic solvent may be achieved by liquid-liquid extraction, evaporation at high pH and subsequent solvent addition, solid-phase extraction on anion exchange resins and subsequent elution with solvents, or aqueous derivatization followed by extraction (Section II.B.3.a). 

Direct sample introduction without any derivatization is used as an alternative to derivatization and is most commonly applied to VFA. GC introduction techniques include direct aqueous and solvent injections, ° " ° headspace, and more recently solid-phase microextraction (SPME) Specially designed capillary GC columns with polar phases are typically followed... 

The analytical chemistry of phenolic compounds was summarized in detail by several authors including [53,69,87,88]. From these articles it can be concluded that the extraction of phenolic acids can be achieved using methanol and ethanol (mostly with different proportions of water), ethyl acetate, acetone, chloroform, pressurized low-polarity water [89] or, rarely, with water itself. In some samples, snch as in wine or clear juices, direct HPLC injection can be applied after several minor alterations [90]. But in a majority of the studies, aqueous methanol or ethyl acetate was used as extraction solvent. As a general treatment, free phenolics were extracted with diethylether from the extract and the water phase was treated with HCl or in common with NaOH under nitrogen to liberate the insoluble-bound esterifled phenolic acids. The use of solid-phase cartridges is also another application to remove undesired compounds. To avoid losses in... 

Fig.6. Schematic drawing of the solid sample and reagent carrier used to monitor enzymatic activity. A gas permeable membrane was applied also directly over the IrTMOS. The reagent strip was 5 mm long and 3 mm wide at its widest part. The holes in the polyethylene test plate were 1.2 mm (sample injection) and 2.5 mm (for the activator strip) in diameter, respectively. The activator strip had a diameter of 2.2 mm.

Directly before injection of the sample, the sample must once again be filtered through a membrane filter (pore size 0A5 pm) in order to remove any solid matter present. 

The most often used and promising techniques for obtaining solutions for direct IC injection of food samples are accelerated solvent extraction (ASE), supercritical fluid extraction (SEE), solid-phase extraction (SPE), and UV photolysis and pyrolysis. The ASE technique basically employs the principles of traditional solvent extraction but at higher temperature and pressure, in which conditions solvents show better extraction properties. 

Solid-phase microextraction (SPME) is a solvent-free sample preparation technique. The volume of the extraction phase is very small compared to the sample volume. The extraction is not exhaustive, but is based on equilibrium between the sample and the extraction phase, which is located on a fiber. SPME involves an adsorption step of the analyte, from a gas headspace or in a liquid sample (direct immersion), and a desorption step, which often is coupled directly with injection in the analytical system. Although SPME is mainly used in combination with GC, it has also been automated for HPLC. Eigure 9.10 shows a schematic representation of an SPME device. 

Quantitation limits (defined as the monomer concentration necessary to produce a peak at least three times the baseline noise or 3% of full scale) for residual monomers (such as vinylchloride, butadiene, acrylonitrile, styrene and 2-ethylhexylacrylate) as low as 0.05 ppm have been reported for solution headspaee, as eompared to 1 ppm for direct solution injection GC [43], Quantification of volatiles by solid headspaee sampling can be challenging. Solid headspace provides about 10-fold more sensitivity than solution headspace. 

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