Solid desorption/injection

Gas chromatography is based on the distribution of a compound between two phases. In gas-solid chromatography (GSC) the phases are gas and solid, the injected compound is carried by the gas through a column filled with solid phase, and pmrtitioning occurs via the sorption-desorption of the compound (probe) as it travels past the solid. Superimposed upon the forward velocity is radial notion of the probe molecules caused by random diffusion through the stationary phase. Separation of two or more components injected simultaneously occurs as a result of differing affinities for the stationary phase. In gas-liquid chromatography (GLC), the stationary phase is a liquid coated onto a solid suppx>rt. The mathematical treatment is equivalent for GLC and CSC. 

Solid-phase microextraction (SPME) consists of dipping a fiber into an aqueous sample to adsorb the analytes followed by thermal desorption into the carrier stream for GC, or, if the analytes are thermally labile, they can be desorbed into the mobile phase for LC. Examples of commercially available fibers include 100-qm PDMS, 65-qm Carbowax-divinylbenzene (CW-DVB), 75-qm Carboxen-polydimethylsiloxane (CX-PDMS), and 85-qm polyacrylate, the last being more suitable for the determination of triazines. The LCDs can be as low as 0.1 qgL Since the quantity of analyte adsorbed on the fiber is based on equilibrium rather than extraction, procedural recovery cannot be assessed on the basis of percentage extraction. The robustness and sensitivity of the technique were demonstrated in an inter-laboratory validation study for several parent triazines and DEA and DIA. A 65-qm CW-DVB fiber was employed for analyte adsorption followed by desorption into the injection port (split/splitless) of a gas chromatograph. The sample was adjusted to neutral pH, and sodium chloride was added to obtain a concentration of 0.3 g During continuous... 

Diffusive sampler Membrane extraction (MESI) Liquid-liquid extraction (LLE) Solid-phase extraction (SPE) SPE-PTV-GC Solid-phase microextraction (SPME) Headspace GC (SHS, DHS) Large-volume injection (LVI) Coupled HPLC-GC Membrane extraction (MESI) Difficult matrix introduction (DMI) Conventional solvent extraction methods 1 Pressurised solvent extraction methods Headspace GC (SHS, DHS) Thermal desorption (TD, DTD) Pyrolysis (Py) Photolysis Photon extraction (LD) Difficult matrix introduction (DMI)... 

Principles and Characteristics As mentioned already (Section 3.5.2) solid-phase microextraction involves the use of a micro-fibre which is exposed to the analyte(s) for a prespecified time. GC-MS is an ideal detector after SPME extraction/injection for both qualitative and quantitative analysis. For SPME-GC analysis, the fibre is forced into the chromatography capillary injector, where the entire extraction is desorbed. A high linear flow-rate of the carrier gas along the fibre is essential to ensure complete desorption of the analytes. Because no solvent is injected, and the analytes are rapidly desorbed on to the column, minimum detection limits are improved and resolution is maintained. Online coupling of conventional fibre-based SPME coupled with GC is now becoming routine. Automated SPME takes the sample directly from bottle to gas chromatograph. Split/splitless, on-column and PTV injection are compatible with SPME. SPME can also be used very effectively for sample introduction to fast GC systems, provided that a dedicated injector is used for this purpose [69,70],... 

Acid-base equilibrium is very important to inorganic chemical reactions. Adsorption-desorption and precipitation-dissolution reactions are also of major importance in assessing the geochemical fate of deep-well-injected inorganics. Interactions between and among metals in solution and solids in the deep-well environment can be grouped into four types1 2 3 4 ... 

In the 1990s, Pawliszyn [3] developed a rapid, simple, and solvent-free extraction technique termed solid-phase microextraction. In this technique, a fused-silica fiber is coated with a polymer that allows for fast mass transfer—both in the adsorption and desorption of analytes. SPME coupled with GC/MS has been used to detect explosive residues in seawater and sediments from Hawaii [33]. Various fibers coated with carbowax/divinylbenzene, polydimethylsiloxane/divinylbenzene, and polyacrylate are used. The SPME devices are simply immersed into the water samples. The sediment samples are first sonicated with acetonitrile, evaporated, and reconstituted in water, and then sampled by SPME. The device is then inserted into the injection port of the GC/MS system and the analytes thermally desorbed from the fiber. Various... 

Solid phase microextraction (SPME) has been shown to be useful for the determination of chloroform in air (Chai and Pawliszyn 1995). This technique is based upon the absorption of chloroform into a polymer coated on a silica liber. Following equilibration of the liber with the atmosphere, chloroform is released via thermal desorption in the injection port of a gas chromatograph. Sample preparation is... 

Groundwater, liquid, and solid matrices Direct injection of head-space gas (EPA method 5020) or purge-and-trap preconcentration and thermal desorption (EPA method 5030) GC/HSD (EPA method 8010) 0.5 pg/L (ppb, w/v) for groundwater, 0.5 pg/g (ppm, w/w) for low-level soil, 500 pg/L (ppb, w/v) for water-miscible liquid waste, 1,250 pg/g (ppm, w/w) for soil, sludge, and non-water-miscible waste Water 0.93C -0.39 where C = true concentration in pg/L. EPA1986a... 

Argon is analyzed by mass spectrometry (characteristic ion m/z 40) or by gas-solid chromatography. Its concentration can be increased by several times by selective adsorption over a suitable adsorbent followed by thermal desorption of the gas onto the GC injection port. 

The most common technique in determining desorption efficiency is to inject the compound or a solution of the compound directly into the solid sorbent (13). The mixture is allowed to stand overnight and then desorbed and analyzed. Gases and highly volatile compounds are usually introduced as a mixture in air or nitrogen from a SARAN film bag or a cylinder. 

You need to decide the goal of an analysis before developing a chromatographic method. The key to successful chromatography is to have a clean sample. Solid-phase microextraction, purge and trap, and thermal desorption can isolate volatile components from complex matrices. After the sample preparation method has been chosen, the remaining decisions for method development are to select a detector, a column, and the injection method, in that order. 

Solid-phase microextraction (SPME). used as a sample introduction technique for high speed gc, utilizes small-diameter fused-silica fibers coated with polymeric stationary phase for sample extraction and concentration. SPME lias been utilized for determination of pollutants in aqueous solution by the adsorption of analyte onto stationary-phase coated fuscd-silica fibers, followed by thermal desorption in the injection system of a capillary gas chromatograph. Full automation can be achieved using an autosampler. 

The subtraction curve in Fig. 3A (solid) is compared with that obtained by ramping the same column load twice with no probe, and then a third time after injecting the basic probe (dotted curve, Fig. 3B). The near congruence of the curves resulting from the first and third ramps demonstrates that acidic sites are part of the glass structure (silanols and silicates) and are not susceptible to desorption in this temperature range. 

NT OTME Needle trap Open-tubular microextraction Device with a hypodermic needle, whose tip is filled with a solid adsorbent onto which the sample is adsorbed. For desorption, which takes place inside the GC injection port, the carrier gas flow is forced through the needle, entering it through a hole at its side As in-tube (micro)extraction Similar application profile as HS-SPME however, the NT device is more robust and also has a higher capacity, potentially allowing exhaustive extraction. Also useful in P T analysis... 

These semi-preparative methods are useful where identification is required but for quantitative and comparative analytical purposes much more rapid sampling techniques, such as automated headspace and solid phase microextraction (SPME), may be preferred. Both of these techniques give similar results for most volatiles. In the former, the vapour above a heated sample is removed by a syringe or gas flushing and injected onto a GC column, either directly or after trapping on a suitable absorbent and thermal desorption. In SPME, the vapour is absorbed on to a suitable bonded medium on a special needle and then injected into the gas chromatogram. 

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