Injection technique, large-volume

One of the main restricting factors in the remediation of soils is the low aqueous solubility of several organic compounds. It is connected with a resistance to mobilization by conventional pump-and-treat measures. Therefore, the residual concentration of mineral oils in sandy soils is in the range of 3—20 g/kg soil [1]. To further reduce this concentration by conventional extraction-injection techniques large volumes of water have to be treated to remove small quantities of contaminants over a period of years [2]. 

The small dimensions associated with CE preclude the injection of large volumes. The sample may be introduced to the capillary either by a diplacement technique (i.e., pressure, vacuum, or siphoning) or via electrokinetic injection. The majority of commercial instruments apply a pressure differ-... 

Grob, K. On-column injection of large volumes using the retention gap technique in capillary gas chromatography. J. Chromatogr. 1985, 334, 129-155. 

HPLC advantages Amenable to thermally labile, nonvolatile, and polar components Rapid and automated analysis of complex samples Precise and quantitative (with spectral confirmation possible) Quantitative recovery (good sample preparation technique) Sensitive and selective detection (UV, FL, MS) Can tolerate injections of large volumes of aqueous samples (0.5-1 mL)... 

The main aims in environmental analysis are sensitivity (due to the low concentration of microcontaminants to be determined), selectivity (due to the complexity of the sample) and automation of analysis (to increase the throughput in control analysis). These three aims are achieved by multidimensional chromatography sensitivity is enhanced by large-volume injection techniques combined with peak compression, selectivity is obviously enhanced if one uses two separations with different selectivi-ties instead of one, while on-line techniques reduce the number of manual operations in the analytical procedure. 

Several sample preparation techniques are performed inside the inlet system. Large-volume injection can be carried out by a number of methods including programmed temperature vaporisation (PTV). Automated SPE may be interfaced to GC using a PTV injector for large volume injection. SPE-PTV-GC with on-column injection is suited to analysis of thermola-bile compounds. 

The traditional sample volumes in CGC (0.1-3 xL) limit sample preparation possibilities. Specialised large-volume injection (LVI) techniques are designed to load more sample in the GC system (typically 150 xL) by placing a length of uncoated fused-silica tubing in front of the analytical column. This procedure also provides... 

Principles and Characteristics Although early published methods using SPE for sample preparation avoided use of GC because of the reported lack of cleanliness of the extraction device, SPE-GC is now a mature technique. Off-line SPE-GC is well documented [62,63] but less attractive, mainly in terms of analyte detectability (only an aliquot of the extract is injected into the chromatograph), precision, miniaturisation and automation, and solvent consumption. The interface of SPE with GC consists of a transfer capillary introduced into a retention gap via an on-column injector. Automated SPE may be interfaced to GC-MS using a PTV injector for large-volume injection [64]. LVI actually is the basic and critical step in any SPE-to-GC transfer of analytes. Suitable solvents for LVI-GC include pentane, hexane, methyl- and ethylacetate, and diethyl or methyl-f-butyl ether. Large-volume PTV permits injection of some 100 iL of sample extract, a 100-fold increase compared to conventional GC injection. Consequently, detection limits can be improved by a factor of 100, without... 

Various authors have described on-line LC-SFC coupling [947,948]. Coupling of LC to SFC with conventional-size LC columns, where only a small fraction of the peak of interest is transferred to the SFC, allows only for qualitative results, and does not address the need for improved sensitivity in cSFC. Cortes et al. [948] have described relatively large-volume sample introductions (>10 xL) into cSFC, using microcolumn LC in the first dimension. LVI-LC-cSFC provides enhanced sensitivity compared with conventional cSFC injection techniques. LC-cSFC is expected to be of utility in the characterisation of complex samples, and in the determination of components which are thermally labile do not contain significant chromophores or do not have sufficient volatility to be analysed by GC. 

Applications If an extract needs further cleanup, it is possible to couple it with multidimensional chromatographic techniques such as LC-LC or LC-GC. The first chromatographic step can then be used for the on-line cleanup and concentration of the extract, and the second one for the final separation. Large-volume, on-column injection (LVI-COC) is particularly useful for coupled LC-GC in which 100-350 xL fractions of eluent from the NPLC cleanup separation step are transferred on-line to the GC column. For example, on-line removal of high-MW interfering material, such as polymers from a polymer/additive dissolution, can be achieved easily by using SEC before the fraction containing additives is transferred to the GC. 

Using nano LC-MS at submicroliter per minute flow rates requires special attention to plumbing, system dead volume, valve switching, large volume sample injection, precolumn methodology, automation, online sample clean-up, and multichannel parallel operation of a single MS. The techniques discussed below are particularly useful for nano LC-MS-MS applications. 

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