Off-line SPE

Sandra and David [37] have reported on validation studies for SHS-HSGC, Py-HSGC, on-line LVI-GC, on-line SPME-GC, on- and off-line SPE-GC, on-line and off-line derivatisation-GC, SBSE-TD-GC, and PTV-LC(SEC)-GC. 

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... 

Principles and Characteristics Off-line SPE utilises disposable cartridges or discs containing 50 to 2000 mg... 

Applications With the current use of soft ionisation techniques in LC-MS, i.e. ESI and APCI, the application of MS/MS is almost obligatory for confirmatory purposes. However, an alternative mass-spectrometric strategy may be based on the use of oaToF-MS, which enables accurate mass determination at 5 ppm. This allows calculation of the elemental composition of an unknown analyte. In combination with retention time data, UV spectra and the isotope pattern in the mass spectrum, this should permit straightforward identification of unknown analytes. Hogenboom et al. [132] used such an approach for identification and confirmation of analytes by means of on-line SPE-LC-ESI-oaToFMS. Off-line SPE-LC-APCI-MS has been used to determine fluorescence whitening agents (FWAs) in surface waters of a Catalan industrialised area [138]. Similarly, Alonso et al. [139] used off-line SPE-LC-DAD-ISP-MS for the analysis of industrial textile waters. SPE functions here mainly as a preconcentration device. 

Drugs of abuse Drinking water Off-line SPE (Oasis HLB)... 

Features that make online SPE more attractive compared to off-line SPE consist of ... 

Extraction of estrogens and progestagens is mostly performed by off-line SPE (on-line SPE has been reported by Lopez de Alda et al. [60]), using either disks or, more frequently, cartridges. Octadecyl (C18)-bonded silica in both cartridge [7,57] and disk [36] format, graphitized carbon black [35,58,59],Isolut... 

In response to off-line SPE assays in the 96-well plate format, LC/MS analyses that feature the direct injection of plasma with on-line extraction have been reported (Ayrton et al., 1997 Needham et al., 1998). This quantitative process approach eliminates the time-... 

Two articles have described analytical methods for the determination of sorafenib plasma concentrations, involving a similar protein precipitation step with acetonitrile [115, 116], There is at present only one published method for the quantification of lapatinib in human plasma after off-line SPE onto C18 cartridge, followed by evaporation and reconstitution in ACN/5.0 mM ammonium formate pH 3/formic acid (1,000 50 1, v/v/v) [117],... 

The assessment can also be done by an off-line SPE procedure where the cumulative recovery curves of the sorbents are evaluated in different solvents and at different pHs. A suitability test for propranolol imprinted sorbents, to be used in SPE of plasma and urine samples, was developed by Olsen et al. [34]. This was based on the solvent composition required to give a recovery of either 20 or 50% of the extracted material (ES20 or ES50 values) on the blank and the imprinted cartridge. The method was developed in order to allow a rapid estimate of the specific and non-specific contributions to overall binding. 

Graphitized carbon black (GCB) materials are frequently used for off-line SPE as well. GCB behaves as a nonspecific sorbent, but because positively-charged adsorption sites are present, it also acts as an anion exchanger, which is useful for the enrichment of acidic compounds [75]. Examples involve the enrichment of chlorinated phenoxy acids [42], aryloxyphenoxypropionic herbicides [45], sulfonylurea herbicides [50], and imidazolinone herbicides [65]. 

The quantitative analysis of the metabolites in groundwater is an important application of LC-MS. Vargo et al. [121-122] described a method based on off-line SPE and LC-MS-MS, enabling the detection at 0.1-ppb level in groundwater. 

The analysis of some A -methylcarbamates and their metabolites in human urine by off-line SPE, LC-UV-DAD and confirmation by ESI LC-MS [141]. 

A. Lagana, G. Fago, A. Marino, Simultaneous determination of imidazolinone herbicides from soil and natural waters using soil column extraction and off-line SPE followed by LC with UVdetection orLC-ESI-MS, Anal. Chem., 70 (1998) 121. 

Benijts et al. [19] applied negative-ion ESI on an ion-trap system for the determination of the estrogens estrone, estradiol, estriol, ethinyl estradiol, and diethylstilbestrol in enviromnental water samples. With manual off-line SPE on 50-ml samples, the deteetion hmits ranged from 3.2 to 10.6 ng/1. 

An important feature and advantage of on-line SPE, compared with off-line SPE, is direct elution of the analyte from the SPE cartridge into the mobile phase of the LC system. The time-consuming off-line steps of evaporation, reconstitution, and preparation for injection are eliminated and make the online SPE more efficient and fully automated. Since the entire volume of eluate is analyzed, maximum sensitivity for detection is obtained. Some other advantages of this on-line approach include ... 

Many methods are available for removal of organic material from aqueous samples by off-line SPE [7]. Hydrophobic organic material is best extracted by solid poly(sty-rene-DVB) polymers or reversed-phase silica extractants. Polyvinylpyrrolidone (PVP) is an appropriate choice for removal of humic acids, lignins and tannins from water samples. 

The SPE process can be performed either on-line or offline. The procedure using SPE cartridges with a vacum manifold is known as off-line SPE, and the eluate from the cartridge is introduced into the chromatographic instrument after concentration and reconstitution of sample extracts. In... 

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