Autosampler characteristics

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

One of the most basic characteristics of a compound is its molecular mass in this context, much of today s popularity of MS can be traced back to the simplicity of molecular mass readout from API spectra. A common experiment is flow injection analysis (FIA). Here, an autosampler is used to inject an aliquot of dissolved sample into a liquid stream, which is provided by an LC pump, to the MS-detector. FIA offers the advantages of easy automation and fast cycle times of about 30 s per sample. In order to increase throughput, multiprobe autosam-... 

Major manufacturers of HPLC instruments include Waters, Agilent (formerly Hewlett Packard), and Shimadzu, PerkinElmer, Thermo, Beckman, Varian, Hitachi, Jasco, Dionex, Gilson, Scientific Systems (SSI), and Isco. The Internet addresses of these companies can be found in the reference section. HPLC is a mature technology and most manufacturers have highly reliable products with sufficient performance and feature sets to be competitive in the market place. However, there can still be significant differences between the vendors on these performance characteristics on systems (dwell volume, dispersion), pumps (low flow, seal life), autosamplers (carryover, speed, sample capacity, minimum sample volume), and detectors (sensitivity, gradient baseline shift). 

This chapter provides an overview of modern HPLC equipment, including the operating principles and trends of pumps, injectors, detectors, data systems, and specialized applications systems. System dwell volume and instrumental bandwidth are discussed, with their impacts on shorter and smaller diameter column applications. The most important performance characteristics are flow precision and compositional accuracy for the pump, sampling precision and carryover for the autosampler, and sensitivity for the detector. Manufacturers and selection criteria for HPLC equipment are reviewed. 

The application of internal standards or the use of the standard addition method for quantitation is strongly recommended for achieving low relative standard deviations (Nielsson etal., 1994 Poerschmann etal., 1997). After the equilibrium is established, the fibre with the collected analytes is withdrawn from the sample and transferred into a GC injector, either manually or more convenient via an autosampler. The analyte is desorbed thermally in the hot injector from the coating. The fibre material is used for a large number of samples in automated serial analysis. Modern autosampler are capable to exchange the fibre holder to provide automated access to different fibre characteristics according to the analyte requirements. 

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