Autosampler operating principles

This chapter presents an overview of current trends in high-pressure liquid chromatography (HPLC) instrumentation focusing on recent advances and features relevant to pharmaceutical analysis. Operating principles of HPLC modules (pump, detectors, autosampler) are discussed with future trends. 

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. 

SPME is a patented sample preparation method for GC applications (32-36). The solvent-free technique was developed in 1989 by Janusz Pawliszyn (http. /Avww.science.uwaterloo.ca/ -janusz/spme.html) at the University of Waterloo in Ontario, Canada, and a manual device made by Supelco, Inc. has been available since 1993. In 1996, Varian Associates, Inc., constructed the first SPME autosampler. SPME involves exposing a fused silica fiber that has been coated with a non-volatile polymer to a sample or its headspace. The absorbed analytes are thermally desorbed in the injector of a gas chromatograph for separation and quantification. The fiber is mounted in a syringe-like holder which protects the fiber during storage and I netration of septa on the sample vial and in the GC injector. This device is operated like an ordinary GC syringe for sampling and injection. The extraction principle can be described as an equilibrium process in which the analyte partitions between the fiber and the aqueous phase. 

The heart of an autosampler is the six-port valve as well. Its loop can be filled partially or totally as in manual operation. Flushing steps are necessary between the individual samples but they are part of the automation routine. The different liquids (sample, mobile phase, flush solvent) can be separated from each other by the aspiration of air bubbles. Different working principles (and variations thereof) are in use. 

Autosamplers work using the same switching valve principles as the manual system. A metered syringe draws a sample through a needle into a loop, the rotor turns against the stator face, and the sample is flushed onto the column. Carryover of the sample from the needle and from the switching valve can be an issue. Many manufacturers now operate a continuous flow system whereby mobile phase is continually pumped through the loop and sample needle when the system is operational. 

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