Automation provides a better utilization of laboratory resources, unattended and out-of-hours operation and improved precision compared with manual methods. Common approaches to automation differ significantly. Using robotics, samples are processed

(usually) in a similar manner to manual methods. Using flow-processing schemes, samples are extracted in parallel with computer or microprocessor control of solvent management. Sorbent conditioning, sample condition, solvent selection, rinse and elution steps are performed automatically and can be varied for method development. Positive displacement instead of suction is used for solvent control, and advanced units can be programmed to replace sorbent cartridges to increase sample throughput and inject extracts into different chromatographic instruments. On-line analysers with a direct coupling to chromato-graphic instruments are widely used. Solid-phase extraction using short precolumns and a switching valve interface is a routine method for analysis by liquid chromatography. Advanced systems even allow programmed replacement of the sorbent cartridges and unattended 24-hour operation. The recovery and separation steps of purge-and-trap and sorbent trapping of volatile organic compounds from air are easily automated using thermal desorption with cold trapping, if required, for the direct injection of analytes into a gas chromatograph. Major strides have been made in the on-line solid-phase extraction of water samples with solvent desorption directly into a gas chromatograph. This method is not far from becoming routine today.

Table 5 Experimental variables that influence recovery of analytes by solid-phase extraction

• Conditioning solvent (typically 3-5 bed volumes)

n Ensures reproducible retention and flow. Critical step for particle-loaded membranes n Helps to minimize contamination of extracts by sorbent impurities n Replace by sample solvent before processing sample

n More critical for cartridges than discs due to their variable and heterogeneous packing density (channelling) n More critical when the sample volume exceeds the breakthrough volume as typical sampling devices provide too few theoretical plates for flow-independent retention

• Sample properties n Dilute viscous samples with a weak low viscosity solvent to reduce sample processing time n Remove excessive particle matter by filtration or centrifugation to maintain a constant sample-processing rate. Concentrated hydrochloric acid is effective for dissolving inorganic particles in water samples n Add small volume of organic solvent (1 -3% v/v) to large volume water samples to ensure sorbent remains solvated and to maintain a constant (fast) sample-processing rate. Important for particle-loaded membranes n Adjust pH to reduce ionization of weak acids and bases for reversed-phase sampling n Maintain ionic strength approximately constant for samples and standards with reversed-phase sampling conditions. Ionic strength is a critical parameter for ion-exchange extraction n Deproteination of biofluids may be required for acceptable recovery of low molecular weight analytes for reversed-phase sampling n Precipitation of inorganic acids (sulfate, phosphate, etc.) by barium hydroxide is sometimes required for acceptable recovery of organic acids from biofluids using ion-exchange extraction

• Drying time (typically 1-5 min, but sometimes considerably longer)

n Sufficient to remove all sample solvent trapped in the sorbent pores n Excessive drying may result in low recovery of analytes from evaporation or retention in poorly solvated regions of the sorbent

• Rinse solvent (optional)

n Small volume of intermediate strength solvent to elute matrix components. Analytes remain immobilized on the sorbent n Biological fluids, plant extracts and soil extracts often require a rinse step but surface waters may not

• Eluting solvent (ideally 2-3 bed volumes but often larger)

n Should be a strong solvent able to displace all analyte from the sorbent in a small volume n Should normally be volatile and miscible with the sample solvent

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook

Post a comment