Time of sampling is very critical. Cooling, heating, poisoning or separating phases
may be a solution, but this cannot be generalised; it depends specifically on the actual
intentions. If enzyme activities are to be determined, heating may be the worst choice, if
just biomass-concentration is to be determined, immediate filtering may be the best
choice.
Flow injection analysis (FIA)FIA has been used for on-line determination of glucose or to estimate biomass directly or
indirectly by means of an extended Kalman filter (Valero et al., 1990). Chemical oxygen
demand (COD) from a waste water stream was determined using an FIA in the range of
30 to 23,000 mg lā^1 within only 3 to 7 min. FIA is therefore useful in environmental
sciences such as water monitoring and it is increasingly important in down stream
processing. FIA has been applied to detect micro-organisms indirectly by measuring the
concentration of a mediator which is reduced by the organisms. Amino acids, such as L-
lysine were measured and even intracellular enzymes can be determined on-line. Rapid
analyses of antibiotics have been realised by a combination of supracritical fluid
extraction and FIA. DNA and RNA have been quantified in extracts. Metabolic studies of
a lactic acid production based on glucose, lactose, galactose, lactate, and protein
determinations after nutrient pulses were made. Also by-products such as acetate have
been determined on-line with an FIA-technique. An important development is the
combination with cytometry or the estimation of nucleic acids.
More and more are biosensors used as detectors in FIA-systems. The drawbacks of
biosensors as direct in situ sensors, namely low dynamic range, inability to survive
sterilisation, limited life time, etc. are no longer valid ex-situ because the FIA interfaces
the biosensor which can be changed any time and the FIA can provide samples in optimal
dilution. The need for chemicals and reagents can be drastically reduced when employing
biosensors, in specific, when the entire system is miniaturised.
ChromatographyA review on chromatographic methods is beyond the scope of this contribution. Both,
liquid chromatography (LC) and gas chromatography (GC) have been applied in
numerous cases to off-line analyses of biotechnological samples but the on-line
application is only recently developing.
The scope of chromatographic methods is the separation of the individual constituents
of mixtures as they pass through columns filled with suitable stationary phases. The
retention in the column is determined by the interaction between the individual
constituents and the stationary phase. Miniaturised versions using micro machined
instruments or array detectors have recently been reported. A filter as an interface is
usually sufficient.
Mass spectrometryMS has been applied mainly for the on-line detection and quantification of gases such as
pO 2 , pCO 2 , pN 2 , pH 2 , pCH 4 and H 2 S or volatiles (alcohols, acetoin, butanediol). The
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