Sampling and Detection Strategies

Once on the chip, the next step is usually some sort of treatment of the sample such that it is made suitable for analysis and detection. Even aqueous samples should be filtered prior to assay. Typically, sample pretreatment takes the form of various fluid handling steps such as concentration flltration , extractions s , ion exchanges and desaltingS. Combinations and permutations of these steps are also reported. Even degassing can be achieved on chip using ultrasonic-induced cavitations.  

There are many reports in the literature about interfaces and devices for preparation of blood samples on chip. Eor example, Jandik et al. used a laminar fluid diffusion interface to replace centrifugation and consequently reduced the preparation time from between 30-60 minutes to only five minutes . The H-filter has also been used to clean up biological samples. A sample, e.g. blood, is put into a reservoir at one end of one post of the H, and a diluent such as water or saline is placed in the reservoir at the other end. The two parallel laminar streams will flow along the crossbar of the H as a resnlt of hydrostatic pressnre. Smaller, more mobile analyte molecnles will cross the interface between streams qnickly, whilst heavier particles remain in the carrier stream. Conseqnently, by controlling the fluid velocity and the length of the channel, the process can be optimised. There are also devices for separating plasma from blood on a chip .  

Detection can take place withont separation (specific detection) or after separation (nonspecific detection). There are very few specific chip-based sensors that detect only one component to the exclusion of all others. It is more common to have some pretreatment or separation of the sample prior to detection. 

Spectroscopic techniques are popular as a means of detection on chips. Examples include the determination of flavins and DNA by fluorescence. Spectrophotometric techniques are often used for biological samples . Mass spectrometry has also been used. Benetton et al. coupled electrospray ionisation MS to a chip while Sillon et al. developed a low cost mass spectrometer which incorporated the ionisation chamber, filter and detector on the chip. A fibre optic coupler has been developed as a detector. The dual optical fibre configuration (one transmitting, one receiving, (Eigure 10.5)) in the chip forms the microchannel as well as the detector itself and measures refractive index changes but can also be used to measure absorbance . 

Electrochemical detection is common in microchips and can be potentiometric, voltammetric or conductimetric. An ultrathin ISE has been employed on a chip for the 

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