Microchip Point of View

Coupling a microfluidic or microfabricated system to MS appears to be fruitful for both the detection on the chip (microchip point of view) and for the MS analysis (MS point of view). On the one hand, MS is a powerful technique for on-chip detection due to its sensitivity and the amount of information it provides on the sample on the other hand, by using microfluidics prior to the MS analysis, new opportunities for the field of MS are created as it provides better MS capabilities compared to conventional sample preparation techniques. 

A first interest of MS when used in combination with microfabricated structures, or at the outlet of microfluidic devices, is the match in the volume of liquid handled. A typical MS analysis requires less than 1 pL of liquid, for ESI-MS as well as for MALDI-MS techniques. When working with a continuous flow of liquid and ESI-MS, the MS performance is even more enhanced for flow rates down to 50-100 nL min-1 the lower the flow rate, the better the MS analysis. This flow-rate range corresponds to flow-rate values observed in microfluidic devices. Consequently, the technique of MS is easily scalable and exhibits an enhanced response when the sample size is decreased. This is not the case for instance for other detection techniques, such as UV absorbance or amperometry these two techniques require large detection area or volume, which is the opposite of the quest of microfluidics. This first advantage of MS compared to other technique goes together with its high sensitivity. 

Mass spectrometry is a fairly universal technique it is a label-free detection technique and can be applied to any molecule as long as it can be ionized (i.e. that it presents a protonation site for instance). Optical techniques (e.g. UV absorbance and fluorescence-based techniques) require that the analytes present given properties to be detected. They must absorb or emit in the UV or in the fluorescence range. If they do not, they must be coupled to an external moiety (aromatic group, fluorophore) that presents such absorption/emission properties, and this imposes an additional derivatization of the analytes before their detection, but after their separation for instance 

Lastly, MS is compatible with high-throughput analysis. Each analysis, for both ESI-MS and MALDI-MS, is fast and lasts for less than a minute. As already mentioned, MALDI-MS analysis can easily be automated with dedicated software so that hundreds of samples can be analyzed without human intervention. ESI-MS lends itself less to high-throughput analysis as sample preparation and introduction in the ionization capillary are more tedious and time consuming. Still, automation is fully conceivable when coupled to LC for instance. 

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