Microreactors MALDI

Scheme4.88 Microfluidic system for MALDI protein analysis, (a) automated sample pretreatment and injection (b) microreactor (c) microdispenser used to deposit sample into nanovials (d) shallow nanovials on the MALDI target plate and (e) automated MALDI-ToF-MS analysis. Reprinted with permission from [345]. Copyright 2000 American Chemical Society.

The microreactor (Figure 11.1) was designed to carry out on-line MALDI-TOF-MS analyses of products of simple chemical and biochemical reactions (A + B = C type). Therefore, the microreactor consists of two inlet reservoirs (A and B) for the injection of a maximum volume of 5 pL of reagent solutions and a 5 pL outlet pocket (C) where the analyte solution is collected and analyzed. The reaction microchannel in which the reagents react is 80 mm long, 200 pm wide and 100 pm deep. Due to the shape of the powder-blasted channel, the cross-section is approximated to a 150 pm wide by 100 pm deep rectangle. The resulting channel volume is 1.2 pL. 

Figure 11.3 Photograph of the microreactor integrated in a standard MALDI-TOF sample plate. Because of the self-activating character of the microfluidics device, the system can be introduced into the MALDI ionization chamber without any wire or tube for the feeding and flow control.

Schiff Base Formation. The Schiff base reaction (Scheme 11.1), in which primary amines (1 and 2) react with aldehydes (3 and 4) to give imi-nes (5 and 6), was chosen as a primary study of the microreaction unit and its coupling with MALDI-TOF-MS, because it is a straightforward reaction and products are obtained in high yields. The pH control required for the Schiff base formation29 turned out to be not necessary under lab-on-a-chip conditions as a consequence of the high surface-to-volume ratio that characterizes microreactors.36... 

A silica sol-gel monohth containing zeolite nanoparticles has been reported [87]. This device had a high stuface area for the immobilized enzyme, allowing for a high load within the microreactor. A 0.5 pL volume containing 0.2 pg/pL of the proteins (cytochrome c and BSA) was digested within 5 s in the microreactor, as indicated by off-line MALDI-TOF (time-of-flight) MS. The microreactor could be used repeatedly and the enzyme remained active for more than a month when it was stored at temperatures below 4 °C. 

Sim TS et al (2006) Application of a temperature-controllable microreactor to simple and rapid protein identification using MALDI-TOF MS. Lab Chip 6(8) 1056-1061... 

At the end of this chapter, the approach of Brivio and coworkers of using microreactor chips in the vacuum chamber of a MALDI-MS machine is worth mentioning [78,79]. This approach allows real on-line monitoring of reaction kinetics by MALDI-MS, but with only one time point at a time. The procedure for measuring a complete reaction curve is too time consuming and complex to be of practical use, but with dedicated and reconfigured M S equipment may be an option for on-hne studies. 

With respect to MS in combination with microreactors, it can be said that ESI is still the preferred method to couple microreactors (or microfluidics in general) to MS however, recent developments show that the combination with MALDI is also feasible. A new development in the field of MALDI-MS is imaging, which up to now has mainly been used to study biological samples, such as tissue and in particular the effect of drugs on the metabolism in that tissue (see e.g. [83]). The... 

Figure 10.16 (a) Schematic representation of on-chip protocol for protein digestion and identification coupled with MALDI-TOF/TOF-MS. (b) Amplified on-chip microreactor within... 

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