Dilution-free MS analysis of picoliter droplets

An instrument's sensitivity is critical in determining how sample-hungry a given technique is, but the instrument itself is often not the limiting factor. The macroscopic requirements of benchtop handling and the effects of dilution mean that ultrasensitive MS methods can require much more material (by at least 4 orders of magnitude) than is used in each assay—so even a sensitive instrument can require large quantities of sample.

RYAN T. KELLY ET AL.: ANGEW. CHEM., INT. ED. 6832–6835. 2009. COPYRIGHT WILEY-VCH VERLAG GMBH & CO. KGAA. REPRODUCED WITH PERMISSION.

Maintaining concentration. An oil-encapsulated analyte plug is transferred, without dilution, to an aqueous stream that feeds into an integrated ESI emitter.
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Coupling microfluidic chips with ESI would seem a straightforward method to address this problem; unfortunately, the approach has so far met with limited success. But Ryan Kelly and co-workers at Pacific Northwest National Laboratory accomplished an important breakthrough by combining a droplet-based technique with continuous-flow microchannels. By crafting a microchip that seamlessly transfers an oil-encapsulated droplet into the continuous-flow source of a nano-ESI mass spectrometer, they managed to analyze picoliter-level samples with essentially no dilution.

The researchers first encapsulated aqueous droplets in immiscible oil (perfluorodecalin) on a microfluidic PDMS chip. These oil-encased droplets, or plugs, have been used in a variety of analytical techniques but proved very difficult to combine with ESI MS. However, by carefully adjusting flow pressure and the mixing area’s geometry, the researchers were able to transfer the plugs into a continuously flowing aqueous channel that ended at a nano-ESI emitter. The integrated transfer and short transit time in the aqueous channel resulted in almost no dilution of the sample.

The flow to the nano-ESI emitter was stable, and the researchers adjusted the flow to greatly increase sensitivity, reporting an improvement of 3 orders of magnitude better than a related setup recently described in the literature. Moreover, the method directed all of the sample toward the mass spectrometer for analysis, rather than outputting large quantities of sample into a waste stream like many other continuous-flow techniques used at such a small scale.

The methods for droplet transfer and coupling transfer with MS could prove useful in microchip CE separations. The authors also see a potential for using the droplets to encapsulate individual cells, which could then be used for single-cell proteomics studies. (Angew. Chem., Int. Ed. 2009, DOI 10.1002/anie.200902501)