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August/September 2011


chromatography) with oil depletion and droplet merging prior to a second dimension separation (capillary electrophoresis). Importantly, the interface is passive in its operation and employs a pillar array


Figure 3: (A) Schematic of the droplet interface used to connect a first dimension HPLC instrument to a second dimension CE instrument. Oil extraction was achieved using a pillar array positioned at the point of droplet injection into the CE separation channel. (B) Two-dimensional separation profile obtained for the analysis of a 5-component peptide mixture using the 2D interface device. Images reproduced from reference [15].


flows into an electrophoresis channel.[14] Figure 2G shows a schematic of this device which consists of a 250 µm wide segmented flow channel, a V-shaped cross-flow channel and a separation channel. The cross flow channel serves two functions; first it counterbalances the pressure exerted by the segmented flow channel at the interface with the separation channel, and second it allows washing of excess sample from the mouth of the separation channel (effectively terminating the injection). The presence of oil at the interface leads to the formation of a “virtual wall” which disappears when an aqueous droplet occupies that region of space, causing an injection into the cross-flow channel (Figure 2H). The sample is then moved to the mouth of the separation channel and a plug is injected using an applied electric field. Using this device the authors demonstrated the serial injection and free solution separation of a mixture of amino-acids (Figure 2I). However, this format of droplet injection incurs appreciable sample wastage as only a small portion of the droplet is injected. Moreover, the transferred sample is further diluted in the cross-flow channel with the injection volume being difficult to control.


To address some of the aforementioned shortcomings Niu and colleagues subsequently demonstrated a fully functional droplet connector for two-dimensional separations.[15] Specifically, this microfluidic connector utilised droplet generation after a first dimension separation (nano-liquid


situated at the intersection of a droplet delivery channel and a separation channel (Figure 3A). The pillar array actively extracts the oil phase, ensuring negligible transfer of oil into the separation capillary, and enables injection of an entire droplet into the separation channel. The authors demonstrated the efficacy of the interface through two-dimensional separations of peptide mixtures (Figure 3B) and ‘heart cutting’ separations of yeast cell proteins. The ability to finely partition peaks originating from a first separation dimension is significant since it ensures that no chemical or biological information is lost during dimensional transfer. Furthermore, reagents can be added and mixed to the formed droplets, therefore allowing the facile integration of sample preparation.


Droplet injection into separation channels and capillaries has also been demonstrated using digital microfluidic systems. In 2009, Gorabatsova et al. [16] and Abedelgawad et al. [17] described the interfacing of digital microfluidic devices for chip-based electrophoretic systems. Both groups successfully demonstrated injection of sample


from µL-volume droplets and free solution separation of their contents. Interestingly, in the following the year the device reported by Abedelgawad was refined to allow the creation of a multilayer interface where droplets could be dispensed and diluted prior to zone electrophoresis on-chip.[18]


Conclusions It is evident that droplet-based microfluidic tools are beginning to provide new opportunities for the controlled injection, transport and isolation of ultra-small sample volumes. Although their integration with both upstream and downstream analytical separations has been proved much work remains. As has been seen, a key requirement for the delivery of droplets to a continuous flow is effective oil extraction. Current approaches require either complex local treatment of channel surfaces or delicate pressure control. More robust oil extraction techniques are therefore required. Furthermore literature reports have only demonstrated droplet interfacing to separation channels that perform free solution


electrophoresis. The use of capillary gel electrophoresis, although more relevant to fields of proteomics and genomics, introduces more challenges to the design of such an interface. These include the practical problems associated with loading and replacing high viscosity sieving matrices in interface devices with complicated geometries and surface chemistries. Despite these challenges, the use of droplet interfacing tools has opened new paradigms for the analysis of ultra-small volumes. The ability to control and process such volumes in high-throughput is likely to find significant application in the fields of proteomics, genomics, metabolomics and disease diagnosis.


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[2] P.G. Righetti, C. Gelfi, Electrophoresis 1997, 18, 1709-1714.


[3] F. Pereira, S. Hassard, A. Cass, J.M. Nagy, Chimica Oggi-Chemistry Today 2008, 26, 40-42.


[4] S.C. Beale, Analytical Chemistry 1998, 70, 279R-300R.


[5] M.L. Marina, M. Torre, Talanta 1994, 41, 1411-1433.


[6] K.D. Altria, H. Fabre, Chromatographia 1995, 40, 313-320.


[7] D.J. Harrison, A. Manz, Z. Fan, H. Luedi, H.M. Widmer, Analytical Chemistry 1992, 64, 1926-1932.


[8] J.P. Alarie, S.C. Jacobson, J.M. Ramsey, Electrophoresis 2001, 22, 312-317.


[9] Y. Jin, G.A. Luo, Electrophoresis 2003, 24, 1242-1252.


[10] X. Niu, F. Gielen, J.B. Edel, A.J. deMello, Nature Chemistry, 2011, 3, 437-442. [11] A. Huebner, S. Sharma, M. Srisa-Art, F. Hollfelder, J.B. Edel, A.J. deMello, Lab on a Chip 2008, 8, 1244-1254. [12] M. Srisa-Art, A.J. deMello, J.B. Edel, Analytical Chemistry 2007, 79, 6682-6689. [13] J.S. Edgar, C.P. Pabbati, R.M. Lorenz, M.Y. He, G.S. Fiorini, D.T. Chiu, Analytical Chemistry 2006, 78, 6948-6954.


[14] G.T. Roman, M. Wang, K.N. Shultz, C. Jennings, R.T. Kennedy, Analytical Chemistry 2008, 80, 8231-8238. [15] X. Niu, B. Zhang, R.T. Marszalek, O. Ces, J.B. Edel, D.R. Klug, A.J. deMello, Chemical Communications 2009, 6159-6161.


[16] J. Gorbatsova, M. Jaanus, M. Kaljurand, Analytical Chemistry 2009, 81, 8590-8595. [17] M. Abdelgawad, M.W.L. Watson, A.R. Wheeler, Lab on a Chip 2009, 9, 1046-1051.


[18] M.W.L. Watson, M.J. Jebrail, A.R. Wheeler, Analytical Chemistry 2010, 82, 6680-6686.


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