Microfluidic Lab-on-a-Chip Devices for Analytical Chemistry

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Microfluidic lab-on-a-chip devices offer new solutions for analytical scientists. The Column spoke to Dietlev Belder from the University of Leipzig, Germany, about the latest developments in this evolving area of research.

Q. When did you start developing and applying microfluidic lab-on-a-chip devices for analytical chemistry applications?

A: I started this research about 20 years ago when I was head of the Chromatography Department at the Max-Planck-Institute in Mülheim an der Ruhr, Germany. I was carried away by the “gold-rush” atmosphere in lab-on-a-chip technology research. Everything seemed to be possible back then. Andreas Manz, from the Korea Institute of Science and Technology (KIST) in Saarbrücken, Germany, presented inspiring talks at conferences on this topic, in those early days.

Q. Is lab-on-a-chip technology likely to become an important research area for chromatographers?

A: Yes, I think so, but it depends on the application. When chromatographic analysis is seen as part of a larger system, for example, in process analysis, lab-on-a-chip technology makes a lot of sense because of the potential for seamless system integration of the different elements.

Q. You recently coupled chip-high performance liquid chromatography (HPLC) to high-resolution ion mobility spectrometry (HR-IMS) (1). What are the advantages of this technique?

A: Coupling chip-HPLC with HR-IMS kills two birds with one stone. On the one hand, IMS is an ideal compact detector for chip-HPLC, and on the other hand, chip-HPLC increases the resolution of IMS. The coupling of two high‑speed separation methods with orthogonal selectivity means two-dimensional separations are achievable in just a few seconds.

In a recent publication we extended the concept to two dimensional HPLC with two columns on one chip device, utilising deep-UV fluorescence detection and mass spectrometry (MS) simultaneously, and applied it in a proof of concept study to a protein digest (2).

Q. What is novel about your approach?

A: Chip-based HPLC shows excellent separation performance but the detection is challenging if it is not combined with huge and expensive mass spectrometers. On the other hand, IMS has only moderate separation performance but enables highly sensitive detection of gas phase ions while the instrumental effort is low. To put it simply, IMS is a perfect compact detector for gas phase ions with significant weaknesses in resolving power, while liquid phase separation devices show excellent chromatographic resolution but appropriate, compact, and powerful detector technology is missing.

Q. What were the main challenges you encountered in developing this technique?

A: A major challenge was the requirement to use high spray potentials of more than 10 kV at the emitter tip of the chip. This was necessary to transfer the ions to the interface of the electrospray IMS system, which operates at an increased potential of 10 kV. This made a redesign of the IMS system necessary. This was skilfully implemented by our cooperation partner, Stefan Zimmermann from the University of Hannover, Germany – one of the leading experts in IMS. At his Chair of Electrical Engineering, the first IMS system with inverted polarity was developed within the framework of this Deutsche Forschungsgemeinschaft- (DFG-) funded joint project.

Q. What practical applications are being explored?

A: This technology makes portable analysis devices possible where complex samples can be analyzed on-site, as well as real time monitoring of fast chemical processes. Innovative applications include on-chip integration of normal phase HPLC and droplet microfluidics to introduce ethylene glycol as a polar continuous phase for the compartmentilization of n-heptane eluents (3); an integrated lab-on-a-chip approach to study heterogeneous enantioselective catalysts at the microscale level (4); and the on-chip integration of organic syntheses and HPLC–MS to monitor stereoselective transformations at the micro-scale level (5). We have also combined high-pressure chip-HPLC with droplet microfluidics on an integrated microfluidic glass chip that can be hyphenated to MS (6). This is a missing link between two popular microfluidic technologies. It allows, for example, the on-chip compartmentalization of a chromatographic run, similar to the fraction collector in classical HPLC. It also enables the on-chip implementation of post-column processes such as derivatization.

Q. What are the advantages of using temperature gradients and superheated eluents in chip-HPLC applications?

A: Temperature gradients are an interesting alternative to solvent gradients. This is very feasible with chip systems because of the low thermal mass, whereas exact solvent gradients are difficult to achieve because of the small volumes. If temperature gradients are used instead of solvent gradients, gradient elution is possible with purely aqueous systems. This is not only very ecological and economical but also very interesting for coupling HPLC with IR-MS, where eluents containing carbon have to be avoided (7).

Q. Can you offer advice to anyone thinking of incorporating chip-HPLC into their research? What are the potential pitfalls to watch out for?

A: When we started with chip-HPLC, the connection technology proved to be the biggest problem. Solving that took some time, but then development progressed quickly. Chips that are stable under high pressure are also very important. We have very good experience with chips made of glass and fused silica, which have the great advantage that you can view inside, which chemists love. We can monitor the packing process, inspect the packing/quality, and we can also detect compounds by fluoresce or absorbance.

Q. Regarding your research into microchip-based supercritical fluid chromatography (SFC), what are your next steps?

A: This is actually a very challenging field of research. I dream of an SFC chip that uses a flame ionization detector (FID) or electron ionization (EI)–MS detector to perform chiral separation in the blink of an eye. It’s a far-reaching vision but we have started research in this area (8,9).

Q. Are you involved in any other projects that you think will advance the applicability of chip-HPLC?

A: In the DFG-funded collaborative research unit, In-Chem, we are working on the integration of chemical synthesis and analysis on one chip. Here we integrate flow-through reactors with chiral HPLC to monitor chemical processes in real time, to develop new enantioselective catalysts (10,11).

References

1. S.K. Piendl, C.R. Raddatz, N.T. Hartner, C. Thoben, R. Warias, S. Zimmermann, and D. Belder, Anal. Chem.91, 7613–7620 (2019).
2. S.K. Piendl, D. Geissler, L. Weigelt, and D. Belder, Anal. Chem.92(5), 3795–3803 (2020)
3. A.J. Peretzki and D. Belder, J. Chromatogr. A. 1612, 460653 (2020).
4. R. Warias, A. Zaghi, J.J. Heiland, S.K. Piendl, K. Gilmore, P.H. Seeberger, A. Massi, and D. Belder, ChemCatChem.10, 5382–5385 (2018).
5. J.J. Heiland, R. Warias, C. Lotter, P. Fuchs, L. Mauritz, K. Zeitler, S. Ohla, and D. Belder, Lab. Chip17, 76–81 (2017).
6. R. Gerhardt, A.J. Peretzki, S.K. Piendl, and D. Belder, Anal. Chem. 89, 13030–13037 (2017).
7. M. Elsner, M.A. Jochmann, T.B. Hofstetter, D. Hunkeler, A. Bernstein, T.C. Schmidt, A. Schimmelmann, Analytical and Bioanalytical Chemistry403, 2471–2491 (2012).
8. J.J. Heiland, C. Lotter, V. Stein, L. Mauritz, and D. Belder, Anal. Chem.89, 3266–3271 (2017).
9. J.J. Heiland, D. Geissler, S. K. Piendl, R. Warias, and D. Belder, Anal. Chem.91, 6134-6140 (2019).
10. https://www.dfg.de/gefoerderte_projekte/programme_und_projekte/listen/projektdetails/index.jsp?id=251124697
11. http://www.in-chem.de/

Detlev Belder is a full professor of analytical chemistry at Leipzig University. He earned his Ph.D. in chemistry in 1994 at the University of Marburg, Germany. From 1995 to 2006, he headed the Department of Separation Science at the Max-Planck-Institut für Kohlenforschung, Germany. In 2006 he was appointed a professor of analytical chemistry at the University of Regensburg. In 2007 he accepted the offer as a chair of Analytical Chemistry at Leipzig University, Germany. Belder’s research is focused on lab-on-a-chip technology as an enabling science in chemistry. In the Belder laboratories at the University of Leipzig, a broad field of research and application of lab-on-a-chip technology is performed. The Belder Group is known for miniaturized separation techniques, such as chip electrophoresis and chip HPLC. The Belder laboratory also works on detection techniques such as the coupling of microfluidic chips with mass spectrometry or ion mobility spectrometry, as well as on optical techniques, including fluorescence and Raman microscopy, and combining reactors and analysis units on one chip.

E-mail:belder@uni-leipzig.deWebsite:www.uni-leipzig.de/en