Ten Year Views: With David Clemmer

In January, The Analytical Scientist will celebrate its 10th anniversary, and we’re using the occasion as an opportunity to bring the community together and reflect on the field of analytical science as a whole. To do that, we’re speaking with leading figures and friends of The Analytical Scientist to understand how far the field has come over the past decade, what lessons have been learned, which memories stand out, and where we go from here. In this installment, David Clemmer, Distinguished Professor & Robert and Marjorie Mann Chair of Chemistry, Indiana University, USA, discusses charge detection mass spectrometry, electrospray ionization, and the role analytical science must play in addressing environmental problems. 

What are the most significant developments in analytical science over the past 10 years?
 

The broad application of cryogenic electron microscopy (cryo-EM) to image the details of large complex molecules, with resolution that approaches the atomic scale, such as intact viruses, is revolutionary and in the last decade has become routine. Now, we can directly see structural details, and with more sensitivity than ever before. Bioanalytical chemistry has really advanced too, especially in the analysis of genome expression. We have the ability to look at small molecules and lipids. We can see what's happening now (lipids and small molecules), what happened in the recent past (protein and gene expression), as well as what has led us here (genetics and familial history). These factors can be measured very quickly, and to some extent in even single cells, opening a new paradigm for understanding living organisms.

The recent discovery of fast reactions on the surfaces of microdroplets is also revolutionary. Graham Cooks (Purdue), Richard Zare (Stanford), Xin Yan (Texas A&M) and others have opened up the idea that reactions at interfaces can play an incredibly important role – we never knew how fast and how efficient they were and the reactions have transformative potential in other areas of science.

Another advance that has transformative potential was made by Martin Jarrold’s (Indiana University) and Evan William’s (Berkeley) laboratories; their groups have been pioneering charge detection mass spectrometry, enabling mass measurements of all kinds. Martin and I founded Megadalton Solutions based on the only technology that can quickly determine masses beyond the 10 megadalton regime. Unlike charge induction on an Orbitrap, where you only measure the partial charge of a swarm of ions, Martin’s instruments send gigantic ions back and forth through a tube, such that the entire charge of macromolecules and particles is induced as an independent signal. This allows each ion’s exact charge to be determined and when combined with mass-to-charge measurements one determines each ion’s mass. In collaboration with Subhadip Ghatak’s group from our Indianapolis campus, we’ve been measuring the masses of wound associated exosomes, vesicles that weigh in across the tens to hundreds of megadaltons regime; these measurements provide evidence for other organelles in wound fluid that were unknown to exist extracellularly. Why are they being excreted? Why are they there? The existence of new instrumentation enabling mass measurements of such large molecules allows us to begin answering such questions.

Can you expand on why you think fast reactions on the surfaces of microdroplets are revolutionary?
 

You think you know about the nature of water, until you start seeing some of these reactions. It turns out that many different types of reactions are accelerated to an unimaginable extent at these interfaces. We measured a three component Bignelli reaction, discovered more than a 100- years ago and requires hours or even days to accomplish in a beaker. We haven’t published this yet, in the droplet the reaction turns out to be extraordinarily efficient; our results suggest that even a single reagent molecule of each of the three components in a droplet can condense to make the product over the lifetime of the droplet, which is, at most, milliseconds. The reaction is probably occurring in microseconds, but it’s doing it with only the possibility of three reagent molecules in the droplet. From the point of view of chemistry, to control a reaction by controlling the number of molecules down to a single reagent molecule combined with another one in a vessel, in this case a droplet, is unimaginable. 

I often have felt that once invented, measurements and instrumentation are somewhat taken for granted and often it is the application that gets that lion’s share of attention. I hope analytical scientists get the credit for the findings that are to come with the last decades advancements!

And do you think analytical scientists generally get the credit they deserve? 
 

I think that analytical chemists, especially those that are involved with advancing new chemical instrumentation are somewhat modest. For example, when the human genome was sequenced, most of the credit went to the biologists who arguably failed to sequence the human genome with standard techniques. It was really the pioneering work of Jim Jorgensen (North Carolina), Norm Dovichi (Notre Dame) and a few others who first sped the process up. Now that sensitivity, ionization methods, and resolution have all progressed, we have the possibility of looking at the next steps. Finding the double bond in a lipid is a tricky thing to do, but analytical chemists are working hard to advance technology, which will have a big impact on how we think about the cell. 

Thinking back over the last 10 years, which commercialized technologies stand out as being particularly innovative?
 

We did write the IMS-TOF patent, which has been incorporated into commercial instruments. Waters have since licensed those. Dick Smith has taken the IMS into SLIMS to get a really high-resolution ion mobility measurement. And Bruker have also got a new technique called TIMS, which is a nice high-res instrument. And, of course Makarov’s (Thermo’s) Orbitrap provided an easy approach to Marshall’s (Florida State University) revolutionary and incredibly creative FTMS measurements using high-field magnets. But I find myself most attracted to emerging innovations; as an example, Scott McLuckey’s (Purdue) ion-ion reactions continue to surprise me. These measurements have the immediate commercial appeal of being able to spread out and resolve ions that are otherwise unresolvable. 

In the next decade,  the real innovation might be in some of the unexpected chemistry. For example, the widely used electron transfer dissociation method that Hunt’s group pioneered is a type of new chemistry that emerges from negative ions interacting with positive ions. But this is only one example. The approach is again a new type of chemical synthesis – but, in a vacuum, and without solvent (making it green). My guess is that Bruce Merrifield, who won a Noble prize for solid-phase peptide synthesis, would be astonished to learn that McLuckey’s group was synthesizing these types of molecules in milliseconds inside of a mass spectrometer. It will be interesting to see how new strategies (for example AI approaches) take advantage of the wealth of kinetic and thermochemical data from ion-molecule reactions, acquired over the last forty years and used to train theoretical quantum chemical approaches. Because these measurements are free from solvent, they have a unique archival quality that can be exploited as new methods emerge.

Do you have any favorite memories from the last 10 years?
 

I think the first time that I saw hepatitis B, T3 and T4 mass spectra that Martin Jarrold was measuring really blew my mind. I just couldn’t believe that you’d ever see sharp peaks at the right mass! That really surprised me and caused me to reassess what is possible. If you had seen the broad, somewhat ugly, peaks that Martin and our colleague George Ewing made on large water clusters you’d appreciate there was no guarantee that it would be possible to remove salts and residual solvent to an extent that any sharp features would be observed. His group has now shown mass spectra for adenovirus with sharp peaks in the 100 megadalton regime. 

I’m also still impressed by what can be done with tiny tips for electrospray ionization. One of my previous colleagues, Lane Baker, really opened up this field when he put one of his nanopores in front of a mass spectrometer. It’s embarrassing that I didn’t appreciate how profound this was going to be. I think those little tips are going to turn out to be really valuable for trapping things, because such small droplets dry and cool really quickly. My students and I did a back of the envelope calculation a few weeks ago that suggests that the temperature in small droplets can drop by more than 10⁶ degrees per second. This thermal quenching rate is similar to that with cryo EM, where you submerge things into liquid nitrogen, and they cool at a fast rate – allowing structure to be preserved. This suggests that many subtle, transient, structures can be trapped by ESI, and several groups, including ours, in collaboration with David Russell and Art Laganowsky (Texas A&M) are beginning to see this.

What do you think the next 10 years will look like? 
 

I think that analytical chemists are really going to have to step it up. There are enormous challenges in this decade. For example, there is a huge plastic problem, and we’re starting to find astroturf in everything – because we made this material that keeps breaking into smaller pieces when decomposing. There also needs to be a global scientific effort to address how to store carbon, and how to slow down the environmental impact of burning fossil fuels. The measurement community has a unique set of skills to solve some of these problems. From the point of view of a chemist, it is unimaginable that we would burn these fantastic molecules. Chemists have worked for years on energy transfer and how to deliver energy back and forth between molecules, and these techniques need to be reimagined and applied globally. Also, I believe analytical science will play an important role in responsible manufacturing – it is important we think about the whole lifecycle of the materials and the products we use. Finally, I think scientists need to be more aware of political, social, and environmental issues, because technology can’t solve everything!