NEW YORK (GenomeWeb) – Advances in ion mobility spectrometry technology are helping scientists see differences between molecules that look the same through other lenses.
An orthogonal approach to chromatography and mass spectrometry, ion mobility is another separation technique that helps resolve molecules with different spatial structures. That can be useful on its own, and has been for decades, but uptake of ion mobility into commercially available research instrumentation is helping scientists see differences in complex samples, where spatial configuration may be the only measurable between them.
In its simplest form — "drift tube" technology — ion mobility involves an electric field that propels ions from one end to the other through a cloud of gas molecules. "In mass spec, you want a vacuum to reduce collisions," Chris Chouinard, a postdoc at the Pacific Northwest National Laboratory told GenomeWeb. "In ion mobility, you make use of those collisions." Larger or elongated ions collide more frequently, slowing down, while compact molecules move more quickly.
For some molecules with the same mass-to-charge ratio, like lipid isomers, ion mobility can help separate them in a time frame that is orders of magnitude faster than chromatography. At the recent US edition of the Mass Spectrometry: Applications to the Clinical Lab meeting, Chouinard presented data showing just that. In a study of patients with pregnancy-associated conditions like preeclampsia and gestational diabetes, he was able to use ion mobility to help detect levels of two lipid isomers, one with a trans double bond, giving it a more elongated form, and one with a cis double bond.
"Whereas mass spec can't tell the difference between these two, ion mobility pretty easily separates them," he said. Importantly, ion mobility can do it quickly, making it an attractive technology for many applications from clinical screening to molecular testing for law enforcement.
"Especially in biology and specifically in the clinical lab, time is money," Chouinard said. "Doing something with a chromatographic run might take several hours, while ion mobility does it on the millisecond timeframe."
Ion mobility isn't a new technology. Airports have been using it for decades to test for residues from explosives: anytime a carry-on bag is swabbed and analyzed at the security checkpoint, that's standalone ion mobility in action.
But in recent years, major mass spec instrument makers have been developing ion mobility technologies and started coupling them with mass spec for research applications.
"I think it's underutilized," Dustin Yaworsky of Waters told GenomeWeb. "We've been talking about it for a decade and I think the marketplace is just starting to wake up to the benefits. There's a lot of emphasis on mass resolution solving all of our problems, but we are starting to understand the limits of mass resolution alone in these complex samples and matrices."
Waters has developed T-wave, short for travelling wave ion mobility, and incorporated it into the Vion IMS QTof instrument and the Synapt G2-Si; Agilent Technologies offers its 6560 Q-TOF instrument with a drift tube attached to the front end; and both AB Sciex and Thermo Fisher Scientific have developed high-field asymmetric waveform ion mobility (FAIMS), with Sciex branding it as SelexION.
At the University of Florida, Professor Rick Yost is high on FAIMS, a technology that he envisions as a fast, mobile platform for molecular detection. "Nobody has a good commercial device that does what we know [FAIMS] can do," he said, but he's got several applications in mind.
One is a breathalyzer for marijuana. Yost is scientific advisor to a Canadian company called Cannabix that is working on such a device. FAIMS devices can be as small as a book of matches, he said, and water vapor actually helps improve the technique. "It's a hot topic for either medical purposes or recreation," he said. Many states have passed laws prescribing a legal limit for THC in the blood, but don't have a field-ready test for it, he said.
He's also launched a spinout called Breathtec, which is looking for biomarkers in the breath. "Diabetes, fatty liver disease, we know these things impact the breath," Yost said. While it might be a stretch to use breath for a definitive diagnosis, he suggested it could be used as a first-line screening method for something like fatty liver disease, which requires a biopsy for diagnosis.
But for many researchers, using ion mobility as an additional separation technique is the key focus. "One of the major benefits is that the timeframe goes well in between chromatography and mass spec," Chouinard said, who used commercially available LC-IMS-MS technology from Agilent in his study of pregnancy samples. Chromatography takes minutes, while ion mobility takes only milliseconds and mass spec takes microseconds. "People find it's easy to couple because you can do the same chromatography and mass spec while also getting info on ion mobility."
In the pregnancy study, Chouinard said that the two lipid isomers co-eluted chromatographically. Looking simply at mass-to-charge, the lipids appeared to go up in one of the disease groups, he said. "But in the ion mobility spectrum, we saw some isomers actually going down. With mass spec, you're not seeing those subtle differences."
In clinical settings, ion mobility might even offer the opportunity to replace chromatography all together.
At the MSACL meeting, Waters researcher Emmanuelle Claude presented a poster on coupling ion mobility with desorption electrospray ionization (DESI) MS for imaging applications. Specifically, she looked at lipid species in a mouse brain tissue sample, noting that lipids are often closely related in structure, so the ion mobility separation can help.
Yaworsky added that imaging applications often don't incorporate liquid chromatography, so ion mobility can help identify detected ions, as well as contribute to reducing background noise and chemical interference, which can be identified by bioinformatics analysis to clean up the signal.
While TOF MS has been a natural counterpart to IMS, featured in the Waters and Agilent IMS-MS offerings, progress is being made on coupling it with other types of mass spec detectors.
Ion mobility works best with TOF mass specs, Chouinard said, because they're fast acquisition instruments. Coupling a high-resolution instrument that works more slowly, like a Thermo Fisher Orbitrap, is a bit more complicated. "That's not to say it can't be done," he said.
In November, researchers at PNNL led by Yehia Ibrahim and Richard Smith published a paper in Analytical Chemistry describing their efforts to link drift tube ion mobility with an Orbitrap instrument.
"Orbitrap instruments can provide advantages in terms of the much higher mass resolution and accuracy, and have useful ancillary capabilities, for example, for performing MS analyses," the authors wrote.
PNNL is a hotbed of ion mobility development. Chouinard, who studied under Yost at UF, recently decamped to PNNL to work on a new technology that he said could provide a "theoretically unlimited" ion mobility separation path length.
"Size does matter," he said. "Longer drift cells means you get better separation between compounds with similar mobility, which means better resolution. However, you don't want 10-meter-long instruments." Moreover, to maintain a constant electric field over longer distances would require high-voltage electrodes. "There are obvious reasons you'd want to avoid that," he said.
What PNNL has come up with is a new device called structures for lossless ion manipulations (SLIM), which borrows the concept of Waters' travelling-wave ion propulsion to move ions around right angles, in any direction, along serpentine paths. Using printed circuit boards sandwiched together to create the ion path, SLIM uses a series of electrodes to propel the ions through. "In a much more compact size, we can get much longer separations," Chouinard said. Already, SLIM provides a separation path length of more than 12 meters, "an order of magnitude better than commercial platforms," he said. "Multiple passes through the same separation space allows us theoretically unlimited separation path length."
PNNL researchers have used SLIM to separate molecules of interest to the omics fields, such as carbohydrates, peptides, and lipid isomers. "There's some cool work we've done with peptide isomers," Chouinard said. "Minor modifications in the peptides, we can resolve pretty easily." That could be a boon to bottom-up proteomics, where the ability to correctly identify peptides is critical, or studies of disease states brought about by minor peptide modifications.
Now, Chouinard is finalizing a SLIM-based IMS-MS instrument he's built from scratch. The pregnancy sample study is partially being done to provide a data set to validate SLIM with. "Everything we're doing now, we're doing on commercially available ion mobility platforms to learn as much info as we can," he said. "Then, in the upcoming months, the plan is to run the same samples using SLIM to see what other changes we can identify that we couldn't see with the commercial platforms."
At MSACL, an Agilent representative said that he hadn't spoken to any other conference-goers about the firm's ion mobility instrumentation. Chouinard noted that despite his and Yost's talks, a standalone session had not been devoted to ion mobility. But he feels the interest is out there.
"It's not that far in the past that mass spec in the clinic was kind of a radical new idea. I see ion mobility as that next tool that hopefully the clinical world will be quick to adopt," he said.