NEW YORK – A team led by researchers at Vanderbilt University and Bruker have used the mass spec vendor's timsTOF platform for MALDI imaging of mouse tissue.
Detailed in a study published last month in Analytical Chemistry, the effort found that the instrument provided high spatial resolution and molecular specificity while maintaining relatively high throughput, making it a useful middle ground between MALDI-TOF instruments and high-performance FT-ICR systems, said Jeffrey Spraggins, a research assistant professor at Vanderbilt and first author on the paper.
In particular, he noted, the instrument's trapped ion mobility system (TIMS), provides researchers with the flexibility to better manage trade-offs between resolution, throughput, and specificity as a specific experiment demands.
Since Bruker launched the timsTOF platform in 2016, it has seen uptake from a number of leading proteomics researchers and has established itself as one of the highest performing systems for conventional proteomic experiments. Key to the instrument's performance is its TIMS technology, which uses an electric field to trap ions in the device, allowing them to accumulate in parallel. They are then released into the mass spec on the basis of their collisional cross section, which is a function of their size, allowing for high-resolution separations in a very compact device.
While much of method development thus far for the instrument has focused on LC-MS/MS workflows, the TIMS technology also has potential for MALDI mass spec imaging, an area of research where Bruker is generally considered one of the leading players.
At the American Society for Mass Spectrometry annual meeting in June, Bruker launched a version of the instrument, the timsTOF Flex, aimed at MALDI imaging applications. The Vanderbilt study is one of the first to evaluate the system for MALDI imaging.
Generally speaking, ion mobility has great potential for mass spec imaging, due to the fact that imaging workflows are not typically compatible with liquid chromatography, which is used in conventional mass spec experiments to provide sample separation and increased coverage and sensitivity.
The inability to couple LC to imaging mass spec has meant that researchers have had "to deal with sample complexity in the gas phase [of an experiment], within the mass spectrometer itself," Spraggins said. "And the way that has usually been done is with [instrument] resolving power."
This, he noted, is why FT-ICR instruments have traditionally been the systems of choice for mass spec imaging.
"They are kind of the Ferraris of mass spectrometers in terms of performance," he said. "They have the kind of resolving power you need to tease apart these complex mixtures."
FT-ICR instruments, however, are more expensive as well as more complicated to use and maintain than a typical TOF instrument.
"Where ion mobility systems come into play is, if you have good ion mobility performance, you can get back some of that molecular coverage and specificity without needing an FT-ICR," Spraggins said.
Ion mobility is available on instruments from nearly all major life science mass spec vendors, including Waters, Agilent Technologies, Sciex, and Thermo Fisher Scientific, but Spraggins said he and his colleagues were particularly interested in the possibilities presented by the TIMS' ion trapping capabilities, which he said allowed the researchers to tune the ion mobility separation according to their particular need.
"Depending on the experiment, if you don't need a lot of separation and want the system to go as fast as possible, you can use very short TIMS scan times," he said. "But if you really need to go all out in terms of ion mobility performance, you can just slow down the TIMS scan time to get more separation. You have that flexibility."
In the Analytical Chemistry study, the researchers used a prototype version of the timsTOF Flex to analyze mouse kidney and brain tissue as well as whole-body mouse tissue, demonstrating that the instrument could image tissue with 10 µm spatial resolution and at a throughput of 20 pixels per second while providing 40,000 resolving power. They also found that the instrument could quickly resolve isobaric lipid species.
Last year, Spraggins and Vanderbilt researcher Richard Caprioli, senior author on the Analytical Chemistry study and a leader in imaging mass spectrometry, received a $2.4 million major research instrumentation grant from the National Science Foundation to work on development of timsTOF-based MALDI imaging, with the goal of, according to the grant award abstract, making the instrument "the cornerstone molecular imaging platform of the Vanderbilt University Mass Spectrometry Research Center."
Spraggins said that over the course of the grant, which runs five years, he and his colleagues will work to expand "the capabilities of the system in terms of spatial resolution, throughput, and specificity."
Currently, they are using the system for work within the National Institutes of Health's Human BioMolecular Atlas Program (HuBMAP), which aims to map the molecular level of healthy cells and tissues in the human body.
The Vanderbilt team's work within HuBMAP is focused on the kidney, Spraggins said, noting that they are focused primarily on measuring lipids and metabolites. He added that they are typically able to detect and identify between 200 and 300 molecules per experiment at close to single-cell resolution.
Spraggins said that the timsTOF Flex's combination of molecular coverage, spatial resolution, and throughput was key to enabling the effort.
"It can acquire data very quickly and still deliver high spectral quality, which is really important for addressing biological questions," he said.
On Bruker's Q3 earnings call last week, President and CEO Frank Laukien said that the company had begun to see contributions from sales of the Flex system, though he noted that imaging was a relatively small part of the overall market for the timsTOF platform.