Exciting Innovations in Mass Spectrometry

Mass spectrometry (MS) is a powerful analytical tool that can be utilized to identify, quantify and characterize molecules, among other applications. Used widely in life science research, MS technology is constantly evolving as users look to overcome bottlenecks such as sensitivity, speed, resolution and accessibility.

Here, we highlight recent research papers that present innovations in MS workflows, technology and manufacturing. 

Rethinking transfer methods with nanopore ion technology

Ionization is an important step in any MS workflow but the standard method, electrospray ionization (ESI), suffers from a key issue – sample loss. Adopting the conventional ESI technique, researchers using MS will typically find that only ~1% of their sample makes its way into the mass spectrometer.

Researchers led by Dr. Derek Stein, professor emeritus of physics at Brown University, have been working towards a solution for over a decade. "This low transfer efficiency is a fundamental problem, especially when one wants to measure small samples like single cells or single molecules,” Stein told Technology Networks.

In Nature Communications, Stein and colleagues recently showcased a new transfer method using a nanopore ion source which delivers ions directly to the mass filter.¹

“By applying a strong voltage to the nanoscale tip, ions can be pulled directly from the liquid meniscus into the vapor phase. This does not involve the release of charged droplets, so it circumvents the Coulomb explosions that usually result in so much sample loss,” Stein said. “Instead, the nanopore ion source delivers ions directly into the vapor phase by the mechanism of ion evaporation.”

The nanopore ion source uses a tiny capillary – roughly 30 nm in diameter – that is 600 times smaller than the conventional needle used in ESI. “We pull on the ends of a thin quartz tube while a laser heats the middle of the tube. The laser softens the quartz and it stretches out like the cheese on a slice of pizza as you pull it away. The quartz tube thins until it snaps in the middle. With practice, the laser heating and pulling forces can be tuned to give a nanoscale orifice nearly every time,” Stein explained.

In a proof-of-concept study, the researchers tested the novel nanopore ion source in an experiment measuring the mass spectra of 16 amino acids. The technology matched the capabilities of existing transfer methods, with a dramatic reduction in sample loss.

Stein and team are exploring routes to commercialization by redesigning the ion source for testing using commercial MS instruments. The researchers are also investigating how the new transfer method could be utilized for single-molecule sequencing.

Accelerating the use of MSI for single-cell profiling

MS imaging (MSI) is an important tool in spatial biology, which enables researchers to localize biomolecules in intact tissue. However, it’s application in single-cell research has been limited due to resolution issues.

Researchers led by Dr. Ruixuan Gao, assistant professor in biological sciences and chemistry at the University of Illinois, Chicago, believe superabsorbent hydrogels could help tackle the resolution challenge. In Nature Communications, they presented GAMSI (Gel-Assisted Mass Spectrometry Imaging) – a sample preparation and imaging method that harnesses the reversible interaction between analytes and a superabsorbent hydrogel.²

In the study, Gao and colleagues demonstrated how GAMSI can enhance the spatial resolution of MALDI-MSI ~three- and ~six-fold in proof-of-concept experiments using mouse brain slices.

“For the initial demonstration, we have focused on MALDI-MSI’s relative quantification capability because it is widely used by the field and does not require the introduction of additional standards or calibrations,” the authors said.

The GAMSI method involves embedding samples in a hydrogel that reversibly tethers biomolecules. “We note that this reversible anchoring can take place non-covalently or covalently. For lipid GAMSI, for example, native lipids can be tethered to the hydrogel via hydrophobic interactions with the membrane proteins that are covalently anchored to the hydrogel polymer chains,” Gao and colleagues explained. For protein analysis, targeted proteins could be labeled with an antibody that carries a photocleavable mass reporter, which can be anchored covalently to the hydrogel’s polymer chains.

Hydrating the gel causes it to expand, which stretches the sample without impacting its integrity. This expansion increases the spacing between the biomolecules for imaging, which underpins the enhanced spatial resolution. Moreover, GAMSI doesn’t require any modifications to existing MS hardware or pipelines.

While further optimization of the method is required – with instrument detection, sensitivity and throughput recognized as current limitations – Gao and colleagues believe that, eventually, their approach will “accelerate the daily usage of MSI for single-cell profiling of native biomolecules, especially for sub-micrometer spatial lipidomic studies of intact cells and tissues.”

3D-printed mass filters using additive manufacturing

A quadrupole is a commonly used mass filter in MS experiments. It comprises four metallic rods arranged around an axis which creates an electromagnetic field – the properties of which can be altered such that only ions of a particular m/z value reach the chamber end.

Researchers at the Massachusetts Institute of Technology recently created a miniature, 3D-printed quadrupole mass filter (QMF) that is one-quarter the density of comparable filters and could increase accessibility to MS equipment in the field. The work, published in Advanced Science, forms part of Dr. Luis Fernando Velásquez-García’s 20-year quest to create a 3D-printed, portable mass spectrometer.

“We are not the first ones to try to do this. But we are the first ones who succeeded at doing this. There are other miniaturized quadrupole filters but they are not comparable with professional-grade mass filters. There are a lot of possibilities for this hardware if the size and cost could be smaller without adversely affecting the performance,” Velásquez-García said.

Miniaturizing a quadrupole is no easy feat – reducing the filter’s size can impact its performance. “You can’t make quadrupoles arbitrarily smaller — there is a tradeoff,” Velásquez-García said. In a bid to overcome these tradeoffs, the research team adopted an additive manufacturing approach whereby materials are added in layer form. This allowed them to “aggressively iterate the design,” Velásquez-García said.

This photo shows an example of a 3D-printed miniaturized QMF. They can be fabricated in a matter of hours for a few dollars. Credit: Courtesy of Luis Fernando Velásquez-García, Colin Eckhoff, et al.

Velásquez-García and colleagues printed a network of triangular lattices to surround the rods, adding durability to the quadrupole. Electroless plating was then used to coat the rots in a thin metal film such that they conduct electricity. “In the end, we made quadrupoles that were the most compact but also the most precise that could be made, given the constraints of our 3D printer,” Velásquez-García said.

In a proof-of-concept study, the 3D-printed quadrupoles were inserted into a commercial system, demonstrating superior performance to existing miniature filters.

“Since QMFs are at the heart of the ‘analytical engine’ in many other types of MS systems, the paper has an important significance across the whole MS field, which worldwide represents a multibillion-dollar industry,” said Dr. Steve Taylor, professor of electrical engineering and electronics at the University of Liverpool, who did not contribute to the research.

New possibilities

The exciting innovations highlighted provide a mere glimpse into the vast and ever-evolving field of MS. As methods and technologies continue to advance, we can expect new possibilities in fields utilizing MS, such as single-cell biology, spatial biology and portable biomarker-based diagnostics, among other research areas.

References:

1. Drachman N, Lepoitevin M, Szapary H, Wiener B, Maulbetsch W, Stein D. Nanopore ion sources deliver individual ions of amino acids and peptides directly into high vacuum. Nat Comms. 2024;15(1):7709. doi:10.1038/s41467-024-51455-x

2.Chan YH, Pathmasiri KC, Pierre-Jacques D, et al. Gel-assisted mass spectrometry imaging enables sub-micrometer spatial lipidomics. Nat Comms. 2024;15(1):5036. doi: 10.1038/s41467-024-49384-w

3. Eckhoff CC, Lubinsky NK, Metzler LJ, Pedder RE, Velásquez-García LF. Low-cost, compact quadrupole mass filters with unity mass resolution via ceramic resin vat photopolymerization. Adv Sci. 2024;11(9):2307665. doi: 10.1002/advs.202307665