Ion mobility spectrometry (IMS) is an analytical technique that separates charged molecules (ions) based on their movement through a gas. Unlike mass spectrometry, which separates ions solely by their mass-to-charge ratio, IMS introduces a second dimension of separation based on physical size and shape. This dual-separation allows distinction between molecules identical in mass but different in structure. By analyzing the speed at which an ion travels through a buffer gas, IMS provides a unique structural fingerprint for analyzing complex mixtures.
Separating Ions Based on Size and Shape
The fundamental principle of ion mobility relies on the movement of a charged molecule through a gas-filled tube under the influence of a uniform electric field. As the ion is pulled along the tube, it repeatedly collides with the neutral gas molecules that fill the space, acting like friction or air resistance. The frequency and force of these collisions determine how quickly the ion can move, establishing its mobility.
Consider the analogy of a golf ball versus a loosely crumpled piece of paper falling through the air. Although they may have the same mass, the crumpled paper encounters more air resistance because it is bulkier. Similarly, a large, extended ion collides with the neutral buffer gas more often than a compact, folded ion of the same mass.
The time an ion takes to travel the drift tube, known as the “drift time,” is the primary measurement reflecting its mobility. Smaller, streamlined ions move faster, experiencing less resistance and arriving sooner. Conversely, larger ions or those with a more open structure are slowed down by increased collisions, resulting in a longer drift time. This difference in speed links mobility directly to the ion’s three-dimensional structure.
How Ion Mobility Spectrometers Work
An ion mobility spectrometer consists of several core components designed to facilitate gas-phase separation. It begins with an ionization source where sample molecules are converted into charged ions. These ions are introduced in a short pulse into the drift tube, a chamber filled with a neutral buffer gas like nitrogen or helium. A constant electric field is applied across the drift tube, providing the force that pushes the ions toward the detector.
The separation takes place as the ions travel through the buffer gas, with the electric field continually accelerating them and collisions continually slowing them down. Each ion reaches a terminal drift velocity specific to its unique combination of charge, mass, and shape. Ions separate temporally, resulting in a chromatogram-like plot where different ions arrive at the detector at different times based on their mobility.
Ion mobility systems are frequently coupled with mass spectrometers in a technique called ion mobility-mass spectrometry (IM-MS). This coupling provides a two-dimensional separation, first by mobility (size and shape) and then by mass-to-charge ratio. The separation occurs on a millisecond timescale, allowing seamless integration before mass analysis. This combination is highly effective for complex samples, providing a cleaner mass spectrum by separating components that would otherwise overlap.
Solving Analytical Puzzles With Collision Cross Section
The unique advantage of ion mobility lies in its ability to resolve structural differences between molecules indistinguishable by mass alone. Molecules that share the exact same chemical formula and mass are called “isobaric compounds,” posing a significant challenge for traditional mass spectrometry. IMS overcomes this challenge by assigning each ion a definitive physical property known as the Collision Cross Section (CCS).
The CCS represents the rotationally averaged, two-dimensional area of an ion as it interacts with the buffer gas molecules during separation. It is a precise, measurable value that acts as a physical footprint, defining the ion’s shape and size in the gas phase. Because CCS is a fixed physical property, it can be measured experimentally and compared against databases, providing confirmation for molecular identification.
This ability to measure CCS is valuable in distinguishing between structural isomers and conformers. Isomers are compounds with the same atoms but bonded in a different sequence (e.g., fructose and glucose). Conformers are molecules with the same chemical structure but existing in different three-dimensional shapes (e.g., folded or unfolded proteins). Since the CCS value is distinct for each shape, IMS can separate these subtle structural variations. This is significant in drug development.
Essential Uses in Science and Security
The rapid, high-resolution separation capabilities of ion mobility spectrometry have made it an indispensable tool across diverse fields, from scientific research to public safety. In the realm of security, IMS devices are widely used for the high-speed screening of trace compounds at airports and border crossings. The instruments quickly detect and identify minute quantities of explosives, narcotics, and chemical warfare agents by comparing the mobility of sampled ions to an internal library of known threat signatures.
In biological research, IM-MS is a powerful platform for analyzing complex samples in proteomics and metabolomics. Proteomics uses IMS to separate large protein ions and fragments, providing insights into folding and structure. Metabolomics utilizes the technique to separate isomeric metabolites (e.g., sugars or fatty acids), allowing researchers to map out metabolic pathways obscured by structural similarities. IMS is also employed in pharmaceutical quality control to identify impurities, ensuring purity and consistency.