Ion mobility spectrometry (IMS)is a powerful technique used in analytical chemistry to separate and determine the structures of molecules. This method works by ionizing compounds in a sample and measuring their movement through an electric field.
Ion mobility offers several advantages over other spectrometry methods, including speed, portability, and sensitivity. This technology has been used for a wide range of applications, such as environmental monitoring, industrial process control, drug development, and homeland security.
The ion mobility spectrometry process
There are three main types of ion mobility spectrometry: drift tube ion mobility spectrometry (DTIMS), differential mobility spectrometry (DMS), and traveling wave ion mobility spectrometry (TWIMS). All methods separate and identify ions based on their ability to migrate through an electrically charged gas.
In DTIMS, ions are introduced into the ion mobility spectrometer through a sample inlet, where they are exposed to a high voltage using an electrospray ionization system. This voltage creates an electric field that accelerates the ions. They then travel through a drift tube under a constant electric field. The tube is filled with a carrier buffer gas (an inert gas that can carry samples), which flows in the opposite direction to the charged gas.
As ions travel through the chamber, they collide with the buffer gas molecules, which slows them down. Ion mobility spectrometry relies on the fact that different ions will travel at different speeds through a gas. This is because each ion has a different mass, charge, and size. For example, a small, light ion will travel faster than a large, heavy ion. The ions then reach the detector, and by comparing the arrival time of different ions, it’s possible to identify them and quantify their concentration in the sample.
TWIMS is similar to DTIMS, but instead of using a constant electric field, this technique alternates sections of zero and positive electric fields parallel to the ions’ trajectory. DMS (also called field asymmetric-waveform ion mobility spectrometry), on the other hand, uses an asymmetric alternating electric field that runs perpendicular to the direction of the ions’ travel.
Ion mobility spectrometry is a highly sensitive technique that can be used to detect trace amounts of chemical compounds. It can be coupled with other analytical methods including gas chromatography, liquid chromatography, and mass spectrometry.
The advantages and drawbacks of ion mobility spectrometry
Ion mobility spectrometers come in a range of different sizes—from a few millimeters to several meters in length—but many are small and light compared to other types of spectrometers, making them highly portable. As such, ion mobility spectrometers are often ideal for use in field settings, such as environmental analysis or homeland security applications.
Ion mobility spectrometry is also a very fast technique, making it suitable for high-throughput applications. The entire analysis can be completed in just a few minutes. Additionally, the method is extremely sensitive and able to detect trace amounts of contaminants that other spectrometry methods would miss.
Along with mass, charge, and size, the shape of an ion can impact its mobility. As such, ion mobility spectrometry can be used to separate isomers. This is something other methods, such as standalone mass spectrometry, lack.
Ion mobility is also a very fast technique, making it suitable for high-throughput applications.
While ion mobility spectrometry is a powerful tool for identifying and measuring ions in a sample, there are a few potential drawbacks to using this method. One issue is that ion mobility can be affected by temperature and humidity. The separation efficiency of IMS is often measured in terms of resolving power, and this typically decreases with temperature. Excessive humidity can degrade samples and interfere with results.
While IMS is considered reliable in many cases, certain situations—for example, some forensics applications—require that it is followed by a more sophisticated technique, such as gas chromatography-mass spectrometry or high-performance liquid chromatography-mass spectrometry, to confirm the results.
Existing and future applications for ion mobility spectrometry
Ion mobility spectrometry is particularly well-suited for use in security applications. This technique is commonly used to detect trace amounts of explosives, drugs, and other materials in various sample types. It is often used for screening at airports, borders, and other security checkpoints. Ion mobility spectrometers can also be used to detect chemical warfare agents in both military and civil environments.
This technique has been broadly used in medical diagnosis. Various types of biological samples including urine, blood, breath, and feces can be analyzed using ion mobility spectrometry to detect volatile compounds associated with certain diseases. This method also has use in pharmaceutical applications, for example, for analyzing over-the-counter drugs and as part of quality assessment protocols. In recent years, ion mobility has been used in biological studies, including the characterization of supramolecular complexes and analyzing changes in hemoglobin.
This method of analysis is widely used in environmental science. Ion mobility spectrometers are often used to monitor air quality and detect pollutants in water and soil samples. They can also be used to identify unknown compounds in the environment.
Food safety is yet another area in which ion mobility spectrometry has been applied. Most importantly, this method can be used to detect harmful chemicals, drug residues, pathogens, and toxins that could cause illness if ingested. But other interesting capabilities of ion mobility spectrometry include assessing food freshness, determining the levels of odorants in wine, and classifying grades of olive oil. Indeed, food authenticity is a newer field in which this analytical method is proving to be particularly useful.
It’s a versatile technology with a wide range of potential applications. While there are a few potential drawbacks to using this method, such as the impact of temperature and humidity on results, ion mobility spectrometry is still considered reliable in a broad range of use cases. As researchers continue to explore its potential, it’s likely they will discover even more exciting uses for this technology.