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The wonderful world of ambient ionization: how to create ions without preparing a sample?
In the scientific progress in recent years, environmental ionization technology has gradually received attention. This is an ionization technology outside the mass spectrometer and does not require sample preparation or separation. This technology is based on a variety of methods, including extraction from charged spray droplets, thermal desorption and the use of laser desorption, bringing new possibilities to scientific research.
Solid-liquid extraction ionization
Solid-liquid extraction ionization is a technology that uses charged spray to create a liquid film on the sample surface. During this process, molecules on the sample surface are extracted into the solvent. Subsequently, when the primary droplets collide with the surface, secondary droplets are produced. These droplets are the ion sources of the mass spectrometer.
The data shows that desorption electrospray ionization (DESI) is one of the earliest environmental ionization technologies capable of analyzing solid samples in real time.
Another solid-liquid extraction method is desorption atmospheric pressure photoionization (DAPPI), which uses a combination of hot solvent vapor injection and ultraviolet light to directly analyze samples deposited on the surface.
Plasma-based technology
Plasma-based ambient ionization technology creates ions through electrical discharges generated in flowing gases. This process typically requires the use of heat to assist in the desorption of volatile materials from the sample.
For example, protonated water clusters can ionize sample molecules through proton transfer, which is critical for many applications.
One of the most widely used plasma technologies today is Direct Analysis Instant (DART), which has been commercialized and can analyze samples in a routine environment.
Laser-assisted ionization
Laser-assisted ambient ionization is a two-step process. First, a pulsed laser is used to desorb or ablate substances from the sample, and then these substances are interacted with an electrospray or plasma to form ions. Lasers are increasingly used as ionization sources, especially in the analysis of metals and the study of a variety of other materials.
Two-step non-laser method
In the two-step non-laser method, the sample removal and ionization steps are separate. For example, probe electrospray ionization (PESI) allows direct sampling with high salt tolerance and minimal sample consumption, making this technique of great interest in analytical chemistry.
Gas phase ionization technology
As sensitivity continues to improve, analytes in the gas phase (e.g. odors, volatile organic compounds) can be detected even if their vapor pressure is low. In secondary electrospray ionization (SESI), tiny droplets generated by nanoelectrospray in a thermal environment can quickly evaporate and precipitate the required ions.
This method makes it possible to analyze the concentrations of low volatile substances, especially for substances with molecular weights up to 700 Da.
Technical classification of environmental ionization
Ambient ionization technologies are usually divided into multiple categories based on their operating modes, including solid-liquid extraction, plasma ghost technology, two-step methods, and laser technology. Each of these methods has its own merits and can be widely used in different analytical needs.
Commercially available ambient ionization sources
With technological advancements, many commercial ambient ionization sources are now available to scientists, making this technology accessible and cost-controllable.
Environmental ionization technology is reshaping the face of analytical chemistry and promoting rapid development in various fields. Faced with these new opportunities, we can’t help but think: How will future technologies further change our understanding of analytical chemistry?
