The direct examination of raw samples with the least amount of preparation is a big potential application for plasma-based ambient mass spectrometry (AMS).
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The development of several plasma-based ionization sources has brought forth positive benefits such as high sensitivity, quick analysis times, and promising qualitative and quantitative capabilities for a number of applications. For dependable, practical applications, further improvements to plasma-based sources have been achieved.
Mass Spectrometry Methods
For years, researchers have used liquid chromatography-mass spectrometry (LC-MS) to examine the metabolic profiles of animal, human, and plant tissues. When separating analytes from a solution-phase matrix at atmospheric pressure and transferring free ions into a vacuum environment in preparation for mass spectrometry (MS) analysis, ionization techniques like electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) have worked very well. However, a drawback of all atmospheric pressure ionization sources is the time-consuming, sometimes difficult, and expensive process of sample preparation.
While a great deal of fine research has been and is still being published using traditional methods, a significant limitation is that samples must be in a vacuum environment. In order to prevent any negative consequences, samples that are sensitive to vacuum must be appropriately prepared. Additionally, pump-down periods and sample size and shape restrictions due to vacuum chamber dimensions limit sample throughput. As a result, there is a significant market for an alternative MS-based analytical method that can examine materials in a natural setting.
Ambient Mass Spectrometry
Direct analysis in real-time (DART) and desorption electrospray ionization (DESI) are two newly developed techniques that have effectively addressed the challenges of vacuum-based analyses, such as the requirement for vacuum-compatible materials.
Comparing ambient sampling approaches, such as DESI and DART, to the traditional methods for the analysis of medicines and banned drugs, which often include time-consuming sample preparation, there are substantial potential advantages. To detect the presence of pharmaceuticals, for instance, liquid chromatography mass spectrometry or gas chromatography mass spectrometry analysis after solvent extraction, filtering, and analysis are frequently utilized. A major analytical difficulty is the capacity to quickly identify analytes without sacrificing the integrity of the data. The sector of pharmaceuticals stands to benefit greatly from ambient pressure ionization methods with high-throughput analytical capabilities.
Plasma Assisted Ionization
A plasma-based method devised for creating mass spectra from a variety of materials is plasma-assisted desorption/ionization mass spectrometry, or PADI. A variety of analytes, including tiny pharmaceutical compounds and polymers, have been examined using PADI. PADI is one of the more recent plasma-based ambient MS techniques. It uses a very low-power plasma, has a very straightforward and reliable design, is largely unaffected by sample geometry, and can quickly produce diagnostic mass spectra from a variety of materials.
In PADI, the analyte's surface is bombarded with a nonthermal, RF-driven plasma without any charged particle extraction. There are various ways in which the PADI source differs from other direct ionization methods, including DART and other methods like Desorption atmospheric pressure chemical ionization (DAPCI) and atmospheric pressure solids analysis probe (ASAP). First, corona discharges—typically created by delivering a few kilovolts applied to the tip of an electrode—are used in the DART, DAPCI, and ASAP procedures. This type of discharge is distinguished by high-voltage/low-current features that produce ions with energies that match the voltage applied. The PADI source, in contrast, is a nonthermal atmospheric glow discharge with greater current characteristics and a lower working voltage than corona discharges. This helps to preserve the samples and operate at ambient conditions for longer times.
The PADI source generally works at a lower peak-to-peak voltage and power level. As a consequence, a plasma that is genuinely nonthermal or cold is created with an operational temperature that is comparable to the surrounding environment. The self-sustaining glow discharge produces relatively low ion energy, usually less than 5 eV and not more than 20 eV. The nonthermal plasma used in PADI is quite cold on contact and does not heat the material, in contrast to corona discharge and DESI methods.
Consequently, thermally sensitive samples can interact directly with the plasma without the need to remove any highly energetic species, unlike with DART. A more reliable, simpler source is the end result. Second, unlike DART, the analyte's surface is in direct touch with the plasma's active region. In addition to interactions with metastable helium atoms, which are assumed to be the primary desorption/ionization process in DART, this direct contact of the plasma with the sample also causes interactions with energetic ions and radicals. Due to the chemically particular ways in which charged particles and radical species interact with surfaces, this opens up the prospect of managing specificity by making use of the capacity to encourage desirable plasma-surface interactions.
Identifying Products Using Plasma-Assisted Ambient Mass Spectrometry
A variety of over-the-counter medications have been examined using Plasma-Based Ambient Mass Spectrometry. It was discovered that semi-solids might easily yield positive- and negative-ion diagnostic spectra. A number of natural products were able to be distinguished, like nicotine from dried tobacco leaves, allicin, the main thiosulfate in freshly chopped garlic, and the readily decomposing lachrymator propanethial-S-oxide from freshly chopped onions, which is not easily detected by conventional MS.
Future Outlook
Since the advent of ambient mass spectrometry, which eliminated the need for materials to be examined in a vacuum, mass spectrometry has undergone a significant transformation. Particularly intriguing are methods based on plasma-assisted methods, such as PADI. Although the mechanisms creating such spectra still need to be understood better, the potential for plasma-assisted ambient mass spectrometry to grow into a useful and adaptable instrument in the toolbox of material analysis looks promising.
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References and Further Reading
Ratcliffe, Lucy V., Frank JM Rutten, David A. Barrett, Terry Whitmore, David Seymour, Claire Greenwood, Yolanda Aranda-Gonzalvo, Steven Robinson, and Martin McCoustra. "Surface analysis under ambient conditions using plasma-assisted desorption/ionization mass spectrometry." Analytical chemistry 79, no. 16 (2007): 6094-6101.
Rutten, Frank JM, Jasim MS Jamur, and Paul Roach. "Fast and versatile ambient surface analysis by plasma-assisted desorption/ionization mass spectrometry." Spectrosc. Europe27, no. 5 (2015): 10.
Black, Connor, Olivier P. Chevallier, and Christopher T. Elliott. "The current and potential applications of ambient mass spectrometry in detecting food fraud." TrAC Trends in Analytical Chemistry 82 (2016): 268-278.
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