Ambient mass spectrometry

It has been more than 12 years since the original report by Cooks and Takáts¹ on desorption electrospray ionization mass spectrometry (DESI-MS) that unleashed a scientific and intellectual storm in the analytical sciences. MS, typically limited to enclosed environments and dissolved samples, was being freed to explore samples in the open air, without shape and size limitations and with all the power of modern mass spectrometers at its disposal. The possibilities seemed endless, and the response of the scientific community was quick to reflect such interest. A literature search with the term “ambient mass spectrometry” results in hundreds to thousands of matches, with topics ranging from forensics and food authentication to metabolomics, protein analysis and molecular imaging. When MS was carried under “ambient” conditions, the sky was the limit. As Professor Marcos Eberlin put it in his 2010 article, analytical mass spectrometry was now in the “real world”.²

The impact of ambient mass MS in the real world is, indeed, tangible and real. Ambient MS has been used for a variety of unique applications. Among the most cited ambient MS papers, one can find the seminal work by Cooks on DESI,¹ the first report on direct analysis in real time (DART),³ the first report on laser ablation electrospray ionization (LAESI),⁴ low temperature plasma probe (LTP),⁵ atmospheric pressure solids analysis probe (ASAP),⁶ and electrospray-assisted laser desorption/ionization (ELDI),⁷ dielectric barrier discharge (DBD),⁸ desorption sonic spray ionization (DeSSI, later relabeled EASI),⁹ flowing atmospheric pressure afterglow (FAPA),¹⁰ and paper spray,¹¹ among others. Five of these (DART, DESI, LAESI, ASAP and paper spray) are now commercially available. This incredible diversity of ion generation approaches has characterized the field of ambient MS from the beginning, being a testament to the creativity of the teams that have played a major role in the field. Application-wise, highly cited papers include those on the detection of melamine in milk via desorption atmospheric pressure chemical ionization,¹² pesticide testing in food with DESI,¹³ olive oil authenticity assessment using DART,¹⁴ surface imaging via infrared laser ablation metastable-induced chemical ionization (IR-LAMICI),¹⁵ metabolomic fingerprinting for beer recognition,¹⁶ fabric analysis for explosives and drugs,¹⁷ screening for counterfeit medicines,¹⁸ profiling of banknotes,¹⁹ latent fingerprint imaging of trace chemicals,²⁰ and intraoperative cancerous tissue identification.²¹

This themed issue of Analytical Methods on ambient mass spectrometry includes four review articles, one communication and 16 full articles, following the successful 2010 themed issue in our sister journal, Analyst. We could not be more pleased with its diversity, both in terms of topics covered and in the geographical distribution of the author teams. In terms of review articles, Venter and Javanshad (10.1039/c7ay00948h)²² provide us with an updated classification scheme for ambient MS techniques, and a thought-provoking definition for what ambient MS truly is, proposing that the “no sample preparation” concept, often touted in the field, is revised to include proximal and real-time processing. They also propose a sliding scale of “ambience” where methods ranging from those typically used in hyphenated analysis (e.g. liquid chromatography-MS) to those that are “purely” ambient can be placed. In their article, Professor Xinrong Zhang and his team take on the daunting task of reviewing the multiplicity of plasma ion sources reported in the ambient MS literature (10.1039/c7ay00965h),²³ providing a highly valuable guideline for understanding the pros and cons of each of the proposed plasma ionization approaches, which are often preferred due to their degree of instrumentation simplicity and robustness. Professor Jentaie Shiea’s group, who first reported on ELDI, reviews the multitude of laser-based ambient MS techniques (10.1039/c7ay00997f),²⁴ including a total of 19 different approaches that use various types of UV and IR lasers. Professor Kauppila focuses on the application of ambient MS to low-polarity compounds (10.1039/c7ay00817a),²⁵ a task that from an analytical perspective is always incredibly challenging and which has seen numerous attempts to increase the efficiency of ion generation through techniques such as desorption atmospheric pressure photoionization (DAPPI).²⁶

Our goal with this issue has been to showcase recent work from some of the most active groups in the field, with contributions from a variety of countries as a way of starting to sketch what “ambient MS 2.0” might look like. All MS practitioners are aware of the challenges that lie ahead, including those of inter-laboratory repeatability and constancy detailed in some relatively recent reports, such as the VAMAS (Versailles Project on Advanced Materials and Standards) study.²⁷ These will undoubtedly have to be addressed for the field to truly move forward, and for ambient MS to become commonplace in national laboratories, law enforcement agencies and industry. For example, accurate absolute quantitation is still an issue in ambient MS.²⁸ Miniaturization, fieldability and automation are all research directions reflected in this themed issue that will continue to define next-generation ambient mass spectrometric analysis. Finally, we wish to acknowledge the work from all the authors that have contributed to this Analytical Methods themed issue on ambient mass spectrometry. We do hope that you will find this compilation of articles enjoyable and useful.

References

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