REVIEW: Mass Spectrometry Imaging under Ambient Conditions

Mass spectrometry imaging (MSI) has emerged as an important tool in the last decade and it is beginning to show potential to provide new information in many fields owing to its unique ability to acquire molecularly specific images and to provide multiplexed information, without the need for labeling or staining. In MSI, the chemical identity of molecules present on a surface is investigated as a function of spatial distribution. In addition to now standard methods involving MSI in vacuum, recently developed ambient ionization techniques allow MSI to be performed under atmospheric pressure on untreated samples outside the mass spectrometer. Here we review recent developments and applications of MSI emphasizing the ambient ionization techniques of desorption electrospray ionization (DESI), laser ablation electrospray ionization (LAESI), probe electrospray ionization (PESI), desorption atmospheric pressure photoionization (DAPPI), femtosecond laser desorption ionization (fs-LDI), laser electrospray mass spectrometry (LEMS), infrared laser ablation metastable-induced chemical ionization (IR-LAMICI), liquid microjunction surface sampling probe mass spectrometry (LMJ-SSP MS), nanospray desorption electrospray ionization (nano-DESI), and plasma sources such as the low temperature plasma (LTP) probe and laser ablation coupled to flowing atmospheric-pressure afterglow (LA-FAPA). Included are discussions of some of the features of ambient MSI for example the ability to implement chemical reactions with the goal of providing high abundance ions characteristic of specific compounds of interest and the use of tandem mass spectrometry to either map the distribution of targeted molecules with high specificity or to provide additional MS information on the structural identification of compounds. We also describe the role of bioinformatics in acquiring and interpreting the chemical and spatial information obtained through MSI, especially in biological applications for tissue diagnostic purposes. Finally, we discuss the challenges in ambient MSI and include perspectives on the future of the field.

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Screening of cosmetic ingredients from authentic formulations and environmental samples with desorption electrospray ionization mass spectrometry

A rapid and sensitive method based on desorption electrospray ionization mass spectrometry (DESI-MS) has been utilized for the detection of chemicals related to cosmetic products without prior preparation. DESI-MS was used to rapidly screen target compounds at ambient conditions, allowing the investigation of surface-bound residues, complex chemical matrices, and components of authentic cosmetic formulations. This high-throughput method routinely achieved low nanogram detection limits, and quantitative ability was demonstrated with decent linearity and precision. Continuous, direct analysis of cosmetic impurities in drinking water matrices was also validated, implementing thermal assistance into the ionization source design to increase overall sensitivity.

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MALDI, Other Surface Sampling Methods [DESI] Continue to Make Inroads in Clinical, Quantitative Proteomics

"...Beyond the advances in MALDI technology, Stolowitz also noted growing interest in non-MALDI surface sampling techniques, such as the desorption electrospray ionization, or DESI, technique developed by Purdue University researcher Graham Cooks and offered commercially by Prosolia, or the liquid extraction surface analysis, or LESA, technique developed by researchers at Oak Ridge National Laboratory in collaboration with biotech firm Advion.

Like MALDI, these techniques allow researchers to take sample directly from a spotted surface, allowing them to skip time-consuming liquid chromatography steps and improve throughput. Unlike MALDI, though, these techniques can be interfaced with electrospray ionization-based systems like triple quad and QTOF machines, which typically still offer higher sensitivity than MALDI instruments...."


"We were talking to them [Agilent] about surface sampling, and there is some interest there in exploring that, particularly [given that] they have this 6550 QTOF," [Stolowitz] he said. "So I'm trying to talk them into collaborating on putting either a DESI or a [LESA] front end [on it] and looking at what its potential would be as an imaging [mass spec] instrument and also as a surface sampling instrument."

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Surface Distribution of a Seaweed Secondary Metabolite by Imaging Mass Spectrometry

Tiffany Andras and co-workers studied the influence of certain seaweeds on the grwoth inhibition of coral reefs.  Extracts of Phacelocarpus neurymenioides revealed a previously characterized antibacterial metabolite, neurymenolide A, as the main allelopathic agent.The authors used desorption electrospray ionization mass spectrometry (DESI-MS) to visualize and quantify neurymenolide A on the surface of P. neurymenioides, and found the neurymenolide A on all surfaces analyzed, with highest concentrations on basal portions of blades.  

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5 tips for success in DESI imaging

1. Solvent selection is a key step in the method development process for DESI.  Always consider the analyte chemistry when choosing a solvent composition.  Follow electrospray ionization principles.
2. Spray stability is a must in order to achieve reproducible measurements using DESI.  Ensure that the solvent flow rate is stable, the DESI emitter is free of debris and cracks, the solvent delivery line is not pinched restricting flow and there are no air bubbles in the line or syringe.
3. The spot size increases and decreases based on the solvent flow rate and the nebulization gas pressure.  Be aware that as the spot size decreases, a corresponding decrease in sensitivity occurs.
4. DESI spray voltages above 4kV are not recommended.  Discharging will occur that is deleterious to the analyte signal.  The working range is 2-3.5kV.
5. Continuous velocity scanning is recommended for most applications.  In the DESI Motion Control software, use Start Point CV.  

New Publication: Developmental phases of individual mouse preimplantation embryos characterized by lipid signatures using desorption electrospray ionization mass spectrometry

In this work, the profiles of fatty acids and phospholipids (PL) in individual mouse preimplantation embryos and oocytes were acquired using an analytical strategy based on desorption electrospray ionization mass spectrometry (DESI-MS). The methodology avoids sample preparation and provides information on the lipids present in these microscopic structures. Differences in the lipid profiles observed for unfertilized oocytes, two- and four-cell embryos, and blastocysts were characterized. For a representative set of embryos (N = 114) using multivariate analysis (specifically principal component analysis) unfertilized oocytes showed a narrower range of PL species than did blastocysts. Two- and four-cell embryos showed a wide range of PLs compared with unfertilized oocytes and high abundances of fatty acids, indicating pronounced synthetic activity. The data suggest that the lipid changes observed in mouse preimplantation development reflect acquisition of a degree of cellular membrane functional and structural specialization by the blastocyst stage. It is also noteworthy that embryos cultured in vitro from the two-cell through the blastocyst stage have a more homogeneous lipid profile as compared with their in vivo-derived counterparts, which is ascribed to the restricted diversity of nutrients present in synthetic culture media. The DESI-MS data are interpreted from lipid biochemistry and previous reports on gene expression of diverse lipids known to be vital to early embryonic development.

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