Ambient ionization

Image:DESI ion source.jpg

Solid-liquid extraction based ambient ionization is based on the use of a charged spray, for example electrospray to create a liquid film on the sample surface.{{cite journal|last1=Badu-Tawiah|first1=Abraham K.|last2=Eberlin|first2=Livia S.|last3=Ouyang|first3=Zheng|last4=Cooks|first4=R. Graham|title=Chemical Aspects of the Extractive Methods of Ambient Ionization Mass Spectrometry|journal=Annual Review of Physical Chemistry|volume=64|issue=1|year=2013|pages=481–505|issn=0066-426X|doi=10.1146/annurev-physchem-040412-110026|pmid=23331308|bibcode = 2013ARPC...64..481B }} Molecules on the surface are extracted into the solvent. The action of the primary droplets hitting the surface produces secondary droplets that are the source of ions for the mass spectrometer.

Desorption electrospray ionization (DESI) is one of the original ambient ionization sources{{cite journal|last1=Takats|first1=Z.|last2=Wiseman|first2=J. M.|last3=Gologan|first3=B|last4=Cooks|first4=R. G.|title=Mass Spectrometry Sampling Under Ambient Conditions with Desorption Electrospray Ionization|journal=Science|volume=306|issue=5695|year=2004|pages=471–473|issn=0036-8075|doi=10.1126/science.1104404|pmid=15486296|bibcode = 2004Sci...306..471T |s2cid=22994482 }} and uses an electrospray source to create charged droplets that are directed at a solid sample. The charged droplets pick up the sample through interaction with the surface and then form highly charged ions that can be sampled into a mass spectrometer.{{cite journal |vauthors=Takáts Z, Wiseman JM, Cooks RG |title=Ambient mass spectrometry using desorption electrospray ionization (DESI): instrumentation, mechanisms and applications in forensics, chemistry, and biology |journal=Journal of Mass Spectrometry |volume=40 |issue=10 |pages=1261–75 |year=2005 |pmid=16237663 |doi=10.1002/jms.922 |bibcode=2005JMSp...40.1261T |doi-access=free }}

Desorption atmospheric pressure photoionization (DAPPI) is a solid-liquid extraction ambient ionization method that enables the direct analysis of samples deposited on surfaces by means of a jet of hot solvent vapour and ultraviolet light. The hot jet thermally desorbs the sample from a surface and the vaporized sample is ionized by a vacuum ultraviolet light and consequently sampled into a mass spectrometer.{{cite journal |vauthors=Haapala M, Pól J, Saarela V, Arvola V, Kotiaho T, Ketola RA, Franssila S, Kauppila TJ, Kostiainen R |title=Desorption Atmospheric Pressure Photoionization |journal=Anal. Chem. |volume=79 |issue=20 |pages=7867–7872 |year=2007 |doi=10.1021/ac071152g |pmid=17803282 }}

Plasma-based ambient ionization is based on an electrical discharge in a flowing gas that produces metastable atoms and molecules and reactive ions. Heat is often used to assist in the desorption of volatile species from the sample. Ions are formed by chemical ionization in the gas phase.

One proposed mechanism involves Penning ionization of ambient water clusters in a helium discharge:

:He^\ast{} + [(H2O)_\mathit{n}H] ->{} [(H2O)_{\mathit n-1}H]+{} + OH^.{} + e^- .

The protonated water clusters can then protonate the sample molecules via

:[(H2O)_\mathit{n}H]+{} + M ->{} [M{} + H]+{} + \mathit{n}H2O.

For this ionization pathway, the gas-phase acidity of the protonated water clusters and the gas-phase basicity of the analyte molecule are of crucial importance. However, since especially smaller protonated water clusters with n = 1,2,3... exhibit very high gas-phase acidities, even compounds with a rather low gas-phase basicity are readily ionized by proton transfer, yielding [M+H]⁺ quasimolecular ions.{{Cite journal|title = Characterization of Direct-Current Atmospheric-Pressure Discharges Useful for Ambient Desorption/Ionization Mass Spectrometry|journal = Journal of the American Society for Mass Spectrometry|date = 2009-05-01|pages = 837–844|volume = 20|issue = 5|doi = 10.1016/j.jasms.2008.12.020|pmid = 19185515|first1 = Jacob T.|last1 = Shelley|first2 = Joshua S.|last2 = Wiley|first3 = George C. Y.|last3 = Chan|first4 = Gregory D.|last4 = Schilling|first5 = Steven J.|last5 = Ray|first6 = Gary M.|last6 = Hieftje|doi-access = free}}{{Cite journal|title = Critical assessment of ionization patterns and applications of ambient desorption/ionization mass spectrometry using FAPA–MS|journal = Journal of Mass Spectrometry|date = 2016-02-01|issn = 1096-9888|pages = 141–149|volume = 51|issue = 2|doi = 10.1002/jms.3733|pmid = 26889930|language = en|first1 = Martin|last1 = Brüggemann|first2 = Einar|last2 = Karu|first3 = Thorsten|last3 = Hoffmann|bibcode = 2016JMSp...51..141B}}

Besides protonated water clusters, other positively charged reagent ions, such as NO⁺, O₂⁺, NO₂⁺ and CO₂⁺, may be formed in the afterglow region.{{Cite journal|title = Direct analysis in real time—a critical review on DART-MS|journal = Analytical and Bioanalytical Chemistry|date = 2013-09-15|issn = 1618-2642|pages = 63–80|volume = 406|issue = 1|doi = 10.1007/s00216-013-7316-0|pmid = 24036523|language = en|first = Jürgen H.|last = Gross| s2cid=9565130 }} These additional reagent ions are capable of ionizing compounds via charge-transfer processes and, thus, offer alternative routes of ionization besides proton transfer, leading to a broader range of suitable analytes. Nevertheless, these ionization mechanisms may also lead to the formation of adducts and oxidation of the original analyte compounds.

Although most applications focus on the detection of positive ions, measurements in the negative mode are for most of the plasma-based ion sources also possible. In this case, reagent ions, such as O₂^–, can deprotonate the analyte molecules to give [M–H]^– quasimolecular ions, or form adducts with species such as NO₃^–, yielding [M+NO₃]^– ions. Measurements in the negative ion mode are especially favorable when the analyte molecules exhibit a high gas-phase acidity, as it is the case e.g. for carboxylic acids.File:DART schematic small.png

One of the most used plasma-based techniques for ambient ionization is probably Direct analysis in real time (DART), since it is commercially available. DART is an atmospheric pressure ion source that operates by exposing the sample to a gas stream (typically helium or nitrogen) that contains long-lived electronically or excited neutral atoms, vibronically excited molecules (or "metastables"). Excited states are formed in a glow discharge in a chamber through which the gas flows.{{cite journal |author=R.B. Cody |author2=J.A. Laramée |author3=H.D. Durst |journal=Anal. Chem. |year=2005 |volume=77 |issue=8 |pages=2297–2302 |pmid=15828760 |doi=10.1021/ac050162j |title=Versatile New Ion Source for the Analysis of Materials in Open Air under Ambient Conditions |url=https://figshare.com/articles/journal_contribution/3290839/files/5128576.pdf }}

File:IRLDESI Side View.jpg

Laser-based ambient ionization is a two-step process in which a pulsed laser is used to desorb or ablate material from a sample and the plume of material interacts with an electrospray or plasma to create ions. Lasers with ultraviolet and infrared wavelengths and nanosecond to femtosecond pulse widths have been used. Although atmospheric pressure MALDI is performed under ambient conditions,{{cite journal|last1=Laiko|first1=Victor V.|last2=Baldwin|first2=Michael A.|last3=Burlingame|first3=Alma L.|title=Atmospheric Pressure Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry|journal=Analytical Chemistry|volume=72|issue=4|year=2000|pages=652–657|issn=0003-2700|doi=10.1021/ac990998k|pmid=10701247}} it is not generally considered to be an ambient mass spectrometry technique.{{cite journal|last1=Ifa|first1=Demian R.|last2=Wu|first2=Chunping|last3=Ouyang|first3=Zheng|last4=Cooks|first4=R. Graham|title=Desorption electrospray ionization and other ambient ionization methods: current progress and preview|journal=The Analyst|volume=135|issue=4|year=2010|pages=669–81|issn=0003-2654|doi=10.1039/b925257f|pmid=20309441|bibcode = 2010Ana...135..669I }}{{cite journal|last1=Wu|first1=Chunping|last2=Dill|first2=Allison L.|last3=Eberlin|first3=Livia S.|last4=Cooks|first4=R. Graham|last5=Ifa|first5=Demian R.|title=Mass spectrometry imaging under ambient conditions|journal=Mass Spectrometry Reviews|volume=32|issue=3|year=2013|pages=218–243|issn=0277-7037|doi=10.1002/mas.21360|pmid=22996621|pmc=3530640|bibcode=2013MSRv...32..218W}}

Laser ablation was first coupled with mass spectrometry in the 1980s for the analysis of metals using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS).{{cite journal|last1=Gray|first1=Alan L.|title=Solid sample introduction by laser ablation for inductively coupled plasma source mass spectrometry|journal=The Analyst|volume=110|issue=5|year=1985|pages=551|issn=0003-2654|doi=10.1039/an9851000551|bibcode = 1985Ana...110..551G }} The laser ablates the sample material that is introduced into an ICP to create atomic ions.

File:Probe electrospray ionization (schematic).png

Infrared laser desorption can be coupled with atmospheric pressure chemical ionization using laser desorption atmospheric pressure chemical ionization (LD-APCI).{{cite journal|last1=Coon|first1=Joshua J.|last2=McHale|first2=Kevin J.|last3=Harrison|first3=W. W.|title=Atmospheric pressure laser desorption/chemical ionization mass spectrometry: a new ionization method based on existing themes|journal=Rapid Communications in Mass Spectrometry|volume=16|issue=7|year=2002|pages=681–685|issn=0951-4198|doi=10.1002/rcm.626|pmid=11921247|bibcode=2002RCMS...16..681C}} For ambient ionization with a spray, the sample material is deposited on a target near the spray. The laser desorbs or ablates material from the sample that is ejected from the surface and into the spray, which can be an APCI spray with a corona discharge or an electrospray. Ambient ionization by electrospray-assisted laser desorption/ionization (ELDI) can be accomplished with ultraviolet{{cite journal |vauthors=Shiea J, Huang MZ, Hsu HJ, Lee CY, Yuan CH, Beech I, Sunner J |title=Electrospray-assisted laser desorption/ionization mass spectrometry for direct ambient analysis of solids |journal=Rapid Commun. Mass Spectrom. |volume=19 |issue=24 |pages=3701–4 |year=2005 |pmid=16299699 |doi=10.1002/rcm.2243 |bibcode=2005RCMS...19.3701S }} and infrared lasers{{cite journal|last1=Peng|first1=Ivory X.|last2=Ogorzalek Loo|first2=Rachel R.|last3=Margalith|first3=Eli|last4=Little|first4=Mark W.|last5=Loo|first5=Joseph A.|title=Electrospray-assisted laser desorption ionization mass spectrometry (ELDI-MS) with an infrared laser for characterizing peptides and proteins|journal=The Analyst|volume=135|issue=4|year=2010|pages=767–72|issn=0003-2654|doi=10.1039/b923303b|pmid=20349541|bibcode = 2010Ana...135..767P|pmc=3006438}} to the desorb material into the electrospray plume. Similar approaches to laser desorption/ablation into an electrospray are matrix-assisted laser desorption electrospray ionization (MALDESI),{{cite journal |vauthors=Sampson JS, Hawkridge AM, Muddiman DC |title=Generation and detection of multiply-charged peptides and proteins by matrix-assisted laser desorption electrospray ionization (MALDESI) Fourier transform ion cyclotron resonance mass spectrometry |journal=J. Am. Soc. Mass Spectrom. |volume=17 |issue=12 |pages=1712–6 |year=2006 |pmid=16952462 |doi=10.1016/j.jasms.2006.08.003 |doi-access=free }} laser ablation electrospray ionization (LAESI),{{cite journal |vauthors=Nemes P, Vertes A |title=Laser Ablation Electrospray Ionization for Atmospheric Pressure, in Vivo, and Imaging Mass Spectrometry |journal= Analytical Chemistry|volume= 79|issue= 21|pages= 8098–106|year=2007 |pmid=17900146 |doi=10.1021/ac071181r }} laser assisted desorption electrospray ionization (LADESI), laser desorption electrospray ionization (LDESI),{{cite journal|last1=Sampson|first1=Jason S.|last2=Muddiman|first2=David C.|title=Atmospheric pressure infrared (10.6 μm) laser desorption electrospray ionization (IR-LDESI) coupled to a LTQ Fourier transform ion cyclotron resonance mass spectrometer|journal=Rapid Communications in Mass Spectrometry|volume=23|issue=13|year=2009|pages=1989–1992|issn=0951-4198|doi=10.1002/rcm.4113|pmid=19504481}}{{cite journal|last1=Berisha|first1=Arton|last2=Dold|first2=Sebastian|last3=Guenther|first3=Sabine|last4=Desbenoit|first4=Nicolas|last5=Takats|first5=Zoltan|last6=Spengler|first6=Bernhard|last7=Römpp|first7=Andreas|title=A comprehensive high-resolution mass spectrometry approach for characterization of metabolites by combination of ambient ionization, chromatography and imaging methods|journal=Rapid Communications in Mass Spectrometry|volume=28|issue=16|year=2014|pages=1779–1791|issn=0951-4198|doi=10.1002/rcm.6960|pmid=25559448|bibcode=2014RCMS...28.1779B }} laser ablation mass spectrometry (LAMS),{{cite journal|last1=Jorabchi|first1=Kaveh|last2=Smith|first2=Lloyd M.|title=Single Droplet Separations and Surface Partition Coefficient Measurements Using Laser Ablation Mass Spectrometry|journal=Analytical Chemistry|volume=81|issue=23|year=2009|pages=9682–9688|issn=0003-2700|doi=10.1021/ac901819r|pmid=19886638|pmc=2911232}} and laser desorption spray post-ionization (LDSPI).{{cite journal|last1=Liu|first1=Jia|last2=Qiu|first2=Bo|last3=Luo|first3=Hai|title=Fingerprinting of yogurt products by laser desorption spray post-ionization mass spectrometry|journal=Rapid Communications in Mass Spectrometry|volume=24|issue=9|year=2010|pages=1365–1370|issn=0951-4198|doi=10.1002/rcm.4527|pmid=20391610|bibcode=2010RCMS...24.1365L}} The term laser electrospray mass spectrometry has been used to denote the use of a femtosecond laser for ablation.{{cite journal |title=Ambient Femtosecond Laser Vaporization and Nanosecond Laser Desorption Electrospray Ionization Mass Spectrometry |journal=Annual Review of Analytical Chemistry |year=2014 |last1=Flanigan |first1=P. |last2=Levis |first2=R. |volume=7 |pages=229–256 |doi=10.1146/annurev-anchem-071213-020343 |pmid=25014343 |bibcode = 2014ARAC....7..229F }}{{cite journal|last1=Brady|first1=John J.|last2=Judge|first2=Elizabeth J.|last3=Levis|first3=Robert J.|title=Mass spectrometry of intact neutral macromolecules using intense non-resonant femtosecond laser vaporization with electrospray post-ionization|journal=Rapid Communications in Mass Spectrometry|volume=23|issue=19|year=2009|pages=3151–3157|issn=0951-4198|doi=10.1002/rcm.4226|pmid=19714710|bibcode=2009RCMS...23.3151B}} Laser ablation into an electrospray produces highly charged ions that are similar to those observed in direct electrospray.

An alternative ionization approach following laser desorption is a plasma. UV laser ablation can be combined with a flowing afterglow plasma for mass spectrometry imaging of small molecules. and IR desorption has been combined with a metastable ion source.

In two-step non-laser methods, the material removal from the sample and the ionization steps are separate.

Probe electrospray ionization (PESI) is a modified version of conventional electrospray ionization in which the capillary for sample solution transferring is replaced by a solid needle with a sharp tip.PESI was first introduced by Kenzo Hiraoka et al. in 2007 — {{cite journal |author1=Hiraoka K. |author2=Nishidate K. |author3=Mori K. |author4=Asakawa D. |author5=Suzuki S. | title = Development of probe electrospray using a solid needle | journal = Rapid Communications in Mass Spectrometry | year = 2007| volume = 21 | pages = 3139–3144 | doi = 10.1002/rcm.3201 | pmid=17708527 | issue = 18 | bibcode = 2007RCMS...21.3139H}} Compared with conventional electrospray ionization, high salt tolerance, direct sampling, and low sample consumption are found with PESI. PESI is not a continuous process; the needle for sampling and spraying is driven up and down at a frequency of 3–5 Hz.

The analytes are in the vapor phase. This includes breath, odors, VOCs, and other molecules with low volatility that, due to the constant improvements in sensitivity, are detectable in the vapor phase despite their low vapor pressure. Analyte ions are produced via gas-phase chemical reactions, where charging agents collide with the analyte molecules and transfer their charge. In secondary electro-spray ionization (SESI), a nano-electrospray operated at high temperature produces nanodroplets that evaporate very rapidly to produce ions and protonated water clusters that ionize the vapors of interest. SESI is commonly used for the analysis of trace concentrations of vapors being able to detect low volatility species in the gas phase with molecular masses of up to 700 Da.

In the table below, ambient ionization techniques are classified in the categories "extraction" (a solid or liquid extraction processes dynamically followed by spray or chemical ionization), "plasma" (thermal or chemical desorption with chemical ionization), "two step" (desorption or ablation followed by ionization), "laser" (laser desorption or ablation followed by ionization), "acoustic" (acoustic desorption followed by ionization), multimode (involving two of the above modes), other (techniques that do not fit into the other categories).

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(*) Not an acronym.

{{Reflist}} {{Mass spectrometry}}

Category:Mass spectrometry

Category:Ion source