Ambient ionization

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File:Ambient Ionization Diagram.jpg
Diagram of ambient ionization in mass spectrometry indicating desorption/extraction (spray, heat, laser), optional post-ionization (electrospray, chemical ionization, plasma), ion formation, and entry into the vacuum of the mass spectrometer.

Ambient ionization is a form of ionization in which ions are formed in an ion source outside the mass spectrometer without sample preparation or separation.[1][2][3][4] Ions can be formed by extraction into charged electrospray droplets, thermally desorbed and ionized by chemical ionization, or laser desorbed or ablated and post-ionized before they enter the mass spectrometer.[5]

Solid-liquid extraction

Schematic of a DESI solid-liquid extraction ion source: primary charged droplets hit the sample surface and molecules are extracted into the liquid. Secondary charged droplets removed from the surface produce bare ions as the solvent evaporates.

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.[3][6] 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[7] 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.[8]

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.[9]

Plasma-based techniques

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^* + \left[(H_2O\right)_nH] \to \left[(H_2O\right)_{n-1}H]^{+} + OH^\centerdot + e^- .

The protonated water clusters can then protonate the sample molecules via

\left[(H_2O\right)_nH]^{+}  + M \to [M+H]^+ + nH_2O.

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.[10][11]

Besides protonated water clusters, other positively charged reagent ions, such as NO+, O2+, NO2+ and CO2+, may be formed in the afterglow region.[10][11][12][13] 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.[11]

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.[11][13] 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
A direct analysis in real time (DART) metastable ion source for plasma based ambient ionization.

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.[14]

Laser assisted

File:IRLDESI Side View.jpg
Ion source for ambient mass spectrometry employing a combination of laser desorption and electrospray. The sample target is on the left.

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,[15] it is not generally considered to be an ambient mass spectrometry technique.[16][17]

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).[18] The laser ablates the sample material that is introduced into an ICP to create atomic ions.

Infrared laser desorption can be coupled with atmospheric pressure chemical ionization using laser desorption atmospheric pressure chemical ionization (LD-APCI).[19] 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 [20] and infrared lasers[21] 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),[22] laser ablation electrospray ionization (LAESI),[23] laser assisted desorption electrospray ionization (LADESI),[24] laser desorption electrospray ionization (LDESI),[25][26] laser ablation mass spectrometry (LAMS),[27] and laser desorption spray post-ionization (LDSPI).[28] The term laser electrospray mass spectrometry has been used to denote the use of a femtosecond laser for ablation.[29][30] 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.[31] and IR desorption has been combined with a metastable ion source.[32]

Two step non-laser

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

File:PESI.png
Probe electrospray ionization schematic

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.[33] 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.

Table of techniques

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).[3]

Acronym Technique Classification
AFAI[34] Air flow-assisted ionization Extraction
AFADESI[35] Air flow-assisted desorption electrospray ionization Extraction
APGDDI[36] Atmospheric pressure glow discharge desorption ionization Plasma
APPIS[37] Ambient pressure pyroelectric ion source
APTDCI[38] Atmospheric pressure thermal desorption chemical ionization Two-step
APTDI[39] Atmospheric pressure thermal desorption/ionization Plasma
ASAP[40] Atmospheric pressure solids analysis probe Plasma
BADCI[41] Beta electron-assisted direct chemical ionization Two step
CALDI[42] Charge assisted laser desorption/ionization Laser
DAPCI[43] Desorption atmospheric pressure chemical ionization Plasma
DAPPI[44] Desorption atmospheric pressure photoionization Extraction
DART[45] Direct analysis in real time Plasma
DBDI[46] Dielectric barrier discharge ionization Plasma
DCBI[46] Desorption corona beam ionization Plasma
DCI Desorption chemical ionization Plasma
DEFFI[47] Desorption electro-flow focusing ionization Extraction
DEMI[48] Desorption electrospray/metastable-induced ionization Multimode
DESI[7] Desorption electrospray ionization Extraction
DeSSI[49] Desorption sonic spray ionization Extraction
DICE[50] Desorption ionization by charge exchange Extraction
DIP-APCI[51] Direct inlet probe–atmospheric-pressure chemical ionization Two-step
DPESI[52] Direct probe electrospray ionization
EADESI[53] Electrode-assisted desorption electrospray ionization Extraction
EASI[54] Easy ambient sonic-spray ionization Extraction
EESI[55] Extractive electrospray ionization Two step
ELDI[56] Electrospray laser desorption ionization Laser
ESA-Py[57] Electrospray-assisted pyrolysis ionization Spray
ESTASI[58] Electrostatic spray ionization Extraction
FAPA[12] Flowing atmospheric pressure afterglow Plasma
FIDI[59] Field-induced droplet ionization
HALDI[60] High-voltage-assisted laser desorption ionization Laser
HAPGDI[12] Helium atmospheric pressure glow discharge ionization Plasma
IR-LAMICI[32] Infrared laser ablation metastable-induced chemical ionization Laser
JeDI[61] Jet desorption electrospray ionization Extraction
LADESI[24] Laser assisted desorption electrospray ionization Laser
LAESI[62] Laser ablation electrospray ionization Laser
LA-FAPA[31] Laser ablation flowing atmospheric pressure afterglow Laser
LA-ICP[63] Laser ablation inductively coupled plasma Laser
LD-APCI[19] Laser desorption atmospheric pressure chemical ionization Laser
LDTD[64] Laser diode thermal desorption Laser
LDESI[25][26] Laser desorption electrospray ionization Laser
LDSPI[28] Laser desorption spray post-ionization Laser
LEMS[30] Laser electrospray mass spectrometry Laser
LESA[65] Liquid extraction surface analysis Extraction
LIAD-ESI[66] Laser-induced acoustic desorption-electrospray ionization Acoustic
LMJ-SSP[67] Liquid microjunction-surface sampling probe Extraction
LPTD[68] Leidenfrost phenomenon-assisted thermal desorption Two-step
LS-APGD[69] Liquid sampling-atmospheric pressure glow discharge Plasma
LSI[70] Laser spray ionization Other
LTP[71] Low temperature plasma Plasma
MAII[72] Matrix-assisted inlet ionization Other
MALDESI[73] Matrix-assisted laser desorption electrospray ionization Laser
MFGDP[74] Microfabricated glow discharge plasma Plasma
MIPDI [75] microwave induced plasma desorption ionization Plasma
nano-DESI[76] Nanospray desorption electrospray ionization Extraction
ND-EESI[77] Neutral desorption extractive electrospray ionization Two step
PADI[78] Plasma-assisted desorption ionization Plasma
Paint Spray*[79] Paint spray Extraction
PALDI[80] Plasma-assisted laser desorption ionization Laser
PAMLDI[81] Plasma-assisted multiwavelength laser desorption ionization Laser
PASIT[82] Plasma-based ambient sampling/ionization/transmission Extraction
PAUSI[83] Paper assisted ultrasonic spray ionization
PESI[84] Probe electrospray ionization Two step
PS[85] Paper spray
PTC-ESI[86] Pipette tip column electrospray ionization Extraction
RADIO[87] Radiofrequency acoustic desorption and ionization Acoustic
RASTIR[88] Remote analyte sampling transport and ionization relay
REIMS[89] Rapid evaporative ionization mass spectrometry Other
RoPPI[90] Robotic plasma probe ionization Two-step
SACI[91] Surface activated chemical ionization
SAII[92] Solvent-assisted inlet ionization Other
SAWN[93] Surface acoustic wave nebulization Acoustic
SESI[94] Secondary electrospray ionization
SPA-nanoESI[95] Solid probe assisted nanoelectrospray ionization Two-step
SPAMS[96] Single-particle aerosol mass spectrometry Other
SSP[97] Surface sampling probe Extraction
SwiFerr[98] Switched ferroelectric plasma ionizer Other
TDAMS[99] Thermal desorption-based ambient mass spectrometry Spray
TM-DESI[100] Transmission mode desorption electrospray ionization Extraction
TS[101] Touch spray Two-step
UASI[102] Ultrasonication-assisted spray ionization Acoustic
V-EASI[103] Venturi easy ambient sonic-spray ionization Extraction

(*) Not an acronym.

References

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