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Desorption Electrospray Ionization (DESI)

Acronym	DESI
Classification	Mass spectrometry
Analytes	Organic molecules Biomolecules
Other techniques
Related	Electrospray ionization Atmospheric pressure chemical ionization

Desorption electrospray ionization (DESI) is an ambient ionization technique that can be used in mass spectrometry for chemical analysis. It is an atmospheric pressure ion source that ionizes gases, liquids and solids in open air under ambient conditions. It was developed in 2004 by Zoltan Takáts, et al., in Professor Graham Cooks' group from Purdue University and is now available commercially by Prosolia Inc. DESI is a similar ionization technique to Direct Analysis in Real Time (DART) in its versatility, applications and analysis time.

History

DESI was developed in 2004 by Zoltan Takáts, et al., in Professor Graham Cooks' group from Purdue University with the goal of looking into methods that didn't require the sample to be inside of a vacuum. They came across this method due to the fact that it didn't need a matrix and you produce multiply charged ions as in electrospray ionization. It is now available commercially by Prosolia Inc. DESI is a similar ionization technique to Direct Analysis in Real Time (DART) in its versatility, applications and analysis time. With increasing studies into optimization there is a new technique that is Nanospray desorption electrospray ionization, which is a similar technique in which the analyte is desorbed into a bridge formed via two capillaries and the analysis surface.

Principle of operation

DESI is a combination of electrospray (ESI) and desorption (DI) ionization methods. Ionization takes place by directing an electrically charged mist to the sample surface that is a few millimeters away. The electrospray mist is pneumatically directed at the sample where subsequent splashed droplets carry desorbed, ionized analytes. After ionization, the ions travel through air into the atmospheric pressure interface which is connected to the mass spectrometer. DESI is a technique that allows for ambient ionization of a trace sample at atmospheric pressure, with little sample preparation. DESI can be used to to investigate in situ, secondary metabolites specifically looking at both spatial and temporal distributions.

Ionization mechanism

In DESI there are two kinds of ionization mechanism, one that applies to low molecular weight molecules and another to high molecular weight molecules. High molecular weight molecules, such as proteins and peptides show electrospray like spectra where multiply charged ions are observed. This suggests desorption of the analyte, where multiple charges in the droplet can easily be transferred to the analyte. The charged droplet hits the sample, spreads over a diameter greater than its original diameter, dissolves the protein and rebounces. The droplets travel to the mass spectrometer inlet and are further desolvated. The solvent typically used for the electrospray is a combination of methanol and water.

For the low molecular weight molecules, ionization occurs by charge transfer: an electron or a proton. There are three possibilities for the charge transfer. First, charge transfer between a solvent ion and an analyte on the surface. Second, charge transfer between a gas phase ion and analyte on the surface; in this case the solvent ion is evaporated before reaching the sample surface. This is achieved when the spray to surface distance is large. Third, charge transfer between a gas phase ion and a gas phase analyte molecule. This occurs when a sample has a high vapour pressure.

${\ce {{A_{(aq)}^{-}}+S_{(s)}->{A_{(aq)}}+S_{(s)}^{-}}}$ (1)

${\ce {{A_{(g)}^{-}}+S_{(s)}->{A_{(g)}}+S_{(s)}^{-}}}$ (2)

${\ce {{A_{(g)}^{-}}+S_{(g)}->{A_{(g)}}+S_{(g)}^{-}}}$ (3)

The ionization mechanism of low molecular weight molecules in DESI is similar to DART’s ionization mechanism, in that there is a charge transfer that occurs in the gas phase.

Ionization efficiency

Side view of DESI ion source along with a table containing typical values for the geometric parameters

The ionization efficiency of DESI is complex and depends on several parameters such as, surface effects, electrospray parameters, chemical parameters and geometric parameters. Surface effects include chemical composition, temperature and electric potential applied. Electrospray parameters include electrospray voltage, gas and liquid flow rates. Chemical parameters refers to the sprayed solvent composition, e.g. addition of NaCl. Geometric parameters are α, β, d₁ and d₂ (see figure on the right).

Furthermore, α and d₁ affect the ionization efficiency, while β and d₂ affect the collection efficiency. Results of a test performed on a variety of molecules to determine optimal α and d1 values show that there are two sets of molecules: high molecular weight (proteins, peptides, oligosaccharide etc.) and low molecular weight (dizazo dye, stereoids, caffeine, nitroaromatics etc.). The optimal conditions for the high molecular weight group are high incident angles (70-90°) and short d₁ distances (1–3 mm). The optimal conditions for the low molecular weight group are the opposite, low incident angles (35-50°) and long d₁ distances (7–10 mm). These test results indicate that each group of molecules has a different ionization mechanism; described in detail in the Principle of operation section.

The sprayer tip and the surface holder are both attached to a 3D moving stage which allow to select specific values for the four geometric parameters: α, β, d₁ and d₂.

Applications

MALDESI of biomolecules on an ice matrix

Laser Desorption electrospray ionization (LDESI)

Laser ablation desorption electrospray ionization mass spectrometry is an ambient ionization technique applicable to plant and animal tissue imaging, live-cell imaging, and most recently to cell-by-cell imaging. This technique uses a mid-IR laser to ablate the sample which creates a cloud of neutral molecules. This cloud is then hit with the electrospray from above to cause ionization. The desorbed ions are then able to pass into the mass spectrometer for analysis. This method is also good for imaging in applications. The analyses can be desorbed through a pulsed laser irradiation without the need of a matrix. This method is best used with small organic molecules up to larger biomolecules as well.

Matrix Assisted Laser desorption electrospray ionization

Another method good for biomolecules. In this technique, it utilizes Infrared laser ionization to excite the sample molecules to allow for the desorbed ions to be ready for MS analysis. The geometry of the source and the distance between the ESI and matrix will have and effect on the efficiency of the sample compound. This technique can also be used with aqueous samples as well. The water droplet can be placed at the focal point of the laser, or the droplet can be dried to form the solid. Planar samples do not need sample preparation to perform this experiment.

Instrumentation

Schematic diagram of the DESI ion source

See also

• Secondary ion mass spectrometry
• Matrix-assisted laser desorption ionization
• Mass spectrometry imaging

External links

• Purdue Aston Lab - DESI
• Method and System for Desorption Electrospray Ionization - US application 20,050,230,635 
• Method and System for Desorption Electrospray Ionization - WO 2005094389 
• Ionization by droplet impact - US application 20,060,108,539 
• Prof Graham Cooks personal page - Purdue University
• Zoltan Takáts personal page - Imperial College London