Question

1. Electrospray ionization (ESI) is used to create gas-phase ions for mass analysis. The process of ESI is similar to a common method for painting cars called “electrostatic coating.” Look up how electrostatic coating works and explain why this process is so useful for volatilizing and ionizing analyte molecules prior to mass analysis.

2. Matrix assisted laser desorption ionization (MALDI) is another common, soft-ionization technique which uses intense radiation (lasers) for ionization/vaporization prior to mass analysis. Research the MALDI technique. How does MALDI generate ions? What is the “matrix” used for this type of ionization? What types of molecules are analyzed with MALDI? Key difference: ESI vs MALDI à MALDI generates singly-charged ions while ESI generates multiply-charged ions.

3. Finding Neo (Advanced): Can you find mention of LDI (no matrix)? What is the purpose of the matrix in MALDI?

Answer 1

Answer:

How electrostatic coating works?

An addition to the electrostatic coating (or e-coating) process is dipping electrically conductive parts into a tank of paint that is then electrostatically charged. The ionic bond of the paint to the metal creates the paint coating, in which its thickness is directly proportional to the length of time the parts are left in the tank and the time the charge remains active. Once the parts are removed from the paint tank, they are rinsed off to remove any residual paint that is not ionically bonded, leaving a thin film of electrostatically bonded paint on the surface of the part.

In gas-phase sources, the sample is first volatilized before the ionization of the gaseous components takes place in various ways. The sample is vaporized outside the ion source. Examples of ionization methods are chemical ionization, electron-impact ionization, and field ionization.

How does MALDI generate ions?

In mass spectrometry, matrix-assisted laser desorption/ionization (MALDI) is an ionization technique that uses a laser energy absorbing matrix to create ions from large molecules with minimal fragmentation.

MALDI methodology is a three-step process. First, the sample is mixed with a suitable matrix material and applied to a metal plate. Second, a pulsed laser irradiates the sample, triggering ablation and desorption of the sample and matrix material. Finally, the analyte molecules are ionized by being protonated or deprotonated in the hot plume of ablated gases, and then they can be accelerated into whichever mass spectrometer is used to analyze them.

What is the “matrix” used for this type of ionization?

Laser energy-absorbing materials matrix used. The matrix consists of crystallized molecules, of which the three most commonly used are 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid), α-cyano-4-hydroxycinnamic acid (α-CHCA, alpha-cyano or alpha-matrix) and 2,5-dihydroxybenzoic acid (DHB).

What types of molecules are analyzed with MALDI?

It has been applied to the analysis of biomolecules (biopolymers such as DNA, proteins, peptides, and sugars) and large organic molecules (such as polymers, dendrimers, and other macromolecules).

Key difference: ESI vs MALDI: MALDI generates singly-charged ions while ESI generates multiply-charged ions.

What is the purpose of the matrix in MALDI?

They are of a fairly low molecular weight (to allow easy vaporization) but are large enough (with a low enough vapor pressure) not to evaporate during sample preparation or while standing in the mass spectrometer. They are often acidic, therefore act as a proton source to encourage ionization of the analyte. Basic matrices have also been reported. They have strong optical absorption in either the UV or IR range so that they rapidly and efficiently absorb the laser irradiation. This efficiency is commonly associated with chemical structures incorporating several conjugated double bonds, as seen in the structure of cinnamic acid. They are functionalized with polar groups, allowing their use in aqueous solutions. They typically contain a chromophore.