General principles of MALDI-TOF-MS

Posted on Posted in how-it-works

In this post, I would like to describe general principles of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), which is a great tool for exploring the chemical world of organic solar cells, dye-sensitized solar cells as well as solar fuel cells.

In MALDI-TOF-MS, as the name implies, the ionization is induced by a laser (most often, it is a 337 nm nitrogen laser operating at 337 nm) and assisted by a matrix (usually, it is a low molecular weight organic compound absorbing the laser irradiation).
At the first step of an MALDI experiment, an analyzed sample is mixed with an excess of a matrix.

At the second step, the matrix-sample mixture is irradiated with a focused laser beam; absorbed energy of the laser beam causes evaporation (desorption) and ionization of the matrix molecules. Captured by the matrix molecules, the sample molecules are also released into the gas phase and get ionized by the charge-transfer reaction with matrix ions.

However, it is not always necessary to add an auxiliary matrix. In the case when the analyte molecules absorb UV light themselves, they can get charged via the multiphoton ionization, so the sample itself plays the matrix role. This variation of the technique is called laser desorption ionization mass spectrometry (LDI-MS). {Wait, many components of solar cells absorb UV light, don't they?} In another variation, surface-assisted laser desorption ionization mass spectrometry (SALDI-MS), surface, on which the sample is applied, absorbs UV radiation, charges the analyte molecules, and releases them into the gas phase. {Usually, a layer of a transition metal oxide (e.g. TiO2), which absorb UV light, is a part of the photoelectrochemical cell, right?}

After the formation of the ion cloud, the MALDI instrument applies a high voltage pulse between the sample plate and the mass analyzer entrance, accelerating the formed ions into a field-free flight tube. {That’s why the examined sample should always be placed onto a conductive substrate, but that is not a problem for solar cells, they are fabricated on a conductive substrate.} Here, in the flight tube, the ions drifts towards the detector with the velocity proportional to their mass to charge ratio: lighter multiply charged ions impact onto the ion detector earlier; heavier single charged ions, later. Recording the detector signal as a function of time results in the MALDI-TOF mass spectrum of the sample. The operating principle explains the name of a TOF mass analyzer: mass is determined from the ions’ time of flight.

Summarizing the above, I want to point out once again that DSSCs, OPVs, and solar fuel cells are always assembled on a conductive surface and always contain UV-absorbing components, meaning that they can be analyzed by MALDI, LDI, or SALDI-MS directly without any preparatory steps like extraction, scratching, etc.


My next posts will cover how MALDI-TOF-MS can address specific questions related to different type of energy converting cells - organic, dye-sensitized, and solar fuel cells.