In ESI, a sample solution is pushed through a stainless steel needle held at a high potential versus a counter-electrode. When the electric field overcomes the surface tension of the solution at the tip of the needle, the meniscus is pulled to form a cone that can explode in a mist of fine droplets. The droplets enter into the vacuum region of the source through an orifice in the counter-electrode, where they undergo fast solvent evaporation. As a droplet shrinks in size, the charges on the surface get so close to each other that they can induce fission of the initial drop into smaller droplets to minimize the coulombic repulsion. The fission-evaporation cycle keeps repeating until all solvent is removed and naked ions are left in the gas-phase.
In MALDI, a tiny droplet of sample is mixed with an organic matrix that is capable of absorbing UV or IR light and can form appropriate crystals with the analyte. When a laser light is shone onto the crystals, the energy of the photons is absorbed by the matrix molecules that heat up and expand at supersonic velocity into the vacuum region of the source. The plume formed by this miniature explosion drags matrix debris and analyte ions into the gas-phase, where they can be finally analyzed.
Once in the gas-phase, the mass over charge ratio (m/z) of these ions can be determined by a variety of mass analyzers that afford different features and performance. In Fourier transform ion cyclotron resonance (FTICR) analyzers, ions are trapped in a cell located in the most homogenous region of a strong superconductive magnet. When injected in a magnetic field, charged particles are deflected into a circular path, where they orbit with angular frequencies that are inversely proportional to their respective m/z. Appropriate radiofrequencies are applied to the transmitter plates of the cell to induce ions of like m/z to resonate together as a coherent packet. Each m/z packet generates an image current on the receiver plates of the cell, which is recorded, amplified, and transformed to calculate the corresponding m/z. In this way, FTICR analyzers can achieve >1,000,000 resolution and <0.001 Da accuracy in molecular mass determinations.
Tandem techniques can be applied to induce the fragmentation of ions in the gas-phase through impact with an appropriate surface (SID) or target gas (CID), or through interactions with infrared photons (IRMPD) or electrons (ECD). The ensuing fragments are characteristic of the precursor ion and can reveal its chemical structure. In the life sciences, tandem techniques are used to obtain the sequence of proteins, nucleic acids, and complex carbohydrates, and to reveal the position of chemical and biochemical modifications. In combination with gel electrophoresis and liquid chromatography, these techniques enable the identification and characterization of the full complement of proteins (or proteome) expressed by a cell at any given time. Proteomics investigations are the key to elucidate the function of all the genes present in the human genome and understand their interplay in healthy as well as in pathological states.