Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles, identifying and quantifying molecules within a sample. For analysis, molecules must first be converted into ions. This fundamental ionization process transforms neutral molecules into charged species for detection in a vacuum. Various ionization methods exist, each suited for different types of molecules and analytical goals.
Electrospray Ionization (ESI)
Electrospray Ionization (ESI) is a soft ionization technique that converts molecules from a liquid sample into gas-phase ions. The process begins by introducing a liquid sample, often from a liquid chromatography (LC) system, through a fine capillary needle maintained at a high voltage. This high voltage creates a mist of highly charged droplets, which are then assisted by a nebulizing gas like nitrogen.
As these charged droplets move through a pressure and potential gradient, the solvent evaporates, causing the droplets to shrink and their surface charge density to increase. This reduction in size eventually leads to the electric field strength within the droplet becoming strong enough to eject gas-phase ions from the surface. ESI is suited for analyzing large, polar, and thermally unstable molecules like proteins, peptides, oligonucleotides, and pharmaceutical compounds, as it produces ions with minimal fragmentation. Its compatibility with liquid chromatography (LC-MS) makes it widely used in proteomics, metabolomics, and drug development.
Matrix-Assisted Laser Desorption/Ionization (MALDI)
Matrix-Assisted Laser Desorption/Ionization (MALDI) is another soft ionization technique useful for large biomolecules and polymers. The process involves mixing the analyte with a specialized chemical called a matrix, and this mixture is then applied as a dried spot onto a metal plate. The matrix material is chosen for its ability to absorb the laser’s energy at specific wavelengths, commonly in the ultraviolet (UV) range.
A pulsed laser then irradiates the dried sample-matrix mixture. The matrix absorbs the laser energy, rapidly heating up and causing both the matrix and the embedded analyte molecules to desorb and vaporize from the surface. During this rapid expansion, the analyte molecules are ionized in the hot plume of ablated gases. This method is effective for analyzing very large molecules, including proteins, peptides, carbohydrates, and synthetic polymers, with minimal fragmentation, making it valuable for direct analysis of samples like tissue sections.
Comparing ESI and MALDI
ESI and MALDI are both soft ionization techniques, but they differ in their operational principles and optimal applications. ESI requires samples to be in a liquid state, typically introduced as a continuous flow, making it compatible with liquid chromatography (LC-MS) for online separation and analysis. In contrast, MALDI uses analytes embedded in a solid matrix, which is then irradiated by a pulsed laser, making it suitable for direct analysis from surfaces.
The ionization mechanism for ESI involves the formation of charged droplets from a liquid, followed by solvent evaporation and subsequent ion ejection from these shrinking droplets. This process leads to the formation of multiply charged ions, which helps extend the observable mass range for very large molecules. For MALDI, ionization occurs through laser-induced energy transfer from the matrix to the analyte, resulting in desorption and subsequent ionization, predominantly producing singly charged ions.
Regarding analyte suitability, ESI analyzes polar, thermally labile molecules such as proteins, peptides, and oligonucleotides. Its ability to generate multiply charged ions allows for the analysis of very large proteins on mass analyzers with limited mass-to-charge ratio ranges. MALDI is also effective for large biomolecules and polymers, used for very large proteins, nucleic acids, and for direct analysis of solid samples, including tissue imaging. While both are sensitive, ESI can be affected by matrix effects and ion suppression from co-eluting compounds in complex mixtures. MALDI shows high tolerance to contaminants and is used for high-throughput screening.
In terms of compatibility with mass analyzers, ESI is coupled with quadrupole, ion trap, and time-of-flight (TOF) mass spectrometers, benefiting from its continuous ion beam. MALDI is coupled with Time-of-Flight (TOF) mass analyzers due to its pulsed nature. Common applications for ESI include proteomics and metabolomics, where its compatibility with LC-MS allows for detailed analysis of complex biological samples. MALDI finds use in polymer analysis, microbiology, and tissue imaging, where it can directly analyze molecular distributions on surfaces.