Basic Principles of Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry

In recent years matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) has emerged as a tool for microbial identification and diagnosis. During the MALDI-TOF MS process, microbes are identified using either intact cells or cell extracts.

The identification of microorganisms using MALDI-TOF is a fast and efficient method that has proven its importance in clinical microbiology. Therefore, most of the entries of the already existing databases are human pathogens. To create the database, the MALDI Biotyper ® (Bruker Daltonics, Billerica, MA) is used to measure the highly abundant ribosomal proteins and form the molecular fingerprint of the bacteria. The protein extract is deposited on a steel target and covered with a matrix solution HCCAportioned (α-cyano-4-hydroxycinnamic acid, Bruker Daltonik GmbH). The matrix is necessary for the "soft ionization"; it absorbs the ultraviolet light and converts it to heat energy. Each sample is irradiated by 240 brief pulses from an ultraviolet nitrogen laser (337 nm), desorbed from the plate into the gas phase and ionized. The cloud of ionized proteins is then funneled through a positively charged, electrostatic field which accelerates the molecules into the Time Of Flight (TOF) mass analyzer. The velocity at which individual ion flies through the TOF chamber is dependent on their mass-to-charge and time-to-impact of each ion and those properties are used to convert the time-to-impact into a mass-to-charge ratio and then into a mass spectrum. Once a database is ready, a strain of bacteria isolated from a sample can be quickly and efficiently analyzed in the same way, and compared to the database. The MALDI-TOF will give the sample a score, indicating how likely it is that the strain being analyzed is a match for the strain in the MALDI-TOF database.