Method of the Month: MALDI Mass Spectrometry
November 2023
Background
In 2002, a new mass spectrometry method won the Chemistry Nobel Prize for being an improved, innovative method for detecting and identifying small molecules in a sample. Now, this method of mass spectrometry, called matrix-associated laser desorption ionization (MALDI) mass spectrometry, is a valuable tool for a variety of applications. It is commonly used for bacterial identification, infection diagnosis, environmental analysis, and proteomics (the study of proteomes, sets of biological proteins).
Method
Matrix-associated laser desorption ionization (MALDI) mass spectrometry uses a machine called a mass spectrometer to identify molecules by ionizing a sample and determining its mass-to-charge ratio. MALDI has two main steps: ionization and time of flight (TOF), which is further divided into the two categories of separation and detection. Each part of a MALDI mass spectrometer carries out these steps in a detailed sequence.
The ionization step of MALDI begins with the plate, where a small organic molecule, called the matrix, is deposited. The matrix, meant to facilitate the ionization process, is then combined with the analyte, the sample whose chemical constituents are being identified. After the analyte is put onto the plate in the machine, a UV laser at about 337 nm shoots into one of the plate’s wells. This laser causes desorption-charge transfer between the matrix and analyte by absorbing the laser’s energy. It vaporizes the sample, turning it into gaseous ions. One of the advantages of using MALDI is that the laser can vaporize the sample with minimal fragmentation, meaning that each molecule in the sample can separate without breaking into fragments that will skew the data collected at the end of the method. This makes MALDI a soft technique, able to analyze whole molecules with little to no sample modification.
Once ionized, the gas will rise in a vacuumed chamber, called the flight tube or mass analyzer, charged in an electric field so that the ions accelerate to the top of the chamber. At the top of the chamber, there is a time of flight (TOF) detector, ready to collect the time it takes for the vaporized sample to fly up the chamber. Each ion is then categorized by speed: the smaller ions move up the chamber more quickly, reaching the detector first, and the heavier ions move slower, taking more time to reach the detector. Using the measured time and the charge of the chamber’s electric field, the detector can determine each ion’s mass-to-charge ratio, which can then be used to identify any molecule that composed the sample. The final result of this method is a graph created by the computer attached to the detector that displays the specific mass of each particle along the time axis.
MALDI is a technique that continues to grow in value as more scientists practice this method for biological and clinical research.
For more information on MALDI-TOF, check out the following links!
