Mass spectrometers measure the mass-to-charge (m/z) ratios of gas phase ions. Creating gas phase ions is the role of the ionization method. Ionization methods available on the instruments within MSPF are described below. Use this as a guide to determine which ionization method is best suited for your sample.
ESI usually generates multiply-charged ions, [M + nH]n+, for a single compound. It is best suited for small and large molecules that are free from detergents and salts. Consequently, samples for ESI often are dissolved in acetonitrile, methanol, or methylene chloride. The sample in solution is passed through a metal capillary which is biased at hight potential (4-5 kV). The electric field caused the solution to disperse into small droplets. These small droplets undergo a process of solvent evaporization and/or Rayleigh disintegration until gas-phase ions are produced.
Matrix-Assisted Laser Desorption/Ionization
MALDI is the method of choice for higher molecular weight compounds, such as proteins and oligonucleotides. A MALDI mass spectrum usually consists of singly-charged ions, [M + H]+, for each sample component. The sample is mixed with a suitable MALDI matrix, such as sinapinic acid, and allowed to dry on a metal target. A laser beam is directed at the metal surface causing desorption and ionization of sample components.
EI is the classical ionization method in mass spectrometry. EI is best suited to relatively nonpolar, volatile samples. An EI mass spectrum usually contains the molecular ion, M+, and many fragment ions, making EI useful for structural characterization. Gas phase ions are produced by heating the sample, if necessary, followed by ionization with a 70 eV beam of electrons.
CI is a “soft” ionization technique which, like EI, is restricted to relatively volatile samples. However, CI is better suited than EI to ionize more polar compounds. A CI mass spectrum oftan contains a protonated molecule, [M + H]+, and a few fragment ions. The sample is heated to generate gas phase molecules which undergo proton transfer reactions with the CI gas. The choices of reagent gases in order from harshest to softest are: methane, isobutene, and ammonia.
Atmospheric Pressure Chemical Ionization
APCI is a technique which creates ions at atmospheric pressure. A sample solution flows through a heated tube where it is volatilized and sprayed into a corona discharge with the aid of nitrogen nebulization. Ions are produced in the discharge and extracted into the mass spectrometer. APCI is best suited to relatively polar, semi-volatile samples. An APCI mass spectrum usually contained the quasi-molecular ion, [M + H]+. This technique is used as an LC/MS interface because it can accommodate very high (1 mL/min) liquid flow rates.