Key concept: - Mass-to-charge-ratio. The fundamental measurement taken by nearly every mass spectrometer on the market today is not directly the mass of an ion, but rather its mass-to-charge ratio. As we will see in this chapter, different ion sources have different trends in terms of charge imparted, and the charge itself may be positive or negative depending on the experimental setup. Crucial to the measurement and application of the mass-to-charge ratio is the fact that charge is always an integer value.
Most mass spectrometers in use today for biomedical research function by measuring the mass-to-charge ratio (m/z) of ions. In order to make this measurement from a biologically relevant molecule, there are three essential components to a mass spectrometer:
The Ion Source of a mass spectrometer is concerned with applying a charge to a molecule. In History of Mass Spectrometry I mention that mass spec was historically started as a mechanism for the analysis of gas molecules. One function of the ion source is to bridge the gap between liquid phase biological molecules and gas phase molecules that can be easily analyzed by mass spec. Furthermore, as the name implies, the ion source is also responsible for the ionization of molecules - we will see the effect of ionization in Reading Mass Spectra. The ion source is also the source of the charge that analyzed molecules have in the spectra.
Amongst ion sources, two mechanisms are seen frequently - ElectroSpray Ionization (ESI) and Matrix-Assisted-Laser-Desorption/Ionization (MALDI). There are a few key differences between these two methods that will be seen here:
- Source molecule state
- ESI starts with molecules in the liquid state (usually in a buffer)
- MALDI starts with molecules suspended in a semi-solid matrix (essentially in a dried or solid state)
- Ionization state
- ESI may give one or many charges to a molecule (the effect of which will be shown later)
- MALDI tends to give only a single charge to a molecule
The Mass Analyzer is in many cases both physically and functionally the middle of a mass spectrometer. Once a molecule has been charged by the ion source, it now has a specific mass-to-charge ratio. The function of the mass analyzer is to evaluate this ratio for ions passing through the mass spectrometer.
The Mass Detector is responsible for counting the molecules at each mass-to-charge value reported by the mass analyzer. The output of the mass detector is the mass spectra.
Transitions between the stages of a mass spectrometer are often controlled by the optics of the instrument. The optics of a mass spectrometer - often separated as a collection of lenses are not optically the same as eyeglasses or binoculars. Rather the optics of a mass spectrometer are commonly controlled by radio frequency (RF) or electro magnetic (EM) fields. By adjusting field strength as ions (or collections of ions) pass through you can "aim" the beam of ions.