Downstream of ionization (provided this is successful and has been optimized) is the next equally important stage -- the mass analysis. There are a number of methods and instrument platforms that make use of the physical characteristics of charged molecules, and their manipulation through a field, by means of magnetic intensity and/or length of travel. The goal, however, is the same -- resolution of each species of particle with regards to mass and charge (m/z).
Quadrupoles are four precisely parallel rods with a direct current (DC) voltage and a superimposed radio-frequency (RF) potential. The field on the quadrupoles determines which charged ions are permitted to reach the detector, and thus the quadrupoles themselves function as a mass filter.
To dig deeper, the imposed electric field directs the ions to oscillate in concordance with their mass-to-charge ratio and depending on the radio frequency. The m/z of an ion can be correlated with a specific radio frequency, which in turn translates into a mass spectrum obtained by scanning the field.
Quadrupoles have been coupled with Electrospray Ionization (ESI) sources for a long time and continue to be the optimal fit with regard to formation and behavior of the charged particles. In fact, these combinations are the most common in use today, with applications throughout the biotechnology, pharmaceutical, food science, chemical, and manufacturing industries.
There are several primary advantages to the quadrupole electrospray interface model:
- Quadrupoles are extremely tolerant at relatively high pressures, a feature that is well-suited to electrospray ionization since the charged particles are formed under atmospheric pressures.
- Quadrupoles are now capable of routinely analyzing up to a m/z of 3000 Da. This is useful as well since ESI commonly produces a charged particle distribution under 3000 Da.
- With regard to resources, the relatively low cost of quadrupole instruments makes them attractive.
With these and other considerations in mind, it’s no surprise that quadrupole instruments have received broad appeal not only by ESI users but many others as well.
See the article entitled Mass Spectrometry Roundup: Mass Analyzers – Time-of-Flight (TOF) for review of this method.