It’s well acknowledged that each mass analyzer (and ionization source) has unique performance attributes and limitations. The type of configuration, whether it is an ESI-quadrupole or a MALDI-TOF for example, is largely dictated by the flavor of research and the molecules of interest – and sometimes by the instrument or resources available. Regardless, it is important to know what the capabilities of each instrument are to see whether it is right (or at least adaptable) for your research needs. If not, it’s probably time to upgrade components or instruments. Here are a few specifications:
Quadrupole
- Typical mass range – 3000 m/z
- Typical resolution – 2000 – the observed m/z divided by the Δm/z for two ions that can be separated
- Tolerance for high pressure – atmospheric
- Well-suited for electrospray
- Ease of switching between positive and negative ions
- Small size
- Relatively low cost
- Mass range limited to about 3000 m/z
- Poor adaptability to MALDI
Ion trap
- Range 2000 m/z
- Resolution 1500
- Small size
- Medium resolution
- Simple design – Low cost
- Well suited for tandem MS
- Easy for positive/negative ions
- Limited mass range – not as much a problem with current innovations
Time-of-Flight (TOF)
- Range m/z ∞
- Resolution 350
- Highest mass range
- Very fast scan speed
- Simple design – Low cost
- Ease of adaption to MALDI
- Low resolution
- Difficulty of adaption to electrospray
Time-of-Flight (TOF) reflectron
- Range m/z ∞
- Resolution 1500
- Improved resolution relative to TOF
- Very fast scan speed
- Simple design – Low cost
- Resolving power has limited m/z range
- Lower sensitivity compared to TOF
Fourier Transform (FT)
- Range m/z ∞
- Resolution 30,000
- High resolution
- Well-suited for tandem MS
- High vacuum required
- Super conducting magnet required – expensive
- Extensive maintenance
- Massive size
As you can see, the differences between these instrument platforms is significant in terms of capabilities, limitations, adaptability, requirements, and size. Each type has a specific set of skills that allow it to excel in certain applications.
One important point is that often times preferred performance and functionality can be conditional on limitations which may not significantly impact your analysis.
Furthermore, the instrument may have focused use in an area in which targets have been both defined and yet to be defined. Such a scenario might involve small molecule quantitation and discovery using a ESI Triple Quadripole in a clinical lab or one studying secondary metabolites in bacteria.
One step further, an instrument can be applied in a purely discovery oriented role where the source of the sample is defined, such as human blood, saliva, or cerebral spinal fluid, but the nature of the analyte is unknown.
