Accurate Mass
Definition
A mass measured to four or more decimal places, sufficient to compute a unique molecular formula. High-resolution accurate-mass (HRAM) instruments (Q-TOF, Orbitrap, FT-ICR) deliver mass accuracy below 5 ppm, which is the working threshold for formula assignment.
- Precision
- Four or more decimal places
- Output
- Narrows to a unique molecular formula
- Instrument types
- Q-TOF, Orbitrap, FT-ICR
- Typical accuracy
- Below 5 ppm
Common questions
Why is sub-5-ppm accuracy the working threshold for formula assignment?+
At that level of precision, the number of chemically plausible molecular formulas matching a given mass narrows to only one or a very few candidates, especially combined with isotope pattern data.
Is accurate mass alone enough to identify a compound?+
No, it narrows the possible molecular formula but multiple compounds can share the same formula, so accurate mass is combined with retention time, fragmentation, or reference standards for confirmation.
How does accurate mass differ from nominal mass?+
Nominal mass rounds to the nearest whole number using the most abundant isotope, while accurate mass captures the small mass defects specific to each element, giving far more discriminating power.
Related terms
- Base Peak
- The most intense peak in a mass spectrum, normalised to 100 percent relative abundance. All other peaks in the spectrum are reported...
- Mass-to-Charge Ratio (M/z)
- The fundamental MS measurement: the mass of an ion divided by its charge number. A singly charged ion of mass 250 Da...
- Molecular Ion (M+)
- The intact ionised molecule with no fragmentation, formed by removing a single electron from the parent. Its m/z gives the molecular weight...
- Resolution
- The ability of a mass analyser to distinguish two ions of nearly equal m/z, defined as m/Δm at a given peak. A...
- Tandem MS (MS/MS)
- Two stages of mass selection in series with a fragmentation step in between. The first stage selects a precursor ion, a collision...