= Mass analyzer
A <mass analyzer> is the part of a <mass spectrometer> that makes <ions> distinguishable or selectively transmits them according to <mass-to-charge ratio>. The measured coordinate can be position, arrival time or transmission under a specified field setting. A separate <mass spectrometer detector> records the resulting signal.
A <quadrupole mass analyzer> selects trajectories using combined static and time-varying <electric fields>: chosen mass-to-charge values remain on stable paths while others are lost from the transmitted beam.<Quadrupole mass analyser in IUPAC Gold Book>[Q04971, trajectory-stability definition]{ref} A <time-of-flight mass spectrometer> instead encodes the ratio in arrival time after acceleration. Different physical designs can therefore provide the same kind of ionic-mass information through different observables.
An original magnetic example exposes an important assumption. For speed $v$ perpendicular to a uniform <magnetic field> $B$, balancing the <Lorentz force> and circular acceleration gives
$$
qvB=\frac{mv^2}{r},\qquad r=\frac{mv}{qB}.
$$
If the incoming ions have the same selected speed and charge, mass values in the ratio $56/54$ give bending radii in that ratio, about $1.0370$. But if ions instead start negligibly slowly and gain energy from the same <electric potential difference> $U$, substituting $qU=mv^2/2$ gives $r=\sqrt{2mU/q}/B$. Their radius ratio is then $\sqrt{56/54}\approx1.0184$. The initial-condition choice changes the mapping.
Analysis requires this mapping and its assumptions, not just separated peaks. <Mass resolving power> describes the ability to distinguish nearby peak coordinates under a stated width criterion; accurate assignment of those coordinates additionally needs <instrument calibration> and the correct charge-state interpretation.
Back to article page
