Time-of-Flight (TOF) Mass Analysis

Theoretical Background

The ions are rapidly accelerated in the ion source using a high voltage (20 - 25 kV). After this initial acceleration, the ions drift in a field free region until they collide with a detector.

Ep = q*V
The potential energy of a charged particle in an electric field is determined by the charge on that particle (q) multiplied by the magnitude of the voltage (V). This potential energy is converted into kinetic energy.

Ek= ½mv2
The kinetic energy of particle is equal to the mass of the particle (m) multiplied by the square of the velocity (v) divided by 2. By setting the potential energy and kinetic energy equal, the "time-of-flight equation" can be simply shown:

Ep = Ek
qV = ½mv2
Then, the velocity is replaced by distance divided by time (d/t).

qV = ½m(d/t)2
2V = md2/qt2
(2Vt2)/(d2) = m/q = m/z
Thus, the mass-to-charge ratio of each ion can be measured.

Data Acquisition

Spectra are acquired by transient recording. The amount of signal from the detector is recorded over time after each laser shot. The final mass spectrum is presented as an average of the individual time recordings. In these spectra, it is important to consider the signal-to-noise ratio (S/N) of the peaks. Ion signals with poor S/N values may not be reliable; on the other hand, it is important to prevent detector saturation, which will occur if the S/N values of lower mass ion signals are too high. Also, the S/N values must be carefully monitored for isotope ratio measurements.