Ratios by Time-of-Flight Instruments

Upon acceleration through an electric potential of V volts, ions of unknown m/z value reach a velocity v = f2zeV/m] ). The ions continue at this velocity (drift) until they reach the detector. Since the start (to) and end (r) times are known, as is the length d of the drift region, the velocity can be calculated, and hence the m/z value can be calculated. In practice, an accurate measure of the distance d is not needed because it can be found by using ions of known m/z value to calibrate the system. Accurate measurement of the ion drift time is crucial. 

Ions in a TOF analyzer are temporally separated according to mass. Thus, at the detector all ions of any one mass arrive at one particular time, and all ions of other masses arrive at a different times. Apart from measuring times of arrival, the TDC device must be able to measure the numbers of ions at any one m/z value to obtain ion abundances. Generally, in TOF instruments, many pulses of ions are sent to the detector per second. It is not unusual to record 30,000 spectra per minute. Of course, each spectmm contains few ions, and a final mass spectrum requires addition of all 30,000 spectra to obtain a representative result. 

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