Continuum background reduction in orthogonal-acceleration time-of- flight mass spectrometry with continuous ion sources

The continuum ion background in an inductively coupled plasma time-of- flight mass spectrometer (TOFMS) has been reduced by 2 orders of magnitude by using energy discrimination (ED). A potential barrier placed in front of the ion detector effectively discriminates between higher-energy signal ions and the lower-energy background ions. The signal-to-noise ratio was increased tenfold and detection limits of 0.4-4.2 ppt were achieved for 11 elements ranging from Li to U. The resolving power was observed to degrade from 1520 to 1230 with the addition of the potential barrier. The residual background count rate was found to be limited by neutrals formed after exiting the ion reflectron via charge exchange with the background gas. This ED method can be employed to effectively reduce the continuum ion background in any TOFMS that uses orthogonal acceleration with a continuous ion source.