Resolution limitations from detector pulse width and jitter in a linear orthogonal-acceleration time-of-flight mass spectrometer

Abstract

Recent and ongoing advances in timing electronics together with the development of ionization techniques suited to time-of-flight mass spectrometry (TOF-MS) have contributed to renewed interest in this method of mass analysis. Whereas low resolving powers ( m/Δ m < 500) were once an almost unavoidable drawback in TOF-MS, recent developments in instrument geometries have produced much higher resolving powers for many ion sources. The temporal width of detector pulses and jitter in timing electronics, however, lead to contributions to peak widths that are essentially independent of the mass-analyzer ion optics. The effective detector pulse width (Δ t d ≈ 1–10 ns typically) can be a limiting factor in the development of high resolution time-of-flight (TOF) instruments with modest drift lengths (∼ 1 m). It also reduces the mass resolution more seriously for light ions. This article presents a method for distinguishing the instrumental “ion arrival-time” resolution ( R o) of a linear TOF mass analyzer from that which is locally measured at a particular mass, limited by the broadening of the detector pulse width and electronics. The method also provides an estimate of Δ t d, that is useful in determining the temporal performance of the detection system. The model developed here is tested with data from a recently constructed orthogonal-acceleration TOF mass spectrometer equipped with a commercially available transient recorder (a LeCroy 400-Msample/s digital oscilloscope) from which we obtained R o = 4240 ± 100 [full width at half maximum (FWHM)] and Δ t d = 3.0 ± 0.1 ns (FWHM).