Ion desorption, detection & dissociation in time-of-flight mass spectrometry
Westmacott, Garrett R.
The results of everal fundamental experiments on desorption/ionization and fragmentation in time-of-flight mass spectrometry are described. The yield of desorption of secondary. molecular ions of valine was determined as a function of primary ion mass with a tandem time-of-flight mass spectrometer. The primary ions consisted of alkali metals Li$\sp+$, Na$\sp+$, K$\sp+$, Rb$\sp+$ and Cs$\sp+$ with impact energy on the valine target ranging between 3.5 and 10keV. The yield measurements for both negative and positive molecular valine ions were found to correspond to the cube of nuclear stopping power, and low mass ions, like H$\sp-$ and C$\sp-$ were found to scale linearly with the nuclear stopping power. Secondary-ion and secondary-electron emission yields from surfaces bombarded with large molecular ions were measured in a tandem time-of-flight mass spectrometer. The primary ions were produced by matrix-assisted laser desorption/ionization and ranged in mass from 6000u to 110 000u, and in energy from 5 to 25keV. The yields were measured for surfaces of stainless steel and CsI. For a given energy, the secondary electron yield decreases rapidly for increasing mass but remains greater than $\sim$10%; in contrast, the efficiency of secondary ion production remains close to unity. For high velocities, the electron yield is significantly higher for the CsI surface, but for velocities corresponding to less than $\sim$0.4eV/u, the emission is rather insensitive to the surface. An alternative method for the analysis of metastable-ions in a reflecting time-of-flight instrument is demonstrated. Post-source decay analysis usually involves collecting fragment ion spectra in segments, decreasing the mirror voltage for each segment. The present method leaves the mirror voltage fixed and increases the accelerating voltage to acquire each segment. The central advantage is that the mass calibration depends sensitively on the mirror voltage but only weakly on the accelerating voltage. The method of post source decay analysis is also demonstrated with precursor ion selection using a Bradbury-Nielsen ion gate. Finally, the forth experiment involves measuring molecular ion yields as a function of laser fluence for matrix-assisted laser desorption/ionization. This experiment was done on two different instruments. One is a linear time-of-flight mass spectrometer where both analog and pulse-counting detection methods are used and compared. The second instrument is an orthogonal-injection time-of-flight instrument with collisional cooling and pulse-counting detection. It is an improvement relative to the linear instrument because it ensures single-ion counting. The results for both instruments and detection methods were consistent. The ion yield versus laser fluence was found to have a steep slope on a log-log plot (between 6 and 9) for a range of fluence covering almost an order of magnitude. Also no evidence was observed for an ion desorption threshold at low laser fluence ($\sim$10J/m$\sp2$).