Schematic of the femtosecond-laser-driven time-of-flight mass spectrometer system

# Femtosecond Ionization Mass Spectrometer

High resolution time-of-flight mass spectrometers (TOFMSs) have broad potential application in planetary exploration, such as rapid isobar identification and detailed isotopic measurements of volatile species, which provide better understanding of the origin and processing of light elements (C, H, O, N, and S), the noble gases, and prebiotic/biogenic organic compounds. For example, to mass-separate formaldehyde (H2CO, 30.0106 Da) and ethane (C2H6, 30.0470 Da), a mass resolution of $m/\Delta m$ > 824 is needed. Indeed, the 2013 NASA Decadal Survey lists mass spectrometers on 7 of the 13 priority planetary missions and emblemizes them as a Recommended Technology Investment (“Mass spectrometers would benefit significantly from a science instrument technology program aimed at improving basic instrument performance characteristics.”). TOFMSs could utilize various ionization sources to produce ions or fragments from neutral parent molecules. However, it is challenging to achieve ultra-high mass resolution in the TOFMSs without increasing the size and complexity of the instrument. For example, pulsed laser desorption/ionization (LDI), effectively used for solid sample analysis with mass resolution up to several hundred Da, is typically limited by the few-nanosecond laser pulse duration (Δt). Filament-based electron guns have long been coupled to quadrupole and ion trap mass spectrometers for gas sample analysis via electron impact (EI) ionization, but less commonly to TOFMS since such guns are limited to producing µs to ms duration ion pulses.

To address this challenge, an ultrafast, high density EI source has the potential to produce picosecond (ps) or femtosecond (fs) ion pulses from gas samples, which will not only improve mass resolution of TOFMSs by 10-100x, but also reduce analysis times through enhanced ionization yield. Here we discuss a series of advances toward an ultrafast laser-triggered EI source, including demonstration of a UV source, demonstration of UV photoemission on copper, design of an ultrafast electron gun, and calculations of the expected ion yield. Further, the ultrafast laser used to trigger the EI source could potentially also be used for LDI in the same instrument package, enabling the possibility of performing both gas analysis and solid sample analysis in a single compact instrument.

##### Ronny Huang
###### Machine Learning Researcher

My interests include adversarial methods, generalization, and sequence modeling.