Sensitive Mass Spectrometer for Time-Resolved Gas-Phase Chemistry Studies at High Pressures

We report a new experimental apparatus for direct time-resolved probing of high-pressure gas-phase chemical reactions by photoionization mass spectrometry. The apparatus uses a laser photolysis flow reactor, capable of operating at P = 0.3-100 bar and T = 300-1000 K. We initiate reactions in homogeneous gas mixtures by the photolysis of an appropriate radical precursor using laser pulses at repetition rates of 1-10 Hz. The reacting mixture is continuously sampled into a vacuum chamber, ionized by vacuum-ultraviolet (VUV) photons from laboratory-based discharge lamps or from a synchrotron beamline, and analyzed by a custom mass spectrometer. Soft near-threshold ionization by tunable synchrotron radiation enables spectroscopic quantification of many key intermediates and products of chemical reactions. A novel ionization scheme in the high-density region of the sample jet expansion increases the experimental sensitivity 100-fold, compared with the existing instruments, without compromising mass resolution. A 40 kHz pulsed reflectron time-of-flight mass spectrometer achieves simultaneous detection of all ionized species with 25 μs time resolution. We demonstrate the capability of this apparatus by probing the ethyl radical oxidation reaction using very dilute (<10¹² molecules·cm^-3) ethyl concentrations at pressures up to 25 bar.