Line‐of‐sight mass spectrometry: principles and practice

Abstract

Line‐of‐sight mass spectrometry (LOSMS) defines a focal area on a sample surface such that only species originating from within that area are detected by a mass spectrometer. Line‐of‐sight gas trajectories are established between a focal area and the ionisation volume using two apertures with cryo‐pumping walls to prevent all other trajectories from reaching the ionisation volume. Because of the geometry, LOSMS is area specific, angle resolving and capable of detecting neutral, radical and charged species with approximately equal sensitivity and can be used to measure the speed distribution of gases. The history of LOSMS is reviewed, and the different aspects are drawn together to give a coherent description of the technique. The general operating principles are used to predict how a gas microscope with a spatial resolution of 25 µm and a gas camera with an angular resolution of 0.014° could be built. Three types of experiment that can be carried out using LOSMS are described: temperature‐programmed desorption, sticking probability measurements and general monitoring of the gas phase above a surface where the temporal variation of the gas pressure and sample temperature can be arbitrarily complex. Procedures for accurately locating the line‐of‐sight focal area are described. The angle‐resolving ability of LOSMS is demonstrated, showing that the angular distributions of two gases, acetone and 1,2,3‐trifluorobenzene, from a solid surface (silver) and a liquid (1‐ethyl‐3‐methylimidazolium ethyl sulfate [C2C1Im][EtSO4]) surface are cosine distributions as predicted by Knudsen's cosine law. Copyright © 2015 John Wiley & Sons, Ltd.