Ambient ionization for methane quantification in simulated Martian atmosphere using miniature inductively coupled plasmas

Summary form only given. The fundamental importance of understanding the origins of life has encouraged searching for extra-terrestrial organic molecules. Methane (CH₄) is the simplest organic molecule and has been sought on Mars for many years. Additionally, the recent developments in ambient ionization techniques enable the ionization of samples in their native environment, without sample preparation or separation, providing sensitive, selective, rapid, direct, and high throughput analysis technique for a wide range of compounds. Interestingly, intrinsic conditions of the Martian atmosphere provide favorable conditions for the formation of plasmas at ambient conditions. The Martian atmosphere consists of 95.32% carbon dioxide, 2.7% nitrogen, 1.6% argon, and trace amounts of other constituents such as oxygen, carbon mono oxide, water vapor and methane. The atmospheric pressure on Mars varies from around ~0.2 Torr on Olympus Mons´s peak to over ~8.7 Torr in the depths of Hellas Planitia, with a mean surface level pressure of ~4.5 Torr. In this study, a miniature inductively coupled plasma (ICP) source was developed as an ambient ionization source, operating under conditions similar to those that exist on Mars. The miniature ICP was self-sustaining at a net power of -1.7 W. The analytical performance of this method in identification and quantitative analysis of methane was investigated by a quadruple mass spectrometer. The operating conditions were optimized in terms of gas pressure, power consumption and the skimmer voltage. The masking effect of 0+ (M.W. 15.9994 g/mol) on CH₄⁺ (M.W. 16.042 g/mol) prevented the monitoring of the ionic methane peak at m/z of 16 with the current setup because of the 1 amu resolution of the mass spectrometer. As an alternative, CH₃⁺ at an m/z of 15 was used to monitor the methane concentration. Calibration curves with excellent linearity were obtained for CH₃^{+ at m/z of 15 at the pressure range of 4 to 16 Torr, resulting in detection limits of as low as 149 ppb. The results introduced a potential method for monitoring the Martian atmosphere for trace amounts of methane.}