We are injecting pure methane gas into a GC-MS system, fitted with a 15m 0.32mm ID and 30 micron df. We see separated peaks for Nitrogen, Oxygen and then as broad mystery peak with molecular ion of 29.3 m/z. A library search matches the mass spectrum with ethane, nitro. There are peaks that look like methane in the spectrum alongside the 29m/z peak, however it’s at 15.5, 16.3 and 17.5 m/z. The ratios are also way off the NIST reference spectrum. Has anyone experienced the same problem?