Methane Non-Methane Measurement Techniques in Ambient Monitoring Systems

Request to the experts of this forum to kindly help me determine the commercially available techniques to measure the Methane and Non-Methane contents in Ambient Air as a part of AAQMS.

The specs I have from one company says its FID (Flame Ionisation and Detection). I wish to ask, if this is the only available and most reliable commercial method to continuously analyze Met Non-met in ambient air. Are there other techniques available for continuous online measurement of MET NMET? What are the major advantages of FID over others, if any?

Regards
Ritika