Mid-infrared spectroscopic investigation of methylamines by a continuous-wave difference-frequency-generation-based system.

A laser spectroscopic system based on a cw difference-frequency generation source with a ratiometric multipass absorption detection scheme was employed for high-resolution spectroscopic investigation of gas-phase monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA). Possible application of the system as a noninvasive human breath analyzer for renal and liver diseases is targeted. The system operates in the fundamental C-H stretch absorption region around 2740-2860 cm(-1). A detection sensitivity of 2 x 10(-6) cm(-1) Hz(-(1/2)) (for signal-to-noise ratio SNR=1) is achieved, corresponding to detection limits of 900 ppb (parts in 10(9)) for MMA, 450 ppb for DMA, and 120 ppb for TMA in mixtures containing H2O and CO2 with concentrations of up to those present in human breath (2% and 5%, respectively). Future developments are discussed to further improve these detection limits that are currently still about 2 orders of magnitude higher than required for direct methylamine monitoring in human breath.