  We demonstrate an online (in-situ) sensor for continuous detection of oil
contamination in compressed air systems complying with the ISO-8573 standard.
The sensor is based on the photo-acoustic (PA) effect. The online and real-time
PA sensor system has the potential to benefit a wide range of users that
require high purity compressed air. Among these are hospitals, pharmaceutical
industries, electronics manufacturers, and clean room facilities. The sensor
was tested for sensitivity, repeatability, robustness to molecular
cross-interference, and stability of calibration. Explicit measurements of
hexane (C${_6}$H$_{14}$) and decane (C${_{10}}$H$_{22}$) vapors via excitation
of molecular C-H vibrations at approx. 2950 cm$^{-1}$ (3.38 $\mu$m) were
conducted with a custom made interband cascade laser (ICL). For the decane
measurements a (1 $\sigma$) standard deviation (STD) of 0.3 ppb was
demonstrated, which corresponds to a normalized noise equivalent absorption
(NNEA) coefficient for the prototype PA sensor of 2.8$\times 10^{-9}$ W
cm$^{-1}$ Hz$^{1/2}$.
