Liquid-phase broadband cavity enhanced absorption spectroscopy (BBCEAS) studies in a 20 cm cell

Sensitive liquid-phase measurements have been made in a 20 cm cell using broadband cavity enhanced absorption spectroscopy (BBCEAS). The cavity was formed by two high reflectivity mirrors which were in direct contact with the liquid-phase analytes. Careful choice of solvent was required to minimise the effect of background solvent absorptions. Measurements were made on the broad absorber Sudan black, dissolved in acetonitrile, using a white LED light source and R > or = 0.99 cavity mirrors, leading to a cavity enhancement factor (CEF) of 82 at 584 nm. The sensitivity as measured by the minimum detectable change in the absorption coefficient (alpha(min)) was 3.4 x 10(-7) cm(-1). Further measurements were made on the strong absorber methylene blue dissolved in acetonitrile at 655 nm. A white LED was used with the R > or = 0.99 cavity mirrors, leading to a CEF of 78 and alpha(min) = 4.4 x 10(-7) cm(-1). The use of a more intense red LED also allowed measurements with higher reflectivity R > or = 0.999 cavity mirrors, leading to a CEF of 429 and alpha(min) = 2.8 x 10(-7) cm(-1). The sensitivity was limited by dark noise from the detector but nevertheless appears to represent the most sensitive liquid-phase absorption measurement to date.