Tolerance analysis of a micro-optical detection system for on-line monitoring of lubricant oils

We present a functional prototype of a micro-optical detection unit for both absorbance and
laser-induced fluorescence analysis in fused silica capillaries. Absorbance detection allows
concentration measurements ranging from 0.6 micro-M to 12 mM, whereas fluorescence detection
enables concentration measurements from 6 pM up to 0.6 mM. We make use of non-sequential
optical ray tracing simulations combined with statistical design methodology to perform a
complete tolerance analysis for manufacturability and we demonstrate experimentally that the
efficiency of the system is insensitive to reasonable misalignment and fabrication errors of its
building blocks. The possibility of using standard fabrication techniques to prototype and
replicate this miniaturized plastic detection system, as well as its wide measurement range
make it a good candidate for applications where low-cost and portable systems are needed to
measure small volumes of liquid samples with low-level concentrations. As an example we
demonstrate the use of the system for the optical characterization and differentiation of
nanoliter volumes of lubricant oils.