Detection Limits for Chiral Amino Acids Using a Polarization Camera

The detection of biosignatures on a planetary surface is of high scientific interest, and enantiomeric excesses of organic molecules are one such signature. Enantiomeric excesses can be detected by their polarizing effects on transmitted light. As part of instrument development work for a microscopic imager, the Cold Lightweight Imager for Europa (C-LIFE), we assess the potential of polarization measurements to quantify enantiomeric excesses. We investigated the optical rotation of the amino acids serine and phenylalanine for a range of enantiomeric abundances. Measurements were made with mixtures of serine and phenylalanine as well as Europa-relevant salts to determine how well these combinations can be detected. We also conducted a small number of measurements on samples of bacteria taken from glacial environments. We found that concentrations greater than 10−3 M are needed to detect serine and concentrations greater than 10−4 M are needed to detect phenylalanine, with larger concentrations needed for smaller enantiomeric excesses. Salts do not have a significant effect on the optical rotation. Optical rotation of bacterial samples were detected at concentrations >106 cells ml−1. Systematic errors in our polarization detector limited our sensitivity to optical activity changes of ∼0.008°, leading to an inability to distinguish enantiomeric abundances separated by 5%, but tests show that improvements to our laboratory technique can yield a factor of 20 improvement in sensitivity.