  Photoacoustic (PA) imaging systems are spreading in the biomedical community,
and the de-velopment of new PA contrast agents is an active area of research.
However, PA contrast agents are usually characterized with spectrophotometry or
uncalibrated PA imaging systems, leading to partial assessment of their PA
efficiency. To enable quantitative PA spectroscopy of contrast agents in vitro
with conventional PA imaging systems, we have developed an adapted calibration
method. Contrast agents in solution are injected in a dedicated non-scattering
tube phantom imaged at different optical wavelengths. The calibration method
uses a reference solu-tion of cupric sulfate to simultaneously correct for the
spectral energy distribution of excitation light at the tube location and
perform a conversion of the tube amplitude in the image from ar-bitrary to
spectroscopic units. The method does not require any precise alignment and
provides quantitative PA spectra, even with non-uniform illumination and
ultrasound sensitivity. It was implemented on a conventional imaging setup
based on a tunable laser operating between 680 nm and 980 nm and a 5 MHz
clinical ultrasound array. We demonstrated robust calibrated PA spectroscopy
with sample volumes as low as 15 {\mu}L of known chromophores and commonly used
contrast agents. The validated method will be an essential and accessible tool
for the de-velopment of new and efficient PA contrast agents by improving their
quantitative characterization.
