A group at King Fahd University of Petroleum and Minerals (Dhahran, Saudi Arabia) has created a laser-induced-breakdown spectroscopic (LIBS) instrument that can be used to measure the level of chromium in synthetic hair dyes.1
Good reasons for measuring hair dye
The instrument is potentially important because chromium is carcinogenic (as well as damaging to the kidney and liver); the safe permissible limit for chromium in hair dye is 1 part per million (ppm). A conventional detection technique, inductively coupled plasma mass spectrometry (ICPMS), requires very large and complex instrumentation, while LIBS is potentially smaller and simpler.
The 266-nm laser source delivers 8 ns pulses with a maximum pulse energy of 50 mJ at a 20 Hz repetition rate; a fiber-optic cable terminating in a lens collects the light from the resulting plasma and sends it to a grating spectrometer that uses an intensified charge coupled device (iCCD) array for sensing.
Synthetic hair dyes are in powder form; when struck by a laser pulse, they quickly disperse and blow away. This led the researchers to create more-easily analyzed pellets out of hair-dye powder mixed with a potassium bromide (KBr) binder.
After first calibrating the system using pellet samples having chromium concentrations of 40, 60, 80, and 100 ppm, the researchers moved on to measuring the chromium concentrations in three different brands of hair dye purchased in Saudi Arabia.
Success for LIBS, fail for the hair dyes
With an ICPMS system providing reference measurements, the researchers measured concentrations of 11, 9, and 5 ppm respectively using their LIBS apparatus, compared to 9.8, 8.1, and 4.6 ppm with the ICPMS. The minimum detectable level of chromium using LIBS was 1.2 ppm -- approximately the same as the 1 ppm safe level for hair dye.
REFERENCE:
1. M. A. Gondal et al., Applied Optics, Vol. 53, No. 8, p. 1636, 10 March 2014.