On the compositional analysis of Coal using calibration free laser induced breakdown spectroscopy

Abstract

In this contribution, we present analysis of the total carbon contents and the major constituents in the Reference Coal sample of known concentration (SARM 20) and the Pakistani Thar Coal samples using laser induced breakdown spectroscopy (LIBS). A high-power Q-switched Nd:YAG Laser, (532nm, 5ns pulse width and 10 Hz repletion rate) was used to produce the Coal plasma in air and the optical emission spectra were registered using a HR 4000 spectrometer covering the wavelength region from 200 nm to 1100 nm. The emission spectrum of the laser produced plume contains the atomic lines of Ca, Si, Fe, Ti, Mg, Na, K, Li, Al and C along with the traces of Ba, Sr and Mn. The intensities of the observed spectral lines were corrected for self-absorption using the internal reference line self absorption correction method. The excitation temperature was calculated using the Boltzmann plot method and the Stark broadened Hα line profile was used to determine the electron number density. The calibration free LIBS (CF-LIBS) technique with a fixed slope method was utilized for the quantification of the carbon and other elements whereas, a laser-ablation time of flight mass spectrometer (LA-TOF-MS) was used to obtain the mass spectra of the same samples. All the elements identified in the optical emission spectrum have been also observed in the LA-TOF-MS mass spectrum. The results reveal that the Thar Coal samples collected from Sindh, Pakistan are comparable to the Reference Coal samples. It is further demonstrated that CF-LIBS and the LA-TOF-MS are efficient, reliable and complementary techniques for analyzing the Coal samples for the total carbon content, elements forming the ash and even for the detection of the trace elements at the ppm level.