High enhancement factor in low-power unipolar discharge arc assisted laser induced plasma spectroscopy

Abstract

The enhanced laser induced breakdown spectroscopy is highly desirable for the quantitatively detection with low limit of detection (LOD) and high accuracy. Higher enhancement factor is always obtained from discharge assisted laser-induced breakdown spectroscopy with enough higher energy injection in comparison to the normally standard laser induced breakdown spectroscopy (LIBS), which means dangerous with high voltage and elevated power. Here, a low power (10 W) injected unipolar discharge arc is proven to greatly enhance the LIBS with a large enhancement factor up to 13.8 when the discharged electric arc followed by a laser induced plasma are simultaneously applied on a silicon wafer, which is comparative or larger than those results with high-power (kW) and high voltage (kV) being applied. Furthermore, the enhanced technique is applied on an actual qualitative detection of cadmium in soil. The lines of Cd II (214.44 nm) from soil is enhanced as compared to a LIBS measurement. Accordingly, the LOD of the typical emission line is reduced to 23.5 ppm by the unipolar discharge arc assisted laser induced breakdown spectroscopy (UD-LIBS), which is 2.45-fold lower than that from the normal LIBS. The pre-ionization of the environment and the surface by the modulated discharge arc to directly make the Inverse Bremsstrahlung (IB) absorption with higher absorption coefficient dominate the initial stage of plasma formation rather than the multi-photon absorption process, thus make both electron density and the temperature of plasma raising are the main mechanism of spectral enhancement, which is further proved by the evolution dynamics images of two plasma overlapping processes under an in-situ intensified charge-coupled device (ICCD) based photographic system. The simple, portable, commercial highly enhancement way is greatly benefit to the actual applications.