Quantitative analysis of soil organic carbon using laserinduced breakdown spectroscopy: An improved method

Standard laser-induced breakdown spectroscopy (LIBS) offers a potentially rapid, accurate, field-portable, and low-cost technique for the measurement of C content in soil samples. Neutral and singly ionized Fe lines form significant interferences, however, that can compromise the LIBS C measurement. In this study, the 247.8-nm line of atomic C (C I) was examined in detail to assess the effect of potential elemental interferences. These interferences and their spectral and temporal signatures were evaluated using control graphite and Fe oxide samples. A combination of high dispersion and appropriate time gating of the LIBS signal was found to generate very high signal/noise ratio spectra using low laser powers and therefore, allowed accurate determination of the C content down to the subpercent level in the presence of Fe interferences. Although the contribution of one singly ionized Fe line cannot be entirely eliminated, a method to quantitatively assess the contribution of this line was developed. The new technique was tested on soil samples with organic C content in the 0.5 to 3% (w/w) range, and a strong correlation of the LIBS C signal with measurements made by the thermal oxidation, dry combustion method was observed. The findings reaffirm the utility of standard LIBS for rapid and accurate laboratory measurement of soil C and the potential use of standard LIBS with further study and development for assessing soil C in situ.