Optimizing the binder percentage to reduce matrix effects for the LIBS analysis of carbon in coal

Quantitative analysis of elements by laser-induced breakdown spectroscopy (LIBS) is significantly affected by matrix effects in coal. Coal powder was mixed with the KBr binder and pressed into pellets to reduce matrix effects. Four groups of mixed-pressed pellet samples were prepared from nine different coal types and different percentages of the KBr binder (KBr accounts for 0 wt%, 30 wt%, 60 wt%, and 90 wt%, respectively). To optimize the percentages of the KBr binder in the mixed-pressed pellets, the influence of KBr binder on laser-induced plasma was investigated in detail. The results indicate that the plasma excitation temperature decreases with increase of the KBr binder concentration. The difference of the excitation temperature between the nine different coal samples was minimal when KBr accounts for 60 wt% in the mixed-pressed pellets. The relative standard deviation of the excitation temperature is 4.26%. The matrix changes from coal to KBr when the percentages of KBr are higher than 60 wt%, which was confirmed by scanning electron microscopy images of the ablated crater. Finally, Si and K were individually chosen as the internal calibration element to construct the calibration curves of carbon. Better results were obtained when Si I 288.16 nm was used as the internal standard. The correlation coefficients R2 of the four groups of mixed-pressed pellet samples are 0.835, 0.893, 0.983, and 0.903, respectively. Hence, the appropriate percentage of binders needs to be carefully confirmed to reduce matrix effects in quantitative analysis of coal by LIBS.