  Measurements of carbon content in coal using laser-induced breakdown
spectroscopy (LIBS) is limited by its low measurement precision and accuracy. A
spectrum standardization method was proposed to achieve both reproducible and
accurate results for the quantitative analysis of carbon content in coal with
LIBS. The proposed method utilized the molecular carbon emissions to compensate
the diminution of atomic carbon emission caused by matrix effect. The
compensated carbon line intensities were further converted into an assumed
standard state with fixed plasma temperature, electron density, and total
number density of elemental carbon, which is proportional to its concentration
in the coal samples. In addition, in order to obtained better compensation for
total carbon number density fluctuations, an iterative algorithm was applied,
which is different from our previous standardization calculations. The modified
spectrum standardization model was applied to the measurement of carbon content
in 24 bituminous coal samples. The results demonstrated that the proposed
method had superior performance over the generally applied normalization
methods. The average relative standard deviation, the coefficient of
determination, the root-mean-square error of prediction, and the average
maximum relative error for the modified model were 3.44%, 0.83, 2.71%, and
12.61%, respectively, while the corresponding values for the normalization with
segmental spectrum area were 6.00%, 0.75, 3.77%, and 15.40%, respectively,
showing an overwhelming improvement.
