To obtain quantitative measurements of the elemental composition of the sample under examination with the LIBS technique, it is necessary to construct the calibration curves of the spectral lines of interest (intensity of the line as a function of concentration of the chemical species). In the case of coal, the calibration procedure is particularly difficult due to the high variability in the chemical composition of the sample. This contribution describes the experimental activity carried out in order to determine the optimum time delay between laser pulse and the beginning of acquisition of the light spectrum emitted by the plasma, necessary to obtain the calibration curves of some elements (C, H, Al, Si, Ca, Na, K) in the case in which spectrometers are used with a long integration time.

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