Evaluation of calibration approaches for quantitative analysis of sludge by laser induced breakdown spectrometry
Sludge is a semi-solid organic waste from sewage treatment stations, whose disposal is a serious environmental issue. Application as a soil fertilizer is feasible due to its high organic matter and nutrient (e.g. Ca, Fe, Zn, and Cu) contents, but it critically depends on a previous chemical analysis. In this sense, laser-induced breakdown spectrometry (LIBS) deserves attention for skipping sample decomposition and due to the multi-elementary capacity. LIBS quantitative analysis requires proper calibration approaches to overcome matrix effects, mainly for complex samples as sludge. This work aims to propose an analytical method to directly determine Ca, Fe, Zn, and Cu in sludge by LIBS, after the evaluation of different calibration approaches: external calibration (EC), internal standard calibration (ISC), multi-energy calibration (MEC) and slope ratio calibration (SRC). EC, carried out using four certified reference materials (CRMs) with a similar composition to sludge samples, did not yield accurate analytical results. The differences between sample and standard ablated masses were successfully compensated for using carbon as the internal standard, except for Cu. MEC yielded accurate results for all analytes (relative errors <5%), requiring only a single standard for calibration. Its main drawback is the need for at least 3 interference-free emission lines. The simplest and more practical calibration approach, also applicable with a single standard, but requiring only the response monitored at a single emission wavelength, was SRC, which yielded accurate results, as demonstrated by agreement with certified values at the 95% confidence level and relative errors <21%.