Characterization of metal–humic acid complexes by polyacrylamide gel electrophoresis–laser ablation-inductively coupled plasma mass spectrometry Original Research Article

A new speciation methodology based on the use of 1D-PAGE (one dimensional-polyacrylamide gel electrophoresis), IEF (isoelectric focusing electrophoresis) and 2D-PAGE (two dimensional-polyacrylamide gel electrophoresis) as separation techniques and LA-ICP-MS (laser ablation-inductively coupled plasma mass spectrometry) as detection system have been applied to study the distribution of metal–humic acids (metal–HA) complexes in environmental samples. The use of 1D-Tris borate method using low current (10 mA) and Tris borate as trailing ion previous to LA-ICP-MS measurements, allowed to obtain the distribution of metals–HA complexes from a compost sample maintaining metals–HA complexes stability. It was observed that metals were associated with the smallest HA size fraction (around 3 kDa), validating results obtained by high performance size-exclusion chromatography (HP-SEC), cross-flow field flow fractionation (FIFFF) and ultrafiltration. IEF–LA-ICP-MS profiles indicated that there are two main regions: (A) with 3 < pI < 4.5 and (B) with pI > 5. The biggest metals-HA signals are obtained in A region (low pIs) corresponding to low molecular weight (MW) compounds which are predominant in our compost sample indicating a low stabilization of organic matter is low. These results are coincident with 1D-Tris borate PAGE–LA-ICP-MS results. The use of 2D-PAGE method followed by LA-ICP-MS confirmed the results obtained by 1D and IEF as PAGE methods and added information about humic acid molecular size distribution.