Contrast solution for the characterisation of biological samples by electron or correlative microscopy

A solution comprising:

• a) a heteropolyacid with the general formula H3PM12O40, wherein M is tungsten or molybdenum,
• b) a lanthanide salt with the general formula Ln(III)Ax*nH2O, wherein Ln is an element belonging to the series of lanthanides other than promethium, A is an organic or inorganic anion, x is a whole number greater than or equal to 1, and n is a whole number greater than or equal to 0;
• c) a solvent consisting of a water/organic solvent mixture or an organic solvent, wherein said organic solvent is selected from a ketone and an alcohol and, in the case of a water/organic solvent mixture, is present in an amount comprised from 1 to 99% (v/v) relative to the total volume of the mixture in the case of ketone, or in an amount comprised from 71 to 99% (v/v) relative to the total volume of the mixture in the case of alcohol.