Contrast Solution

<p>The invention relates to a solution comprising: a heteropolyacid and a lanthanide salt in a hydroalcoholic solvent. The solution may optionally also comprise an organic or inorganic buffer with a pH comprised in the interval of 4-6, or a strong acid or base. The object of the present invention also relates to a use of said solution and any products of the reaction between the components thereof as a negative stain for biological samples, a kit containing the components or precursors of said solution and a method for preparing biological samples for analysis by transmission electron microscopy which uses said solution in at least one step.</p>