Thiomersal (also called thimerosal) is a mercury containing organic molecule that has been widely used as preservative for vaccines and eye drops. Other applications of this substance have been for ear drops, storage and cleaning solutions for contact lenses, and in tattoo inks. It is very effective, even in very low concentrations, against a wide range of microorganisms and viruses.

To reduce the risk for consumers the maximum concentration of mercury in the products is limited by the authorities. Typical limit values in the European Union are 0.007% (as Hg) in eye products (EU Commission Regulation 1223/2009) or up to 25 μg thiomersal per vaccine injection dose if vaccines are distributed in multi-vaccine containers. 

Polarography or voltammetry can be used to accurately determine the concentration of thiomersal in vaccines or other cosmetic and pharmaceutical solutions (such as eye drops). The method is simple to perform, specific, and free of interferences. 

Vaccine against diphtheria, tetanus, pertussis

884 Professional VA.
Figure 1. 884 Professional VA.

The sample and the supporting electrolyte are  pipetted into the measuring vessel. The determination of thiomersal is carried out with an  884 Professional VA using the parameters specified  in Table 1. The concentration is determined using  external calibration with a calibration curve recorded  from five standard concentrations. 

Table 1. Parameters for voltammetric analysis of thiomersal in vaccine
Parameter Setting
Working electrode HMDE or DME
Mode DP – Differential Pulse
Deposition  none
Start potential -0.2 V
End potential -0.6 V 
Peak potential thiomersal -0.45 V
  • Working electrode: Multi-Mode Electrode pro with standard glass capillaries 
  • Reference electrode: Ag/AgCl/KCl (3 mol/L) reference electrode with electrolyte vessel. Bridge electrolyte: KCl (3 mol/L)
  • Auxiliary electrode: Platinum rod electrode
Determination of thiomersal in vaccine sample.
Figure 2. Determination of thiomersal in vaccine sample.

The determination of thiomersal can be carried out in a simple and straightforward manner with voltammetry. The method is selective and free of interferences. 

Table 2. Results of the thiomersal determination with the 884 Professional VA
Sample Thiomersal [mg/L] 
DTP vaccine 102
Author

Metrohm AG

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