During the post-certification stability monitoring of the certified reference material (CRM) BCR-551 (DNPHderivatives dissolved in acetonitrile), a decreased concentration of one of the analytes of this CRM, formaldehyde DNPH-derivative, was detected in reference samples (stored at -70 °C) while normal "on sale" samples (stored at -20 °C) remained stable. This behaviour is contrary to the expectation of better stability at lower temperatures. Apparently, the formaldehyde DNPH-derivative reacts with dinitrophenylhydrazine (DNPH) left-over from the synthesis phase, to produce two new substances. These substances have been elucidated as C13H12N8O8 (substance 1) and C20H16N12O12 (substance 2) which, based on their structure, are suggested to be produced consecutively: DNPH + formaldehyde DNPH-derivative -> substance 1 substance 1 + formaldehyde DNPH-derivative -> substance 2 Since acetonitrile freezes at -45 °C, reference samples are frozen at -70 °C while normal samples are still liquid at -20 °C. We believe that this leads to a cryo-concentration of the solutes above the eutectic point and thus to an increased reaction rate in the reference samples. This case demonstrates that care should be taken when extrapolating stability results towards conditions that never have been tested, especially if phase transitions are involved, even at temperature as low as -70 °C. Furthermore, a slower degradation rate at lower temperatures can be overcompensated by a higher concentration due to cryo-concentration above the eutectic temperature.

PEREZ ALCAIDE Juan Andres;  VANERMEN Guido;  NOTEN B;  LINSINGER Thomas; 

2016-02-09

SPRINGER

JRC97838

0949-1775,   

https://publications.jrc.ec.europa.eu/repository/handle/JRC97838,   

10.1007/s00769-015-1185-5,