Investigation Of The Effects Of Buffer Solutions On NDMA Formation

N-nitrosodimethylamine （NDMA），the simplest molecule in structure among allthe nitrosamines, has aroused widespread concern due to its high carcinogenic,mutagenic and teratogenic risks. It is well known that the NDMA formation reactionis highly dependent on pH value, therefore, a specific buffer has to be used toregulate the suitable pH value for almost all the investigations concerning NDMA.However, little attention has been paid to the influence of buffer matrices and severalcommon anions on the formation of NDMA, in spite of their widespreademployment during these studies. Therefore, the investigation of the influence ofbuffer and some common anions will be helpful to minimize potent analytical errorsand systematicly understand NDMA formation mechanisms.In this paper, the effects of four commonly used buffer solutions（Na₂HPO₄/C6H8O7, Na3（C6H5O7）/C6H8O7, NaH₂PO₄/NaOH and NaH₂PO₄/Na₂HPO₄）,as well as HCO₃^-and Cl^-on NDMA formation were investigated. Moreover, thepossible influence mechanisms were conducted.1. Investigation of the influence of four common buffer matrices on the formationof NDMA and reaction mechanism.The effect of buffer solutions on NDMA formation from the nitrosation ofdimethylamine （DMA） by sodium nitrite （NaNO₂） was investigated at pH6.4in fourkinds of buffer matrices, i.e., Na₂HPO₄/C6H8O7, Na3（C6H5O7）/C6H8O7,NaH₂PO₄/NaOH and NaH₂PO₄/Na₂HPO₄. Experimental results indicated that the fourbuffer solutions had different influences on NDMA formation. Moreover, thephosphate composition played a more significant role in inhibiting NDMA formationcompared to the citrate composition, and the inhibitory effect of phosphate also wasproved to be in a dose-dependent manner. A further mechanistic study indicated that itwas the interaction between NaH₂PO₄and NaNO₂rather than DMA that lead to theinhibitory effect of phosphate buffer. This conclusion is consistent with the theoreticalresults. Based on these results, the effects of phosphate buffer on NDMA formationwere also been investigated in1,4,10, and50mM buffer solutions, respectively（with the pH in the range of6.5to8.0）. The experimental results demonstrated thatthe inhibitory ability of phosphate buffer increased when the buffer concentrationsincreased and the pH value decreased. 2. Investigation of the effects of HCO₃^-and Cl^-on NDMA formation andcorresponding reaction mechanism.As HCO₃^-and Cl^-are significant anions in vivo, batch experiments on studyingtheir impacts on NDMA formation were performed. During these experiments, theconcentrations of HCO₃^-and Cl^-were controlled at1、5and10mM with the pH inthe range of6.5to8.0. Experimental results indicated that the effects of HCO₃^-andCl^-on NDMAformation are the opposite. The HCO₃^-exhibited an obvious promotionon NDMA formation which became stronger with the increasing concentration anddecreasing pH value of the reaction system. The mechanistic studies showed that theconcentrations of NO₂^-and DMA were both decreased with the addition of HCO₃^-,which indicated that HCO₃^-could react with both of them. Therefore, it can beinferred that an active intermediate may be produced from the reaction of HCO₃^-withDMA or NO₂^-, which leaded to the increase of NDMA. In consistent with the effect ofphosphate buffer, Cl^-was also found to inhibit the formation of NDMA. Moreover,the inhibitory effect became more significant with the increasing concentration of Cl^-,while it became weaker with the increasing pH. The further mechanistic studiesillustrated no interactions of Cl^-with DMA, but the NO₂^-concentration increasedwith the addition of Cl^-. As can be seen from previous studies, N₂O₃is the directnitrosating agent during NDMA formation from the reaction of NaNO₂and DMA. Itis possible the interaction between N₂O₃and Cl^-can account for the decreased NO₂^-,which leads to the decrease of nitrosating agent amount. This may be the cause of Cl^-inhibiting NDMA formation.