The reactant compound (1-2g) was exposed to NaNO2 4.00 g (13.6 mmol) and 40 mL of DMF. Aliquots were taken periodically by the removal of 1 mL of the reacting mixture and doing a mini liquid/liquid extraction using 2 mL of DCM and washing four times with 3 mL of water in a 5 mL screw cap vial. As the DCM layer stays at the bottom, the water portion is easily drawn off. It contains a small amount of NaNO2 and an even smaller amount of HBr. It can be discarded down the sink after each wash.
The DCM layer is then dried over anhydrous MgSO4 and transferred to a GC-MS vial by passing it through a Pasteur pipette that had been plugged with a small amount of cotton.
I used anhydrous MgSO4, but the monohydrate is also adequate. Any greater than 1 water per molecule of MgSO4 will not effectively dry the DCM. For example, MgSO4.3H2O or MgSO4.7H2O will not work.
This method quenches the reaction as it washes off the reagent which is water soluble. As the reaction is stopped the proportions of the reactant and product organic compounds will remain the same. In my case the GC-MS was out of order for several weeks and I continued to take aliquots in this way and label them. When the GC-MS was back online, I ran all of the samples and was still able to collect excellent data (although I had to top-up the DCM in some of the vials).