During my PhD, I prepared a compound library of α‑nitroisobutyranilides by substituting α‑bromoisobutyranilides. This reaction was done using NaNO2 in DMF at room temperature. I monitored the reaction by GC-MS to collect rate data to characterize the mechanism of the reaction. The method I used to take the aliquots is simple and highly transferable to other reactions where a reactant is placed into DMF with a water soluble reagent.
α‑Nitroisobutyranilides from α‑bromoisobutyranilides
| 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. | 
5 mL screw cap vial and prepared cotton Pasteur pipettes.
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| 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). | 
Labelled GC-MS vials
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