Quantitative Analysis of Hydration Using Nitrogen-14 Nuclear Quadrupole Resonance.

Hydration is a quite common process in pharmaceutical solids. Sometimes it is desirable, as it stabilizes the crystal structure; in other cases it is unwanted, as it changes the physical and chemical properties of drugs. We here use (14)N NQR spectroscopy to quantitatively analyze hydration of a model compound, 5-aminotetrazole. (14)N NQR has some great advantages compared to other routinely used techniques to study hydration, like a very simple spectrum, single point calibration, and no need for special sample preparation, but the method's great disadvantage is a rather small sensitivity. Nevertheless, here we demonstrate that (14)N NQR, although being significantly less sensitive than XRD, NIR, and also (35)Cl NQR, is still capable of providing excellent quantitative accuracies. We can achieve errors <1% of the total amount, provided good temperature stabilization is implemented, which then allows long experimental times. We also present results obtained with a SLSE pulse sequence, which is a less robust approach but allows the use of much shorter measuring times (∼200×) and could be used for quantitative real time monitoring of hydration or dehydration.