Nuclear
magnetic resonance (NMR) longitudinal rotating frame relaxation
time (T1ρ), rarely used in low-field NMR, can be
more effective than conventional T1 and T2 relaxation
times to differentiate polymorphic forms of solid pharmaceuticals.
This could be attributed to T1ρ sensibility to structural
and molecular dynamics that can be enhanced by changing the strength
of the oscillating magnetic field (B
1
) of spinlock pulses. Here, we compared the capacity of T1, T2, and T1ρ to differentiate
inactive (A) and active (C) crystalline forms of the World Health
Organization essential drug Mebendazole. The results showed that T1 and T2 values of both forms were statistically
identical at 0.47 T. Conversely, T1ρ of both forms
measured with weak spinlock B
1
fields, ranging from 0.08 to 0.80 mT were statistically different
in the same spectrometer. The T1ρ also has the limit
of detection to detect the presence of at least 10% of inactive A
form in the active C form. Therefore, T1ρ, measured
with weak spinlock B
1
fields
can be an effective, streamlined, and complementary approach for characterizing
not only solid active pharmaceutical ingredients but other solid-state
materials as well.