Handheld NMR Systems and Their Applications for Biomolecular Sensing - Point-of-Care Diagnostics on a Chip - page 184

Fig. 8.10 Measured excitation signal duty cycle versus control voltage V C

8.4 NMR Experiments and NMR-Based Biomolecular

Sensing

8.4.1 Proton NMR Experiments

NMR is performed on protons of hydrogen atoms in a 2-L water sample using
the palm system. Figure 8.11 shows a measured, down-converted NMR signal. The
repeated ringings, which are the result of the proton excitations using a CPMG pulse
sequence [ 6 ], constitute the NMR signal. It decays with characteristic time called
T 2 , one of the key parameters in NMR experiments [ 6 ], which we use in our NMR-
based biomolecular sensing, as seen shortly. T 2 D 100 ms is extracted from the

exponentially decaying envelope of the NMR signal, shown as a dotted line. The
repeated spikes between the ringings are due to the coupling of the large excitation
signals, but they do not compromise the observation of the NMR signal (ringings),
as they occur at different time instances. The spin-mass (the minimum mass of water
that produces a detectable NMR signal; a smaller minimum mass corresponds to a
higher spin-mass sensitivity) sensitivity is 2.5 times higher than that of the 2-kg
portable system and 150 times higher than that of the state-of-the-art commercial
system [ 1 ].
Figure 8.12 a shows a measured, down-converted NMR signal obtained in a
proton NMR experiment (5 L water sample) done with the 1-chip system,

from which we obtain T 2 D 722 ms. The spikes coupled from the large excitation

signals are more pronounced, but once again, they do not hamper the observation