## 3 Applications of CWFP in Steady State Regime  
The first application of SSFP/CWFP sequences in low-field TD-NMR was the enhancement of SNR for quantitative analysis [12]. Compared to dynamic nuclear polarization and hyperpolarization [31] techniques, CWFP improves SNR without using sophisticated instrumentation and/or special chemical agents [12]. Therefore, it can be applied in any standard low-field NMR spectrometer. The enhancement in SNR using the CWFP sequence occurs because the magnitude of signal is not dependent on T\({}_{1}\), but is dependent on the T\({}_{1}\)/T\({}_{2}\) ratio (Equation (3) for \(\theta\) = 90\({}^{\circ}\) and \(\Phi\) = (2\(n+1\))\(\pi\)). Therefore, the pulse interval T\({}_{\rm p}\) can be as short as 100 \(\upmu\)s. Normally, T\({}_{\rm p}\) is used between 300 to 500 \(\upmu\)s depending on T\({}_{2}\)*. A greater enhancement is observed for samples with longer T\({}_{1}\). This way, a one order of magnitude SNR gain is easily obtained for a sample with T\({}_{1}\approx 1\) s using the same average time of FID or echo signals, using conventional pulse sequences with a 5T\({}_{1}\) waiting time. Figure 4 shows an enhancement of more than one order of magnitude in SNR of a single corn seed in 15 s [13]. Figure 4A shows FID and echo acquisition, using pulse sequence \(\pi/2-\tau_{\rm e}-\pi\), where the echo is observed at 2\(\tau_{\rm e}\), with \(\tau_{\rm e}\) = 3.5 ms. The FID and echo signal data were acquired with 16 scans. For the CWFP experiment, Figure 4B, more than 16,000 scans were accumulated in the same total time of 15 s.  
Figure 4: **(A)** Free induction decay (FID) and spin echo (SE) signals for a corn seed obtained in 15 s with a spin-echo sequence with \(\tau_{\rm e}\) = 3.5 ms, 16 scans. **(B)** CWFP signal for the same sample and the same acquisition time of 15 s (16,664 acquisition windows of 3T\({}_{\rm p}\)), using 90\({}^{\circ}\) pulses, T\({}_{\rm p}\) = 0.3 ms, and \(\Phi\) = 5\(\pi\). Adapted from Publication [13], Copyright (2003), with permission from Elsevier. Figure 3: Simulated magnitude profiles of CWFP signals. **(A)** CWFP and CP-CWFP profiles and **(B)** CWFPx-x and CP-CWFPx-x profiles. Solid lines represent the profiles for \(T_{p}\) = 300 \(\upmu\)s and dashed lines represented \(T_{p}\) = 200 \(\upmu\)s. Adapted from Publication [24], Copyright (2015), with permission from Elsevier. Another advantage of CWFP in quantitative analyses is the direct correlation between the \(\,\mathrm{I}\,\mathrm{M}_{\mathrm{ss}}\,\mathrm{l}\) value and the concentration \(\mathrm{M}_{0}\) for samples with the same \(\mathrm{T}_{1}/\mathrm{T}_{2}\) ratio, enabling samples with similar chemical composition to be measured quantitatively. Figure 5 shows the linearity correlation between \(\,\mathrm{I}\,\mathrm{M}_{\mathrm{ss}}\,\mathrm{l}\) and concentration of vegetable oil in CCl\({}_{4}\) (Figure 5A), and hydrogen content in several organic solvents (Figure 5B). CWFP signals are less demanding than standard pulse sequences in flip angle calibration, \(\mathrm{B}_{1}\) homogeneity, and magnetic field drift, which are common problems in low-field TD-NMR [13]. CWFP was also used for quantitative analyses in flowing samples [11; 32]. Experimental and theoretical studies showed that CWFP is much more dependent on flow than SSFP [11]. CWFP is sensitive to a smaller magnetic field gradient than SSFP, dependent on frequency offset and flow direction. Therefore, tailored sensitivity can be produced by the tuning frequency offset, which is a useful property for fine flow monitoring and flow control [11]. [PERSON] [33] proposed the use of these characteristics for real-time measurement of proportion and flow-rate of multicomponent complex fluid in high speed. The main advantages are the improvement in the SNR and the steady state signal depends on the fluid velocity, which is virtually insensitive to self-diffusion effects. Another example of quantitative measurements in flow using the CWFP regime is the determination of oil content in seeds.