Principle of Oil &Moisture Content Testing

The hard pulse echo sequence originating from CPMG sequence with only one echo count, namely only one 180° pulse after the 90° pulse, shown in Figure 1, is applied to measure the Oil &Moisture content in seed by means of NMR technology. After 90° pulse, FID signal appears and after 180° pulse, echo signal appears.

Figure 1 Testing Oil &Moisture content of seed by applying CPMG sequence

In CPMG sequence, the line composed of all echo peak points, whose decaying speed is proportionate to exp（-t/T2）, embodies the decaying process of spin-spin(T2) relaxation. T2 is the transverse relaxation time of the hydrogenous substances in samples. Supposing there exist different kinds of hydrogen in a sample, the signal magnitude M can be expressed as

Since there exists hydrogen in both Moisture and oil, the magnitude M should be expressed as follows.

We consider the detected NMR signal of seed as merely that of Oil &Moisture Content, between which, there is a marked difference of decaying speed, namely, their relaxation time T2 differs a lot. Solidity, irreducible Moisture and bound Moisture have short relaxation time, while oil and free Moisture have long relaxation time. Since Moisture in seed can be counted as irreducible Moisture, its relaxation time T2_Moisture is far less than oil’s relaxation time T2_oil. When t is zero, the FID signal M, composed of hydrogen both from Moisture and oil, independent of either T2_Moisture or T2_oil, will be detected. After τ, signal M can be expressed as

When τ is much larger than T2_Moisture yet far less than T2_oil, P_water=exp(-τ/T_water) is approximately zero while P_oil=exp(-τ/T_oil) is not. Namely, at that time, signal of irreducible Moisture has completely decayed and the echo peak signal is entirely contributed by the hydrogen from oil. Since FID signal is the sum signal of irreducible Moisture and oil, and peak value of the first echo is merely the signal of oil, we can get the signal of irreducible Moisture as the difference of FID signal and echo signal.

According to ISO10565: 1998, the standard τ is 3. 5ms when testing Moisture and oil content in seed.

Based on the linear correlation, which can be calculated by means of calibration, between signal magnitude and mass of oil and/or Moisture, oil and/or Moisture content of sample can thus be easily got.