Process NMR

Solid Fat Content by Time-Domain NMR  - Printable Version (pdf)

Solid Fat Content (SFC) is generally accepted analysis of fats and oils in the food industry. The traditional extraction methods for SFC determination are slow, irreproducible and require additional chemicals. Direct measurements of SFC by NMR (nuclear magnetic resonance) provides quick and accurate determination of SFC value. NMR as method of fats and oils analysis is included in the following international standards:

  • AOCS Cd 16b-93 revised in 2000 (in USA)
  • ISO 8292 (in Europe)

Solid Fat Content determination by NMR is based on direct ratio measuring between the solid and liquid parts of the sample.

After excitation of the sample by powerful 90o RF pulse FID (Free Induction Decay) is detected. FID is the signal accompanies relaxation process of hydrogen proton magnetic spins back to equilibrium state after disturbing. FID contains contributions from both solid and liquid parts. The protons of liquid content are more rapid in comparsion with protons of solid. It provides more fast signal decay from solids than from liquids. Thus, these contributions can be distinguished.

Calculation of SFC value using Free Induction Decay

Fig. 1. Principle of SFC determination using FID

The SFC value is determinated by taking two measuring points on FID. FID amplitude at point S corresponding to total solids plus liquids and at point L corresponding liquids only The specific ratio can be found using equation shown on Fig. 1. This ratio is considered as the SFC value.

An F-factor (f) allows prediction of FID amplitude value immediately after RF pulse as it is impossible to obtain initial NMR signal amplitude direclty during so-called dead-time. Dead-time is a short time (< 10 us) period caused by the decay of powerful RF pulse in resonance circuit of NMR sensor. F-factor increases the measurements accuracy and is determined during calibration procedure.

Portable NMR spectrometer Spin Track from Resonance System Ltd. is ideal instrument for SFC measurements because of fine technical characteristics which make measurements very reproducible and accurate. Small weight and mobility allow its usage in production routine measurements and in advanced laboratories for complicated investigations. Spin Track fulfills the requirements of international standards like AOCS Cd 16b-93, AOCS Cd 16-81 revised in 2000, ISO 8292, IUPAC 2.150.

The volume of the sample is 1-3 ml. SFC measurements cycle for 6 point melting curve using special container for samples and tempering:

  • investigated compound is melted at 80..100oC and held for 15 min. 6 samples are prepared.
  • temperature of all samples is maintained at 60oC for 5-15 min.
  • temperature of all samples is maintained at 0oC for 60 min.
  • Each sample is held at required temperature for measurement (typically 10oC / 15oC / 20oC / 25oC / 30oC / 35oC) for 30 -35 min.
  • Every sample is placed into sensor and measurement process is started. Full cycle requires 110 min. Every NMR measurement takes not more then 6 s. It is possible to use only one sample for whole cycle with consequently holding at all temperatures but this approach requires approximately 230 min.

The procedure of calibration can be performed using the set of original calibration samples developed by Resonance Systems Ltd.

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