Protein testing methods at the Canadian Grain Commission

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Appendix – The CNA Procedure practiced at the CGC/GRL

The instrument is calibrated against 150 mg of EDTA (usually - the actual weight is flexible, as long as it is weighed accurately). Samples of grain are ground in a U-D Cyclone grinder, fitted with a 1.0 mm screen, and are analyzed as received. Sample weight for analysis is 250 mg. Again, the weight is flexible, provided that it is accurately recorded, but the standard error per test increases with sample weights of less than 150 mg, while sample weights in excess of 300 mg are difficult to handle. The sample is weighed into a piece of tin foil, and is then wrapped tightly in the foil, to exclude air. Weighed, wrapped samples are then loaded onto the carousel, and analyzed in sequence. Up to 34 samples can be loaded onto the carousel. Results are expressed on a constant moisture basis, either 13.5% or moisture-free. Moisture is determined on all samples by the AACC Approved Method No. 44-15A. We are developing calibrations for all frequently-analyzed materials on the NIRSystems Models 5000 and 6500 for moisture.

This is the method. The procedure for employing the method here includes:

1. Set the instrument to "wake up" - this is computer-controlled
2. Run up to 20 blanks, until the instrument is stabilized - this happens before the staff arrives. Calibrate the blank level
3. Calibrate with 6-8 EDTA samples. EDTA contains 9.59+/- 0.02% nitrogen. The EDTA is dried overnight at 100 degrees C and cooled before use
4. After calibration run 2-3 EDTAs to check accuracy
5. Test CWRS and CWAD check samples
6. Test precision samples (12) in duplicate twice a week
7. Test "routine" samples (up to about 120 per day - each test takes about 3 minutes)
   1. Tare a piece of tin foil and transfer weight to the instrument
   2. Add approximately 250 mg of ground sample (U-D Cyclone grinder, with 1.0 mm screen)
   3. Tightly twist sample into the foil by hand, to make a pellet
   4. Weigh the pellet and transfer the weight to the instrument
8. Test PC (Protein Check) every 20 samples
9. Test every 10^th sample in duplicate

Maintenance is carried out every day according to the instrument software. Further maintenance is carried out periodically according to the Manufacturers' manual.

The coefficient of variability (our measure of analytical precision) is normally about 0.6% of the mean, as compared to about 1.2 % for the Kjeldahl method.

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