2.2.2.4 Dry Matter by Near Infrared Reflectance Spectroscopy >>

2.2.2.3 Laboratory Dry Matter by Microwave Drying to Constant Weight

Reference:
None.

Scope:
This procedure is applicable for the determination of laboratory dry matter in ground, air-dry or partially dried (³ 85% dry matter) forages. Samples dried by this procedure are not appropriate for subsequent fiber, lignin, or acid detergent insoluble nitrogen analysis.

Basic Principle:
Moisture is evaporated from sample by microwave radiation. Total dry matter is determined gravimetrically as residue remaining after drying.

Equipment:
Microwave oven with minimum of 600 watts, turntable preferred but not essential Top loading electronic balance, accurate to 0.01 g Pans, paper or microwavable glass or plastic

Reagents:
None.

Safety Precautions:

• Periodically test the oven for radiation leaks. Microwaves are absorbed by the body and can produce damaging heat effects, especially on the lens of the eye.
• Cardiac pacemakers may fail in presence of microwave radiation.
• Do not place any metal item or aluminum foil into the oven. Sparking or crackling in the oven indicates the presence of metal.

Procedure:

1. Dry empty pans in microwave for 3 min at full power.
2. Weigh empty pans on top loading balance and record weight (W4) to nearest 0.01 g.
3. Weigh forage (100 to 200 g) into pans, recording weight of pan and sample to nearest 0.01 g (W5).
4. Place sample in oven and microwave for about 3 min at full power.
5. Remove sample and mix.
6. Weigh sample and container recording weight to 0.01 g.
7. Return sample to microwave at 50% power for 1 min and re-weigh container recording weight to nearest 0.01 g.
8. Repeat steps 6 and 7 until no weight loss occurs during drying interval. Do not allow sample to char. Record final weight of pan and dried sample (W6) to nearest 0.01g.

Comments:

• Be careful not to char or burn samples. As samples approach dryness, reduce drying time and/or power setting. Discard darkened or burnt smelling samples and start over. Charring affects dry matter determination.
• Samples with higher moisture contents require longer drying times. Increase the number of drying intervals, not the time per drying interval or microwave power level, as these may lead to charred or burned samples.
• A glass of water placed in the microwave oven during drying will reduce the possibility of charring samples.

Calculation: Percent Laboratory Dry Matter (Lab DM)

% Lab DM = (W6 - W4/W5 - W4) X 100

• Where W4 = tare weight of pan in grams
• W5 = initial weight of sample and pan in grams
• W6 = dry weight of sample and pan in grams

Quality Control:
Include at least one set of duplicates in each run if single determinations are being made.

An acceptable average standard deviation among replicated analyses for moisture or dry matter is about ±0.10, which results in a warning limit (2s) of about ±0.20 and a control limit (3s) of about ±0.30. Plot the results of the duplicate analyses on an R-control chart (Appendix D) and examine the chart for trends. Results outside the 95 percent confidence limits warn of possible problems with the analytical system. Results outside the 99 percent confidence limits indicate loss of control, and results of the run should be discarded. If more than five or six points in succession fall on one side or the other of the 50 percent line, it is a strong indication that something has changed and is cause for investigation.

2.2.2.4 Dry Matter by Near Infrared Reflectance Spectroscopy >>

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