2.2.2.2 Laboratory Dry Matter by Oven Drying at... >>

2.2.2.1 Laboratory Dry Matter by Oven Drying for 2 hr at 135°C

Reference:
Drying of feeds at 135oC for 2 hr. (930.15) Official Methods of Analysis. 1990. Association of Official Analytical Chemists. 15th Edition.

Scope:
This procedure is applicable for the determination of dry matter on ground air-dry or partially-dried (³85% dry matter) forages with low volatile acid or alcohol content. Samples dried by this procedure are not appropriate for further chemical analyses. This procedure is recommended for developing forage dry matter calibration for NIR.

Basic Principle:
Moisture is evaporated from the sample by oven drying. Dry matter is determined gravimetrically as the residue remaining after drying.

Equipment:
Forced-air drying oven at 135o ±2oC. Oven should be equipped with a wire rod shelf to allow the circulation of air. It should be vented and operated with vents open.
Analytical electronic balance, accurate to 0.1 mg Aluminum dish (pan), ³50 mm diameter, £40 mm deep, covered Desiccator

Reagents:
None.

Safety Precautions:

• Use standard precautions when working around electrical equipment or glassware.
• Make sure that all electrical equipment is properly grounded and installed and maintained by qualified electricians.

Procedure:

1. Dry aluminum dish with cover at 135o C ±2o C for at least 2 hr.
2. Cover dish and move to desiccator. Immediately cover desiccator and allow dishes to cool to room temperature. Do not allow dishes to remain in desiccator more than 2 to 3 hr.
3. Weigh dish with cover (W4) to nearest 0.1 mg, removing one at a time from desiccator and keeping desiccator closed between container removals.
4. Add approximately 2 g ground sample to each dish. Record weight of dish with cover and sample (W5) to nearest 0.1 mg.
5. Shake dish gently to uniformly distribute the sample and expose the maximum area for drying.
6. Dry samples in oven preheated to 135oC for 2 hr.
7. Move samples to desiccator, placing cover on each dish as it is transferred. Seal desiccator and allow to cool to room temperature. Do not allow samples to remain in desiccator for more than 2 to 3 hr.
8. Weigh dish with cover and dried sample (W6), recording weight to nearest 0.1 mg.

For NIR calibration replace step 4 above with:

• 4.1) Load NIR sample cup placing one scoop of forage (previously dried to 90-95% dry matter or greater, ground with cyclone mill to pass 1 mm screen, thoroughly mixed) on each third of the glass surface. Fill to leave adequate space for cell backing.
• 4.2) Scan sample on NIR instrument and store spectra.
• 4.3) Immediately remove sample from NIR instrument and weigh 2 g forage from sample cup to an aluminum dish. Record weight of dish with cover and sample (W5) to nearest 0.1 mg.

Comments:

• Time and temperature described in procedures must be adhered to closely.
• Samples should be placed in drying oven so that air can circulate freely. Containers should not touch each other.
• Do not overload oven.
• Slide the desiccator lid open. Do not place the lid on the counter top with the grease side down. The grease will pick up dirt, preventing formation of a seal.
• Seals should be kept clean and well greased and the lid should always slide easily on or off. If the lid "grabs," it is time to remove the old grease and apply fresh lubricant.
• If a lid can be directly lifted off the desiccator, either the desiccator was not properly sealed or, more likely, it needs fresh lubricant.
• Rubber stoppers in the lid should always be pliable.
• Sample dishes should not be packed excessively tight in a desiccator. Air movement is necessary to cool sample dishes. Dishes should not touch each other.
• The desiccator lid should be left open for minimal amount of time.
• Desiccant should be checked and dried periodically. Replace desiccant twice annually or more often depending on use. Use of desiccant with color indicator for moisture is recommended.
• Open a loaded desiccator very slowly after samples have cooled. A vacuum forms during cooling, and abrupt opening results in turbulence which can blow samples out of uncovered containers.

Calculation: Percent Laboratory Dry Matter (Lab DM)

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

• Where W4 = tare weight of dish (with cover) in grams
• W5 = initial weight of sample and dish (with cover) in grams
• W6 = dry weight of sample and dish (with cover) 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.

2.2.2.2 Laboratory Dry Matter by Oven Drying at... >>

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