2.1.1 Total 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 content. Samples dried by this procedure
are not appropriate for subsequent fiber, lignin, or acid
detergent insoluble nitrogen analysis. Volatile acids and
alcohols may be lost from fermented samples.
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:
- Dry aluminum dish with cover at 135o C ±2o C for at
least 2 hr.
- Cover dishes and move to desiccator.
- Immediately cover desiccator and allow covered dishes to
cool to room temperature. Do not allow dishes to remain
in desiccator more than 2 to 3 hr.
- Weigh dishes with cover (W4) to nearest 0.1 mg, removing
one at a time from desiccator and keeping desiccator
closed between dish removals.
- Add approximately 2 g ground sample to each dish. Record
weight of dish with cover and sample (W5) to nearest 0.1
mg.
- Shake dish gently to uniformly distribute the sample and
expose the maximum area for drying.
- Insert samples with lids removed to the side into
preheated oven at 135oC and dry for 2 hr after oven has
returned to temperature.
- Move samples to desiccator, place cover on each dish,
seal desiccator and allow to cool to room temperature. Do
not allow samples to remain in desiccator for more than 2
to 3 hr.
- Weigh dish with cover and dried sample (W6), recording
weight to nearest 0.1 mg.
For NIR calibration replace step 5 above with:
- 5.1 Load NIR sample cup
placing one scoop of forage (previously dried to 90 to
95% dry matter or greater, ground with cyclone mill to
pass 1 mm screen, thoroughly mixed) on each third of the
glass surface to ensure that portions of different
subsamples are scanned. Overfill the sample holder and
scrape off any excess. Press back into holder until tight
and level.
- 5.2 Scan sample on NIR
instrument and store spectra.
- 5.3 Immediately remove
sample from NIR instrument and weigh 2 g forage from
sample cup to aluminum dishes. Record weight of dish with
cover and sample (W5) to nearest 0.1 mg.
Comments:
- Time and temperature must be adhered to closely.
- Samples should be placed in drying oven so that air can
circulate freely. Containers should not touch.
- Slide the desiccator lid open. Do not place the lid on
the countertop 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.
- 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.
- Desiccator lid should be slid open for the removal of
each container and closed during weighing. Leaving the
lid open allows samples to absorb moisture.
- Desiccant should be checked and dried periodically.
Replace at least twice annually. Use of desiccant with
color indicator for moisture is recommended.
Calculation: Percent Total Dry Matter (Total
DM)
W6 - W4
% Total DM = -------- X 100
W5 - W4
- Where W4 = tare weight of dish in grams
- W5 = initial weight of sample and dish in grams
- W6 = dry weight of sample and dish in grams
- Calculation: Percent Total Moisture: % Total Moisture =
100 - % Total DM
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 ±0.20 and a control limit
(3s) of ±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.
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