2.1.2 Total Dry Matter by Oven Drying at 100°C
for 24 hr or 105°C for 16 hr
References:
- Moisture in Peat. (967.03) Official Methods of
Analysis. 1990. Association of Official Analytical
Chemists. 15th Edition.
- Faichney, G.J. and G.A. White. 1983. Methods for the
analysis of feeds eaten by ruminants. Commonwealth
Scientific and Industrial Research Organization,
Melbourne, Australia.
- Windham, W.R., J.A. Robertson, and R.G. Leffler. 1987. A
comparison of methods for moisture determination of
forages for near infrared reflectance spectroscopy
calibration and validation. Crop Sci. 27:777-783.
- Goering, H.K. and P.J. Van Soest. 1970. Forage fiber
analyses (apparatus, reagents, procedures, and some
applications). ARS/USDA Handbook No. 379, Superintendent
of Documents, US Government Printing Office, Washington,
D.C. 20402
Scope:
This procedure may be used for determination of dry matter on
forage samples or for dry weight determinations of fiber residues
following fiber extraction. 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.
Basic Principle:
Moisture is evaporated from sample by oven drying. Total dry
matter is determined gravimetrically as residue remaining after
drying. Weighings may be made on hot sample or after cooling in
desiccator.
Equipment:
Forced-air drying oven at 100oC (or 105oC), capable of
maintaining temperature at ±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. Aluminum dish (pan), ³50 mm
diameter, £40 mm deep, covered if desiccator used Crucibles,
porcelain, low wide form, 50 mL, Coors #1, covered if desiccator
used Top loading electronic balance, accurate to 0.1 mg
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:
Hot Weigh Method
- If only moisture is to be determined on the sample, use
an aluminum dish. If ash determination is to follow on
the dry matter residue, use a porcelain crucible. Dry the
appropriate containers and three empty crucibles or
dishes to be used for warming balance at 100oC (or 105oC)
for at least 2 hr.
- Warm balance by sequentially placing the three empty
crucibles or dishes on balance for 20 sec each.
- Removing one at a time from the oven, weigh container
(W4), recording weight to nearest 0.1 mg. Weigh rapidly,
recording minimum weight (as soon as balance has
stabilized, usually within 15 sec after removing from
oven). Whenever weighing is interrupted, balance should
be re-warmed according to step (2).
- After all containers have been weighed, allow balance and
sample containers to cool.
- Tare empty container to zero and weigh (W7) approximately
2 g ground sample into each container or weigh
approximately 2 g into each container and record weight
of sample and container (W5) to nearest 0.1 mg.
- Shake container gently to uniformly distribute the sample
and expose the maximum area for drying.
- Place samples into an oven which has been preheated to
100oC (or 105oC) for at least 3 hr. Oven should return to
temperature within 1 hr after samples in containers have
been placed into it.
- Leave uncovered samples in oven for 24 hr at 100oC or 16
hr (or overnight) at 105oC.
- Individually remove containers from oven and hot weigh
containers with dried sample as described in steps (2)
and (3). Record weight (W6) to nearest 0.1 mg.
Comments:
- Use a forced-air oven so that drying is more rapid and
uniform and temperature drop is minimized during
weighing.
- Samples should be placed in drying oven so that air can
circulate freely. Containers should not touch each other.
- The balance must be located next to the oven; carrying
samples any distance will allow cooling and absorption of
moisture.
- Containers should be removed from oven one at a time and
immediately weighed.
- Use of computer software to electronically record weight
can reduce variance in weights due to operator
differences in determining minimum weight.
Procedure:
Cold Weigh Method
- If only moisture is to be determined on the sample, use
an aluminum dish with cover. If ash determination is to
follow on the dry matter residue, use a porcelain
crucible with cover. Dry the appropriate container at
100oC (or 105oC) for at least 2 hr.
- Cover containers and rapidly move to desiccator.
Immediately cover desiccator and allow containers to cool
to room temperature. Do not allow containers to remain in
desiccator more than 2 to 3 hr.
- Weigh container with cover (W4) to nearest 0.1 mg,
removing one at a time from desiccator and keeping
desiccator closed between container removals.
- Tare balance and weigh (W7) approximately 2 g ground
sample into each container or weigh approximately 2 g
ground sample into each container and record weight of
container with cover and sample (W5) to nearest 0.1 mg.
- Shake dish or crucible gently to uniformly distribute the
sample and expose the maximum area for drying.
- Place samples (with covers removed to the side) into oven
that has been preheated to 100oC (or 105oC) at least 3 hr
prior to use. Oven should return to temperature within 1
hr after samples have been placed into it.
- Leave uncovered samples in oven for 24 hr at 100oC or 16
hr (or overnight) at 105oC.
- Move samples to desiccator and place cover on each
container as it is transferred. Seal desiccator and allow
to cool for at least 1 hr but not more than 2 to 3 hr.
- Weigh container with cover and dried sample (W6),
recording weight to nearest 0.1 mg.
Comments:
- Samples should be placed in drying oven so that air can
circulate freely. Containers should not touch each other.
- Desiccator 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.
- 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.
- 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.
- 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.
- Desiccator lid should be left open for minimal amount of
time.
- Desiccant should be checked periodically and dried.
Replace twice annually. Use of desiccant with color
indicator for moisture is recommended.
Calculation: Percent Total Dry Matter
(Total DM)
If empty container is tared to zero in step 5
(hot weigh) or step 4 (cold weigh)
W6 - W4
% Total DM = -------- X 100
W7
- Where W4 = tare weight of container (with cover) in grams
- W7 = dry weight of sample in grams
- W6 = dry weight of sample and container (with cover) in
grams
If empty container is not tared to zero in step 5 (hot weigh)
or step 4 (cold weigh)
W6 - W4
% Total DM = -------- X 100
W5 - W4
- Where W4 = tare weight of container (with cover) in grams
- W5 = dry weight of sample and container (with cover) in
grams
- W6 = dry weight of sample and container (with cover) 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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