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 maybe used for determination of laboratory dry
matter on ground air-dry or partially dried (³ 85% dry matter)
forage samples. This procedure is also applicable for dry weight
determinations of fiber residues following acid detergent or
neutral detergent extraction.
Basic Principle:
Moisture is evaporated from the sample by oven drying. Laboratory
dry matter is determined gravimetrically as residue remaining
after drying.
Equipment:
Forced-air drying oven at 100oC (or 105oC), capable of
maintaining temperature at ±1oC. 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 with 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 container and three crucibles to be used to
warm the balance at 100oC (or 105oC) for at least 2 hr.
- Warm balance by sequentially placing three empty
crucibles 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 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 the 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 addition of
moisture.
- Containers should be removed from oven one at a time and
immediately weighed.
- Use of computer software to electronically record weight
is recommended to 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 containers at
100oC (or 105oC) for at least 2 hr.
- Cover containers and 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 container and weigh (W7) approximately 2 g ground
sample into container with cover or add approximately 2 g
ground sample to each container and record weight of
container with cover and sample (W5) to nearest 0.1 mg.
- Shake container gently to uniformly distribute the sample
and expose the maximum area for drying.
- Place samples with covers removed to 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, placing 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 the drying oven so that air
can circulate freely. Containers should not touch each
other. Air movement is necessary to cool sample dishes.
- 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.
- 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.
- 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.
- 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 reclosed before weighing. Leaving the
lid open allows samples to absorb moisture.
- Desiccant should be checked and dried periodically. It
should be replaced twice annually. Use of desiccant with
color indicator for moisture is recommended.
Calculation: Percent Laboratory Dry Matter (Lab DM)
If empty container is tared to zero in step 3 (hot weigh) or step
4 (cold weigh)
% Lab DM = W6 - W4 / W7 X 100
- Where W4 = tare weight of container (with cover) in grams
- W7 = initial 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 3 (hot weigh)
or step 4 (cold weigh)
% Lab DM = (W6 - W4/W5 - W4) X 100
- Where W4 = tare weight of container (with cover) in grams
- W5 = initial weight of sample and container (with cover)
in grams
- W6 = dry weight of sample and container (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.
|