1. Sample Preparation
Reference:
- Animal Feed: Sample Preparation. (950.02)
Official Methods of Analysis. 1990.
Association of Official Analytical Chemists, 15th
edition.
- Plants: Preparation of Sample. (922.02)
Official Methods of Analysis. 1990.
Association of Official Analytical Chemists,
15th edition.
Scope:
This method is applicable to the preparation of forages for
laboratory analysis.
Basic Principle:
Laboratory sample preparation is the process of converting the
sample received at the laboratory into a homogeneous material
suitable for analysis. This process generally involves drying
and/or grinding.
Most forage samples received at a laboratory fall into one of
the following categories:
- those dry enough to grind and analyze immediately (sample
90 to 95% or more dry matter).
- those dry enough to be coarsely ground (to pass a 4 to 6
mm sieve) but too wet to be finely ground.
- those samples which need to be partially dried before the
sample can be coarsely ground (sample dry matter less
than approximately 85%).
These sample types must be handled differently. See the
following flow chart (Figure 1). The sample type, the type of
grinder to be used and the fineness of the grind will determine
how "wet" a sample can be ground. Most forages can
easily be ground to pass a 4 to 6 mm sieve using a cutting mill
(Wiley or equivalent) at 80 to 85% dry matter without problems
(sticking in the mill, moisture loss, etc.). However, when using
a cyclone mill to grind forage samples to pass a 1 mm sieve, most
samples need to be 90 to 95% dry matter to grind properly.
Samples at approximately 85% or greater dry matter that are
too large to grind in their entirety to the fineness desired for
analysis are first ground through a large mill to pass a 4 to 6
mm sieve. The coarse ground sample is then reduced in a gated
riffle splitter. When necessary, the reduced sample is partially
dried (method 2.2.1) and the
sample is ground again to the fineness desired for analysis.
Precautions are taken during sample preparation to avoid
sample contamination and retain sample identity. All equipment is
kept clean and maintained on a regular schedule.
Prepared samples are stored in airtight containers away from
heat and light.
Equipment:
- Laboratory forage chopper (Hege 441, Wiley No 1, or
equivalent)
- Cyclone pulverizing mill (Udy or Cyclotec are required
for samples using NIR)
- Cutting mill, Wiley or equivalent Riffle sample splitter,
gated with 2 pans, 1-inch riffles
- Sample containers, Hedge clippers, hand Dishpans, plastic
Plastic sheet, 3 by 3 feet Flat-blade spatula, stainless,
12-inch Plastic bags, Ziplock or Whirlpak or equivalent
- Freezer and refrigerator
Reagents:
None.
Safety precautions:
- Wear hearing protectors when operating a grinder. The
noise level is on the threshold of being hazardous.
- Wear a dust mask and avoid inhaling dust when handling
samples.
- Observe all safety and operating instructions supplied by
the manufacturer of the grinding equipment.
- Do not insert fingers or objects into the grinding mill.
Procedure:
Preparing samples greater than approximately 85% dry matter
for grinding
- Remove sample from shipping container and discard any
roots from plants and brush off dirt particles. Note and
report removed material and any other sample
manipulation.
- Chop samples of whole plants into about half-inch pieces
using either hand clippers or the laboratory forage
chopper. Include any ears attached to corn plants. When
using the laboratory chopper, be sure to brush any sample
adhering to the sides of the chopper into the receiving
tray. Cored hay samples do not require chopping.
- Grind entire sample to pass 4 to 6 mm sieve in Wiley or
other large mill.
- Reduce the sample size in a gated riffle splitter to
amount desired for laboratory subsample. Transfer the
remainder to a plastic bag, seal and label.
- If the sample is too wet to grind (less than 90% dry
matter), dry the reduced sample using the forced-air oven
(method 2.2.1.1) or
microwave oven (method 2.2.1.2).
- Grind the reduced sample to fineness desired for analysis
in appropriate grinder (see section 1.1.1 and 1.1.2).
- Thoroughly mix the ground sample. Transfer to an airtight
container and label immediately.
Preparing samples less than approximately 85% dry matter
for grinding
- Remove sample from shipping container and discard any
roots from plants and brush off dirt particles. Note and
report removed material and any other sample
manipulation.
- Chop samples of whole plants into about half-inch pieces
using either hand clippers or a laboratory forage
chopper. Cut open stalk or corn cob pieces to facilitate
drying. Include any ears attached to corn plants. When
using the laboratory chopper, be sure to brush any sample
adhering to the sides of the chopper into the receiving
tray. Silages and haylages generally have average
particle lengths less than 1 inch and do not require
chopping.
- Place the chopped sample into a clean dishpan or on a
clean plastic sheet. Mix thoroughly.
- If the entire sample cannot be dried, reduce the sample
size by making a cone of sample and quartering. Save
opposite quarters. Repeat mixing, coning and quartering
until the volume is reduced to an appropriate size. Make
certain that representative ratios of stem and leaf occur
in each pile.
- Transfer reduced sample to a tared container for drying.
- Transfer the remainder to a plastic bag, seal, label and
refrigerate.
- Dry the reduced sample using either the forced-air oven
(method 2.2.1.1) or
microwave oven (method 2.2.1.2).
- Grind the partially dried sample to fineness desired for
analyses in appropriate grinder (see method 1.1.1
and 1.1.2).
- Thoroughly mix the ground sample. Transfer to an airtight
container and label immediately.
Comments:
- Samples to be scanned by NIR must be dried by the same
method used to develop the calibration equation.
- Keep all equipment clean to avoid contaminating one
sample with another.
- Care should be taken with the cyclone grinder not to heat
the sample during the grinding process. Heating may
affect subsequent chemical analysis.
- Equipment should be maintained on a regular schedule. The
condition of the sieves, rotors, blades or other grinding
surfaces should be monitored and regularly recorded in a
designated log book.
- Transfer samples quantitatively. Do not leave sample
portions in grinder or splitter.
- Store ground samples in airtight containers away from
heat and light.
Calculations:
See appropriate dry matter method for dry matter calculations.
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