8. Crude Fat (Ether Extract) in Forages >>

7. Total Ash in Forages

Reference:
Ash of Animal Feed. (942.05) Official methods of Analysis. 1990. Association of Official Analytical Chemists, 15th Edition.

Scope:
This procedure is applicable for the determination of ash in all types of dried, ground forages and feeds. It is not applicable for ash determination in liquid feeds or feeds high in sugar content.

Basic Principle:
A dried, ground sample is ignited in a furnace at 600oC to oxidize all organic matter. Ash is determined by weighing the resulting inorganic residue.

Equipment:
Crucibles, porcelain, low wide form, 30 mL, with covers numbered with furnace-proof ink
Muffle furnace with pyrometric controller
Analytical balance, sensitive to 0.1 mg
Desiccator, with vented lid
Drying oven

Reagents:
None.

Safety Precautions:

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

Procedure:

1. Remove crucibles with cover which have been dried for at least 2 hr at 100oC from oven, to desiccator. Cool, and record weight of crucibles with cover to the nearest 0.1 mg (W1).
2. Weigh 1.5 to 2.0 g of sample into the crucible, recording weight of crucible with cover and sample to the nearest 0.1 mg (W2).
3. Ash in furnace at 600oC for 2 hr after the furnace reaches temperature.
4. Allow crucibles to cool in furnace to less than 200oC and place crucibles with cover in desiccator with vented top. Cool and weigh crucible with cover and ash to the nearest 0.1 mg (W3).

Comments:

• Time and temperature described must be adhered to closely.
• Samples should be placed in ashing furnace so that air can circulate freely. Crucibles should not touch each other.
• 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 a 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.
• Crucibles should not be packed excessively tight in a desiccator. Air movement is necessary to cool crucibles. Crucibles 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 crucibles.
• If determining ash after fiber analysis, set furnace at 500OC and ash until carbon-free and grey ash color (3 to 5 hr). Lower ashing temperatures require longer ashing times.
• Higher temperatures will melt glass and ruin filter crucibles. A practical maximum service termperature for pyrex glass is 510OC and the annealing temperature is 560OC.

Calculation: Percent Ash, DM basis

% ASH (DM basis) = (W3 - W1)X 100 / (W2 - W1 ) X Lab DM/100

• W1 = tare weight of crucible in grams
• W2 = weight of crucible and sample in grams
• W3 = weight of crucible and ash in grams

Quality Control:
Include one or more quality control (QC) samples in each run, choosing QC samples by matching analyte levels and matrices of QC samples to the samples in the run. 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 ash 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 control sample(s) on an X-control chart and examine the chart for trends. Results outside of upper or lower warning limits, ±2s (95 percent confidence limits), are evidence of possible problems with the analytical system. Results outside of upper or lower control limits, ±3s (99 percent confidence limits), indicate loss of control and results of the run should be discarded. Two consecutive analyses falling on one side of the mean between the warning limits and the control limits also indicate loss of control.

8. Crude Fat (Ether Extract) in Forages >>

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