Soil and Water Conservation Laboratory

Standard Operation Procedure

Homogenization of dry solid-phase samples

Purpose

To use an impact ball mill to grind soil samples to a <250 µm fineness for elemental concentration and/or stable isotope analysis, or for Mid Infrared (MIR) scanning.

Materials and Equipment

Aluminum foil Kimwipe 250 mm sieve Paper Towels Spontex Sponge/Scrubbers Retsch MM200 Mill (Fig. 1)

o Millo 2 stainless steel grinding jars with lids (Fig. 2)o 1 or 2 grinding balls per jaro Wrench

Mortar and Pestle Spatula Ethanol/Acetone Scintillation vials with caps Beach sand Appropriate lab attire

Preliminary Preparation

- For soil total carbon analysis, samples must be oven dried at a 105° C, generally 24 hours.

- To decrease risk of carbon contamination to sample, use aluminum foil.

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Figure 1. Retsch MM200 Mill.

Figure 2. Stainless grinding jars and balls.

Procedures

Sample Preparation

1. Wipe down the bench work surface with clean water and a paper towel, and cover your work area with aluminum foil.

2. Fill up to no more than one-third of the grinding jar with the sample.3. Place one ball inside grinding jar and close with the lid. The lids have a friction seal on

the jar (just sit on the jar). Using one ball allows the ball to move more freely in the jar, grinding evenly.

Milling/Homogenization

4. Turn on the Retsch MM200 Mill. The power switch is located on the back of the mill.5. Set the mill’s frequency and time using labeled buttons on the front of the mill.6. Set the frequency to 25 cycles. This is the maximum amount of frequency that can be

reached with the Retsch Mill.7. Set the time for the mill to grind to the desired time. 45 seconds is a useful time to

consider at first - if samples are coarser, increase the time to about 50 seconds (or until sample is milled into a homogenous powder); other samples will require less time.

8. Lock the sample cups in the arms of the mill. To lock the samples in place, tighten by hand the black wheel first, and then use the wrench to gently tighten the silver locking wheel. Be careful not to jostle the arms too much.

9. When grinding, both jars with balls must be in the mill to keep the mill balanced!!! Press the start button to begin grinding.

10. To unlock the sample after grinding, unscrew the silver locking wheel with the wrench the opposite way in which you tightened it, and then by hand loosen the black wheel.

Final Processing

11. Pass the powdered sample through the 250 µm sieve into an appropriate labeled scintillation vial. This can be done using a folded piece of clean aluminum foil by first sieving the sample onto the piece of aluminum foil and then pouring carefully into container. (Use mortar and pestle to grind finer if needed. A spatula can also be used to help pour the sieved sample into its new vial). Ensure that the entire sample passes through the 250µm sieve.

Cleaning

12. Clean the mortar and pestle between uses using kimwipes and ethanol.13. Rinse and scrub grinding jars with water for every new sample.14. Dry grinding jars, once with paper towel for most of the water and then again with a

kimwipe to make sure that the cups are dry.15. Sometimes clayey soils tend to stick the bottom of the sample cups and are hard to scrub.

A suggestion is to, at the end of the day, briefly ball mill beach sand in the grinding jars.

Prepared by Leena Muller on 9/12/2010. 2 of 2