Alpha Stat Consulting

March, 2003

Do you have the right scoop?

Scoops and spatulas are used for mixing and for sampling loose, solid material such as soils and powders. They vary by size and shape. Which one below has the right shape for taking a correct, cross-stream sample: A, B, or C?

Whether in the lab or in the field, being confronted with a conical "pile" of material is common. The question is how to get as representative a sample as possible. A frequent solution is to take a convenient sample by using a scoop or spatula. In the lab, a sample from the top is often taken from a vial or from a small pile. With a large pile in the field, samples are frequently taken from the bottom; rarely does anyone climb to the top to get a sample! And large fragments tend not to be included if the sampling tool is too small to contain them. This type of convenience ("grab") sampling is not correct because not every part of the lot has an equal chance of being in the sample. In other words, these types of samples are not random. These samples will not be representative, especially if there is variation from top to bottom due to gravitational effects on the shape, size, or density of the material.

In the sequence above, the spatula, A, will over-represent material near the bottom of where it is sampling and under-represent material at the top, since material on top will fall off. The round-bottom scoop, B, will over-represent material at the top of where it is sampling and under-represent material near the bottom, since most of that material will be missed. If a large pile in the field can be rearranged to make a narrow "line," then a correct, cross-stream sample can be obtained with a "squared off" front loader. Of course for the lab, the "squared off" scoop, C, is the answer.

How it works: When a thin line of solid material is laid out in the lab, a "square off" scoop, like C above, is used to cut across and perpendicular to the "line" of material.

Advantages

If used properly, the squared off tool is a big improvement over a scoop or spatula of any shape used to take a convenient "grab" sample from a pile, especially if there is variation from top to bottom due to different particle shapes, sizes, or densities.
The flat bottom and flat sides ensure that, as the scoop is moved through the thin line of material, particles on the top and bottom are selected with the same probability.

Limitations

The sides of the scoop must be high enough to ensure that the full height of the material is collected.
The sides of the scoop must be parallel to each other and perpendicular to the bottom.
It is difficult to find scoops with this shape on the market.

Moral: The squared off scoop protects us against variation in the material from top to bottom.

More information about sampling equipment and procedures for solids, liquids, and gases is available in the Short Course April 24-25, 2003, at Texas Tech University and in the short introductory book A Primer for Sampling Solids, Liquids, and Gases, by Dr. Patricia L. Smith.

Patricia L. Smith, Ph.D., Owner
Phone: 806.798.3420
Fax: 281.749.8158
E-mail: alpha@alphastat.com
Web: www.alphastat.com

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E-mail: alpha@alphastat.com