Representative Sampling - An Introduction To Particle Segregation And Spin Riffling

Is the entire size distribution represented in the small scoopful you just put into your analyzer?  Probably not, unless you split the original sample on a Rotary (spinning) Riffler.

Particle Segregation
It is often found that divided solids (powders, granules, etc) segregate by size when they are agitated or shaken. Small particles tend to collect towards the bottom of the container, larger ones on top (the so-called "Brazil Nut Effect". Heaps tend to accumulate larger particles towards the outside as they roll over smaller particles. Therefore, when taking sub-samples it is important to eliminate the effects of such segregation. If this precaution is not taken, the results of particle sizing, surface area measurements and, at times, porosity measurements can be severely biased. When inaccurate results are scaled up to industrial sized applications, entire processes can fail to achieve their desired end results.

Good Sampling Practice
The "Golden Rules" for sub-sampling put forth by Allen [1] simply state that the sample(s) should be taken when the powder is in motion (i.e. a powder stream), and the entire cross section of the entire stream shall be sampled many times. If this is not done when the container is loaded, it should surely be done when the container is emptied. A Rotary Riffler does just that.

A Superior Sampling Device
The superior performance of rotary riffling over all other methods of powder sampling is demonstrated by the data below [1]:

When compared to the random variation of 0.075, the rotary riffler is clearly superior, and should be used whenever possible [1], and is probably the best instrument available for the subdivision of powders having heterogeneous particle-size distributions[3], and produced the smallest overall inaccuracy levels of five protocols assessed for splitting soil samples [4].

[1] T.Allen (1981) Particle Size Measurement , 3rd ed, Chapman & Hall.
[2] A.A.Khan (1968) MSc Thesis, Bradford University
[3] H.G.Brittain (2002) Pharm. Tech., July ‘02, 67-73 [external link]
[4] R.W.Gerlach et al (2002) J. Chemometrics, 16 , 321-328

Instruments for Representative Sampling
Quantachrome offers two devices to ensure representative sampling and both use the superior spin (rotary) riffling technique; the Rotary Micro Riffler™ and Sieving Riffler™. For more details specific to each model, please click here.

Application Note
We will be happy to send you a copy of our application note no. 58000-05 "Representative Sampling for Surface Area Measurements". Please complete this short request form.

NIST Recommended Practice Guide
For details on how to obtain your free copy of NIST's Special Publication 960-1 "Particle Size Characterization" which includes a section on sub-sampling powders, please email Quantachrome at qc.powders@quantachrome.com.

A Note on Standard Methods
Because of copyright restrictions, Quantachrome cannot provide copies of standards published by other organizations such as ASTM International.  However they are available for purchase online, together with those of ISO and DIN, at techstreet.com.