Thief probe

The most commonly used devices for sample retrieval in blending homogeneity testing are thief probes. Several different sampling thieves have been developed and used for many years in the pharmaceutical industry.72 A major problem with most thieves is that the retrieved sample is not representative of the true concentration at the location from which the sample is supposed to be obtained. Contamination with powders from other locations in the mixture during probe insertion causes these sampling errors. Also, nonuniform flow of different components into the sampling cavity can skew the sample concentration. 

A vertical core sample is a correct delimitation of a solid flat pile if it represents the whole thickness. Using a thief probe to obtain it, however, produces an extraction error because the thief cannot extract material at the very bottom (Figure A.2). 

FIGURE 2 A thief probe can take a vertical core sample but cannot get material from the very bottom of the container. 

Two-dimensional sampling is very common, and even though correct, core samples are difficult to obtain. A thief probe, for example, does not extract a core but rather material only at the probe windows. Of course, taking material from various levels is better than taking a grab sample from the top, as is often the case with a three-dimensional lot. But we... 

Harwood C, Ripley T. Errors associated with the thief probe for bulk powder sampling. J Powder Bulk Solids Tech 1977 l(2) 20-29. 

Thief/spear probe One or more cavities are stamped Thief samplers belong to... 

Figure 5b) an outer rotating sleeve. The thief is inserted into sample with the cavities closed, once opened the sample fills the hole. The cavities are closed and the thief is withdrawn. It must be ensured that samples are withdrawn from different locations sampler (has one or more cavities along the probe) and end sampler (has a single cavity at the end of the probe), which are the most common used for stored non-flowing material [10]. 

FIGURE 5 Stationary bulk sampling (a) low-volume powder sampler (b) thief/spear probe sampler (c) pneumatic lance sampler [7]. 

Figure 15-4 Systematic sampling errors introduced by a side-sampling thief, (a) Initially layered configuration of large (light) and small (dark) particles are noticeably disturbed as the thief entrains particles during insration. (b) This type of thief relies on free flow of particles to fill a cavity when a slot is opened in the side of the sampling tube. Consequently, fine and freely flowing particles are overrepresented by this probe, and fine particles are transported to regions whCTe they were not placed originally.

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