Orientation and Training
Food and Drug Administration
The objective of sample preparation in trace analysis is to obtain a sample of convenient size (usually 5-100 g) whose composition accurately reflects the composition of the bulk sample from which it was taken (usually 2-25 kg). The nature of mycotoxin contamination makes this objective more difficult to attain than for most other types of trace analysis. Because the molds that produce mycotoxins do not grow uniformly on the substrate, contamination of the commodity is usually far from uniform (See Section 7.5 References 19, Chapter 49, Method 977.16). However, when food is prepared from that commodity, the mycotoxin present in perhaps only a small portion of the commodity may be spread throughout the entire lot of food.
Sample preparation can be divided into three steps using three types of equipment. Particle size reduction can be accomplished using Vertical Cutter/Mixers, such as Hobart VCM, Urschel Comitrol, and Robot Coupe VCM. Sample size reduction may be accomplished using a Jones divider (riffle). Mixing may be done with a planetary mixer or V-shell blender. All three steps are carried out for all samples prepared for mycotoxin analysis, but not necessarily in the order given, or with the particular equipment mentioned. In some cases, the grinding equipment combines the particle size reduction with the mixing step. In other cases, the steps have to be performed separately because the grinding operation does not thoroughly mix the sample composite. Chose the proper equipment for each sample; a properly ground and mixed sample is called a “composite.” Following compositing of regulatory samples, two one-quart portions of the composite should be placed in two containers – one for an “original” analysis, and one for a “check” analysis. A one-quart container may be saved for surveillance samples in case a follow-up compliance or regulatory sample is needed for a positive analytical finding. Check analyses are not performed on surveillance samples that are not large enough to support regulatory action as per the Investigations Operations Manual (IOM) Sample Schedule Chart Six or the compliance program. The remainder of the sample composite is disposed. In the case of a violative sample, the remaining original and check portions should be sealed and returned to the Sample Custodian for a reserve.
The samples themselves may be grouped in two categories: those that are moist or relatively high in oil content and yield a paste when ground (e.g. peanuts, tree nuts, dates), and those that are dry and yield a powder or dry particles when ground (e.g. corn, small grains). For guidance on the preparation of nut samples, consult see Section 7.5 References 20-22. Dry samples present a different set of problems that come from two sources. The kernel or grain of a dry commodity is often not the same hardness and density throughout, and the mold may grow or produce mycotoxin preferentially on one part of the kernel or grain. When the sample is ground, the softer portion of the kernel or grain is often ground to a smaller particle size than the harder portion. These smaller particles migrate to the bottom of the sample container when the sample is handled. Consequently, the sample withdrawn for analysis may not be homogeneous, and the analytical result may be inaccurate. To minimize these problems, one needs to mix the sample thoroughly and withdraw the portion for analytical determination as soon as practical after mixing. Note: Aspergillus sp., penicillium sp. will not grow under refrigeration or freezing, but fusarium sp will grow under refrigeration or freezing. The trainer will discuss the selection and proper use of the sample preparation equipment in the laboratory.