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The sampling procedure is easy and obvious, when it comes to targeted sampling of products with visible failure. But, as already mentioned in Chapter 3.2, Part II, sampling of deliveries of large lots of liquids, granules or failure parts is more complicated.

One risk, which has to be avoided, could be called convenience error. That means samples are preferred which are easily available and others which demand more effort are neglected. Example: A coating failure issue is persisting a whole weak erratically. The process has to be reviewed by taking samples. Although, it is more convenient to pick up just the samples that fall actually out of the machines at the end of the week, it is recommended to select also samples from the beginning of the week although they might be already stowed away in another building.

A failed sampling procedure sometimes produces baffling and surprising results. The author asked one of his laboratory personnel to compare three paint samples by ATR-FT-IR spectroscopy that were provided by three different manufacturers. The result was that the three samples were identical. A close look at the spectra instantly revealed what went wrong. All three spectra contained broad and intense absorption peaks of water. The operator had not realized that the three paint samples were water-borne. He had measured the specimen simply by dropping a representative aliquot onto the ATR crystal, as is the standard procedure for liquid samples. However, water (which is the main solvent) exhibits strong and broad absorption peaks which overlay all spectroscopic features of the paint itself. The correct procedure would have been to prepare a dried film of the paint. This kind of sample treatment, however, is not useful for TOF-SIMS analysis. Surface-active additives which migrate to the uppermost layer of the film whilst the film is drying, form a homogeneous layer on top that masks the main ingredients. This example shows that sample preparation is one of the key operations for correct results. It should be performed by experienced analysts who have the analytical background to determine whether a standard routine is suitable for the sample to be investigated or whether it has to be modified.