10.5. Nordtest approach in practice: Determination of acrylamide in snacks by LC-MS

Nordtest approach in practice: Determination of acrylamide in snacks by LC-MS [1] 
http://www.uttv.ee/naita?id=18163

https://www.youtube.com/watch?v=P94xWjC6Og0

Some comments on this example:

1. We use relative uncertainties in this example. The reason is that the concentration in these samples is quite high and in LC-MS analysis variability of the results is often roughly proportional to the values of the results.

2. In this example we assume that the matrixes of the used CRMs – potato chips and crisp bread – are sufficiently similar. This means that both reproducibility and bias obtained with these matrixes are similar. In such case the calculated uncertainty is applicable to both of these matrixes. If afterwards we will analyse some matrix that is different from potato chips and crisp bread then we cannot apply to that result the uncertainty estimate that we obtained here. Assessing whether a new matrix is sufficiently similar to the one used for bias evaluation is usually based on experience.

3. The uncertainty estimate u_crel that we have obtained is a relative uncertainty, so it is assigned to a result C_A by the following formula:

(10.8)

In principle it can be assigned to a result of any magnitude, however, it would not be correct to apply this uncertainty to analysis results that are very different from the one presented here. The difference should not be higher than 3-4 times.

When we use the Nordtest approach for uncertainty estimation then in general we can assume that the number of degrees of freedom is sufficiently large so that the k = 2 uncertainty can be assumed to have roughly 95% coverage probability.

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[1] Please note that in the tables of the slide “Measurements with the CRMs” the measurement units must be µg/kg, not mg/l.

The slides of this example and the calculation files – both the initial and the solved file – are available from here: