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All laboratories use modifications of the Kjeldahl method. These modifications differ with respect to choice of catalyst procedures for digestion, distillation and determination of the ammonia.

It is not only important to know which methods can be used to measure nitrogen or protein values, but it is also important to have insight in the reliability and precision of these data. To obtain that insight you will study the results of an interlaboratory trial in which protein content s of several products is determined by various experienced food labs. Study the results and answer the following questions.

Not completely right. There are more right statements. The CV within are not higher than 5%. This can be considered as a small within laboratory variance.

Not completely right. There are more right statements. Note that in general the CV within is low. But there are some laboratories that provided very different results for their duplicate analysis.

Not completely right. You selected some statements that are not right. Realize that this question is about the differences WITHIN laboratories. You could assume that each laboratory uses the same analysis method for their duplicate analysis. So, the use of methods or conversion factors does not differ within a laboratory.

Right!

a

Most laboratories claim that their CV within is lower than 1%. Now interpret the results of the interlaboratory trial. Which statements are true? The differences within the laboratories….

b
c
d
e

are rather small, but for some foods still higher than expected.

are high for some laboratories, e.g. lab no 10 and lab no 15.

can be partly due to the use of different nitrogen - protein conversion factors.

can be partly due to methods modifications (e.g. the choice of catalyst in the Kjeldahl method).

can be partly due to poorly validated methods in one or more laboratories.