Quality Assurance Challenge 3

For the first time this year, we would like to invite you to participate in the Analytical Challenge, which is a series of puzzles to entertain and challenge our readers. The special ABC feature ‘Analytical Challenge’ has established itself as a truly unique quiz series, with a new scientific puzzle published every second month. Readers can access the complete collection of published problems with their solutions on the ABC homepage at http://www.springer.com/journal/00216. Test your knowledge and tease your wits in diverse areas of analytical and bioanalytical chemistry by viewing this collection.

In the present challenge quality assurance is the topic. And please note: there is a prize to be won (a Springer book from our catalogue up to a value of €75,-). Please read on...

Meet the quality assurance challenge

An analytical laboratory is asked to check the quality of polystyrene. This type of test is actually required in some industries; for example, the residual monomer concentration must not exceed a specified limit in polystyrene used for pharmaceutical purposes (such as in sealing caps of pill bottles).

In this Challenge, the specification defines a maximum monomer concentration of 0.480 m%. The styrene concentration in polystyrene is determined by headspace chromatography. A round robin analysis performed among eight laboratories, each conducting five replicate measurements, determined the standard deviation of the method to be s=0.010 m%. Four standard samples were analyzed using headspace GC, and four replicate determinations were conducted for each sample. The following results were obtained:

Standard	m% of monomer
1	0.469	0.419	0.445	0.436
2	0.333	0.342	0.348	0.398
3	0.436	0.400	0.415	0.421
4	0.308	0.321	0.301	0.315

Daily quality control of production batches of polystyrene is achieved via two replicate determinations. The Pearson criterion is applied to the maximum difference allowed between the results (${\left| {x_{{\max }} - x_{{\min }} } \right|} = D{\left( {P{\text{;}}\,n_{{\text{j}}} } \right)} \cdot s$). The Pearson factor for two replicate determinations is D(P=95%; n_j=2)=2.77 [1].

Answer the following questions

• a. Are there any outliers among the results obtained for the standard samples?

• b. Are the variances of the four series homogeneous?

• c. Is the required precision of the method reached? (Is there a difference between the standard deviation of the standard samples and that of the method?)

• d. What is the maximum allowable mean of the analytical results which must not be exceeded if the production batch is to pass the quality check?

• e. An analysis of two production batches of polystyrene gave the following results:

  Batch	Replicate 1 (m% styrene)	Replicate 2 (m% styrene)
  1	0.456	0.466
  2	0.459	0.466

  Is it appropriate to calculate the mean of the values?

  Should the batches be approved for sale?

We invite our readers to participate in the Analytical Challenge by solving the puzzle above. Please send the correct solution to abc-challenge@springer.com by April 15, 2006. Make sure you enter “Quality Assurance Challenge 3” in the subject line of your e-mail. The winner will receive notice via e-mail and his/her name will be published on the ‘Analytical and Bioanalytical Chemistry’ website at http://www.springer.com/journal/00216 and in the Journal. Readers will find the solution and a short explanation on the ‘Analytical and Bioanalytical Chemistry’ website after April 15, 2006, and in the Journal (Issue 385/5) .

The next Analytical Challenge will be published in Issue 385/1, May 2006. If you have enjoyed solving this Challenge you are invited to try the previous puzzles on the ‘Analytical and Bioanalytical Chemistry’ website (http://www.springer.com/journal/00216).