General

In 2001, the SENAI-CETIND started to build up a PT scheme (PEP-CETIND) for external quality assurance in water analysis with metals parameters. In 2005 we are extending the PEP-CETIND to other organic parameters. The PT scheme is open to all laboratories that are interested in testing their analytical performance. The conduction of our external quality assurance system is based on the requiriments of ISO Guide 43 and thus also correspond to the international criteria for proficiency testing in chemical laboratories.

Samples

The water samples are prepared at the SENAI-CETIND by gravimetric formulation. They consist of synthetic samples, which is assembled from pure water, pure reagents and pure standard chemicals from NIST (National Institute of Standard & Technology).
Reference concentrations are calculated by combination from the gravimetric data of sample preparation and results obtained from valited methods usually used to measura the analýtes. In Each series correctness of the reference values, homogeneity and stability of the PT samples are checked.
For homogeneity purpose samples are randomly selected and analysed for the target compounds before sample dispatch. For stability purpose samples are analysed since the sample preparation date until the closing date to return the results

Since PEP-CETIND PT scheme use the so-called Confidence Ellipse plot, this implies that per parameter at least two samples are distributed for analysis. These two samples are practically identical for the parameter to be analysed. There is only a slight difference in concentration between the two samples, by adding known amounts of the parameter.

Confidentiality

The participation in the PEP-CETIND PT scheme is treated confidentially. A randomly number is given to each laboratory as an identifier code and only the laboratory knows its own number. Your results are treated strictly confidential. In the final series report each participant is assigned by its identifier code.

Sample dispatch

The samples are mailed to the participants by air fright. Within Brazil samples packages are usually delivered within two to three working days. The delivery time for packages into Non-Brazil countries does not consider times for customs clearance and is about two to five working days. On the day of the dispatch we will send you a message by e-mail informing that the samples were sent. The exact dispatch dates are listed in the series schedule (section 1).

Analysis of samples

The selection of the analytical method is up to the participant. The laboratory should make three measurements in each sample and inform the analytical method used in the result form. Although we check the stability of the samples by measurements within each series, we recommend analysing the samples as soon as possible after the receipt. This recommendation particularly applies to the parameters total phosphor and mercury.

Reporting resultos to the PEP-CETIND PT provider

The results and uncertainties must be transmitted to the SENAI-CETIND within four weeks after sample dispatch. We will inform you about the exact closing date in the instruction letter attached to the samples. In the week of the closing date, we send a reminder email to those laboratories whose results have not arrived. The results should be report directly in the PEP-CETIND site(http://www.cetind.fieb.org.br/PEP). For this purpose each participant will receive a member login and a respective password to access the PEP result form. The web site will be open to enter results as soon as samples are dispatched. The results can be corrected, if necessary, until the closing date. After that the web site is automatically closed to new result enters.

Data evaluation and series reports

The participants will be evaluated by the three methodologies: z-score, En number and confidence ellipse plot.
The z-score allows to the participant evaluate its accuracy regarding to the reference value and regarding to the group average. The En number enables the participants to evaluate their measurement uncertainty adequacy. The confidence ellipse plot enables to obtain a good indication if deviating results of participants are caused by systematic errors and/or by relatively large random errors. In this case, correction actions could be applied with more efficiency.
The final report of each PEP-CETIND series will be sent to the participants within 4 weeks of the closing date. To guarantee the confidentiality, the results reported are related only with their laboratory randomly code. A list of participants is given in the final report. If any company do not want to be named in the list, let us know by phone, by email or by letter.
The results of the whole group of participants are displayed both graphically and in the table format. An individual assessment of the performance of each participant is given in the table format and specific commentaries are given to each laboratory. The following section shows examples for results presentation by using table and plot format.

Graph 01
Flags Legend

Graph 02
Results and uncertainties reported by participants for Iron - Sample B

Graph 03
Z-Score for Iron - Sample B

Graph 04
Example of confidence Ellipse plot for zinc