The Outcome of a PT Scheme Specifically Designed to Evaluate Soil Sub-Sampling Effectiveness

Identifying and Combatting Inappropriate Laboratory Practices
Oral Presentation

Prepared by K. Middlebrook
Canadian Association for Laboratory Accreditation, Suite 102, 2934 Baseline Rd., Ottawa, ON, K1C 5V5, Canada

Contact Information:; 613-233-5300


The Canadian Association for Laboratory Accreditation (CALA) has operated a Proficiency Testing (PT) Program for environmental testing laboratories since 1991. Several years ago CALA expanded on this program to include the insertion of specifically designed "Challenge" samples into the routine PT rounds. This involves replacing, unknown to the participant, one of the four samples normally provided for each PT type with one that will provide an additional challenge to participants. These challenges have included the addition of a known interference, an inappropriate preservation, and the intentional mis-labelling of one of the samples. This presentation will describe one of the challenges that was recently published in the journal, Accreditation and Quality Assurance. This challenge involved CALA's routinely scheduled metals in soil PT. One of the four samples was prepared in a fashion where the between-bottle homogeneity was the same as the other three samples but with very poor within-bottle homogeneity. As such it was a test of the participants' ability to obtain a subsample for subsequent digestion and analysis that was quantitatively representative of the entire sample. This presentation will describe this challenge, the results observed and the possible causes and consequences.