Say for example, you have a sample that you are comparing to both CCME and Ontario Reg. 153 and have results from a sample that expired under the recommendations of one of these guidelines, but still within acceptable timing of the other.
Hold times are based on recommendations in reference methods, which in many cases tend to be associated with the US EPA, Canadian federal or provincial documents.
Ideally, hold times are based on studies including a significant number of samples for which stability is monitored over time. Depending on when certain regulations are updated, the guidelines associated with hold times may change, as new studies more accurately represent the stability of target contaminants. As such, the suggestion is to take into consideration the date of the reference document publishing and follow the hold time that utilizes the most recent reference or study.
Alternatively, when in doubt, it may be prudent to adopt the shorted hold time of applicable regulations. Following a different set of guidelines than those outlined in the regulation under who's jurisdiction the project falls, may mean that the regulator can reject the data as being unreliable.