When the method describes a holding time as "expires in 7 days", how is this holding time interpreted (i.e., does the holding time expire on the 7th day at 23:59 or on the 7th day at collection time)?
Holding times should be from the documented time a sample was collected, not based on midnight of the last day. For example, if a semivolatile water sample (with a holding time of 7 days) was taken at 8:30 am on Tuesday, October 15, 2013, then the laboratory can extract that sample at any time up until 8:30 am on Tuesday, October 22, 2013 and be considered within the method's holding time.
Holding times are distinctions in the methods to try to reduce the effect of degradation on the results due to analyte decomposition, reaction, evaporation and any number of other effects of time on a sample matrix. However, a line has to be drawn somewhere for unacceptable losses and the method's holding time represents the best data available as to when the analysis is still completely valid. If an exact sample collection time is known, then the exact time should be used in calculating the holding times. If the collection time is recorded to the hour, the holding time should be interpreted to the hour. If the collection time is recorded to the hours and minutes, the holding time should be interpreted to the hours and minutes.
Ideally, laboratories should try to do the required preparation steps (i.e., extraction, digestion, etc.) in advance of the hold time's expiration so as to avoid hold time problems.