6-Monoacetylmorphine (6-MAM) is a labile metabolite of diamorphine due to hydrolysis of the ester bonds. It may undergo deacetylation to morphine during storage. Buprenorphine, codeine, fentanyl, hydromorphone, methadone, morphine, oxycodone, oxymorphone and tramadol were all stable in frozen plasma stored for almost 3 months and subjected to two freeze-thaw cycles (Musshoff et al. 2006). Free concentrations of morphine, codeine and methadone were moderately stable in frozen urine preserved with sodium fluoride over a year (Moody et al. 1999). However, total morphine concentrations under similar conditions may be less stable (Moriya and Hashimoto 1997). Long-term storage in preserved whole blood stored at room temperature showed significant increases and decreases (Giorgi and Meeker 1995) over 1 to 5 years. These results suggest important differences in stability between free and conjugated species.

The stability of glucuronidated morphine is of importance because ratios of free and total morphine are sometimes used for interpretive purposes. Marked differences in glucuronide stability exist between antemortem and postmortem blood. Morphine-3-glucuronide was stable in refrigerated antemortem blood preserved with sodium fluoride, but unstable in postmortem blood under the same conditions (Carroll et al. 2000). Hydrolysis of the glucuronidated species to free morphine increases with temperature, storage time and degree of putrefaction. Other studies have confirmed the stability of morphine-3-glucuronide in refrigerated antemortem blood and plasma for up to 6 months (Skopp etal. 2001). Storage of postmortem specimens at —20°C prevented in vitro hydrolysis of the glucuronide.

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