Ashing of bone: errors due to loss of CO₂ and their correction

Ashing is widely used to determine weight fraction of water-free bone that is mineral, but no standard procedure exists and the range of techniques used spans a range of temperatures and times over which the amount of weight loss is variable. We show that variability is largely due to progressive loss of CO₂ from CO₃ ^2- ions in the apatite crystal lattice, beginning at 600 ℃, typically used for ashing. We test the effect of varying temperature, time, and weight of sample and develop a reliable method, using small samples.

Replicate samples of bovine cortical bone were tested at 500 ℃, 600 ℃, and 700 ℃ for times ranging up to 24 h. We also tested samples of multiple humans at what we concluded to be the optimal conditions.

Varying conditions of ashing resulted in variations in apparent ash weight % by up to 7%. Samples between 5 and 20 mg heated to 600 ℃ for 1 h gave results agreeing with generally accepted values, but with much smaller variability. Ash wt% values for multiple human bone samples differed by up to 4.8%, but replicate data for individuals agree to ± 1 wt%.

In conclusion, a satisfactory method is given for ash weight determination using small samples, and yielding highly reproducible data. If accepted widely, ash weight values between laboratories could be used to study variations due to diet, age, drug treatment, and disease.