P is for Phosphate
Phosphate has the chemical formula PO4. It is commonly known an important nutrient. In the world of stable isotopes, phosphate is important because it is an important component of bioapatite, and the oxygen contained within phosphate is difficult to alter through the process of fossilization.
The reason why oxygen is phosphate is not likely to be altered is because the chemical bond between it and phosphorus is very strong. The unfortunate result of this is that it is also difficult to break this bond in the laboratory. The procedures to isolate oxygen from phosphate require dangerous chemicals and extreme temperatures.
However, because this oxygen is seldom altered, this means we can rely on its isotopic value to represent the original oxygen that was in the bioapatite when the animal was alive. Because of this, sometimes it’s worth the struggle in order to get good results.