If you are using a 13C-glucosamine internal standard though, you may encounter problems with detection reproducibility. Matching the concentration reasonably well with the amount in the sample should give you no more than about 1% error, which should be tolerable unless you need extreme precision in the quantitation.
Alternatively, try using deuterated glucosamine as an internal standard instead. In the EICs, you can readily distinguish glucosamine from either the 13C isotope or the deuterated version. The principle is the same in either case.
The advantage of using the deuterated internal standard is that the natural abundance of deuterium is much lower than that of 13C.