The column eluent was monitored using a Sciex 5500 QTrap mass spectrometer using electrospray ionization operating in positive mode. The mass spectrometer was programmed to monitor the following multiple reaction monitoring (MRM) m/z transitions: 195.15 → 138.10 and 198.00 → 140.00 for temozolomide and IS, respectively. Calibration curves for temozolomide were computed using the area ratio peak of the analysis to the internal standard by using a quadratic regression with 1/x2 weighting for plasma and 1/x weighting for artificial CSF, both with a calibration range of 0.005–1.0 µg/ml, and dilutions up to 1:10 (v/v) were accurately quantitated.