1. General Procedure
The mass spectra are acquired using the QTrap linear ion trap mass spectrometer (AB/MDS Sciex, Toronto, Canada). The protein sample is injected into the sample loop and delivered to the mass spectrometer using 65% acetonitrile/0.1% formic acid in water at 20 µl/min. The curtain gas is set at 25 (arbitrary units) while ion source gas 1 is set at 20. The ion spray voltage is set at 5500 V and declustering potential is set at 20 V. The scan rate is 4000 amu/s with step size of 0.12 amu.
2. Mass Accuracy and Resolution
The mass accuracy is typically 0.5 Da for m/z value. The resolution is around 2000. Resolution=M/delta M
3. How to Read the Spectrum?
The ESI mass spectrum normally consists of consecutive peaks of multiply charged molecular ions obtained through protonation (M+zH)z+. The average molecular weight of the protein can be calculated based on the measured mass-to-charge ratio (m/z) and the number of charges (z).
For example:
MW1=(m/z) x 10 - 10 (10 positive charges)
MW2=(m/z) x 11 - 11 (11 positive charges)
MW3=(m/z) x 12 - 12 (12 positive char16 November, 2007 x z - z The average MW=(MW1+MW2+MW3+...........MWn) / n
