**4. Acknowledgment**

This work was supported by the Swedish Research Council (621-2010-5322), EU Marie Curie Program (PIRG-GA-2007-205302) and Reumatikerförbundet (R-85481). The mass spectrometers were obtained by grants from the Swedish Research Council (342-2004-4434) and from Knut and Alice Wallenberg's foundation (KAW2007.0118).
