ESI sample for experimental section
 
 
 

    Electrospray ionization was performed on a Finnigan TSQ700 triple quadrupole mass spectrometer Finnigan MAT, San Jose, CA) fitted with a Finnigan ESI interface.
Samples were introduced into the electrospray interface through an untreated fused-silica capillary with a 50 um i.d. and 190 um o.d. A myoglobin and MRFA mixture was used for tuning and routine calibration of the instrument. The mixture was made in the following manner: stock solutions of myoglobin and MRFA were prepared by dissolving 0.9 mg of horse skeletal muscle myoglobin in 1 ml of a solution of 50:50 methanol: water containing 1% acetic acid, and o.6 mg of MRFA was dissolved in 1 ml of a solution of 50:50 methanol: water containing 1% acetic acid. The calibration and tuning standard was made from the two stock solutions: 50 ul of myoglobin and 12 ul of MRFA  were diluted with 538 ul of a 50:50 methanol:water solution in a polypropylene vial. The tuning mixture was infused into the mass spectrometer at a rate of 3 ul/min on a Harvard Apparatus (model 22, South Natick, MA) syringe pump.