A sample submitted for electrospray has to be free from all salts, detergents and acids except acetic and formic acids. Small amounts of triflouroacetic acid (0.1% or less) and in some cases ammonium acetate are also permissible.

Acceptable solvents are water, methanol, acetonitrile or any combination of these with above mentioned permitted acids. If another solvent is required please speak with lab personnel.

The minimum sample quantity is 20 uL of a solution containing 20 pmole per uL. At least 20 times this concentration or more is preferable.

The sample is analyzed by injecting 20 uL of the sample into a loop where a carrier solvent (typically 50:50 water:acetonitrile with 1% acetic acid) is used to transfer it into the mass spectrometer.

The ionization process routinely places multiple charges on a compound so that the mass to charge ratio generally falls below 1500 u. Small molecules below 1000 u generally have a single charge. The charging is caused by addition of a proton for each added charge. In short, if you have a peak at 500 u and has one charge, then the molecular weight is 499 u.

Below is a mass spectrum of insulin obtained from injecting 20 uL of 188 pmoles per uL solution.

Both peaks in the mass spectrum are from insulin (Molecular weight 5733.6 u) where the peak at 1147.6 has 5 charges and the peak at 1434.2 has 4 charges.

This mass spectrum can then be analyzed by a computer and a reconstructed mass spectrum can be generated. Such a spectrum for the above mass spectrum is shown below.

Notice that the resulting reconstructed mass spectrum shows a peak that contains no charge so is the measured molecular weight of 5733 u.