A positive ion ESI-MS of PAI-2 ΔCD-loop from pQE9 in 10 mM ammonium acetate (pH 6.8) containing 0.1% formic acid. The m/z spectrum shows a Gaussian-type distribution of multiply charged ions ranging from m/z 2700 – 4500. Charge states are indicated. The monomeric (A) and dimeric (B) forms of the protein gave a measured molecular mass of 44496 Da (± 0.89) and 88961 Da (± 0.84), respectively. Mass was calculated using MassLynx MS software (Waters) and caesium iodide was used for external calibration.