Folded and unfolded Proteins in their native state, tend to produce multiply charged ions covering a smaller range of charge states. These charge states tend to have fewer charges than an unfolded protein would have, due to the inaccessibility of many of the protonation sites. Increasing the sampling cone voltage may provide sufficient energy for the protein to begin to unfold and create a wider charge state distribution centering on more highly charged ions in the lower m/z region of the spectrum.

Folded and unfolded states Holo and apo proteins

Positive or Negative Ionisation? If the sample has functional groups that readily accept a proton (H + ) then positive ion detection is used e.g. amines R-NH 2 + H +  R-NH 3 + as in proteins, peptides If the sample has functional groups that readily lose a proton then negative ion detection is used e.g. carboxylic acids R-CO 2 H  R-CO 2 – and alcohols R-OH  R-O – as in saccharides, oligonucleotides

Follow link: Training and education WEB Articles Vestling, M.M. USING MASS SPECTROMETRY FOR PROTEINS J. Chem. Ed. 2003, 80, 122

Atmospheric Pressure Chemical Ionization

zV 1/2 mv 2 =cost