1. A method for calibrating two or more mass spectra comprising: recording the mass spectra of at least two sample ions; estimating the frequencies of the ion species from the mass spectra to form two frequency lists; matching the frequencies of the ions between the two lists, wherein a frequency shift is modeled by the equation, f ≈ A + A + 1 2 ⁢ A ⁢ ( 4 ⁢ B ⁢ m z ) 2 ⁢ ( m / z ) = A m / z + B A , wherein m represents mass, z represents charge, f represents frequency, A is proportional to the magnetic field strength, and B is proportional to the space-charge effect, and determining the optimal frequency that best superimposes one list of frequencies on to the other.