Fourier transform ion cyclotron resonance mass spectrometry has the ability to achieve unprecedented mass measurement accuracy (MMA); MMA is one of the most important features of mass spectrometric measurements as it affords extraordinary molecular specificity. However, due to space-charge effects, the attainable MMA depends drastically on the total number of ions trapped in the ICR cell for each and every individual measurement. Even through the use of automatic gain control (AGC), the total ion population is not able to be held constant between measurements. Multiple linear regression calibration in conjunction with AGC is utilized in these experiments to formally account for the differences in total ion population in the ICR cell between the external calibration spectra and experimental spectra. This ability allows for the extension of dynamic range of the instrument while allowing mean MMA values to remain less than 1 ppm. Additionally, multiple linear regression calibration is used to account for differences in total ion population in the ICR cell between measurements as well as relative ion abundance of a given species, which also affords mean MMA values at the parts-per-billion level. Initial studies were carried out on ammonium-adducted PPG-1000 oligomers with a m/z range of 732-1312. Additional experiments were carried out utilizing both ammonium-adducted PPG-1000 oligomers and a peptide mixture.