Meaning
Variations in the transmission efficiency of ions of different masses through a mass spectrometer can distort the recorded signal intensities. This mass discrimination occurs when the analyzer or detector preferentially detects light ions over heavy ions, or vice versa. This instrumental bias can lead to inaccurate quantification of high-molecular-weight additives in polymer samples.
Instrumental Distortion
The physical cause of the bias often lies in the quadrupole field settings, the time-of-flight extraction voltage, or the detector’s response characteristics. During the analysis of polymer additives like light stabilizers, mass discrimination can make high-mass compounds appear to be at much lower concentrations than they actually are. This distortion complicates the profiling of multi-component additive packages where compounds span a wide mass range.
It requires the operator to apply mass-dependent correction factors to the raw data.
Additive Quantification
Correcting for this bias is necessary when comparing the ratio of a monomer to its oligomers. Without correction, the polymer’s molecular weight distribution will appear skewed toward the lower end.
Calibration Procedure
To measure and correct for the bias, laboratories run a reference standard mixture containing known concentrations of compounds across the entire target mass range. This calibration curve allows the software to calculate a correction factor for each mass region. Routine calibration ensures that the mass spectrometer provides stable and reproducible results over months of continuous operation.
