Spectral Skewing

[farzanabaksh]

Hello
Can someone explain what spectral skewing is?

Farzana
Thanks

[ods-at-pacific]

Spectral skewing is an artifact of scan speed and chromatographic peak width. As an analyte elutes from the chromatographic column, the concentration of the analyte changes a function of time. At first the concentration increase, then passes through a maximum, and then decreases. The measured ion current for a given m/z value is a function of the amount of analyte that is present at the time the ion current for that m/z value is measured. If the mass spectrometer scans from low m/z to high m/z, then, on the front side of the chromatographic peak the amount of analyte in the ion source when the ion current at a low m/z value is measured will be less than the amount that is present when the ion current for a high m/z value is measure. If under constant partial pressure condition, the ion current for the ion with a low m/z value and that for the m/z value is about the same, the two peaks would not have the same intensity if these ion currents were measured as function of a chromatographic elution.

The skew would be in opposite direction on the back side of the chromatographic peak because the amount of analyte would be high when the ion current for the low m/z value is measured and low when the ion current was being measured for the high m/z value.

Spectral skewing only occurs in scanning type mass spectrometers where a continuos beam of ions is being monitored one m/z value at a time. The instruments that exhibit spectral skewing are transmission quadrupole and double-focusing mass spectrometers. Spectral skewing is not an issue with quadrupole ion trap or TOF mass spectrometers.

Faster data acquisition rates can minimize spectral skewing; however, faster scanning can lead to poor quality of mass spectral data due to the reduced amount of time measuring each individual value. Reducing the rate in a temperature program can help reduce spectral skewing by broadening the chromatographic peak so that the rate of change in the amount of analyte is reduced; however, this can lead to longer analysis times. Changes can also be made the mobile phase flow rates, but this can lead to longer analysis time and poorer chromatographic resolution.

If the chromatographic peak represents a single compound, then spectral averaging can compensate for spectral skewing.

All transmission quadrupole mass spectrometers scan from low to high m/z values with the exception of the Agilent instruments which scan from high to low m/z values.