Why are electron capture negative ion mass spectra not reproducible? An ion source problem

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Environmental Science

Background:

• Electron capture negative ion mass spectrometry is crucial for analyzing environmental organic compounds.
• Variations in mass spectra can arise from instrument-specific ion formation or transmission efficiencies.

Purpose of the Study:

• To investigate the causes of observed differences in electron capture negative ion mass spectra between a VG 30-250 and an HP 5985B gas chromatography/mass spectrometry system.
• To determine whether ion formation or ion extraction efficiencies are responsible for spectral discrepancies.

Main Methods:

• Experimental determination of thermal electron populations in both ion sources.
• Analysis of ion extraction efficiencies by monitoring ion transmission profiles across varying lens potentials.
• Theoretical ion trajectory evaluation using SIMION simulations.

Main Results:

• Ion source temperature and pressure were found to be insignificant factors in spectral differences.
• Thermal electron populations were similar in both instruments, indicating comparable electron capture reaction efficiencies.
• The HP 5985B's extraction lens significantly suppressed low-mass ions, unlike the VG 30-250 which transmitted ions of all masses equally.

Conclusions:

• Ion extraction efficiency, specifically the suppression of low-mass ions by the HP 5985B, is the primary cause of spectral differences.
• The VG 30-250 exhibits superior ion transmission efficiency for both low- and high-mass ions compared to the HP 5985B.