Peak shape, IRMS

So how does the IRMS get its stability Collector slits are several times the width of the ion beams. This gives a flat-topped peak shape (Fig 6) which makes the ion current intensive to drift. The main source of drift is temperature variation which both affects the electronic components used for mass selection and caused expansion and contraction of mechanical parts. Simultaneous measurement of ion beams using a double or triple collector is more precise than sequential measurement by mass scanning with a single detector. Finally, frequent comparison of sample gas under identical conditions also contributes to stability. Ion beam stability is more important than resolution for isotopic measurements. 

Unlike ordinary mass spectrometers, the goal of peak focusing in IRMS is broad peaks with wide flat tops. This allows peaks to be centered on the detectors during measurements and ensures that small inaccuracies in focusing do not affect the ion current. A typical peak shape for CO2 measurements is shown in Fig. 7.8. 

Fig. 7.8 Peak shape in IRMS of carbon dioxide (www.monitorinstruments.com)...

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