"Isotopes"
"an IonSource.Com tutorial"

Slide 6

Mass Spectrometer Operation

Boosting Response by Lowering Mass 
Spectrometer Resolution

 

Changing the operating resolution impacts a mass spectrometers calibration.  It is important to operate a mass spectrometer at the resolution it was calibrated at.  If the mass spectrometer is calibrated at isotopic resolution and then the operating resolution is changed the calibration will shift accordingly.  A common trick used with quadrupole instruments in LC/MS to boost sensitivity is to lower the operating resolution.  This is OK if one understands that the calibration will change slightly.  A practical experiment can be performed to demonstrate this principle.  Select a small window around a peak of interest that is drawn down to near baseline resolution of the isotopes, see Figure A..  Begin to lower the operating resolution and the isotopic cluster will begin to merge and march off to the left on the mass scale, see Figure B.  As the resolution is lowered the number of counts associated with the base peak should steadily increase. 

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Figure A

Figure B

In the spectra above we have demonstrated  how we can increased the response of the mass spectrometer by lowering the resolution.  There are three things that happen when the resolution is lowered: 1.) Resolution of the isotopes is lost as demonstrated in Figures A and B, 2.) The peak apparently shifts to the left, see Figure B, 3.) The relative number of counts goes up.  One can either accept the mass inaccuracy or better yet recalibrate the mass spectrometer at the lower resolution.  The common reason why mass spectroscopists  lower the operating resolution is to boost response with the rational that while they lose isotope information and mass accuracy they may be able to observe a species that would not otherwise be observable.

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