Effects of saturation on peak shape B-51
• If the first isotope maker is above the best fit line compared with the
second isotope marker, then too much dead time correction is being
applied. Increase Np multiplier by 0.1, and repeat steps 3 through 7.
Result: In the spectrum, the first and second isotope peaks of the tris
(trifluoromethyl) triazine ion should be within 1 mDa of each other on
the residual output. This value of Np is typically 0.6 to 0.8 and should be
used from now on.
Effects of saturation on peak shape
The time to digital converter (TDC) records the time at which an ion strikes
the detector. MassLynx uses this time value to determine the mass of the ion.
As the number of ions increases, saturation of the TDC causes the peak shape
to characteristically change as an increasing numbers of ions are not detected.
At the onset of saturation, the peak shifts a little to lower mass. This is
followed by a failure to increase recorded intensity because the ion signal
continues to increase. There will then be a sudden, sharp, high-mass cut off.
Finally, the detector will appear to “ring”, causing a secondary peak to appear
on the high mass side of the peak of interest.
Figure titled “Schematic diagram of peak shape saturation effects:” on
page B-52 represents the characteristic transition from unsaturated signal (A
and B) to saturated signal (C, D, and E), which appears when tuning using the
real time peak display in continuum mode. Since the recorded intensity of the
saturating ions appears to reach a constant value, it is important to recognize
the onset of saturation by the change in peak shape and position.
When acquiring data in centroid mode, MassLynx detects when saturation
occurs and flag the saturated peaks with a question mark. These peaks do not
show valid accurate mass data.
To see the (?) peak flags:
1. In the Spectrum window, click Display > Peak Annotations.
2. In the Spectrum Peak Annotation dialog box, select the Peak Flags
check box.
3. Click OK.
Note: The (?) peak flag appears only on single centroid spectra. The (?)
peak flag does not appear in combined or continuum spectrum.
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