A "Little" Mass Spectrometry:
Accurate Mass Data...
We have both magnetic (Autospec and VG70) and Time-of-flight (TOF) mass
spectrometers (Micromass
GCT and LCT). We have more data on the
precision and accuracy of the magnetic instruments and are
just beginning to understand our TOF instruments.
Accurate Mass with Magnetic Instruments: We have identified unknowns by GC/MS (EI and CI),
solid probe, FAB, etc. for over 17 years.
It is critical to set appropriate limits in processing the data. In
order to set these limits, I kept data on known components present in the
samples being analyzed over a
period of several years (see Figure 1, 181 points). My data is shown in graph1.
I had no idea whether or not the results were reasonable. When I talked to
people (other users, manufacturers),
they indicated their data was much better than I was obtaining. However, no one
offered any statistical
documentation to support his or her claims. Furthermore, I noticed when
observing other mass spectrometrists
within our corporation and those at laboratories of mass spectrometry
manufacturers, they tended to pick
scans that supported the proposed structure and discounted ones that did not
support the proposed structure.
That didn't surprise me, I often "catch" myself doing the same thing
when confirming the molecular formula of
a known compound. Unfortunately, this must be human nature. However, you can't
"pick and chose" when
determining the molecular formula of an unknown!
Trying to better understand our process, I tabulated results from the
Journal of Organic Chemistry (197
different mass spectrometry laboratories, 1280 points). The results (6.4 ppm,
one standard deviation) are
shown in graph2. This data made me feel reasonably good
about our results, since I was doing capillary GC/MS.
Most of the data obtained in the journal was primarily obtained via solid
probe. The latter approach should
yield results that are more accurate and precise than GC/MS since data can be
averaged
and the signal is usually more intense.
Click Here to Download Excel V2.1
"Zipped" Spreadsheets of ppm-error Values.
Accurate Mass with Time-of-Flight (TOF) Instruments: We (Curt Cleven and myself) don’t have
extensive information on the long-term precision and accuracy of our TOF
instruments. However, we
have performed some basic studies to understand the limitations of the LCT and
GCT. Our instruments
are based on TDC (time-to-digital converters), which affects the accuracy and
the precision of the
instruments. The TDC approach has two significant
limitations:
1) When two or more ions hit the detector in one flight cycle the TDC counts them as one event.
2)
When two ions hit the detector within a certain time interval,
the TDC does not count the second ion
(dead time loss)
The LCT and GCT correct for this problem via software. Our ASMS
poster session addresses these
limitations and Excel ”program” models
the effect.
This limitation can be avoided by using an ADC but there are other limitations to this alternate approach.
