Wednesday, August 15, 2012

OFN Sensitivity Specifications – Are they of any Value or Just a Game

Aviv Amirav, Professor of Chemistry at Tel Aviv University and Director – Aviv Analytical
August 15 2012


Sensitivity is a prime specification of GC-MS which is often used to decide which system "is better". Usually GC-MS sensitivity is specified with 1 pg OFN RSIM on its molecular ion m/z=272 and currently the quadrupole GC-MS sensitivity specifications of all the major vendors are in the range of 400 to 800 in RMS units.

However, these “sensitivity” specifications should not be used for GC-MS sensitivity comparisons or evaluations since the vendors employ misleading procedures and OFN is a non representative “easy to analyze” sample.  

In this post we discuss the "tricks" vendors probably use to increase their OFN specifications.

How do the vendors mislead us in their OFN specifications?

The OFN sensitivity specification is poorly defined, evaluated and compared, due to large variability in the way of noise measurements. Unfortunately, due to competitive pressure the major vendors use “tricks” that prevent proper system comparison including:

  • Use software that eliminates single ion noise.
  • Use software that measures noise in a small, ill-defined window.
  • Use software that measures noise in a selected time window with minimal noise.
  • Create new “definitions” of RMS noise that makes it smaller.
  • Use different sources of OFN
  • Use high “Threshold” to eliminate single ion noise.
  • Use different scan ranges (and speed).
  • Employ artificially small mass window of ±0.05 amu that reduces the noise but also increases the LOD thus misleading.
  • Employ multiple injections until one injection passes the specs.
  • Wait for several days until the vacuum background noise subside.
  • Test electro-polished ion sources that are deteriorated after their first cleaning.
  • Test ultra clean system that develops vacuum background in time.

The Figure below, for example, shows a chromatogram from one of the vendors brochure in which the "measured signal to noise" is reported as 904 (RMS) while it should be about 30. Probably they used some or all of the first four "tricks" as above and clearly such reported value is misleading.

OFN is too Easy - The OFN Gap

OFN is inert, non-polar, stable, volatile, has dominant molecular ion, has exceptionally low vacuum background and no ion source peak tailing. Thus, the generated OFN specification mislead GC-MS users that find that in reality they are unable to detect a one picogram of many standard compounds, particularly those that are the bottleneck of their analysis method. We named this gap between specifications and real life situation as “the OFN Gap”   
OFN Specifications of the Aviv Analytical 5975-SMB GC-MS with Cold EI

The Aviv Analytical 5975-SMB GC-MS with cold EI has no vacuum background noise, thus, it can reproducibly provide for 1 pg OFN signal with zero RSIM noise as shown in the figure below hence S/N>1,000,000. Should we use this number as a specification? No, since no one will believe it and although it is true, it does not reflect on LOD and does not overcome the OFN gap issue. The subject of better sensitivity specifications and evaluation will be discussed in a future note.

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