comparsion scan and sim mode

[ockie]

Is there anybody who know: how much intensity increasing is visible in the sim mode compared to scan mode (in %)?

thanks,
ockie

[lmh]

there isn't a single answer to this. If you are using an elderly ion trap, there is very little improvement at all in SIM compared to scan (because the trap is filled with total ions in both cases, and sensitivity depends on trap capacity). On some more modern ion-traps, as I understand it, there have been big improvements in the wave-forms used to eject unwanted ions during trap filling, so in SIM mode you can fill the trap with the right ion, and not rubbish. Someone else might be able to answer better. In this case I'd expect a modern ion trap to do much better in SIM in situations where the signal is a mixture of many ions, but not much better in a pure chromatographic peak.

In single quads, the increase in sensitivity in SIM over scan depends on how much extra time the instrument has to look at the ion of choice in SIM compared to scan. This can depend on features such as the scan-speed chosen by the manufacturer.

But to give a very rough idea, I would be upset if I didn't get a 100-fold improvement in SIM compared to scan on our elderly single quad system.

[Peter Apps]

The total signal in SIM is less than it is in scan (OK I'm ignoring scan times and dwell times) - but SIM is more selective and so the signal:noise is much better in SIM - if you can work on one of the major ions, and if it is an ion that is rare in the background and in interfering peaks you might reasonably expect an improvement of 4 orders of magnitude or better in usuable signal. SIM a minor ion (say the M+ of an alkane for illustration) and SIM will give smaller peaks than scan.

Peter

[Ron]

There is no simple answer to your question. The first thing to be aware of that sensitivity in GCMS is not just peak height, it is signal to noise ratio. If you have one system with a signal of 1000 counts and an average noise level of 10 counts, and a second system with a signal of 10,000 counts with the same sample, and the average noise level of 500 counts the first instrument is more sensitive even though the signal is a factor of 10 lower.

The major factor in increased sensitivity in SIM mode in modern MS systems is decreased noise, not increased signal. Modern electronics are much better than those of 10 or 15 years ago, and there is not much increase in signal, the major factor is reduction in the noise level. The most common detector is an electron multiplier, and this supplies an instantaneous measurement, any time averaging or accumulation must be done in the firmware. There may be a multiplier applied to the signal to change scaling, but as a rule the signal will not be larger in SIM mode than in scan, the background noise will be lower, leading to an increased signal to noise ratio.

Older instruments with less advanced, higher noise electronics may have a higher signal level, but the noise is also significantly reduced, and the reduction in noise is a major factor in the increased signal to noise ratio.

The sensitivitly increase in a newer mass spectrometer running SIM mode will probably be less dramatic than in an older instrument. instead of a 100 times increase in sensitivity you are more likely to see a 5 to 10 times increase in sensitivity.