Aviv Amirav, Professor of Chemistry at Tel Aviv University and Director – Aviv Analytical
This state of affair raises the question of "what is more important for sensitivity evaluation, including for MS-MS, is it signal alone (number of ions) or signal to noise ratio".
We used this software for the simulation of several situations in which the signal to noise remains the same but the number of ions is increased. The main point to realize is that for the same signal to noise ratio, if we increase the signal by a factor X, the number of noise ions also increases, but by a factor of X^2 which is not intuitive. However, while the noise level (baseline) increases linearly with the number of ions the noise fluctuation which is the noise in S/N, increases only as the square root of the number of baseline ions.
|Figure 1. Sparse ion chromatogram with 5 signal ions and one noise ion (upper) compared with the same chromatogram but with 10 times higher signal and 100 times greater number of noise ions hence having the same signal to noise ratio in RMS.|
|Figure 2. Sparse ion chromatogram with 20 signal ions and 20 noise ions (upper) compared with the same chromatogram but with 10 times higher signal and 100 times greater number of noise ions hence with the same signal to noise ratio in RMS.|
In Figure 3 we show a situation in which a larger increase in both number of signal ions and noise is used for the same S/N. This simulation demonstrates another unexpected effect of reduced limit of identification via increased signal (for the same chromatographic peak S/N) since while both chromatograms have the same total ion chromatogram (TIC) signal to noise ratio, the much larger increase in baseline number of MS ions implies that the generated mass spectrum will be hampered by noise ions and library search or any other form of identification will be impeded.
|Figure 3. Sparse ion chromatogram with 10 signal ions and 10 noise ions (upper) compared with the same chromatogram but with 100 times higher signal and 10,000 times greater number of noise ions hence having the same signal to noise ratio in RMS.|
Increasing the number of signal ions without corresponding increase in chromatographic peak signal to noise ratio does not improve the value of that chromatogram.
We suggest using a chromatogram with few compounds that include an easy to analyze compound such as OFN plus additional compounds with increasing difficulty such as n-C16H34, methylstearate, cholesterol and n-C32H66 which elute near the column bleed plateau. This way the evaluation will measure the TIC signal to noise ratio, which is the ratio of total ion signal - the signal averaged on several masses for several compounds as well as baseline noise which is an average noise of all masses in the scanned mass range of 50-500 amu. Such evaluation also includes the effect of peak tailing, molecular ion abundance, transfer line cold spots and all other relevant GC-MS parameters for standard analysis.
The Signal and Noise Simulation software can be downloaded and installed for free using the following link:
Signal and Noise Simulating Software (50MB download)
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