Signal-to-noise ratio (S/N): How much louder must the signal be than the noise for it to be ‘visible’?

The signal-to-noise ratio (S/N) is the ratio of the signal intensity of the target to the intensity of the background noise. It answers a very practical question: by how much must a peak ‘rise above’ the background before we can be confident that it represents a genuine substance rather than a fluctuation in the noise? The higher the ratio, the cleaner the target peak appears in the spectrum and the easier it is to identify and quantify; if the ratio is too low, the signal will be drowned out by noise, leading to false negatives, or even if the peak is barely visible, it cannot be accurately quantified. The common definition of the detection limit (where the signal is three times the noise) is essentially a threshold drawn for the signal-to-noise ratio.

Where does the noise come from? It includes both the background from the instrument’s electronics and chemical background, as well as ion suppression caused by co-existing components. Consequently, there are two approaches to improving the signal-to-noise ratio: firstly, amplifying the ‘signal’—by increasing ion yield and enhancing ion transport efficiency; secondly, reducing the ‘noise’—by suppressing background and ion suppression, and optimising sampling and signal averaging. Truly effective solutions often address both aspects simultaneously.

In mass spectrometry imaging, the signal-to-noise ratio directly determines whether an ‘ion image’ is clear. If the signal is weak and impure, the image will appear smudged, with blurred boundaries, or may even display spurious distributions. Neo Source INSTRUMENT’s MSI DPI combines photochemical secondary ionisation (PSI) with DESI desorption; on the one hand, this ‘illuminates’ non-polar compounds that would otherwise remain unionised, whilst on the other, it further enhances the signal intensity of polar components. Consequently, the official website describes it as ‘significantly improves the imaging signal-to-noise ratio’—with overall metabolite signals enhanced by 1–3 orders of magnitude, and cholesterol signals in mouse brains increased by approximately 205-fold.

In online mass spectrometry, the signal-to-noise ratio is similarly achieved through ‘signal enhancement and noise reduction’. The New Source ProC series utilises high-transmission-efficiency ion lenses to improve ion yield, whilst employing an MCP detector to achieve a gain of 10⁶ and effectively shield against pulse interference signals, thereby yielding spectra with a high signal-to-noise ratio even at ppb concentrations. Both MSI DPI and MSI LDPI can be retrofitted onto mass spectrometers from Agilent, AB SCIEX and Thermo.