AGILENT Technologies has dropped the measure of signal-to-noise rario (S/N) from some instrument descriptions, saying that the measure is no longer meaningful. From now on, the company will cite the ‘ instrument detection limit’ specification for the 7000B triple quadrupole gas chromatography/mass spectrometry (GC/MS) system.
The move is a response to the greatly improved performance of the technology, says Agilent. “Ten or 20 years ago, S/N was a reasonable performance indicator. Baseline noise is now so low that is no longer the case”, says the company’s GC/MS marketing manager Terry Sheehan. “Different baseline segments can change the ratio by five or even tenfold, and dividing an MS signal by a noise value which is approaching zero has no correlation to the system’s true performance.”
Agilent now cites the instrument detection limit (IDL), which is based on system precision and which correlates directly with ion count, in its performance description for the 7000B. “Ion count is the real measure of MS sensitivity” adds Sheehan.
The IDL approach follows the guidelines of the International Union of Pure and Applied Chemistry (IUPAC), the US Environmental Protection Agency (US EPA), and other organisations. By this method, performance is calculated from a series of automated injections, to which the Student’s T-test is applied. A further advantage, says Agilent, is that IDL confirms the performance of each component, from the autosampler through the detector.
Although the S/N ratio is still cited by other instrument makers, and even for the time being by Agilent for other devices in its portfolio, the company indicated that the measure is of little value above a ratio of about 50.