How Really Sensitive is your Next Instrument - Instrument Detection Limit as Alternative to Signal-to-Noise Ratio

Andrey Rossin, Analytical, Eldan Electronic Instruments, Petah Tikva, Israel

Sensitivity is one of the most important criteria in the process of selection of the new instrument for the lab. Historically Signal-to-Noise Ratio (SNR) was used for sensitivity estimation, everybody knows - the higher SNR - the better.

Comparing SNR for the same type of instrument for the last 10 years shows unbelievable advancement in sensitivity - it looks like sensitivity became at least 1000 times better for this period of time. The current numbers for SNR are amazing: 150,000 for 1pg of Reserpine or even higher, while 10 years ago these numbers used to be 10 or 20 for the same amount of Reserpine.

In reality people that have more than 10-years old instrument in their lab expect from the brand-new instrument 1000 times better sensitivity based on SNR specifications, but real sensitivity is barely 50 times better or so. So maybe SNR is not that perfect scale for the sensitivity at the end of the day.

There are multiple reasons for SNR not being perfect - mainly because there are many ways to measure Noise: Peak-to-Peak, RMS, RMS x N, ASTM, etc. Also Chromatogram baseline can be manipulated in different ways: smoothing, baseline shift, baseline threshold procedures can be applied to improve SNR.

Introducing Instrument Detection Limit (IDL) - the procedure of evaluating the true sensitivity based on Peak Area RSD of multiple injections when the concentration is close to detection limit. IDL shows real analytical performance instead of number-crunching capabilities.

Multiple vendors already started to publish IDL specifications aside with SNR, some even ditched the SNR totally and publish IDL only sensitivity specs. Let's put an IDL in our analytical vocabulary for better understandidng of instrument sensitivity.