How Accurate and Precise are SIFT-MS Measurements?

Martin Perkins

18th June 2019

Selected Ion Flow Mass Spectrometry, SIFT-MS, VOCs, Voice200, Volatile Organic Compounds,

There is some confusion out there, as to how precise a tool, Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) is. Time for a spot of clarification and while we’re at it, we’ll deal with the matter of SIFT-MS accuracy too.

Here are the facts about SIFT-MS accuracy:

  • The Voice200ultra is a very precise analytical instrument capable of generating RSDs of 2% and better.
  • SIFT-MS, like all analytical instruments, is inherently inaccurate until it is calibrated against a standard. When this is done, the instrument’s accuracy is wholly dependant upon the accuracy of the standard. Traceable standards rock!

Do we have data to back up our claims concerning precision? Yes, we do.

Here are some links to some application notes, that include precision data:

Rapid Determination of Volatile Compound Content Using Multiple Headspace Extraction-SIFT-MS.

Automated Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) – particularly page 2.

High-Throughput Analysis of Volatile Compounds in Air, Water and Soil Using SIFT-MS.

If you prefer to see data from an independent source, take a look at this Webinar (one of many on the Syft Technologies website) – this shows the great repeatability that Joe Wicks obtained while working with SIFT-MS at Intertek.

If you don’t want to sit through the whole presentation, skip to 30 minutes for the precision data.

Spoiler Alert!
Joe shows results with RSDs less than 0.5% for 36 measurements of HCHO in air! Good luck finding a better method than that!

One thing I will say is that one of the most important things we have worked to establish over the last 4 years (because we don’t take anything at face value) is the precision of the instrument. You can do nothing worthwhile if an instrument is imprecise. Given that we don’t use internal standards in any of the methods, the precision is at least as good as you will see for LC methods.

To summarise, SIFT-MS compares well with other analytical techniques in terms of precision and is capable of great accuracy when calibrated with a good quality standard, and we have the data to back this up!

All pretty conclusive, so where does the confusion come from?

Just like other analytical data from GC-MS and LC-MS, our data was all generated with reference to standards, in practice, 99% of SIFT-MS measurements, will always be made in this way.

The theory of SIFT-MS tells us that the technique is capable of making absolute measurements, so long as we know the rate constant for the target analyte. This proves to be true in practice.

If you don’t have a standard to hand, so long as the Voice200ultra has the appropriate rate constant in its library, it will give you a rough indication of the concentration of the analyte in your sample (plus/minus 20%). A GC-MS can’t match this ability.

This can be handy on occasion however, in the real World, where measurements are expected to be bullet-proof, no-one is going to put their name to data unless it is measured against a traceable standard.

Back in the mists of time, when every SIFT-MS was built in somebody’s garage, physicists and engineers, who knew little of the disciplines of analytical chemistry, made a big thing about the ability of SIFT-MS to make absolute measurements and promoted it as a much bigger benefit than it really is.

Finally, one individual (who should know better) has been spreading some fake news on SIFT-MS precision, presumably due to some warped sense of self-interest.

We know who you are and if you are reading this; beware! – If it happens again, you will be struck-off the Anatune Christmas card list!

But don’t take my word for it. Why not attend our forthcoming SIFT-MS Interest Group Meeting (9th/10th July 2019 in Cambridge), where you can pose your questions to experienced users from the likes of the National Physical Laboratory, Imperial College and the University of Strathclyde.

Please click here to register or CONTACT US for more information.