Looking for a needle in a haystack? Non-target screening

Recent developments in analytical chemistry in combination with data handling appear to be a major gain for the water sector, in particular for those of us having to keep an eye on water quality. See for yourself: CAS, the international register of chemical substances, currently recognises more than 153 million unique chemical compounds. Fortunately, they are not all in daily use. The EU has 22,253 unique substances registered in its REACH authorisation system for industrial substances. Then we have crop protection agents, medicines, drugs, disinfectants, etc. Many of these substances can end up in surface waters or groundwater after use, where they may or may not be transformed, for example by bacteria or in other natural processes. This then results in multiple transformation products. Water managers and producers of drinking water therefore have an enormous task ensuring that water quality is not impaired by all these substances.

Monitoring programme?

But how do you know which substances to keep an eye on? You can of course start by including the 22,000 REACH substances in your monitoring system. Until recently the analytical chemist then had to acquire standards for all 22,000 substances, set up a measurement programme, and ensure that the methods for all these substances were of sufficient quality for reliable data to be obtained. And that was just the REACH substances. So pretty much an impossible task.

High-resolution mass spectrometry

For a number of years there has been a technique available that links high-resolution mass spectrometry with the liquid chromatography separation technique. The separation technique, as the term suggests, separates the different substances present in a sample. This is shown by means of series of peaks (a chromatogram: each peak is a substance). The mass spectrometry (MS) then

Each substance has a specific mass: for example, water consists of two hydrogen atoms (each with mass 1) and an oxygen atom (with mass 16), so water (H2O) has a mass 18. This is called the nominal mass of water.

. If you use MS at high resolution, you can find out very precisely what the (accurate) mass of a substance is. For water the nominal mass is 18, whereas its accurate mass is 18.0153 (which is due to the presence of isotopes, but that’s another story). The nominal mass for the ammonium ion NH4+ is also 18, but the accurate mass is 18.0385. A high-resolution MS allows you to distinguish these accurate masses, and consequently the substances, from one another and therefore identify them reliably.

Databases and software

And now comes the best part: if you do not know what you should be looking for beforehand, you are analysing a sample, simply set the high-resolution mass spectrometer to turn out the most likely substances that go with the very accurately determined masses corresponding to the peaks in the chromatogram. This is possible because there is now software available that can find out which substance every accurate mass most probably is by seeing whether that mass appears in all kinds of different databases, not just the one that the supplier of the mass spectrometer has used, but also open access databases on the internet. This allows the analyst to discover which substances he has in his sample and at the same time new substances are discovered that were not previously known to occur in, for example, water samples. So if your MS analysis suggests that particular substances are present in your samples, you can acquire a standard for those substances for reliable determination of their concentrations, instead of having to do so for 22,000 or more substances. This broad screening technique, known as non-target screening, is rapidly becoming a fantastic tool for the water sector.