Do You See a Horse or a Zebra’s Shadow?

Martin Perkins

2nd February 2016

Accurate Mass GC-MS, Accurate Mass Q-TOF-MS, Applications Laboratory, Dan Carrier, GC/Q-TOF, Multiflex, MultiFlex GC/Q-TOF, NIST, Single Quadrupole GC/MS,

Things aren’t always what they seem!

Last month in the lab, we have been working on taint application that has proved to be very interesting.  It is a perfect illustration of how the MultiFlex GC/Q-TOF can avoid the risk of being lead up a blind alley in the miss-identification of a critical taint component.

So here is the story…

The customer has a problem with an unpleasant pencil-like taste in potable water.  They have run the sample on a GC-single quadrupole MS and, using EI data alone, found a reasonable hit in the NIST library; this matches the unknown to the structure shown in Figure 1.  This structure has an empirical formula of C12H18O; with loss of a radical, this gives an accurate mass of 178.1357.

Blog 2

Figure 1:  At the top you can see how closely the single quad EI spectrum of the unknown compound (the red trace) matches the library entry for the compound C12H18O (the blue trace).  The structure of the matching library entry is shown below.

Despite this, the customer had his suspicions and so asked us to do some work running his samples on the high-resolution accurate mass instrument we have in our lab; to see if we could confirm the structure.

We were very surprised to see that, from accurate mass data the GC/Q-TOF suggested am empirical formula of C11H14O2 with an accurate mass of 178.09883.  The accurate mass that we measured on the instrument was within 1ppm of this mass at 178.09869.

We can (and will) go further in confirming the identity of this compound and we will do this by running some MS/MS experiments to understand the structure better.

So, do you see a horse or is it a shadow of a zebra?

This is a perfect example of why accurate mass matters!