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A blog on the latest applications, articles, & research on chromatography solutions in sample preparation, Ion Chromatography (IC),
High Pressure Liquid Chromatography (HPLC), Ion Chromatography-Mass Spectrometry (IC-MS), Gas Chromatography (GC),
Gas Chromatography-Mass Spectrometry (GC-MS and GC-MS/MS), and software (Chromeleon CDS, LIMS, and ProteinCenter).

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GC-MS/MS Results Made Ultra Simple

  
  
  
  
  
  

USRM, GC-MS/MS, TSQ Quantum XLS Ultra, Thermo Scientific

In the beginning … that is too far back.  Long, long ago in a galaxy far, far away… OK, too dramatic.  You get the picture, when gas chromatography was in its infancy there were detectors (non-mass spec detectors); TCD, FID, then ECD, PID, NPD, FPD, ELCD (aka Hall detector), DID, and PDD (there may be more but they’re not coming to mind; these are the ones I’ve used over the years).  They were great, ranging from universal detection to specific to certain functional groups.  Many of them were very sensitive.  

All of them have one issue in common; if you have a coelution then you cannot tell what you are looking at.  If you have a retention time shift due to matrix then it is difficult to work with the data.  If you were good at working with this data it took you years of dedicated focus and training. 

Enter mass spectrometers.  They started out huge but could do things the other detectors couldn’t, like tell you if there was something coeluting and be universal.  These have gotten a lot smaller and a lot more sensitive with greater precision.  There are a lot of different mass spectrometer technologies available today.  By far the most pervasive is the single quadrupole.  These things are great!  They are small, relatively inexpensive, and can be very sensitive.  However, they run into interference problems too especially at very low levels with lots of matrix.  This is where triple quadrupole GC-MS/MS systems come in.  They are great at finding targeted compounds but even they can have problems with interference as you attempt to trade increase in signal with selectivity of the compounds in heavy matrix.

With all of these problems how do you simplify the data?  One solution is to increase the resolution on the first quadrupole Q1.  This provides the additional separation of the compounds away from the matrix.  First question I get is: does that work for all compounds? The answer yes it works, however, the results are much more pronounced with those compounds that have a mass defect as compared to the matrix.  I sometimes get a blank stare when I say mass defect, especially from those that are used to running only single quadrupole GC-MS.  Do not worry, you did learn this in school (It might have been awhile or you stayed out late the night before).  The concept of mass defect is really quite simple, no matter what others might tell you.  All compounds have an exact mass, not just the nominal mass you are used to in single quads.  The exact mass will either be more or less than the nominal due to the atoms that make it up (that is the defect, defect from nominal). 

So, for example, organochlorine pesticides that are analyzed by GC-MS have chlorine atoms, making the exact mass less than the nominal (negative mass defect).  Matrix background usually has little to no halogens, making the exact masses greater than nominal (positive mass defect).  This combination allows the TSQ Quantum XLS Ultra greater ability to separate the target from the matrix by increasing the resolution before the ions enter Q2 for fragmentation.  The result is significantly more selectivity.  This actually gives you two levels of selectivity, the first is higher resolution (mass defect) and the second is structural seen in other triple quadrupole systems.    

To learn more download the White Paper here; Ultra Selective Detection

Comments

Dear Eric, this selectivity enhancement works well for analytes in concentration ranges of ug/kg. However, for the analysis of dioxins on a ng/kg level, this is not a good option. By decreasing the mass window of the Q1 (0.7 -> 0.4) you will also loose sensitivity and will get a problem with detection limits.
Posted @ Thursday, May 24, 2012 9:58 AM by Harald Dahlmans
Harold, You make a good point and I completely agree with you. We found similar results. You do need an amount of signal that can tolerate the minimal loss when moving to the higher resolution settings. There are a couple other things to keep in mind. First, the TSQ Quantum XLS Ultra has a wider dimension for ion path through the rods allowing more ions at any resolution setting. Second, the sample clean-up procedure for dioxins is quite significant in order to remove any potential interference. There will always be some matrix but between the required clean-up and chromatography most is take care of.
Posted @ Thursday, May 24, 2012 10:22 AM by Eric Phillips
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