Impossible MS identification and quantitation of two similar molecules

Hello,

 

I work with a rapidfire coupled to an ultivo triple quadrupole MS, therefore there is no separation.

The main purpose is to monitor the biocatalysis reaction and to quantitate the substrate and the product.

The substrate molecular weight is 411.1 Da and the product molecular weight is 412.1 Da.

 

I tried two methods:

  • 1st method:
    • Polarity +
    • Fragmentor: 118 V
    • Product ions:
      • Substrate:
        • 412.1 => 366.1 as a Quantifier (Collision energy 17 V)
        • 412.1 => 306 as a Qualifier (Collision energy 29 V)
      • Product:
        • 413.1 => 367.1 as a Quantifier (Collision energy 17 V)
        • 413.1 => 350.1 as a Qualifier (Collision energy 21 V)
  • 2nd method:
    • Polarity +
    • Fragmentor: 118 V
    • Product ions:
      • Substrate:
        • 412.1 => 306 as a Quantifier (Collision energy 29 V)
        • 412.1 => 239.7 as a Qualifier (Collision energy 41 V)
      • Product:
        • 413.1 => 350.1 as a Quantifier (Collision energy 21 V)
        • 413.1 => 200 as a Qualifier (Collision energy 45 V)

In each case, when I work with one of the two molecules only, and I look for one of the two molecules with the MS method, the results are good (RSDRepeatability = 2%, linear response).

 

When there is only one molecule in my solution, but I look for the two molecules with the MS method the results are rather bad (RSDRepeatability = 23%, nearly quadratic response). The system finds substrate in the product solution or product in the substrate solution. I’m sure there is no substrate in product or product in substrate.

 

When I work with the molecules and look for both, the results are also bad.

 

I think that the MS is lost and takes the substrate response for the product and vice versa, probably because the molecular weights are close. Does anybody know how I can manage to do that analysis? Or is it because the Ultivo Triple Quadrupole is not sensitive enough for that kind of task?

 

Thank you very much

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