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Hi,
For a compound, the average molecular weight is 301 and mono isotopic
molecualr weight is 300.
in LC/MS/MS, the parent ion(M+1) is obtained at 301.2 and daughter ion
at 159.0, and the same were used in MRM (301.2/159.0).
so in the calculation of concentration of stock solution, which
molecular weight should be used as the molecular weight of the free
form?
whether average molecular weight or mono isotopic molecular weight?
COA does not mention the moelcular weight.
The formula used during the calculation is:
calculated concentration
(Molecular weight of Free form/Molecular weight of salt form)*(amount
weighed/volume in ml)*(potency/100)*1000.
thanks in advance.
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Hi Debbie,
The relation of the average v/s isotopic mo wt is rather confusing.
For any molecue, the molecular wt is calculated by adding the sum of
the atomic wts of the elements its empirical formula contains wherein
the values taken will be of the most abundant isotope. Thus there will
be a single wt. Yes, the average wt is the one where the atomic wt
taken in the calculation is the avarage of the all of the abundant
isotopes. Now, since for the case menioned herein the difference is
rather high (I guess for most of the case the diff will be in the
first dzicmal place).
Now, the observed m/z in LC-MS will always be the monoisotopic one you
have to take the same I think. Else you can also manully calculate the
average wt by averageing the isotopic patterns of both precursor ion
and the product ion that you selected for the MRM and can compare the
difference. Theoretically, they should come same.
Thanks and Best Regards,
Amrit
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Hi Debbie,
This is what I found on WIkipedia, go through one especially 2nd
paragraph, and I think taking average molecular weight make more sense
according to article. "The monoisotopic mass is the sum of the masses
of the atoms in a molecule using the unbound, ground-state, rest mass
of the principal (most abundant) isotope for each element instead of
the isotopic average mass.For typical organic compounds, where the
monoisotopic mass is most commonly used, this also results in the
lightest isotope being selected. For some heavier atoms such as iron
and argon the principle isotope is not the lightest isotope. The term
is designed for measurements in mass spectrometry primarily with
smaller molecules. It is not typically useful as a concept in physics
or general chemistry. Monoisotopic mass is typically expressed in
unified atomic mass units (u), also called daltons (Da).
The mass spectral peak representing the monoisotopic mass is not
always the most abundant isotopic peak in a spectrum despite it
containing the most abundant isotope for each atom. This is because as
the number of atoms in a molecule increases the probability of the
entire molecule containing at least one heavy isotope atom increases.
For example if there are 100 carbon atoms in a molecule each of which
has an approximately 1% chance of being a heavy isotope the whole
molecule is highly likely to contain at least one heavy isotope atom
and the most abundant isotopic composition will no longer be the same
as the monoisotopic peak.The monoisotopic peak is sometimes not
observable for two primary reasons. First the monoisotopic peak may
not be resolved from the other isotopic peaks. In this case only the
average molecular mass may be observed. In some cases even when the
isotopic peaks are resolved, such as with a high resolution mass
spectrometer, the monoisotopic peak may be below the noise level and
higher isotopes may dominate completely."
Varun Kapoor
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Amrit
Is your questions relating to which ion to choose for analytical
evaluation? Evaluation of the spectrum of signals and the MRM that
provides you with the best signal to noise ratio in a practical sense
will yield the more selective and sensitive method development for
identification and quantification. Use the most stable of the isotopes
that provides a coherent MRM signal also. Does that help?
--
Sanjeev Thohan, PhD
SARx Consulting
SARxconsult.-at-.Gmail.com
http://www.linkedin.com/in/sanjeevthohan
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Hi Sanjeev,
The question was not of mine but it was the answer from my side for
somebody asking which out of the isotopic and average MW is to be used
in the PK calculation.
Indeed, in MRM the ultimate aim is to generate the signals to overrule
the noise. and same was my answer for it, the most abundant isotope is
eventually the most stable one. But for the query of the person
concerned the answer was use the most abundant isotopic peak as well
as the averaged values for the same for the calculation and observe
the difference. Virtually there should not be any difference.
Anyway thanks for the information.
Amrit
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