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الموضوع: Mass Spectrometry Isotopes

  1. #1
    عضو ذهبي الصورة الرمزية اسراء
    تاريخ التسجيل
    Aug 2011
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    افتراضي Mass Spectrometry Isotopes

    Since a mass spectrometer separates and detects ions of slightly different masses, it easily distinguishes different isotopes of a given element. This is manifested most dramatically for compounds containing bromine and chlorine, as illustrated by the following examples. Since molecules of bromine have only two atoms, the spectrum on the left will come as a surprise if a single atomic mass of 80 Da is assumed for Br. The five peaks in this spectrum demonstrate clearly that natural bromine consists of a nearly 50:50 mixture of isotopes having atomic masses of 79 and 81 Da respectively. Thus, the bromine molecule may be composed of two 79Br atoms (mass 158 Da), two 81Br atoms (mass 162 Da) or the more probable combination of 79Br-81Br (mass 160 Da). Fragmentation of Br2 to a bromine cation then gives rise to equal sized ion peaks at 79 and 81 Da.
    bromine vinyl chloride methylene chloride
    The center and right hand spectra show that chlorine is also composed of two isotopes, the more abundant having a mass of 35 Da, and the minor isotope a mass 37 Da. The precise isotopic composition of chlorine and bromine is:
    Chlorine: 75.77% 35Cl and 24.23% 37Cl
    Bromine: 50.50% 79Br and 49.50% 81Br

    The presence of chlorine or bromine in a molecule or ion is easily detected by noticing the intensity ratios of ions differing by 2 Da. In the case of methylene chloride, the molecular ion consists of three peaks at m/z=84, 86 & 88 Da, and their diminishing intensities may be calculated from the natural abundances given above. Loss of a chlorine atom gives two isotopic fragment ions at m/z=49 & 51 Da, clearly incorporating a single chlorine atom.Fluorine and iodine, by contrast, are monoisotopic, having masses of 19 Da and 127 Da respectively. It should be noted that the presence of halogen atoms in a molecule or fragment ion does not change the odd-even mass rules given above.
    To make use of a calculator that predicts the isotope clusters for different combinations of chlorine, bromine and other elements Click Here. This application was developed at Colby College.
    Isotopic Abundance Calculator
    C H N O SSi
    Molecular Ion
    100%
    M + 1
    M + 2


    Two other common elements having useful isotope signatures are carbon, 13C is 1.1% natural abundance, and sulfur, 33S and 34S are 0.76% and 4.22% natural abundance respectively. For example, the small m/z=99 Da peak in the spectrum of 4-methyl-3-pentene-2-one (above) is due to the presence of a single 13C atom in the molecular ion. Although less important in this respect, 15N and 18O also make small contributions to higher mass satellites of molecular ions incorporating these elements.
    The calculator on the right may be used to calculate the isotope contributions to ion abundances 1 and 2 Da greater than the molecular ion (M). Simply enter an appropriate subscript number to the right of each symbol, leaving those elements not present blank, and press the "Calculate" button. The numbers displayed in the M+1 and M+2 boxes are relative to M being set at 100%.

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  2. #2
    مشرفة الاقسام الاكاديمية الصورة الرمزية تمارا احمد
    تاريخ التسجيل
    Jul 2011
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    افتراضي رد: Mass Spectrometry Isotopes

    Thanks a lot

    عضو في نادي ماركا الأكاديمي


  3. #3
    مراقب عام الصورة الرمزية Eiman
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    افتراضي رد: Mass Spectrometry Isotopes

    شكرا اسراء
    من جد وجد ....... ومن سار على الدرب وصل

  4. #4
    عضو ذهبي الصورة الرمزية نداء محمد
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    Aug 2011
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    افتراضي رد: Mass Spectrometry Isotopes

    Thank you

    عضو في نادي ماركا الأكاديمي


  5. #5
    عضو مميز الصورة الرمزية قمر بلحاج
    تاريخ التسجيل
    Jul 2012
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    افتراضي رد: Mass Spectrometry Isotopes

    جزاكم الله خيرا

    عضو في نادي ماركا الأكاديمي


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