Ev 675 x 103 kj mol 1

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Unformatted text preview: •  observe peaks represen7ng the loss of CH3 (15 Da), CO (28 Da), etc. •  the CI spectrum gives us (almost exclusively) the quasimolecular ion peak [M+1]+ •  no fragmenta7on observed •  observe peaks represen7ng electrophilic addi7on of the sample with the reagent gas primary ions CH3+ + CH4 → C2H5+ + H2 CH4 + C2H5+ → C3H5+ + 2H2 CHM3122 Mass Spectrometry 11 (B) Desorp7on ioniza7on methods •  sample molecules are emiked directly from a condensed phase into the vapour phase as ions •  used for large, non ­vola7le, or ionic compounds •  usually provides only the molecular mass (no fragmenta7on) CHM3122 Mass Spectrometry 12 Field Desorp7on Ioniza7on (FD) •  sample is applied to a metal emiker whose surface is covered in carbon microneedles (anode) •  very high voltage gradients at the 7ps of the needles remove an electron from the sample, an the ca7on is repelled away •  rela7vely sod ioniza7on technique, so the resul7ng molecular ions have likle excess energy (so, no fragmenta7on) CHM3122 Mass Spectrometry 13 Let’s compare: EI, CI, and FD CHM3122 Mass Spectrometry 14 Fast atom bombardment ioniza7on (FAB) •  sample is dissolved in a “matrix” (solvent) that protects the sample from excessive radia7on damage •  the sample + matrix are bombarded with high energy xenon or argon atoms (6 to 10 eV) •  generally observe [M+1]+ and/or [M ­1]+ peaks •  useful for very large/fragile molecules with low vapour pressures (e.g., polysaccharides, pep7des of 10 ­20 kDa) •  fragmenta7on pakerns generally reflect the “building blocks”, like glycosidic or pep7de bonds  ­ ­> sequencing •  disadvantage: interference from matrix ions CHM3122 Mass Spectrometry 15 Laser desorp7on ioniza7on •  a pulsed laser beam is used to ionize the sample (usually in a mixture) •  in a matrix: « Matrix ­ Assisted Laser Desorp7on Ioniza7on », MALDI •  measure masses up to 3 kDa •  likle fragmenta7on •  interference from matrix ions •  adducts formed with matrix ions, making it difficult to analyze an unknown sample CHM3122 Mass Spectrometry 16 (C) Evapora7ve Ioniza7on Methods Ions or neutral molecules in solu7on are isolated by evapora7ng the solvent (i)  thermospray mass spectrometry –  the solu7on is heated in a capillary tube –  stream of fine droplets is fed to the ion source –  unpopular: mostly replaced by electrospray CHM3122 Mass Spectrometry 17 (C) Evapora7ve Ioniza7on Methods (ii) Electrospray Mass Spectrometry (ES) –  also called atmospheric pressure ioniza1o...
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