Ch12_MS - Ch 12: Mass Spec Summary The mass...

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Unformatted text preview: Ch 12: Mass Spec Summary The mass spectrometer: ionization acceleration H3C sample H3C H3C CH H3C CH3 ionization bonding orbital - e– 2 e– bond E = 2(e–) x Eorbital = 2Eorbital bonding orbital 1 e– bond E = 1(e–) x Eorbital = 1Eorbital weakened bond H3CCC3C 3C H3 HH H H3 3C CHHCCH3C3 CH3 H3 CCH CHCH3 C HH C HCH3 H3CCC3C 3C H3 HH H H 3 3C H3C CH3 H3C CH H3C CH B CH CH3 ionization - e– H3C CH H3C CH3 H3CCC3C 3C H3 HH H H 3 3C CHHCCH3C3 CH3 H3 CCH CHCH3 C HH C HCH3 H3CCC3C 3C H3 HH H H3 3C H3C CH H3C detector CH3 separation (deflection) X H3C * spectrum 43.1 (std) 43.0542 (HRMS) * Intensity 58.1 (std) 58.0777 (HRMS) m/z (mass / charge) The mass spectrum: Mass Spec Summary most intense peak is called the "base peak." it is the most stable of all fragments. The mass of base peak may or may not be useful for any given problem parent peak (aka molecular ion) is the mass of the parent compound, before any fragmentation. It has lost an electron, but that mass is insignificant. The compound's molecular formula can be determined from this value. Intensity of each peak. Reports the abundance of each fragment the more stable a fragment, the higher its intensity. See "base peak." I Mass of charged fragment. Common masses are given on attached table. See NOTE below. 10 50 100 m/z 150 200 m/z = mass/charge = mass to charge ratio In this course, will only see charges of 1+ As a consequence: m/z = mass/1 = mass m/z = m+1 (parent ion having one 13C isotope) NOTE: Not all m/z values provide useful information. m/z (parent) - m/z (other peak) = mass of the neutral fragment lost. Common values are listed on the accompanying table. Nitrogen Rule If M+ has an odd m/z value, the compound contains an odd number of nitrogen atoms. Common Neutral Fragments Although MS does not detect neutral fragments, their mass is reported indirectly by the x term of m/z = M+  ­ x. Common masses are given in Table 1. Table 1: Mass and identity of the x term of [M ­x]+ Value x Likely neutral fragment Likely parent compound 1 H. . 15 + (14n) (CH2)nCH3 [alkanes] 18 H2O [alcohols and straight chain aldehydes] . 27 HC=CH2 . 31 CH2OH [1˚ alcohol] . 31 + (14n) O(CH2)nCH3 [ethers] . 35 37 35, 37 Cl [ Cl and Cl] . 41 CH2=CHCH2 . 43 + (14n) [CH3(CH2)nC=O] [ketones] . 59 + (14n) [CH3(CH2)nO ­C=O] [esters] . 79 81 79 Br [ Br and Br] Note: there is no difference in mass for a neutral or charged fragment (the mass of an electron is trivial). This implies that Table 1 values can also correspond to m/z values. ...
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This note was uploaded on 08/15/2010 for the course BCH 100 taught by Professor Staff during the Spring '08 term at UC Riverside.

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