Mass Spectrometry

1. Mass Spectrometry

2. Prelab and Datasheet Reading Technique �P� in lab text, p. 160-171 Carey, Chapter 13 (~4 pages at end of chapter) Notebook � nothing Datasheet Handed out in class Due at end of lab period (so come prepared and ready to go to work).

3. Mass Spec - Introduction Very different from IR and NMR Absorption of electromagnetic energy Sample can be recovered and reused Mass spectrometry Records what happens when an organic molecule is hit by a beam of high-energy electrons Sample is completely destroyed

4. Mass Spec - Introduction What does a mass spectrum tell us? Molecular weight Molecular formula Either directly or in conjunction with other kinds of spectra such as IR or NMR Fragmentation pattern Key pieces of what the molecule looks like (such as methyl, ethyl, phenyl, or benzyl groups

5. Mass Spec - Introduction We hit an organic molecule with a beam of electrons (usually 70-75 eV) That removes an electron from the molecule resulting in the molecular ion (a radical cation) The molecular ion then fragments in smaller radicals and cations The cations are detected by the MS instrumentation

6. Mass Spec - Introduction

7. Mass Spec - Instrumentation

8. Mass Spec - Instrumentation

9. An Example � Hexane (mw = 86)

10. Hexane Fragmentation

11. Fragmentation Guidelines Cleavage generally favors the formation of the most stable cations and radicals tertiary > secondary > primary > methyl resonance stabilized cations often formed small, neutral molecules often eliminated (water, carbon monoxide, ammonia, ketene) Also depends upon the relative bond strengths of the bonds being broken

12. Common Fragment Ions

13. Isotopic Clusters Especially useful for identifying chlorine and bromine Chlorine has two isotopes: 35Cl and 37Cl in about a 75/25 ratio (avg MW = 35.45) So compounds that have a chlorine will have 35Cl (75%) and 37Cl (25%) The molecular ion peak will show as two peaks in a 75/25 ratio, 2 mass units apart

15. Isotopic Clusters Bromine also has two isotopes 79Br and 81Br (avg MW = 79.9) Molecules that contain bromine have a 50/50 chance of getting one or the other Molecular ions of compounds containing a single bromine will have two peaks, 2 mass units apart, of about the same relative intensity

17. More Things to Look For Iodine � does not have an isotope cluster, but often shows a major peak at M-127 due to loss of I Alcohols will often have a peak at M-18 (loss of water) Nitrogen rule � if a compound has an odd number of nitrogens, the MW is odd (and M+) If the number of nitrogens is even (or zero), the MW will be even Many compounds do not show a good M+ peak Alcohols, highly branched compounds

24. Interpreting Spectra If an IR is present, examine for functional groups If an NMR is present Validate IR functional groups via chemical shift data Look for characteristic shifts and splitting patterns Examine the mass spec Look for the presence of Cl, Br, I, and N Determine the MW of the compound from M+ Look for common fragments and patterns Subtract known atoms from M+ to determine C and H Postulate a molecular formula Postulate a structure consistent with the known data