2Mass Spectrometry Used to identify organic compounds mainly through molar mass and identification of major fragments.A mass spectrometer is often used as the “detector” for a gas chromatograph (GC-MS)A mixture is separated and a total ion chromatogram (TIC) obtained. A TIC is the GC-MS equivalent of a gas chromatogram.The MS then allows identification of the components without having to rely on a comparison of retention times.
4Mass SpectrometryAnalysis time once the GC has been set up (column chosen, gas flow set, oven program finalized) is about 20 minutes.Not too expensiveOur benchtop model was $30K.Compounds that can be analyzed are those that can be analyzed on a GC:Low to moderate boiling liquids (and, of course, gases)
5Mass Spectrometers analyze particles…cations!, not photons. Impact from a high-energy electron knocks an electron from the organic compound and creates the Molecular ion (aka parent ion)a cation with an unpaired electron M+•It has the m/z of the molecule, so the value of m/z in most cases is the value of the molar mass (provided z, the charge of the cation, is +1)
6Mass Spectrometers analyze cations, not photons Impact from a high-energy electron also causes fragmentation.Base peakis the most intense m/z peakis the most stable fragmentOther fragments form as well, some from complex rearrangements
7Mass SpectrometersThe MS detects the m/z of each fragment by directing the ionized particles through a magnetic quadrupoleThe trajectory of the ions in the magnetic field bends as a function of the mass of the particle and its charge:
8Mass Spectrometers The magnetic quadrupole thus acts as a mass filter. For a given magnetic field strength, light particles are bent more and heavier particles less.By changing the magnetic field strength, m/z can be focused on the detector (an electron multiplier) in succession and yield a mass spectrumA plot of signal vs m/z
9OCCC’s GC-MS Shimadzu GC GC-17A version 3 Restek Rtx-XLB 30 meter fused silica capillary column with 0.25 mm inner diameter and a 0.25μm coating (the liquid phase is low polarity…and proprietary) for a column volume of 1.5 mLColumn operates from 30° - 340°C
10OCCC’s GC-MS Shimadzu MS parameters GCMS-QP5000 quadruple mass filter requires an operating pressure of approximately 2 Paquadruple mass filterelectron multiplier detectorturns on after 2 min to clear solventset up for 2 scans of m/z per second
11Fragmentation Patterns - Benzene Ring Odd m/z suggests N may be present.Peaks at 51 and 77 are very typical of the benzene ring.
12Fragmentation Patterns - the benzylic cation, the allylic cation Extraneous peaks happen.
16Fragmentation Patterns - halogens Chlorine has 3:1 ratio of 35Cl : 37ClLook for peaks at 35 and 37Look for a gap of 35.Look for M:M+2 ratio of 3:1.Bromine has 1:1 ratio of 79Br : 81BrLook for peaks at 79 and 81Look for a gap of 79.Look for M:M+2 ratio of 1:1.IodineLook for peak at 127.Look for a gap of 127.
17Rule of Thirteen Once you have the molecular weight (MW)… Divide MW by 13 and express the result as an integer (n) and a remainder (r).If your compound is a hydrocarbon, its formula is approximated as CnHn+r.You may then find elements of unsaturation from the molecular formula.
18Rule of ThirteenIf you have hetero atoms in your compound, adjust the formula to accommodate:For O, add O and subtract CH4.For N, add N and subtract CH2.For Cl, add Cl and subtract either C2H11 or C3H-1.Again, calculate elements of unsaturation from the formula.
19How to Analyze a Mass Spectrum Identify the molecular ion peak, if present. Note if the MW is odd.Mark the base peak and show the fragment which gives rise to it.Mark significant fragment peaks and identify the fragments giving rise to them.Mark peaks or gaps that indicate the halogens, Cl (M : M+2 peaks = 3:1), Br (M : M+2 peaks = 1:1), I (M or gap = 127).The mass of the compound with Br will be M+1.