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CHM3122 (7)

mass spec

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University of Ottawa
Kathy Focsaneanu

• two essential steps ◦ making ions ◦ separating ions by mass (mass/charge ratio: m/z) • M+ = molecular ion (also known as radical cation) • we can give the molecular an excess of energy which may cause fragmentation of molecular ion (m+ and n+) smaller pieces of original molecule, therefor smaller ion • in most cases, the Z (charge of individual species) is +1. therefore separation can be separated based on the actual mass. if dication, will be based on half the mass because it would be +2. • displays the number of ions to create a spectrum • slide 3: tallest peak in the spectrum is at 77, known as the base peak. on the vertical axis, the unit is % of base peak - intensities are relative. assign the tallest peak (base peak) of the spectrum at 100%. base peak in sample is not the molecular ion. it's actual the peak where it says 121. this suggests that fragmentation is more likely for this particular molecule instead of surviving intact to the detector. the horizontal axis is not g/mol. • slide 4: 4 steps. in most cases, the 4 stages are present in every single mass spectrometer. steps 2 and 3: how to make ions and how to separate them. key steps that dictate whether you know what the molecule is or not. • slide 5: ionization techniques ◦ a) gas phase method ▪ popular. 1st to be developed and relatively cheap. but there are major disadvantages ▪ must work in gas phase - get sample molecule in gas phase, must be volatile. min vapour pressure of 10^-6 Torr. ▪ works really well for non-ionic organic compounds (no salts) and low molecular masses (<1000 Da) ◦ electron impact (EI) ▪ electron beam - extremely high energies (70 eV) ▪ introduce sample vapour - bombarded by highly energized electron beam. the collision end up ejecting molecules. leaves behind a radical cation/molecular ion. ▪ creating a molecular ion with an excess of 55 eV - fragmentation is a problem because of this. ▪ number 1 consequence is fragmentation ◦ chemical ionization (CI) ▪ "softer" technique ▪ indirect ionization, not going to bombard molecule directly ▪ use of a reagent gas - use electron beam and shoot the gas and create ions from the methane and shoot those ions at the target molecule. ▪ CH4 > CH4 .+ + e- CH4.+ + M > CH3. + [M-H]+ ▪ [M+1]+ is the quasimolecular ion ▪ transferring lower quantities of energy (<5 eV) ◦ comparison of EI and CI spectra ▪ top: lots of fragmentation. base peak is not molecular ion. base peak = 165. lost a CH3, most popular ion was the molecule without a methyl group (base peak) ▪ bottom: extra peaks are adducts. instead of simply transferring an atom, you can get a reaction. the peaks should be small. ▪ if you wanted to eliminate the adducts, use less methane. the concentration of adducts is generally proportional to the partial pressure of the methane you use. ▪ CI spectrum gives you the quasimolecular ion was 117, therefore 117 - 1 = 116. molecular is so fragile that none of it reached the detector which is why it didn't show up on the spectrum. ◦ b) desorption ionization methods ▪ condensed phase ▪ appropriate for molecules which cannot be analyzed by gas phase i.e. big molecules and non volatile. no fragmentation ◦ field desorption ▪ metal emitter covered with micro needles. coat micro needles with sample molecule. turn on the voltage (extremely high) which turns metal needle into anode. electrons is stripped from molecule. creates molecular ion. because it is an anode. electrostatic repulsion lifts molecule away because it is positively charged. ▪ advantage gives you a very clear molecular ion peak. disadvantage is there is no fragmentation which means there is no extra info other than the molecular mass. ◦ fast atom bombardment (FAB) ▪ uses a beam of atoms. dissolve sample in a "matrix" (solvent, gel, etc). the purpose of the matrix is to protect molecular to minimize fragmentation and stabilize molecule. bombard sample with atoms (xenon or argon). volatilize and ionize molecules at the same time. depending on the matrix, proton transfer to and from. 2 quasimolecular peaks representing addition or subtraction to and from the matrix. peaks around (one on either side) depending. useful for large biomolecules. ▪ can get some fragmentation at very predictable points ▪ disavantage is the matrix itself (see extra peaks) ◦ laser desorption ▪ firing a laser, but similar to FAB ▪ limitation for MALDI is 3kDa, small compared to FAB. ◦ c) evaporative ionizatin ▪ i) thermospray: grossly unpopular ▪ ii) electrospray (ES) ▪ capillary, dissolve sample in a solvent (volatile solvent). comes down to increasingly narrow tube. creates droplets containing ions. over a short period of time, you have a volatile solvent that is evaporating, droplets become smaller. individual cations are coming closer together because solvent is evaporating away. more and more electrostatic repulsion. repulsions overcomes adhesion properties of the solvent and you get a coulombic explosion generate isolated ions. ▪ disadvantage: aqueous solvent, formation of adducts with other cations present in the solution (explains really big peak). • ion separation methods (3rd stage) ◦ analyzer: separate ions ◦ magnetic sector (oldest, cheapest, most popular) ▪ send in stream of positive ions. apply a magnetic field to the path of those ions. magnetic field will being cations. deflecting force is Bvz (b is force of magnetic field, v is velocity and z is charge) since it is mostly similar, B is the dictation - greater deflation force, larger curve in their trajectories. lighter ions curve more, heavier ions curve less. both hit the detractor at another point. count the number of collisions at the detector on the specific position and plot them. relative proportions. ◦ quadrupole ▪ 4 parallel rods with a hole down the middle. apply an alternating current to the 4 individual rods. molecular ion is fed into the mouth of the 4 rods. the positively charged ion will be attracted to one of the negatively charged electrodes (cathode). in the next instant, we change the polarity of the 2 rods (because it is alternating). the ion then becomes attracted to the changed cathode. then the polarities change again, so molecular ion moves again. constantly changing the trajectory of the cation as impasses through the 4 rods - creates a spiral trajectory. makes resonant ions towards the detector. the smaller the ion, the longer it travels through the tube. the bigger the ion, the faster it travels through the tube. ◦ time of flight ▪ lighter ions travel thorough the chamber faster than fatter ions. mass range is unlimited. can get an exact mass. disadvantage is that it's expensive. part 2 • slide 27: the isotopomers
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