Analytical Chemistry CEM 434
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This 37 page Class Notes was uploaded by Ladarius Rohan on Saturday September 19, 2015. The Class Notes belongs to CEM 434 at Michigan State University taught by Greg Swain in Fall. Since its upload, it has received 37 views. For similar materials see /class/207679/cem-434-michigan-state-university in Chemistry at Michigan State University.
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Date Created: 09/19/15
Chapter 20 Molecular Mass Spectrometry Read pp 550570 Problems 202567111213 MS is probably the most widely applicable of all the analytical tools available Elemental composition of samples of matter Structures of inorganic organic and biological molecules Qualitative and quantitative composition of complex samples Structure and composition of solid surfaces Isotopic ratios of atoms in samples WPWP Mass Spectrum 4 am peak mi Spectral features Hard ionization depend on method M Rciaiiv nbundmc o RX 153 g of Ionization 2quot 91 Cum quot i 39 l i i 40 60 an IUD 20 140 lle mz Most ideal to have m the molecular ion m n to remain intact so u i M39 can determined cHrH239ngii M 0W BIS pcnk be 5 w l CliCH1cH 5 Soft Ionlzatlon E 40 i T M l 1 157 m 70 m l 98 I12 l I l i i i i i i i I if i 60 80 IO IZO MD 100 MR FIGURE 202 Mass spectrum 039 1 dscanol from a a hard ionizatlon source electron impact and b a soft ionization source chemicai ionization Ion Sources The Starting Point l V TABLE 201 Ion Sources for Molecular Mass Spectrometry Basic Type Name and Aaonyni Ionizing Agent Gas phase Electron impact EI Energetic electrons Chemical ionization Cl Reagent gaseous ions Field ionization F1 Highpotential electrode Desorption Field desorption FD Highpotential electrode Electrospray ionization ESI High electrical eld Matrixassisted desorptionionization MALDI Laser beam 7 Plasma desorption PD Fission fragments from 751C Fast atom bombardment FAB Energetic atomic beam ii Secondaryion mass spectrometry SIMS Energetic beam of ions Thermospray ionization IS High temperature Formation of gaseous ions Gas phase sources sample is vaporized and then ionized 103 Da limit Desorption sources sample is converted directly to gaseous ions Applicable to nonvolatile and thermally unstable samples 105 Da limit Electron Impact Ionization Hard E e 7 Electron slil Heater First acccl slit and m bGas Filament Fncus Sm accel SliE BZIII e l g 1quot To mass FIGURE 203 An electronimpact ion lj 1 analyzer source Adapted from R M Silverstein and F X Webster Spectrometric In J I i 133 328321235333 612an Ye ion 39v I r b A ade g Reprinted with permission of John 7777777777777777777 gt Wiley amp Sons Inc M e39 M39 2e39 Energetic electron beam causes molecules to lose an electron become ionized due to electrostatic repulsion Electron Impact Spectra Complex mass spectra are useful for compound identification LE 202 Some Typical Reactions in an ElectronImpact Source lecular ion formation gmentatlon ABCD e gtABCDH 26quot ABCD39 gtA BCD39 A39 BCD gtBc D B A CD AB A 3 D C AB CD39 C Jr W B 39 AD ABCD gtADBC BC ABCD ABCD gtABCD392quot gt BCD39 ABCDA arrangement followed by fragmentation 39 lljsion followed by fragmentation Interaction with the electron beam ionizes the molecule but it also leaves the molecule in a highly excited vibrational and rotational state Relaxation occurs by extensive fragmentation giving a large number of positive ions of various masses that are less than the mass of the molecular ion Typical Electron Impact Mass Spe tra 4 390 am ptak CIIZCI2 HECI39 34 u an 7 MW 84 M CHZClZ g no 5 u g 40 2 u z 20 I I I v I I I I I u 0 20 1D 40 o 60 70 an 90 I00 IIn mZ a 100 M 7 H20 and Cll2CH31 x a g CHFOM nupmk M llzonnd cm E E 5quot M 7 H10 5 E Molccular ion pm as FIGURE 204 Electron Impam mass spectra of a methylene chloride and b 1 pentanol Isotapi ic Abundances TABLE 20 Natural Abundance of Isotopes 0 Some Comman Elements Most Abundant Alumina of other Isotopes R Elemem Bump m 100 Parts ume Mm Alp Hydrogen H ZH Carbon C uC Nllmgen N N n I50 I70 30 Sulfur S 338 S Chloris JSCI J7Cl Bromine r mBr Silicon Si Si 3quotSi 39Fhmnll FL phmpnums quot17 sodium PM and iconquot 39quotn have no nudixional mlnrally emuring we Hams mm mll bc m av zg 01103 C moms Chemical Ionization Soft CH4 e39 9 CH4 2e CH4 CH4 9 CH5 CH3 CH5 MH 9 MH2 CH4 Spectra contain mainly the molecular ion peak 39M1 most abundant There is no extensive fragementation MALDI leniaati ari Monoclonal antibody IgG Exuacrion grid 4 u Laknr bcum Dcsnrbed matrix and analymmns 3 o MW 3 Mau139x Caiion 2 Sample plmc h 39 0 1 00000 200000 FIGURE 207 Diagram Oi the MALDI Drama The analyie is uniforml dis arsed in a matrix and placed on a metal other ions The analer can he prolonated be deprot Hated or rorm adducts before entering the TOF analyzer Atom transfer reactions happen in the matrix to give multiply charged Electrospray Ionization Cylindrical elECIIOdS Elecuoslatic lenses First pumping stage Quadrupole mass spcctromtter Second Pumping slag FIGURE 209 Apparatus for electrospray ionizaiion From J B Fenn et al Science 1989 65 246 Reprinted with permission Electrospray Ionization MS Data 100 a W Human palmlhylmd McImmrlcukinZ hormone l L w Is 549 N 14 505 m 5 7 M 15 O 0 1 t ml mz a mu l39 Mehuman gmth hnrmnne Eminc albumin Lil 22255 44 135300 quot 3999 111 35 0 J D quotIquot 400 600 no 000 um moo quot2 ml H1 FIGURE 2010 w 1 mn From R D Smith at at Anal Chem 1990 E2 852 Copyr gm 1990 American Chsm cal Socinly Other Ionization Methods Field Desorption Fast Atom Bombardment Mass Analyzers MB GHC nnulyzcr Euugy mmlplml e p 39 f mm 0 EM Sill 11 a u dnnhle focus Diru on focal plane d Inn culleulux Some exit sm lun wurcc FIGURE 2014 NielJohnson deslgn ofa doublefocusing mass spectrometer Chapter 16 Infrared Spectrometry Read pp380398 Excitation of vibrations and rotations in a molecule Transitions from one vibrational state to another 25 to 50 pm or 4000 to 200 cm1 midIR Much lower in energy than electonic excitation 530 nm 375 x1019 Jphoton or 226 kJmol 43 em 46 x1020 Jphoton or 27 kJmol Problems 1612358 Types of Vibrations gtlt gtlt Symmetric Asymmetric a Stretching vibrations a gt4 Inplane rocking Inplane scissoring K gtlt Outofplanc wagging Outotlplane twisting b Bending vibrations Figure 162 Types of molecular vibrations Note indi cates motion from the page toward the reader indicates motion away from the reader Dipole Changes During Vibrations and Rotations A molecule must undergo a net change in dipole moment as a consequence of its vibrational and rotational motion in order to absorb IR radiation Only then can the alternating electric field of the radiation interact with the molecule and produce a change in the amplitude of one of its motions When two charges q and q are separated by a distance R then a dipole moment exists Directed from negative toward positive end p Debye Cm Aq R HCI O C O quot Classical and Quantum Mechanical Picture of Two Atoms in a Bond Vibrating 1 H r 39 I 1 2 r o oa I 1 1 l quot 2 39 l I I Dissocnauon energy 39 i I I E l II n 33 gt I a a o 3 I 5 a l E v 5 l E Energy level 9quot I 5 vibrational 3quot 2 quantum number A 0 11 0 lt Displaccment y gt Interatomic distance r gt a b Figure 163 Potential energy diagrams Curve 1 harmonic oscillator Curve 2 anharmonic oscillator Fky AEhu LVL E 12ky2 ML 2 i kltm1m2gt m 275 it 275 mlmz U An Absorption Example 0 C O A linear symmetric molecule Predicted of vibrations for a linear molecule 3N5 Predicted of vivbrations for a nonlinear molecule 3N6 CO2 gt 3 X 3 5 normal modes 00 gt10 Symmetric inactive Asymmetric 2330 cm391 43 um O O O Degenerate bending motions 667 cm391 15 um Instrumentation Sources weakly intensell 1 Nernst glower rare earth oxides 2 Glowbar SiC rod Detectors must be stable have fast response time and be highly sensitive 1 Thermal transducers temperature changes 2 Thermocouples junction of two metals with a resistance that 3358 3553itsgggjgggg ggjggo g a changes with temperature Energy arbitrary units 3 Pyroelectric changes in temperature cause polarization in InStrumentSf mUSt have material to change 900d fOCUS39ng and 4 Photoconductors incident photons collection opticsll cause charge separation internally Wavelength Selector Interferometer Remember Dispersion instruments were used in the past but they were slow slow scanning and highly susceptible to noise poor sensitivity FT instruments now used zero I retardation Signal sampling interval 0 gure 167 Time domain signals for the threevinterferorneters contained in a urier transform infrared instrument Curve A infrared signal curve B white so or 7 7 quot quot 39 quot ht signal curve C laserfringe reference signal curve D squarewave electrical 1131 formed from the laser signal From P R Gri i rs Chemical Infrared Fourier nsform Spectroscopy p 102 New York Wiley 1975 Reprinted by permission of aim Wiley 39ons Inc Advantages of Fourier Transform Spectrometers Very high light throughput fewer optical components Jaquinot advantage High resolution lt001 cm391 All wavelengths of light reach the detector simultaneously multiplex advantage Fast speed and improved sensitivity SN ratios 7I Principles of Fourier Transform Optical Measurements 207 Frequency domain Time domain mm PV V2 Time 3 a Frequency d One cycle V1 3 quot2 E v Frequency 6 Time b Time RE 742 Timedomain signal of a source made up I any wavelengths Fixed mirror neters Movable mirror v V V F A B 0 CD H M SourccA 1 A 0 A 11 I Beamsphtter Sample I Distance cm s 2 1t O 1A 2A M 5cm FIGURE 743 Schematic of a Micheson interferometer itluminated by a monochromatic a Detcctor o 91 source Typical FTIR Spectrometer HeNe Dessicant quot quot Shield Source coil Interferometer at mirror Beam splitter 39 Interferometer 5cm mirrors 2 IR detector Laser fringe detector Optical stop Interferometer Purge cover at mirror wmdows Sample area Adjustable toroidal window Figure 168 A singlebeam FTIR spectrometer Courtesy ofPerkinElmer Norwalk C72 W3 W 5705 M 7177 a WMWMQ 1 J 9 I 39 gf a 71M gt 39 39 i D 4 h a l w t zf gm I i0 HID 2 msiio ajl 3quot 402d mud4a az Q 540m f5 V 5V g kiwi lhwuc 4 0 M w P 39 w A Maw Way Q0 N O o I485 if 65 kiwi W Wm Z quot 41 4 4 4 3 2 4 e e e a S 2 21 2 K3 g 39 83 pg 8413 g Introduction to Voltammetrv Read pp 716728 and 737742 Problems 25234713 Voltammetry involves measurement of a current at a working electrode as a function of the applied potential The measured current is proportional to the analyte concentration in solution OX Au Pt carbon 11639 Red Electrode Electrolyte Solution Currents in Electrochemical Cells Remember all electrochemical reactions take place at the electrodesolution interfacell Electrode Surface layer Phy 1 Electron transfer Phy l sica state Chemical change reaction sica srare Chemical gt39 Change reaction Solution bulk Mass r transfer Masx 1 Red trahfer Figure 228 Steps in the reaction Ox ne Red at an electrode Note that the surface layer is only a few mol ecules thick Adapted from A It Bard and L R Faulkner Benmchemlcal Methods p 21 New Yark Wiley 1980 Reprinted by pare mission aflolm Wiley amp Sons Inc Current limited by i charge transfer resistance ii mass transport resistance and ohmic solution resistance Modes of mass transport i diffusion ii convection and iii migration i current aQat nFAarea cm2aCat flux molscm2 a Name Triangular E Voltammetric Methods Waveform Type vohzmmetry P lnmgraphy Lineups Vollammury Durmenual 1 pu s polurogmphy Squurwzvc voitammetry Cyclic vollammciry Figure 251 Potential excitation Stgnals used in voilzmmetry Start the potential scan at a potential where the reactant is stable and scan through a region where redox chemistry occurs Everything that happens occurs at the working electrode solution interface Et Ei ot t time s and o We nstruments gnd Cells Counter electrode Figure 25 2 A all bridge Stirring bar Figure 256 A threeelectrode cell for hydrodynamic V I R voltammetry Three electrode cell Current flows between the working and counter electrodes while the potential of the working electrode is controlled versus the reference electrode Microelectrode Preparation carbon ol ro lene fiber 8pm p yp py resin 10 to 30mm resin and carbon powder glass 35 pm in diameter and 500 pm in length I 39 39i L fquot l n I r O FIgure 2 Scheme 01 standard carbon ber elec rode elechic wire Tvloicgl Voltgmmetric Dag Fe3CN6393 e H Fe2CN6394 E0 0225 v vs SCE Small microelectrode Large macrOEIECtmde Eve 1004 V 20 lemng cnmmk 2 6 800 A g g F 601 10 lt 1 E 4410 5 0 AEp IEpCEpal o 0059Vn 00 710 39 39 710000 02 704 706 03 710 lpclpa 1 SW vs SCE Figure 255 mneapscan voltammagram fox the educ 5 ion of a hypothetical species A to give a pmducl r 20 5 7 08 0 702 6 04 02 Potcntial V vs SCE 0 E1 2 E for re d OX rx n 39 Figure 2520 Cyclic VDItammogram fax a solution max is 39 60 mM in K3F2CN6 and 10 M in KN03 meP T m 4 n F Doc 0 r0 d I s k el 6 Ct r0 d e Kissingzrand w H Heinemnn chem Educ 1953 so 702 With ynmlssitm ip const x n32ADo12Coo12 Currents in Electrochemical Cells When currents are allowed to flow in electrochemical cells this means that net reactions are taking place at each electrode Equilibrium concentrations as dictated by the Nernst equation are not necessarily achieved on the time scale of the voltammetric measurement E iR where R is the resistance in the cell Some types of resistance that can limit the current flow are chargetransfer resistance mass transport resistance and solution ohmic resistance E cell Ecathode Eanode IR i or kC where C is the analyte concentration Concentration Depletion of Reactant Concentration at 10 20 Distance fmm surface x mm x 103 a Electrode Surface Concentration gt 2 Figure 258 Concentration dis tance pro les during the diffu sioncontrolled reductioh of A to give P at a planar microelectrode a Eappl 0 V b Eappl POim Z in Figure 255 elapsed time 1 5 b and 10 ms 1 ms 5 ms 10ms a CF 30 0 10 20 Distance from surface x mm x 103 30 40 Diffusion is the sole mode of mass transport Flux molcmZs Docm2s x aCax Potential Step Measurements it nFADOCTctWZ 39 Eappl Potential Current pA gt Time gt Time gt a b Figure 257 Current response to a stepped potential for a planar microelectrode in an unstirred solution a Excita tion potential b Current response Step the potential from a value where the reactant is stable to a value at which the reaction occurs at a mass transport limited rate Nature of the Measurement SECRETION FROM INDIVIDUAL CELLS 83 Carbon Fiber Microelectrode Stimulating Pipette Ch aff H mm nce Mast PC12 pancreatic cells 10um J Figure I Cartoon of typical cell microclcctrode arrangement for electrochemical measurement of exocytosis The beveled tip ofa glassencased carbon ber microelectrode is placed in direct Contact with the surface oI a single chroma in cell adhering to a culture plate A nearby pressure ejection micropipetle is used to administer nL volumes of stimulating solutions directly onto the cell Measurements could be made from secretory cells or actual neurons
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