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Gas-Chromatography/Mass Spectrometry (GC-MS) Interpretation of EI Spectra

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Gas-Chromatography/Mass Spectrometry (GC-MS) Interpretation of EI Spectra Gas-chromatography/mass spectrometry (GC-MS) Interpretation of EI spectra Jeremy Keirsey CCIC MSP Mass Spec Summer Workshop August 17, 2015 Gas Chromatography Important goals in chromatography - Achieve the best separation - Little band-broadening – narrow chromatogram peaks – efficient column - “Dynamic range” - Separate and detect the “small” in the close vicinity of the “big” - Reproducibility - Stable peak positions, retention times Even with the best column diffusion always plays a role! Parameters to control diffusion Particle size (Column type) Flow rate The Van Deemter Equation HETP = A + (B/u) + (Cs + Cm) × u HETP = height equivalent to a theoretical plate -a measure of the resolving power of the column H = L/N The Van Deemter Equation HETP = A + (B/u) + (Cs + Cm) × u •HETP = height equivalent to a theoretical plate, a measure of the resolving power of the column [m] (Height = Length/number of plates=L/N) •A = Eddy-diffusion parameter, related to channeling through a non-ideal packing [m] •B = diffusion coefficient of the eluting particles in the longitudinal direction, resulting in dispersion [m2 s−1] •C = Resistance to mass transfer coefficient of the analyte between mobile [m] and stationary phase [s] •u = Linear Velocity [m s−1] Original paper: Van Deemter JJ, Zuiderweg FJ and Klinkenberg A (1956). "Longitudinal diffusion and resistance to mass transfer as causes of non ideality in chromatography". Chem. Eng. Sc. 5: 271–289 Youtube: https://www.youtube.com/watch?v=8i_4-OMCANE The Van Deemter Curve Resolution Solute moving through a column spreads into a Gaussian shape with standard deviation σ. Common Measures of breadth are: 1) The width w½ measured at half-height 2) The width w at the baseline between tangents drawn to the steepest parts of the peak (inflection points). Derive Van Deemter for Resolution HETP = L/N N = L/N substitute H = σ2/L N = L2/σ2 convert length to time (utility) 2 2 N = (tr) /(σ) relate σ to w1/2 (2.35σ) and w (4σ) 2 2 N = 16tr /w 2 2 N = 5.55tr /w1/2 GC coupled to Mass Spectrometry (MS) Autosampler MS Analyzer: GC oven GC controller -Quadrupole -GC Column -Ion trap -Time of flight (TOF) -Orbitrap GC Columns • Two Common Formats • Packed columns (most common with bonded liquid coating) • Open tubular (typically long columns with small diameters) Packed Columns Open Tubular (end on, cross section view) Polyimide Coating Column Wall (fused silica) Mobile phase Stationary phase (wall coating) GC Columns • Advantages of Open Tubular Columns • Best resolution (negligible A term, small C term in Van Deemter Equation) • More robust • Better sensitivity with many detectors (due to less band broadening vs. lower mass through column) • Column Selection • High resolution (thin film, 0.25 mm diameter, 60 m) vs. higher capacity (thick film, 0.53 mm diameter) • Stationary phase based on polarity GC Stationary Phase Retention factor (k’) describes the migration of the analyte on the column k’= (tR – tM)/tM Selectivity factor (α) describes the separation of 2 species, A and B, on the column α = k’B/k’A Resolution (R) = ¼ √N (α-1/α) (k’/k’+1) efficiency selectivity retention GC Adjustments • k is adjusted by changing temperature (higher T means smaller k) • Selection of stationary phase affects k and α values • The α values are adjusted by changing column (will work if there is a difference in solute polarity) • example: separation of saturated and unsaturated fatty acid methyl esters (FAMEs). CH3 C18:0 FAME bp = 369°C O H3C O CH O 3 C18:1 FAME bp = 352°C H C 3 O • Retention of C18:0 and C18:1 FAMEs on RTX-5MS columns is very similar (due to similar boiling points) • Retention on more polar columns (RTX-50MS) is greater for the more polar unsaturated FAMEs • Main concerns of stationary phase are: polarity, functional groups, maximum operating temperature, and column bleed (loss of stationary phase) • More polar columns suffer from lower maximum temperatures and greater column bleed-derivatize? • Changing carrier gas has no effect on retention GC Column Options Type Functional Groups Polarity RTX-1MS 100% dimethyl Non-polar RTX-5MS 5% diphenyl/95% dimethyl Low polarity RTX-50MS Phenyl methyl More polar Stabliwax-MS PEG High polarity Chirality Columns β-cyclodextrin GC Column Options RTX-1MS = NonPolar RTX-5MS = Low Polarity Saturated Hydrocarbons Ethers Olefinic Hydrocarbons Ketones Aromatic Hydrocarbons Aldehydes Halocarbons Esters Mercaptans Tertiary amines Sulfides Nitro compounds without α-H atoms CS2 Nitrile compounds without α-H atoms Long lifetime and very low bleed at high operating temperatures. Column specifically tested for low-bleed performance. Temperature range: -60 °C to 350 °C Temperature range: -60 °C to 350 °C. GC Column Options RTX-50MS = Medium Polarity stabliwax-5MS = High Polarity Alcohols Polyhydroxyalcohols Carboxylic acids Amino alcohols Phenols Hydroxy acids Primary and secondary amines Polyprotic acids Oximes Polyphenols Nitro compounds without α-H atoms Nitrile compounds without α-H atoms Low bleed Low bleed but rugged enough to with stand repeated cycles without Temperature range: 40 °C to 320 °C. retention time shifting. Sensitive to water and oxidation. Temperature range: 40 °C to 250/260 °C. GC-MS Applications - Environmental monitoring - Food, beverage, flavor, and fragrance analysis - Forensic and criminal cases confirmation test - Biological and pesticides detection - Fermentation control - Security and chemical warfare agent detection - Astro chemistry and Geo chemical research - Medicine and Pharmaceutical Applications - Petrochemical and hydrocarbon analysis - Clinical toxicology - Energy and fuel applications - Industrial use - Academics http://www.omicsonline.org/open-access/gc-ms-technique-and-its-analytical-applications-in-science-and-technology-2155-9872.1000222.pdf GC-MS Projects in the MS&P https://www.youtube.com/watch?v=7yxWBOPNkJ8 GC-Chromatogram EI Spectrum The Nitrogen Rule • Compounds* that contain even number of N atoms have even number of nominal molecular weight • Compounds* that contain odd number of N atoms have odd number of nominal molecular weight But what about singly protonated molecules and accurate molecular weights?? * Common organic compounds Ion Stabilities • Even electron ions are more stable than odd electron (radical) ions How about protonated molecules: even electron or not? And how about ions formed by electron impact (EI) ionization? Selected Isotope Ratios and 13C Contributions Accurate elemental masses: C: 12.000000 H: 1.007825 O: 15.9949 N: 14.003 Cl: 34.9688 Br: 78.9183 S: 31.9720 Use of Isotope Ratios to Distinguish Structures Increasing 13C contribution in polyalanines 100 13 375.2118 (exact mass) Increasing C contribution 80 375.4263 (average mass) in polyalanines 60 40 (Ala)5 20 C H O N Relative Intensity Relative 15 29 6 5 0 374 376 378 380 382 m /z 100 1975.9541 (exact mass) 80 60 1977.1722 (average mass) 40 (Ala)25 C90H129O26N25 20 Relative Intensity Relative 3568 (nominal mass) 0 1976 1978 1980 1982 1984 m /z 100 3569.8663 (exact mass) 80 3572.0062 (average mass) 60 40 (Ala)50 C150H252O51N50 20 3568 (nominal mass) Relative Intensity Relative 0 3574 3576 3578 3580 3582 m /z 13 100 a Increasing C 8 c 374.4183 (average mass) 7 80 average mass 6 contribution in + [Ala5+H] 60 5 polyalanines 4 (dalton) 40 374.2040 (exact mass) Deviance 3 2 20 mass nomfrom inal 1 Relative Intensity Relative exact mass 0 0 374 376 378 380 382 0 2000 4000 6000 8000 m /z m /z 100 b 3572.9742 (average mass) 80 + [Ala50+H] 60 40 3570.8741 (exact mass) 20 Relative Intensity Relative 0 3570 3572 3574 3576 3578 m /z [Ala120+H] GC Chromatogram- an example for you EI Spectrum- give the top and bottom a try Characteristic Isotope Distribution of Selected Transition Metal Elements 190 Origin of m/z 190? 255 Metal element? Transition Metal? Isotope Ratios to Identify Chemical Compositions…No Accurate Mass! Isotope Ratios to Identify Chemical Compositions…No Accurate Mass! Isotope Ratios to Identify Chemical Compositions…No Accurate Mass! 10B: 25% 11B: 100% What Happened? Renormalized Isotope Distribution in the M+. Region 43 15 86 Try these!! 105 77 182 …and these!! 129 102 117 Halogen? …and this!! Use Isotope Distribution in the M+. Region …and these!! GC-MS Projects in the MS&P https://www.youtube.com/watch?v=7yxWBOPNkJ8 Untargeted GC-MS Analysis of Spider Silk Droppings -the hunt for hormones Untargeted GC-MS Analysis of Plants Exposed to Crude Oil Targeted GC-MS Analysis of Caffeine in Urine-Full Scan Targeted GC-MS Analysis of Caffeine in Urine-SIM Targeted GC-MS Analysis of SCFAs (Short Chain Fatty Acids) in Mouse Feces acetic acid propanoic acid butyric acid 4-methyl valeric acid Quantitative GC-MS Analysis with Xcalibur Xcalibur Method Setup-DSQ II Xcalibur Method Setup-Trace GC Ultra Xcalibur Method Setup-Autosampler Xcalibur Processing Method Setup Xcalibur Processing Method Setup-Identification Xcalibur Processing Method Setup-Detection Xcalibur Processing Data Xcalibur Processing Data Xcalibur Exported Results Make Discoveries 700 140000000 600 120000000 500 100000000 400 80000000 60000000 300 40000000 200 y = 118779x - 3E+06 R² = 0.9991 20000000 100 0 0 0 200 400 600 800 1000 1200.
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