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Chromatography_-_in_tandem - CHROMATOGRAPHY CHEM3001(C30J...

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CHROMATOGRAPHY CHEM3001- (C30J) Chemical Analysis II Donna A Minott-Kates
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Tandem techniques Previously chromatographic effluents were collected after passing through a non-destructive, non-selective detector Each fraction was independently analysed by NMR, IR, MS or electroanalytical instruments for structural information In modern systems chromatographs are operated in tandem with a selective “detector” Effluent is continuously monitored Most common hyphenated system: GC- MS Process under computer control for storage and manipulation of chromatograms & spectra Digitised libraries available for data comparison Eg National Institute of Standards Technology - NIST/EPA/NIH Mass Spectral library (NIST 08) DMK: CHEM3001 - Chromatography (2009-10)
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NIST/EPA/NIH Mass Spectral Library Contents of the NIST/EPA/NIH (NIST 08) Mass Spectral Library DMK: CHEM3001 - Chromatography (2009-10) Library Name Library Description Library Contents mainlib Main EI MS library 191,436 spectra replib Replicate spectra 28,307 spectra nist_salts EI Salt Library 717 spectra nist_msms MS/MS Library 14,802 spectra of 3,898 positive and 1,410 negative ions nist_ri Retention Index Library 293,247 Kovats RI values for 44,008 compounds
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Mass spectrometry Small sample inserted into MS Neutral molecules passed through ionising beam Ions accelerated by electric field Magnetic sector exposes ions to field perpendicular to travel direction Ions deflected/focused on detector at end of tube Light ions deflected too much; heavy ions not deflected enough → these ions fail to reach the detector Mass (m/z) of ions selected dependent on varied field strength → mass spectrum (continued) DMK: CHEM3001 - Chromatography (2009-10) Source : Harris, D.C. 2007. Quantitative Chemical Analysis , 7th Edition. W.H. Freeman and Company, New York. p475
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Mass spectrometry Electron multiplier detector initiates an electron cascade between series of dynodes (current amplified) Measured current allows development of mass spectrum as signal intensity vs m/z Mass spectrum of unknowns compared to digitised library Resolving power low ; differences of m/z resolved at 0.1 Note: System maintained under high vacuum DMK: CHEM3001 - Chromatography (2009-10) Source : Harris, D.C. 2007. Quantitative Chemical Analysis , 7th Edition. W.H. Freeman and Company, New York. p475
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Mass spectrometry Electron Ionisation (EI) Ionising source - electron beam (70 eV) Energy greater than ionisation energy (12-15 eV) for molecules Molecular ion, M + . , generated M + . fragments due to excess residual energy (~1 eV) imparted by beam M + . ion may be absent from mass spectrum or seen as small peak Ion fragments responsible for mass spectrum Chemical Ionisation (CI) Reagent gas eg CH 4 , NH 3 , fills ionisation source chamber High energy electrons (100-200 eV) ionise gas CH 4 + e - → CH 4 + .
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