Instrumental Lecture 11

Instrumental Lecture 11 - Chapter 11: Atomic Mass...

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Unformatted text preview: Chapter 11: Atomic Mass Spectrometry (Inorganic MS) Mass Spectrometers ICP-MS Spark Source MS Glow-Discharge MS Elemental Surface Analysis by MS Laser Ablation ICP-MS Atomic Mass Spec processes Atomization (sample intro) Conversion to ions Separation based on m/z ratio Detection In other forms of MS (GC-MS or MS of organic compounds), sample introduction does not involve making atoms, just getting molecules into the high vacuum system Basic MS design components Note high vacuum Advantages of Atomic Mass Spec over Optical Atomic Spectrometry 1) Detection limits are better, sometimes several orders of magnitude better 2) Very simple spectra 3) Ability to measure isotope ratios Disadvantages 1) Equipment cost 2) Instrument drift 3) Isotopic interferences Atomic Mass Spec has been around for a long time but one of the most significant advances in this field occurred recently, the development of ICP-MS Detectors = Transducers for Mass Spec 1) Electron Multipliers transducer of choice Discrete dynode electron multiplier works like a photomultiplier tube ions electrons many electrons typical current gain 107 Cu/Be surface Continuous dynode electron multiplier glass doped With Pb, gains from 105 to 108 2) Faraday Cup historically important, not extremely sensitive, slow Separation of ions = Mass Analyzers 1) Quadrupole Mass Analyzers Important device that acts as a mass filter passing only ions of certain m/z Separation of ions = Mass Analyzers 1) Quadrupole Mass Analyzers Important device that acts as a mass filter passing only ions of certain m/z 2) Time of Flight Mass Analyzers Ions are accelerated into a field-free drift tube approximately 1 m long & separated based on kinetic energy ( 1 30 s) 3) Double-Focusing Mass Analyzer employs electrostatic analyzer & magnetic sector analyzer ICP-MS showing various sample introduction modes, ICP torch, mass analyzer & detector ICP-MS Developed early 1980's Low detection limits (0.1 to 10 ppb) High selectivity Good precision (2 - 4%) & accuracy m/z range 3 - 300 = 90% of periodic table Resolution 1 m/z Dynamic range 106 Approximately 10 sec measurement time Various sample introduction methods ICP Mass Spectrum of rock sample produced by Laser Ablation 6 Ce lines 2 Ce isotopes Comparison of ICP optical emission spectrum & ICP-MS spectrum for 100 ppm cerium (Ce) Spectroscopic Interferences 1) Isobaric interferences element isotopes with same m/z (e.g. 113In+ & 115In+ overlap with 113Cd+ & 115Sn+) 2) Polyatomic ion interferences formed from interactions of species in plasma 3) Oxide & Hydroxide species interference MO+ or MOH+ formed in plasma 4) Matrix effects similar to optical atomic spectrometry Multielement Analysis Good linearity Isotope Ratio Measurements ICP-MS makes measurement of isotope ratios for a sample become relatively easy in many cases. This is a powerful technique that can identify elements from different sources, allow use of tracers, etc. Chapter 26: An Introduction to Chromatographic Separations Column Chromatography Migration Rates Distribution Contstants Retention Times Selectivity Factor Zone Broadening & Column Efficiency Optimizing Performance Resolution ...
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This note was uploaded on 02/13/2012 for the course UML 84.314 taught by Professor Ryan during the Fall '11 term at UMass Lowell.

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