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Unformatted text preview: CHEM*3440 Chemical Instrumentation Topic 12 Mass Spectrometry Fundamental Scheme In general there are four steps associated with a mass spectroscopic experiment: ! generate gas-phase molecules from analyte (solid, liquid, solution, etc.) " ionize those molecules # separate the ions based on mass $ detect the ion beam Numerous experiments are distinguished from one another by the way they handle stage 1 (LC-MS, GC-MS, SIMS, MALDI-TOF, etc.) Different mass spec instruments are distinguished from each other by the way the accomplish both stage 2 and stage 3. Fewer options for stage 4 exist and this is common to most instruments. Source - Sample Injection The mass spectrometer needs gaseous ions to analyze. No problem if sample is already gaseous or if experiment occurring in vacuum chamber. Heated injection port most common way of dealing with liquids and solutions. Normal boiling point < 500 C. Maximum port temperature ~ 350 C. Pressure ~ 10-5 Torr. Source - Ionization Once we have produced gas phase molecules, we can start to look at the ionization process. With some techniques, ionization occurs in conjunction with the vapourization step (we ! ll discuss them later). If it does not, then we need to create the ions. Electron Impact ionization (EI) Chemical ionization (CI) Thermal ionization (TI) Electron Impact Ionization Neutral gas phase molecules drift into ionization region. Energetic electron beam (50 - 100 eV) intersects molecules. Collisions create cations. Field extracts ions from ionizer and focuses and accelerates (500 - 1000 eV) ion beam towards mass analyzer. EI Fragmentation It only takes about 5 to 15 eV of energy to ionize the molecule. The rest of the electron energy is available to induce molecular bond rupture. Fragmentation with ET is extensive. This is useful since by studying the fragmentation pattern, one can discern the likely structural components of the original molecule and develop a model for its structure. Fragmentation with EI is so extensive, however, that the parent ion the ion arising from the original molecule is often not detectable. EI really blows apart the target molecules. Chemical Ionization Chemical Ionization (CI) is a means of softening the fragmentation effect of EI. A buffer gas, often methane CH 4 , is added in a large excess (~x100 fold) over the analyte. The electron beam ionizes the methane, gas phase reactions produce molecular ions such as CH 5 + which collide with analyte molecules (M) to gently produce the analyte ions (MH + ). Can perform CI in same unit with EI except for much higher pressure. Must provide additional pumping speed to move extra buffer gas out of ionizer so as to not degrade vacuum in analyzer....
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