{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

CHEM3440Lec12F06 - Fundamental Scheme In general there are...

Info icon This preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
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)
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
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. Ionization process much gentler and it tends to produce ions with an extra proton mass.
Image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern