Mass Spectrometry Flashcards
what is an isotope?
atoms that differ by the number of neutrons.
(hydrogen, deuterium, tritium - carbon 12 and carbon 13)
what is the difference between nominal mass, atomic mass, molecular mass?
Nominal Mass: atoms - the integer mass of the most abundant isotope
molecules - integer mass of a species using the most abundant isotope of each constituent.
Atomic Mass: weighted average of the masses of the isotopes of an element.
Molecular Mass: sun of atomic masses listed on the periodic table.
what are the three inlets and their advantages and disadvantages?
Direct Infusion: Advantages - smaple enter directly in the MS, faster and simple - no concern of sample loss.
Disadvantages - spectra can be complicated may have interferences.
Chromatography (LC or GC): Advantages - LC or GC eluent goes from the column to 1 or more detectors. simpler spectra
Disadvantages: more complicated - method development required.
in general, how does electron ionization work?
electrons emitted from the hot filament are accelerated through 70 V before interacting with the analyte molecules.
molecular ions can:
lose energy with collisions with other molecules and stay intact
Retain enough energy that they break into fragments
what is the molecular ion? how does it differ from other ions like Na+?
molecular ion: has an electron added or removed (outer shell is no longer closed)
differs from ions like Na+ because it loses an electron to have closed shells (stable)
why does the molecular ion sometimes fragment?
molecular ion has so much energy / unstable it can fragment
- may result in no molecular ion peak seen in spectra.
what are fragment or daughter ions? why do they form?
fragmentation occurs in a very predictable manner
charged fragments are called “daughter ions”
what does the precursor fragment into?
dissociative result: charged and neutral fragments.
non-dissociative result: ionized parent molecule.
in general terms, how does chemical ionization work?
produces less fragmentation than electron ionization. (softer & gentler)
ionization chamber is filled with a reagent gas (eg methane) at 1 mbar pressure.
energetic electrons convert CH4 into a variety of reactive products.
in general terms, how does electrospray work?
gentle minimal fragmentation
molecule often picks up protons and salts from LC mobile phase
what does a mass analyzer do?
High resolution: can determine m/z to many decimal places (allows differentiation between moieties with same nominal mass.
Time of Flight: widely used to study large molecules such as proteins (more resolution and separation)
High Throughput: used when you have many samples to process (used when you know what you are looking for)
Quadruple: can select ions of a specific m/z to pass through to the detector.
How does a quadrupole sort ions?
atoms and molecules are ionized by the source.
ions with a specific mass-to-charge ratio are filtered by applying a combination of DC and RF voltages to each quadrupole.
