Mass Spec Flashcards
Analysis steps of mass spec
- atomization, 2. ionization, 3. mass separation, 4. detection and processing
what happens in atomization
atoms or molecules made into a gas phase
what happens during ionization
the addition of energy to create singly charged positive ions
mass separation occurs based on what
mass to charge ratio
3 advantages of icp ms
- detection limits lower by 3 orders of magnitude 2. simple spectra 3. measurable isotopic ratios
3 disadvantages of icp ms
- expensive instrumentation 2. instrumental drift of 5-10%, 3. prone to interferences
components of a mass spec
inlet, gaseous ion source, mass analyzer, ion transducer, signal processor, mass spec (all in a vacuum)
4 types of atomic mass spec analyzers
quadrupole, double focusing ms, laser microprobe, secondary ion microbe
what allows the introduction of ions into the vacuum chamber
sampling cone, and skimmer cone
3 types of mass analyzers for icp ms
- quadrupole, 2. time of flight, 3. double focusing analyzers
mass range of a quadrupole mass analyzer
3000-4000
how does a quadrupole work
voltages of the same polarity are applied to opposing pole sets (stabilizing and destabilizing forces) and the ions are sequentially attracted and repelled allowing only certain ions to pass through
2 advantages of the time of flight mass anlyzer
- unlimited mass range, 2. lower resolution
How does a time of flight mass analyzer work
sample ions are accelerated to a uniform KE into the drift tube
what are the 2 devices for focusing the ion beam in the double focusing mass analyzer
electrostatic and magnetic
which mass analyzer has the highest resolution
double focusing
what are the the detectors of ms
electron multipliers and faraday cup
what are the two electron multipliers used as detectors for ms
discrete dynode, and continuous dynode
what is the current gain up to in the discrete dynode
107
what is the current gain up to in the continuous dynode
10^5 to 10^8
how does the faraday cup detector work
collected electrode is surrounded by a cage to prevent the escape of reflected ions and secondary electrons
what are the 2 advantages of faradays cup
- inexpensive, 2. electrically/mechanically simple
what are the three disadvantages of faraday’s cup
- needs high impedence amp 2. slower response time 3. less sensitive`
typical mass range of icp ms
3-300 amu
resolution of a typical icp ms
1 amu
dynamic range of the icp ms
10^6
mass range of the agilent 7500
5-260 amu
resolution of the agilent 7500
0.6 to 0.7 amu
dynamic range of the agilent 7500
10^9
2 general types of interferences in ms
spectroscopic and matrix
two causes of spectroscopic interferences
- isobaric 2. polyatomic ions
when is it most common for polyatomic ions to cause spectroscopic interferences
below a mass to charge ratio of 82
plasma and atomosphere are examples of what type of intereference
spectroscopic
plasma and matrix species are examples of what type of intereference
spectroscopic
how can spectroscopic interferences be corrected?
a blank or the selection of different analytical isotopic mass
when are matrix interferences noticeable
greater than 500 ppb
how can matrix interference be corrected
diluting solutions and separating out the offending species
2 general types of ion sources of ms
gas phase and desorption
what does the gas phase ionization source do
vaporize sample
what does the desorption ionization source do
convert to gas and then ionize
3 types of gas phase ion sources
electron impact, chemical ionization, and field ionization
3 types of desorption ion sources
matrix assisted desorption ionization, electrospray ionization, secondary ion mass spec
what is a hard ionization source
breaks chemical bonds to get fragmentation
what is a soft ionization source
low energy causes little fragmentation
is electron impact (EI) soft or hard
hard
is chemical ionization (ci) soft or hard
semi-hard
is matrix assisted desorption ionization (maldi) soft or hard
soft
is electrospray ionization (ei) soft or hard
soft
is secondary ion mass spec (sims) soft or hard
soft
in order to use a gas phase ionization source, the bp of the sample must be
below 500 degrees celsius
are desorption sources used for liquid or solid
both
how does the electron impact source work
bombard has phase molecule with electrons, electrons are lost, and a radical ion is formed, and fragmentation occurs
3 advantages of the electron impact source
higher currents, increased sensitivity, and fragmentation
2 disadvantages of the electron impact source
- molecules must be vaporized, and thermally stable. 2. weak or absent m+ peak
how does the ci ion source work
reagent gas is bombarded with electrons and produces ions of reagent. ions collide with analyte molecule causing ions
2 advantages of ci
- molecular ion survives, 2. simple spectra
how does madli work
the analyte is mixed with a matric, and a laser is used to irradiate the mix. the matrix absorbs laser energy. as a result, matrix molecules ionized, analyte and matrix vaporized
how does esi work
analyte solution pumped through a stainless steel capillary, and a spray of fine droplets that are charged by the electrode needle. the droplets evaporate, and the density increases. analyte with charge goes to detector
how does sims work
bombard sample surface with primary ion beam
