mass spec quiz 4

general idea of mass spec

blow apart molecules, look at the pieces, and identify them by piecing them back together in data analysis

flow of mass spec

1. create gas ions

2. separate ions based on mass-to-charge ratio

qualitative analysis

moelcular ion M+ just the moelcule but missing an electron, gives the molecular weight

fragmentation pattern

quantitative analysis

intensity of the peaks/area of the peaks is proportional to concentration

atomic mass spectrometry

• record the frequency of an atom ionization by the mass-to-charge ratio of the ions formed

mass selector

need to separate our ions so that the detector can detect ions one type at a time

general principles of mass spec

1. analyte molecules are atomized, vaporized, and ionized

2. ions are accelerated into the mass separator/analyzer with potential difference between metal plates

3. ions are separated by m/z

4. separated ions are detected

magnetic sectors

• based on the interactions of charged particles with magentic fields

• charges in a magnetic field circulate

• can scan potential or magnetic field strength to get different m/z at certain r

• usually scan with varying current

double focusing

• uses one slit to select for velocity and kinetic energy

• exit slit, one m/z on detector

• detector with spatial information

• makes resolution overall better than one magnetic sector alone

performance characteristics of magnetic sector

• resolution: r < 2000 for singe and double focusing <100,000 it is independent of mass

• mass range is typically 10 - 10,000 so good for synthetic products, metabolites, and peptides

Time of flight mass analyzer

accelerate ions through a potential and measure how long it takes ions to get to the end of the flight tube. The heavier the ions, the harder to accelerate.

performance characteristics of time of flight

resolution: 5,000 for linear and 20,000 with reflectron

m/z up to 40,000 but some with MALDI up to 500,000

low sensitivity since there are many ions in flight

reflectron

an electric field that reverses the direction of flying ions in the mass analyzer section so that they head towards the detector, compensates for different kinetic energies of the ions

quadrupole

4 poles, no magnets, all based on electric field. A direct current and radio frequency field is applied to let one m/z through at a time (other ions will neutralize). Can scan the applied voltage to choose which m/z reaches the detector

positive voltage in quadrupole

vertical rods form high pass filter and low m/z ejected.

negative voltage in quadruploe

horizontal rods for low pass filter and high m/z are neutralized

performance characteristics of a quadrupole

resolution: 1,000 lower than others for speed

mass range: up to 4,000

other: compact, rugged, inexpensive

ionization sources

vaporize and ionize molecules so that ions do not react and happens in vaccuum so that ions do not react with each other

electron ionization

1. electron from filament approaches molecule M and distorts electron cloud

2. electron passes by and then molecules electron cloud ejects and electron

   analyte + e- → analyte + 2e-

Hard source

chemical ionization

1. electrons ionize reagent gas, then react the reagent gas with analyte to create ions

reagent gas most comonly is methane

2. EI of reagent gas to form ions

3. reaction of reagent gas to produce adducts

4. reaction of reagent adducts with analyte molecules

has phase soft source

MALDI: Matrix-assisted laser desorption/ionization

desorption relies on energy introduced to the solid or liquid sample to directly form gaseous ions

1. low concentration of analyte is mixed in a matrix on a metal plate

2. laser focused on plate

3. matrix strongly absorbs the laser

4. matrix and analyzer are desorbed and ionized into ion plume