Chemistry 311: Topic 3 - Mass Spectrometry

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1 Mass Spectroscopy: A technique used to measure the mass-to-charge ratio of molecules and atoms. Often characteristic ions produced by an induced unimolecular dissociation of a molecule are measured. These characteristic ions can be used to finger print a molecule for identification or identity confirmation. Essential Mass Spectroscopy Components: Source of Ions: o Electron Impact Source o Chemical Ionization Source o Atmospheric Pressure Ionization (API) o Electrospray Ionization (ESI) o Inductively Coupled Plasma (ICP) o Matrix Assisted Laser Desorption Ionization (MALDI) o Fast Atom Bombardment (FAB) Mass analyzer: o Magnetic Sector and Electrostatic Sector o Time of Flight o Quadrupole o Ion Trap o Fourier Transform Ion Detection device: Data Acquision system:

2 Information Resources: Web Links: American Society for Mass Spectrometry (ASMS) General Intro to MS: Education resources: Isotope Pattern Calculator: General Web Source of Spectroscopy

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4 Basic Physics Fundamentals: Ion Kinetic Energy (KE) = qv = zev : Where; e = 1.6 x coulombs o Fixed Energy can be transferred to ion 1 electron volt (ev) = x Joules = 96.5 KJ/mol o Ion beam accelerated has significant energy KE = ½ m v 2 = zev o Different mass ions have different velocities Magnetic Fields exert a force on a moving electrical charge that is perpendicular to the direction of motion.

5 Mass Analyzers: Magnetic Sector and Electrostatic Sector Source Magnetic Sector Detector m/z = B2 r 2 e 2V Where; m/z = mass-to-charge ratio B = magnetic field strength r = radius of curvature e = x coulombs V = Accelerating Voltage Mass Analyzers: Magnetic Sector and Electrostatic Sector

6 An Electrostatic Sector Distinguishes ions based on Kinetic Energy, when coupled with a Magnetic sector significant improvements in mass resolution can be obtained. Resolution magnetic sector 2000, for reverse geometry below resolutions of 10,000 to 100,000 obtainable.

7 Mass Resolution (R): R = m/ m Two ions are considered to be separated in the valley between them is 10% or less. To separate ions with a mass of (N 2 + ) and (CO + ); Mass N 2 + Mass CO + delta mass absolute Average Resolution Required 2500 Similarly, successful separation of these ions indicates a resolution > 2500 High Masses require much more separating power; Mass Mass delta mass absolute Average Resolution Required 4631 Mass Analyzers: Magnetic Sector and Electrostatic Sector

8 Advantages o Very high Resolution obtainable o Ions are separated in space (VIP Isotope Ratio MS) Disadvantages o Massive ( lbs) Transport, space required, etc. o Very Expensive ($500, million) o High Vacuum (<10-5 torr) and High Voltages ( ,000 V) required o Highly trained operators o Operating cost., ie electrical, cooling water, operator cost.

9 Supplementary Lecture covered: Mass analyzer: Magnetic Sector and Electrostatic Sector Time of Flight Quadrupole Ion Trap Fourier Transform Mass Spectroscopy: Gaseous ions move in a circular motion in a strong magnetic field. The angular frequency of this motion, the cyclotron frequency depends only upon the inverse of the m/z value, in a fixed field. Ions trapped in a magnetic field can absorb energy from an ac electric field if the applied frequency matches the cyclotron frequency. The absorbed energy increases the velocity of a specific m/z ion. The movement of ions induces a capacitor current that is dependant on the velocity of ion movement. This induced current can be monitored as an image current. Since the entire process is linked to the m/z of the ion, m/z specific information can be obtained. Very expensive instruments, with very high possible resolution > 10 6

10 Ionization and Ion Sources: Electron Impact Most common source of ions is the electron impact source. A gaseous vapor of analyte (M) is introduced into a small chamber, held under vacuum. Electrons are generated by emission from a heated filament and then accelerated by a potential difference, typically 70 ev. These electrons collide with the neutral analyte molecules and transfer energy to the analyte, inducing ionization and fragmentation. M M M M e - e - e - e - e - e - The magnitude of energy transferred to the analyte is usually sufficient to induce ionization and often induces additional fragmentation. Some of the fragments are also ions. The population of ions generated in this manner are sampled. M + e - 70 ev M +* + e - from M + e - (70 ev energy transferred) M +* [M-X] + + X [M-X] + X + [M-Y] + + Y

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13 Ionization and Ion Sources: Chemical Ionization Ion molecule reactions can also induce ionization in a neutral molecule. A convenient route to producing a gaseous reagent plasma is to bombard a gas with electrons to produce a steady state concentration of reagent ions. If the density of the reagent molecules is much larger than the analyte molecules, only reagent analyte interactions occur. Typically a similar source to an EI source is used. Methane is the most common reagent gas. e - e - e - e - e - e - CH 4 CH 4 CH 4 CH 4 CH 4 CH 4 CH 4 M M CH 4 CH 4 M CH 4 CH 4 M CH 4 CH 4 CH 4 CH 4 CH 4 CH 4 CH 4 CH 4 + e - 70 ev CH 4 +* + 2e - CH CH 4 CH 5 CH CH 4 C 2 H 5 CH CH 4 C 2 H 4 CH CH 4 C 2 H 5 + CH 3 + H 2 + H 2 + H 2 + H C 2 H CH 4 C 3 H H 2 Abundance is Pressure dependant,ch 5 + (48%), C 2 H 5 + (41%), C 3 H 5 + torr Chemical Ionization Source Analyte Reactions:

14 M + CH 5 + MH + + CH 4 Most common reaction RH + CH 5 + R + + CH 4 + H 2 M + CH 3 + {M + CH 3 } + Saturated hydrocarbons (RH) can lose a H Polar molecules can also form a complex MH +, R +, and {M + CH 3 } + are sometimes called psuedomolecular ions. In addition to methane, Isobutane (CH 3 ) 3 CH and ammonia NH 3 are also used as chemical ionization gasses.

15 Electron Capture Negative Chemical Ionization (ECNI): Electron Capture is not a chemical ionization process, however, standard chemical ionization conditions also produce a significant population of thermalized electrons. CH 4 + e - 70 ev CH 4 +* + e - 70-IE ev + e - thermal, etc, etc.. M + e - thermal M - * M - [M-X] - + X [M-X] + X - An analyte molecule (M) will normally only form a significant population of M - if the electron affinity (EA) is positive, or in other words the anion is more stable than the neutral species. Polyhalogenated species generally have high electron affinities, whereas, many sources of chemical noise, ie., unsubstituted hydrocarbons generally do not. The rate and efficiency of electron capture reactions can approach the collision limit, therefore, much more efficient than EI. Net effect is a very sensitive technique fg level for select analytes. Biologically important analytes, steroids, neurotransmitters, amino acids, etc. can be determined after derivation to add high EA functional group, ie., -C 6 F 5

16 Fast Atom Bombardment (FAB): A beam of energetic atoms are focused on a sample dissolved in a non volatile solvent (glycerol) and the molecular ions produced are accelerated into the mass spectrometer.

17 Matrix Assisted Laser Desorption Ionization (MALDI): An intense laser beam ( W/cm 2 ) is focused on a sample. The matrix is selected to have a very strong absorption of the laser λ. Get desorption, desolvation and charge transfer to the analyte. Proteins up to mass 300,000 Daltons have been ionized in this manner.

18 Electrospray Ionization (ESI): A solution is sprayed through a metal tube with a high voltage applied to it. Ions are ejected from the solution.

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