Components of Mass Spectrometer Sample inlet : bring the sample to the lower pressure of MS. Ion source : the sample molecules are transformed into gas phase ion which are accelerated by the electromagnetic field. Mass analyzer : sepatate the sample ions based on their m/z ratio. Detector : count the ions. Data system : the signal is recorded by PC (mass spectrum)

Sample Introduction Gas sample : use of a simple inlet system (larger reservoir) Less-volatile materials : the system can be designed to fit within an oven which can heat the sample to increase the vapor pressure Nonvolatile samples : use of direct probe method. The sample is placed on a thin wire loop or pin on the tip of the probe, which is then inserted through a vacuum lock into the chamber and positioned close to the ion source. Gas chromatography – mass spectrometry (GC-MS) High-performance liquid chromatography – mass spectrometry (LC-MS)

Electron Ionization (EI)

Mass Analysis 1. The magnetic sector mass analyzer

The greater the value of m/z, the larger the radius of the curved path. The analyzer tube of the instrument is constructed to have a fixed radius of curvature. Thus, The magnetic field strength is continuously varied.

2. Double-focusing mass analyzers The particles leaving the ionization chamber do not all have precisely the same velocity, so the beam of ions passes through an electric field region before or after the magnetic sector. In the presence of an electric field, the particles all travel at the same velocity.

3. Quadrupole mass analyzers A set of four solid rods is arranged parallel to the direction of the ion beam. A direct-current (DC) votage and a RF is applied to the rods, generating an oscillating electrostatic field in the region between the rods. Ions of the correct m/z ratio undergo a stable oscillation of constant amplitude.

Detection and Quantitation : The Mass Spectrum

Base peak : The most abundant ion peak Molecular ion (M+) : largest m/z value Isotope peak also appear (M+1, M+2)

Metastable ion peak Ions with lifetimes on the order of 10-6 may disintegrate into fragments while they are passing into the analyzer region of the mass spectrometer. The fragment ion does not follow an abnormal flight path on its way to the detector. This ion appears at an m/z ratio that depends on its own mass as well as the mass of the original ion from which it formed. M1 is the mass of the original ion and m2 is the mass of new one

Determination of Molecular Weight

Determination of Molecular Weight

Determination of Molecular Weight

Determination of Molecular Formulas 1. Precise mass determination High-resolution mass spectrometer can be used to determine the precise molecular weights of substances with four or five decimal places of an ion’s m/z value and also allows one to know the exact molecular formula.

2. Isotope ratio data Another method of determining molecular formulas is to examine the relative intensities of the peaks

Example (mass 42) C3H6 and CH2N2 For propene M+1 peak : (3 x 1.08) + (6 x 0.016) = 3.34% M+2 peak : 0.05% For diazomethane M+1 peak : 1.08 + (2 x 0.016) + (2 x 0.36) = 1.87% M+2 peak : 0.006%

3. Calculation of halogen isotope For Cl2 : M 35Cl x 35Cl = 1 x 1 = 1 M+2 (35Cl x 37Cl) + (37Cl x 35Cl) = (1 x 0.33) + (0.33 x 1) = 0.66 M+4 37Cl x 37Cl = 0.33 x 0.33 = 0.11 For Br2 : M 79Br x 79Br = 1 x 1 = 1 M+2 (79Br x 81Br) + (81Br x 79Br) = (1 x 0.98) + (0.98 x 1) =1.96 M+4 81Br x 81Br = 0.98 x 0.98 = 0.96 For BrCl : M 79Br x 35Cl = 1 x 1 = 1 M+2 (79Br x 37Cl) + (81Br x 35Cl) = (1 x 0.33) + (0.98 x 1) =1.31 M+4 81Br x 37Cl = 0.98 x 0.33 = 0.32

Structural Analysis and Fragmentation Patterns 1. Stevenson’s rule The most probable fragmentation is the one that leaves the positive charge on the fragment with the lowest ionization energy. Fragmentation processes that lead to the formation of more stable ions are favored over processes that lead to less-stable ions.

Alkanes

Alkenes

2. The initial ionization event It is easier to remove an electron from a nonbonding orbital n than it is to strip an electron from a p orbital. Similarly, it is much easier to eject an electron from a p orbital in comparison to a s orbital.

3. Radical-site-initiated cleavage : a-Cleavage

4. Charge-site-initiated cleavage : Inductive cleavage Attraction of an electron pair by an electronegative heteroatom that ends up as a closed-shell neutral molecule.

5. Two-bond cleavage Elimination occurs and the odd-electron molecular ion yields an OE+ and an even-electron neutral fragment N : H2O, HX, alkene

6. Retro Diels-Alder cleavage

7. McLafferty rearrangements A hydrogen atom on a carbon 3 atom away from the radical cation of an alkene, arene, carbonyl, or imine is transferred to the charge site via a six-membered transition state, with concurrent cleavage of the sigma bond between the a and b position of the tether.

Aromatic Hydrocarbons