Mass spectroscopy

In a typical MS procedure:  1- a sample is loaded onto the MS instrument, and undergoes vaporization.  2- the components of the sample are ionized by one of a variety of methods (e.g., by impacting them with an electron beam), which results in the formation of positively charged particles (ions)electron beamions  3- The positive ions are then accelerated by a magnetic field  4- computation of the mass-to-charge ratio of the particles based on the details of motion of the ions as they transit through electromagnetic fields, andmass-to-charge ratioelectromagnetic  5- detection of the ions, which in step 4 were sorted according to m/z.

A full diagram of a mass spectrometer The need for a vacuum  It's important that the ions produced in the ionization chamber have a free run through the machine without hitting air molecules.

Understanding what's going on Ionisation The vaporized sample passes into the ionization chamber. The electrically heated metal coil gives off electrons which are attracted to the electron trap which is a positively charged plate. The particles in the sample (atoms or molecules) are therefore bombarded with a stream of electrons, and some of the collisions are energetic enough to knock one or more electrons out of the sample particles to make positive ions. Most of the positive ions formed will carry a charge of +1 because it is much more difficult to remove further electrons from an already positive ion. These positive ions are persuaded out into the rest of the machine by the ion repeller which is another metal plate carrying a slight positive charge M + e -  M + + 2e -

 Deflection  Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on:  the mass of the ion. Lighter ions are deflected more than heavier ones.  the charge on the ion. Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge.  These two factors are combined into the mass/charge ratio. Mass/charge ratio is given the symbol m/z (or sometimes m/e).  For example, if an ion had a mass of 28 and a charge of 1+, its mass/charge ratio would be 28. An ion with a mass of 56 and a charge of 2+ would also have a mass/charge ratio of 28.

In the diagram, ion stream A is most deflected - it will contain ions with the smallest mass/charge ratio. Ion stream C is the least deflected - it contains ions with the greatest mass/charge ratio. Assuming 1+ ions, stream A has the lightest ions, stream B the next lightest and stream C the heaviest. Lighter ions are going to be more deflected than heavy ones.

Detection  Only ion stream B makes it right through the machine to the ion detector. The other ions collide with the walls where they will pick up electrons and be neutralised. Eventually, they get removed from the mass spectrometer by the vacuum pump.  Detecting the other ions  Remember that stream A was most deflected - it has the smallest value of m/z (the lightest ions if the charge is 1+). To bring them on to the detector, you would need to deflect them less - by using a smaller magnetic field.  To bring those with a larger m/z value (the heavier ions if the charge is +1) on to the detector you would have to deflect them more by using a larger magnetic field.  If you vary the magnetic field, you can bring each ion stream in turn on to the detector to produce a current which is proportional to the number of ions arriving.

 The most intense peak is called the “Base Peak”, which is arbitrarily set to 100% abundance; all other peaks are reported as percentages of abundance relative to the “Base Peak”  The Molecular Ion peak has the highest M/z in the spectrum, i.e., the last peak on the right side of the spectrum  The Molecular Ion Peak(s) abundance, i.e., peak height, can be quite small.  The Molecular Ion peak represents the molecular weight of the compound.  Molecular Ion peak is usually not the base peak!  The Integral Molecular Weights reflect the naturally occurring isotopic mixture of the compound.

Molecular Ion Peak (88) Base Peak

The Presence of Nitrogen in the Compound If the Mass / Charge (m/z) ratio for the Molecular Ion peak is “Odd”, then the molecule contains an Odd number of Nitrogen atoms, i.e., 1, 3, 5, etc. Note: An “Even” value for the Mass / Charge ratio could represent a compound with an even number of Nitrogen atoms, i.e., 0, 2, 4 etc., but the actual presence of Nitrogen in the compound is not explicitly indicated as it is with an “Odd” value for the ratio.

Complications may occur because of the possibility of different isotopes (either of carbon or of chlorine or bromine) in the molecular ion. Most elements exist in several isotopic forms: Ex. 1 H 1, 2 H 1, 12 C 6, 13 C 6, 35 Cl 17, 37 Cl 17, 79 Br 35, 81 Br 35 “Average Molecular Weight” The average molecular weight of “All” isotopes of a given element relative to the abundance of the each isotope in nature. “Integral Molecular Weight” The Number of Protons and Neutrons in a specific isotope Each fragment represented in a Mass Spectrum produces several peaks each representing a particular isotopic mixture of the elements in the compound.

The carbon-13 isotope  If you had a complete (rather than a simplified) mass spectrum, you will find a small line 1 m/z unit to the right of the main molecular ion peak. This small peak is called the M+1 peak.  The M+1 peak is caused by the presence of the 13 C isotope in the molecule. 13 C is a stable isotope of carbon and makes up 1.11% of all carbon atoms.

The Presence of Chlorine in a Compound  The two (2) principal Chlorine Isotopes in nature are Cl-35 and Cl-37 (2 additional Neutrons in Cl- 37)  The relative abundance ratio of Cl-35 to Cl-37 is 100 : 32.6 or 75.8 : 24.2 or  3 : 1  Therefore, a Molecule containing a single Chlorine atom will show two Mass Spectrum Molecular Ion peaks, one for Cl-35 (M + ) and one for Cl-37 (M + +2) Note: M + +2 denotes 2 more neutrons than M +  Based on the natural abundance ratio of 100 / 32.6 (about 3:1), the relative intensity (peak height) of the Cl-35 peak will be 3 times the intensity of the Cl-37 peak.

The Presence of Bromine in a Compound  The two (2) principal Bromine Isotopes in nature are Br-79 and Br-81 (2 additional Neutrons in Br-81)  The relative abundance ratio of Br-79 to Br-81 is 100 : 97.1 or 50.5 : 49.5 or  1 : 1  Molecules containing a single Bromine atom will also show two molecular ion peaks one for Br- 79 (M + ) and one for Br-81 M +2 ).  Based on the natural abundance ratio of 100 / 97.1 (about 1:1), the relative intensity of the Br-79 peak will be about the same as the Br-81 peak.

Mass Spectrometry Summary  Fragmentation of Organic Molecules by high energy electrons  Base Peak – 100 % abundance  Molecular Ion Peak – Highest Mass/Charge ratio  Molecular Ion Peak – Last peak(s) on right side of chart  Molecular Ion Peak – Represents Molecular Weight of compound  Molecular Ion Peak – If value is “Odd” the compound contains an odd number of “Nitrogen” atoms  Molecular Ion Peak – If two peaks occur and the relative abundance ratio is 3:1, then the compound contains a single Chlorine atom.  Molecular Ion Peak – If two peaks occur and the relative abundance ration is 1:1, then the compound contains a single Bromine Atom.