Mass Spectrometry

Introduction Mass Spectrometers are instruments which can identify the type of molecules in a sample by creating ions from the specimen molecules. These ions are then accelerated via an electric field and then passed through a magnetic field, which classifies them according to their mass to charge ratio (m/z). The act of creating ions often causes the molecules to be broken into charged fragments which are characteristic of the original substance. The mass of each fragment will be displayed in the form of a spectral plot, and then the compound's mass spectrum can be used for qualitative identification. The process here is that the fragment masses of the molecules can be used to piece together the structure and the mass of the 'original molecule'. The analyzing work, therefore, is that from the 'molecular mass' and the 'mass of the fragments', reference data is compared to find out the identity of the sample. It is possible to do that because each substance's mass spectrum is unique, as long as the parent mass correctly fit the output, or visa versa.

Process Description The basic description of a mass spectrometer is that it contains a sample inlet, an ionization source, an ion accelerator, a mass focuser and a detector. More sophisticated instruments also employ some form of energy filter before the mass focuser in order to achieve more accurate mass assignments. Of course there are many variations of 'mass spectrometer' but for the lack of space here, a look at the conventional and basic mass spectrometer will be sufficient. Samples admitted to a mass spectrometer must exist in the vapour phase, so to make sure that the sample need to be analyzed will stay as a gas, the sample inlet is kept above ambient temperature, and sometimes as high as 400º C. The following steps illustrate: 1. Sample enters the ionization chamber, is heated and turns to gas 2. With a high voltage, a beam of electrons is accelerated 3. With the high voltage electrons, sample molecules are ionized and shattered (producing well-defined fragments)

4. Every fragment is then travels to the accelerator as 'an individual Particle' 5. Under the influence of the accelerating voltage, the charged particles velocities increase in the acceleration chamber 6. The ions enter the magnetic field which only allows those of particular charge to mass ratio to pass through. In order to detect different masses, so that all fragments reach the detector, the magnetic field varies. The ion collide with the detector, amplifying the original signal, which is passed to a computer for processing and analysis. The output is produced in the form of an array of peaks on a chart; this is called the 'mass spectrum'. Every one 'peak' is equivalent to the value of a fragment mass. The more fragments detected with one particular mass, the more intense the peak' will be.

Output Analysis Under certain controlled condition, each substance has a characteristic mass spectrum. That means that it is possible to identify a specimen by comparing its mass spectrum with those of known compounds. In measuring relative intensities of the mass spectra, only then can quantitative analysis be possible. A peak in the mass spectrum representing the unfragmented molecule is called the 'parent ion or molecular ion' and is the highest detected mass, representing the molecular weight of the sample under analysis. However, it is the various other peaks observed in the mass spectrum which reveal the molecule's structure. Sometimes the hardest part during mass spectrometer analysis is finding the parent peak, and thus the molecular mass of the sample. For modern mass spectrometry, computer hardware and software in both instrument control and spectral analyses play vital roles in obtaining the final results.

Najib Altawell

© Altawell 2009