The Atomic Mass

Mass spectrometer

A mass spectrometer is an instrument used to determine the relative atomic mass of an element. It has the ability to determine the relative molecular mass of a compound and the formula of a compound in a spectrographic plate.

A mass spectrograph is a plate used in mass spectrometers to detect and record the relative atomic mass of an element. The spectrographic plate is a detector or recorder. The following is a description of the different parts of a mass spectrometer:

Vaporization

The instrument is evacuated to prevent interference from air. The solid sample is heated until it forms gaseous atoms, which are then vaporized before being introduced into the mass spectrometer.

Ionization

The vaporized sample of an element is introduced into the ionization chamber. This region contains an electric filament that emits fast-moving electrons. These electrons collide with the atoms of the element, causing the atoms to split and leaving only the positively charged nuclei, where the mass of the atom is concentrated.

Acceleration

The ionized particles formed in the ionization chamber are accelerated by two negatively charged plates towards the magnetic field. These two plates are connected to a negative potential, which accelerates the ions as a beam towards the magnetic field.

Deflection

The ion beam is deflected in the magnetic field. The extent of deflection depends on the mass-to-charge ratio ($m/e$). Lighter particles are deflected more than heavier particles. The deflected ions strike the detector, and ions with the same mass-to-charge ratio strike the same mass spot.

Detection

Once the ions strike the detector, the collector converts the intensity of the ions into electric signals. These signals are amplified by the amplifier into a large electric current.

Recording

The electric current operates a pen that moves on paper, tracing the peak of the isotope. The ions are recorded in terms of atomic mass and relative abundance. When the ions fall on the photographic plate, they produce a mass spectrum consisting of a series of lines at different points. The mass of the detected ions is displayed, and the relative abundance is the percentage of an isotopic atom in the element. The ions produce a mass spectrum, recorded as peaks.

Example of mass spectrum

Here, the mass spectrum displays the detected ions as peaks, showing the different mass-to-charge ratios and relative abundances of the isotopes.

Calculation of relative atomic mass

The relative atomic mass of an element is the average value of all the known isotope atomic weights relative to their proportional abundances. The relative atomic mass is more useful in chemistry than the simple atomic mass of an element because it accounts for the existence of isotopic forms of elements. The relative abundance of an isotopic element is applied to determine its relative atomic mass.

For example, if an isotopic element X contains $Y\%$ of $^{a}_{z}X$ and $W\%$ of $^{b}_{z}X$, the relative atomic mass of element X can be determined as follows:

1. Since $Y + W = 100$
2. The length of peaks for each isotopic atom is applied to find the relative atomic mass.
3. Each atom has a peak due to its different mass-to-charge ratio ($m/e$).