TOPIC D: SPECTROMETRY AND SPECTROSCOPY

Mass spectrometry is used to detect isotopes. mass spectrometer uses an ionizing beam of electrons to analyze a sample of an element by turning atoms into ions. The individual ions are then sorted by mass. Since a sample of a single element can contain different isotopes with different masses. a number of distinct ions of different masses can be identified within the spectrum. data can lead to the identification of different isotopes and the calculation of average atomic masses.

Analysis of mass spectra A typical mass spectrum for chlorine is shown below: Like PES, Y-axis: relative intensity (indicates abundance of each isotope) X-axis: mass/charge ratio (equivalent to the mass of each isotope)

Chlorine has two isotopes, with masses of 35 and 37, in a 3:1 ratio (relative peak height). When chlorine atoms pair together in chlorine molecules, there are three possibilities for their mass; Cl 2 70 (two Cl-35 atoms), Cl 2 72 (one Cl-35 atom and one Cl-37 atom) and Cl 2 74 (two Cl-37 atoms). Since Cl-35 is three times more prevalent, molecules with masses that are made from Cl-35 atoms are more abundant compared to Cl-37 atoms.

Task 2D 1. Naturally occurring chlorine molecules, Cl 2, have masses of 70, 72 and 74 amu as seen in the mass spectrum above. They occur in the percentages 56.25%, 37.50% and 6.250% respectively. Use this data to calculate the average atomic mass of chlorine atoms and to find the relative abundance of 35-Cl and 37-Cl isotopes. 2. Sketch the mass spectrum that you might expect to observe if bromine were passed through a mass spectrometer. The two common isotopes of bromine are Br-79 and Br-81 that are known to exist in an approx. 1:1 ratio and like chlorine, bromine is known to form diatomic molecules.

3. The mass spectrum for titanium produces peaks according to the following data. Use the data to calculate the relative atomic mass of Ti. 4. A typical mass spectrum for Mg contains three peaks at m/z values of 24, 25 and 26 respectively. (a) What does the existence of three peaks suggest? (b) The relative intensities of the three peaks (24, 25 and 26) are found to be 63, 8.1 and 9.1 respectively. (i) What do these data tells us about the isotope with m/z = 26? (ii) Calculate the relative atomic mass of magnesium.

Spectroscopy and the Beer-Lambert Law Spectroscopy - the study of the interaction of electromagnetic radiation and matter. Absorption spectroscopy methods involve a sample being exposed to photons of varying energy, and then measuring the extent to which the sample absorbs or reflects that energy. Depending on the magnitude of energy (E) used (which depends upon the frequency (ν) of the photons according to E = hν), we can collect data about substances.

infrared (IR) spectroscopy When covalent bonds are exposed to infrared radiation they absorb that energy and tend to bend, stretch and vibrate. The interaction with the IR is unique for each type of bond, so IR spectroscopy can be used to distinguish between compounds that have different types of covalent bond. Ultraviolet (UV) and visible spectroscopy (PES) As we saw with PES, UV and visible light tends to cause electronic transitions within atoms, so can be used to gather information about electronic configurations. Beer-Lambert Law The Beer-Lambert law is used to relate the concentrations of colored solutions to the amount of visible light they absorb.

Beer-Lambert Law The amount of absorbance is calculated using the formula A = a b c Where, A = absorbance, a = molar absorptivity (a constant that depends on the material tested) b = path length (the length of the sample that the light passes through) c = concentration. When absorbance measurements are made at a (1) fixed wavelength, (2) in a cell of constant path length, both a, and b are constant, and the absorbance, A, will be directly proportional to c. If a solution of a compound obeys the Beer-Lambert law, a plot of absorbance (y-axis) versus concentration (x-axis) gives a straight line with a slope of ab. The y-intercept is zero (the line will pass through the origin of the graph). One can use the graph to read corresponding concentrations and absorption values.