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; • Cl2 70 (two Cl-35 atoms), • Cl2 72 (one Cl-35 atom and one Cl-37 atom) and • Cl2 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, Cl2, 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.