2. Mass Spectroscopy 1 Mass Spectroscopy (Mass Spec) Applying Atomic Structure Knowledge to Chemical Analysis

3. Mass Spectroscopy 2 Spectroscopy is the study of the interaction of electromagnetic radiation with matter. In mass spectroscopy, atoms and/or molecules are exposed to a beam of high-speed electrons. The electron beam knocks electrons off the atoms or molecules and thereby changes them into positively charged ions.

4. Mass Spectroscopy 3 If the sample is an atom, the mass spectra will reveal the different isotopes of the element. If the sample is a molecule, it is broken into several fragments, each of which becomes ionized in the electron beam. After ionization, an applied electric field accelerates the positive ions into a chamber where an applied magnetic field deflects their path. Positive ions of different masses and charges are deflected differently in the field.

5. Mass Spectroscopy 4 Mass Spectroscopy Analysis The amount of deflection in the magnetic field for each ion depends on its mass and charge. The most massive, singly charged ions are deflected the smallest amount. The locations where different ions hit the detector plate can be correlated to their atomic masses and charges.

6. Mass Spectroscopy 5 Mass Spectrometer Basics Selection by magnetic field velocity ++++++++ ---------------- + velocity amplifier recorder or PC Ionization by electron beams + + + + + + + (This must be done in vacuum so the ions can move about freely without hitting air molecules) Accelerating voltage applied + (Ions are accelerated through a series of slits with decreasing voltages) Deflection in a magnetic field Detector + + + (An ions deflection depends on their mass and charge, but most all have a +1 charge)

7. Mass Spectroscopy 6 sample slits positive ions electron beam heating device to vaporize sample accelerated ion beam magnetic field least massive ions most massive ions ion-accelerating electric field Schematic of a Mass Spectrometer

8. Mass Spectroscopy 7 Carbon atom with 4 electrons in its outer orbit Oxygen atom with 6 electrons in its outer orbit Six electrons being shared by oxygen and carbon (or three covalent bonds between oxygen and carbon) Carbon Monoxide Note: Carbon Monoxide has 28 units of mass Note: Carbon has 12 units of mass Oxygen has 16 units of mass Note: CO 1) Molecular formation by atomic collisions 2) Ion Formation Mass Spectrometry Competing Phenomena CO CO Alternate Drawing

9. Mass Spectroscopy 8 Energy (collection of positive ions) 16 unit Note: This ion has a mass of per unit charge12 units Note: This ion has a mass per unit charge Note: This ion has a mass of per unit of charge 28 units Note: This ion has a mass of per unit of charge 14 units Carbon Monoxide 2) Ions formation by electron collisions with atoms and/or molecules CO O + C + CO ++ CO + 1) Molecular Formation2) Ion Formation Mass Spectrometry Competing Phenomena

10. Mass Spectroscopy 9 CO Mass Spectrometry Ionization, acceleration and selection O + C + C O ++ C O + Carbon Monoxide sample from a vacuum process chamber e e CO CO Cold wire Hot wire e e e C O e e CO a detector that counts the ions that pass through the quadrupoles Quadrupole rods direct select mass to charge ions down middle path depending on magnetic fields applied to pole pairs.

11. Mass Spectroscopy 10 Mass Spectra – the Quadrupole A quadrupole (4 rods) is one configuration used for deflecting ions to separate them by mass. A magnetic field is created by the 4 rods inside the steel tube and can be adjusted to cause different ions to reach the detector as the applied magnetic field is changed. 4 steel rods inside ionizing filaments quadrupole mass spectrometer installed in a vacuum application electronics for quadrupole mass spec

12. Mass Spectroscopy 11 Mass Spectra – the Data A mass spectra is sometimes referred to as a “stick diagram” which shows the relative amounts of the different ions as a function of their mass, expressed as the ratio of mass to charge, m/z. Most of the ions formed in the electric field have a +1 charge. Relative abundance Mass to charge ratio, m/z 90 92 94 96 98 100 102 Schematic of a Typical Mass Spectra

13. Mass Spectroscopy 12 Mass Spectra – the Data The y-axis reflects the relative abundance of a particular ion hitting the detector. The ions hitting the detector at each location on the detector produce an electrical current. This current is proportional to the number of ions hitting the detector. The more ions of a given size that reach the detector, the larger the signal for that ion. In this application, the detector acts something like a counter. Relative abundance, % Mass to charge ratio, m/z 90 92 94 96 98 100 102 100 80 60 40 20 0

14. Mass Spectroscopy 13 Mass Spectra – the Analysis The x-axis reflects the mass (m) of each ion as a ratio to its charge (z). Most ions formed in mass spectrometers have a +1 charge. For samples that are atoms, the different mass to charge ratios reflect different isotopes. Isotopes of an element have different numbers of neutrons but the same number of protons. For samples that are molecules, the ion with the highest m/z ratio is called the “parent” ion. This ion is the original molecule with one less electron, and thus has the same mass weight as the original species. Relative abundance, % Mass to charge ratio, m/z 90 92 94 96 98 100 102 100 80 60 40 20 0

15. Mass Spectroscopy 14 Mass Spectra – the Data This is the mass spectra of molybdenum, Mo. Each of the 7 peaks reflect a different isotope of Mo. If they all have +1 charge, they all have masses of 92, 94, 95, 96, 98, and 100. Relative abundance Mass to charge ratio, m/z 90 92 94 96 98 100 102 Using the spectra, which isotope of Mo is the most abundant in this particular sample? The tallest “stick” is the one at m/z = 98. This isotope of Mo is the most abundant in the sample analyzed.

16. Mass Spectroscopy 15 Mass Spectra – the Data Mo is element 42 in the periodic table, and has an average atomic weight of 95.94. If you read the relative abundance of each isotope’s peak, you could calculate the average atomic weight from this spectra and compare the value you get to the published value of 95.94. Relative abundance Mass to charge ratio, m/z 90 92 94 96 98 100 102 Knowing the atomic number of molybdenum is 42, how may neutrons are in the Mo isotope represented by the peak that is most abundant in the sample? How many neutrons are in the peak with the smallest abundance? 98 (protons + neutrons) – 42 protons = 56 neutrons 97 (protons + neutrons) – 42 protons = 55 neutrons

17. Mass Spectroscopy 16 Isotopic masses are used: to determine average atomic mass of elements. to identify a compound’s composition and structure. for archaeological dating. to identify particulates in space (when mounted on a satellite or the space station). to assure safe environments in nuclear powered vessels. to monitor process conditions when fabricating computer microchips. Mass Spec Applications

18. Mass Spectroscopy 17 Many disciplines use mass spectroscopy for chemical identification. Astronomy: analysis of astronomical components of the solar system Electronics: analysis of microchips Environmental: detection of toxic chemical, monitoring of nuclear facilities, analysis of petroleum products, etc. Forensics: toxicology, trace metals, biological materials, etc. Medical: drug abuse diagnosis, analysis of pharmaceuticals and products of genetic engineering Military: mobile mass spectrometers are used to detect liquid chemical warfare agents Mass Spec Applications

19. Mass Spectroscopy 18