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Mass Spectroscopy Mass Spectroscopy I Applying Atomic Structure Knowledge to Chemical Analysis 1 Mass Spectroscopy Mass Spectroscopy • 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. 2 Mass Spectroscopy Mass Spectroscopy • 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. 3 Mass Spectroscopy 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 deflecting the smallest amount. • The locations where different ions hit the detector plate can be correlated to their atomic masses and charges. 4 Mass Spectroscopy Mass Spectrometer Basics (An ions deflection depends on their mass and charge, but most all have a +1 charge) ++ + + + + + + ++++++++ Ionization by electron beams Accelerating voltage applied (This must be done in vacuum so the ions can move about freely without hitting air molecules) (Ions are accelerated through a series of slits with decreasing voltages) Selection by magnetic field velocity + velocity Deflection in a magnetic field ---------------- amplifier recorder or PC 5 Detector + + + Mass Spectroscopy Schematic of a Mass Spectrometer ion-accelerating electric field positive ions accelerated ion beam least massive ions heating device to vaporize sample sample most massive ions slits electron beam magnetic field 6 Mass Spectroscopy Mass Spectrometry Competing Phenomena 1) Molecular formation by atomic collisions Carbon atom with 4 electrons in its outer orbit C O Note: Carbon has 12 units of mass C 2) Ion Formation Oxygen atom with 6 electrons in its outer orbit Note: Oxygen has 16 units of mass O Carbon Monoxide Note: Carbon Monoxide has 28 units of mass Alternate Drawing Six electrons being shared by oxygen and carbon C O (or three covalent bonds between oxygen and carbon) 7 Mass Spectroscopy Mass Spectrometry Competing Phenomena 1) Molecular Formation 2) Ion Formation 2) Ions formation by electron collisions with atoms and/or molecules C Energy + O Carbon Monoxide (collection of positive ions) C ++ O C Note: This ion has a mass of 28 units per unit of charge C O Note: This ion has a mass of 14 units per unit of charge O + + Note: This ion has a mass 16 unit per unit charge Note: This ion has a mass of 12 units per unit charge 8 Mass Spectroscopy Mass Spectrometry Carbon Monoxide sample from a vacuum process chamber O a detector that counts the ions that pass through the quadrupoles O C Ionization, acceleration and selection C ++ C C O O C C C + + O C e + O O e e e e e e Cold wire Hot wire Quadrupole rods direct select mass to charge ions down middle path depending on magnetic fields applied to pole pairs. 9 O Mass Spectroscopy 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 different ions reach the detector as the applied magnetic field is changed. ionizing filaments the quadrupoles electronics for quadrupole mass spec quadrupole mass spectrometer installed in a vacuum application 4 steel rods inside 10 Mass Spectroscopy 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 Schematic of a Typical Mass Spectra 90 92 94 96 98 100 Mass to charge ratio, m/z 11 102 Mass Spectroscopy Relative abundance, % Mass Spectra – the Data • The y-axis reflects the relative abundance of a particular ion hitting the detector. 100 • The ions hitting the detector at each location on the detector produce an electrical current. 80 60 40 • This current is proportional to the number of ions hitting the detector. 20 0 90 92 94 96 98 100 Mass to charge ratio, m/z 102 • 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. 12 Mass Spectroscopy Mass Spectra – the Analysis Relative abundance, % • 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. 100 80 60 40 20 0 90 92 94 96 98 100 Mass to charge ratio, m/z 102 • 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. 13 Mass Spectroscopy Mass Spectra – the Data Relative abundance •This is the mass spectra of molybdenum, Mo. •Each of the 7 peaks reflect a different isotope of Mo. 90 92 94 96 98 100 •If they all have +1 charge, they all have masses of 92, 94, 95, 96, 98, and 100. 102 Mass to charge ratio, m/z 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. 14 Mass Spectroscopy Relative abundance Mass Spectra – the Data • Mo is element 42 in the periodic table, and has an average atomic weight of 95.94. 90 92 94 96 98 100 Mass to charge ratio, m/z 102 • 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. 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? 98 (protons + neutrons) – 42 protons = 56 neutrons How many neutrons are in the peak with the smallest abundance? 97 (protons + neutrons) – 42 protons = 55 neutrons 15 Mass Spectroscopy Mass Spec Applications 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. 16 Mass Spectroscopy Mass Spec Applications 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 17 Mass Spectroscopy 18