39 Use of Isotopes (38/50) -- Introductory Chemistry

39 Use of Isotopes LumenLearning Structural Determination Structural determination using isotopes is often performed using nuclear magnetic resonance spectroscopy and mass spectrometry. LEARNING OBJECTIVES Identify the uses of isotopic labeling in structural determination and the primary techniques used to study isotopically-labeled molecules KEY TAKEAWAYS Key Points - Nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS) detect isotopic differences; this allows the position of labeled atoms in a product ‘s structure to be determined. - Isotopic labeling is a technique used to track the passage of an isotope through a reaction, metabolic pathway, or cell. - With information about the position of isotopic atoms in products, the reaction pathway can also be determined. Key Terms - mass spectrometry: An analytical technique that measures the mass:charge ratio of the ions formed when a molecule or atom is ionized, vaporized, and introduced to a vacuum; may also involve breaking molecules into fragments, enabling structure to be determined. - nuclear magnetic resonance: The absorption of electromagnetic radiation (radio waves), at a specific frequency, by an atomic nucleus placed in a strong magnetic field; used in spectroscopy and in magnetic resonance imaging. - isotope: Any of two or more forms of an element where the atoms have the same number of protons but a different number of neutrons; the forms have the same atomic number but a different mass number. Structural determination utilizing isotopes is often performed using two analytical techniques: nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS). Nuclear Magnetic Resonance Spectroscopy NMR analysis is isotope-dependent, and it often relies on trace isotopes of a molecule for detection. For example, the most abundant isotope of carbon, C-12, is invisible to NMR, whereas the minor isotope C-13 is NMR active, but only comprises 1.1 percent of a given sample of carbon. By replacing C-12 in a molecule with C-13, NMR analysis of that position is greatly enhanced. Similarly, H-1 is an NMR active nucleus, whereas H-2 is NMR invisible, so it is possible to determine where a specific hydrogen atom is by replacing it with H-2 and watching for the disappearance of the corresponding signal. Mass Spectrometry Mass spectrometry is a technique for determining the molecular weight of an ionized molecule and fragments of the molecule that appear when the molecule is ionized. The addition of an isotope will change the observed mass of the parent ion—the molecule that is ionized and does not fragment. Adding an isotope will also change the observed mass of any fragment which contains the isotope. If a fragment does not contain the isotope, then its mass will not be changed. This can reveal information about the position of an isotope in a molecule. Isotopic Labeling Isotopic labeling is used to track the passage of an isotope through a reaction, metabolic pathway, or cell.