Organic Chemistry
Mass spectrometry is a technique used to gain information about a compound's structure and mass. It works by ionizing and breaking compounds into different size cations, which then pass through a magnetic field and are deflected into a detector that measures their mass-to-charge ratio. The process requires a stronger magnetic field to deflect heavier ions. A mass spectrum is the output of a mass spectrometer and displays the mass-to-charge ratio on the x-axis and relative intensity on the y-axis, providing valuable information about the functional groups in a compound. The parent ion, the largest ion detected, can be used to determine the compound's overall mass.
Lesson Outline
<ul> <li>Introduction to Mass Spectrometry</li> <ul> <li>Primary goal: Learn about the mass of a compound</li> <li>Helps uncover the identity of unknown compounds</li> <li>Useful for gaining info on isotopes</li> </ul> <li>The Mass Spectrometer</li> <ul> <li>Compound is vaporized and shot with a beam of electrons</li> <li>Creates cations of different sizes</li> <li>Electron beam breaks bonds, creating parent ion or molecular ion</li> </ul> <li>Acceleration of Ions and Detection</li> <ul> <li>Voltage accelerates ions in a straight path through a vacuum</li> <li>Cations pass through a magnetic field</li> <li>Magnetic field deflects ions towards a detector</li> <li>Detector measures mass-to-charge (m/z) ratio</li> <li>Deflection depends on the mass of the ions</li> </ul> <li>Interpreting a Mass Spectrum</li> <ul> <li>X-axis: mass-to-charge (m/z) ratio or mass</li> <li>Y-axis: relative intensity (0-100)</li> <li>Provides info on ion masses & amounts</li> <li>Helps identify functional groups present</li> </ul> <li>Example of Mass Spec Identifying Functional Groups</li> <ul> <li>Phenyl cations: C6H5 with mass of 77</li> <li>High intensity at 77 indicates possible phenyl groups</li> </ul> <li>Identifying the Parent Ion</li> <ul> <li>Largest ion detected, farthest right major peak</li> <li>m/z ratio reveals the mass of the entire compound</li> </ul> </ul>
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FAQs
A mass spectrometer works on the principle of separating ions based on their mass-to-charge ratio (m/z). A sample is first ionized, typically creating cations. These ions are then accelerated by an electric field into a magnetic field, causing ions to follow curved paths based on their m/z. Lighter ions get deflected more while heavier ions follow a straighter path. The ions are then detected by a detector, and the intensity of the detected ions is plotted against their m/z values to create a mass spectrum. This spectrum helps identify the molecular composition, isotopes, and functional groups present in the sample.
The mass-to-charge ratio (m/z) is crucial in mass spectrometry because it is the primary factor that determines the separation of ions in the mass spectrometer. The m/z value of an ion influences its path while moving through the magnetic field. Ions having different m/z values will have distinct trajectories, allowing them to be separated and eventually detected. This separation, along with the intensity of the detected ions, generates a mass spectrum that can be used to identify substances, isotopes, and functional groups present in the sample.
The detector plays a critical role in mass spectrometry as it senses and records the individual ions that have been separated based on their mass-to-charge ratios. The detector generates an electrical signal proportional to the number of ions it intercepts. The signal is then amplified and processed to create a mass spectrum, which is a graphical representation of the intensity of the detected ions plotted against their m/z values.
A parent ion, also known as the molecular ion or precursor ion, is the ion formed when a molecule in the sample loses or gains (less common) an electron during the ionization process. The parent ion essentially represents the intact, positively charged molecule and its m/z value corresponds to the molecular weight of the sample molecule. The presence and intensity of the parent ion peak in a mass spectrum can provide critical information on the molecular weight of the analyte and help in identifying the compound, studying its isotopes, and determining its functional groups.
Mass spectrometry allows the identification of isotopes and functional groups within a sample by analyzing the mass spectrum. Isotopes of an element have the same atomic number but different atomic masses, resulting in distinct m/z values. The presence of isotopic peaks in a mass spectrum, with specific mass differences and intensity ratios, can help identify the isotopes present in the sample. Functional groups, on the other hand, can be identified by analyzing the fragmentation pattern of ions in the mass spectrum. Upon fragmentation, specific ions are formed that can be associated with certain functional groups. By examining these characteristic ions and their associated m/z values, researchers can deduce the presence of specific functional groups in the sample molecule. This information can be used in conjunction with other analytical techniques to deduce the complete structure of the molecule.
