instrumental analysis Flashcards
What is elemental microanalysis used to determine?
the masses of C, H, O, S and N in a sample of an organic compound in order to determine its empirical formula.
What does an empirical formula show
the simplest ratio of the elements in a molecule
What can elemental microanalydis be determined from?
combustion product masses
percentage product masses
What can mass spectrometry be used to determine?
the accurate GFM and structural features of an organic compound
What happens in mass spectrometry?
In mass spectrometry, a small sample of an organic compound is bombarded by high-energy electrons.
This removes electrons from the organic molecule generating positively charged molecular ions known as parent ions.
These molecular ions then break into smaller positively charged ion fragments.
A mass spectrum is obtained showing a plot of the relative abundance of the ions detected against the mass-to-charge (m/z) ratio.
What can the mass-to-charge ratio of the parent ion be used to determine?
the GFM of the molecular ion, and so a molecular formula can be determined using the empirical formula
What can the fragmentation data be interpreted to gain?
structural information
What is infrared spectroscopy used to identify?
certain functional groups in an organic compound
What happens when infrared radiation is absorbed by organic compounds?
bonds within the molecule vibrate (stretch and bend)
the wavelengths of infrared radiation that are absorbed depend on the type of atoms that make up the bond and the strength of the bond.
What happens in infrared spectrometry?
In infrared spectroscopy, infrared radiation is passed through a sample of the organic compound and then into a detector that measures the intensity of the transmitted radiation at different wavelengths. The absorbance of infrared radiation is measured in wavenumbers, the reciprocal of wavelength, in units of cm-1
what can proton NMR give information about?
the different chemical environments of hydrogen atoms (protons) in an organic molecule, and about how many H atoms there are in each of these environments
How does proton NMR work?
‘H nuclei behave like tiny magnets and in a strong magnetic field some align with the field (lower energy), whilst the rest align against it (higher energy). Absorption of radiation in the radio frequency region of the electromagnetic spectrum causes the ‘H nuclei to ‘flip’ from the lower to the higher energy alignment. As they fall back from the higher to the lower energy alignment the emitted radiation is detected and plotted on a spectrum.
In a proton NMR spectrum, what is the chemical shift (peak position) related to?
the environment of the H atoms and is measured in ppm
What is the area under the NMR peak related to?
the number of 1H atoms in that environment (often given by an integration curve on a spectrum)
What is the standard reference substance used in H NMR spectroscopy?
tetramethylsilane (TMS)
chemical shift value is equal to 0
How can H NMR spectra be obtained?
using low-resolution or high-resolution NMR
Difference between low and high resolution H NMR?
high uses higher radio frequencies than low and provides more detailed spectra
What can happen during high-resolution H NMR as a result of what?
An interaction with H atoms on neighbouring carbon atoms can result in the splitting of peaks into multiplets.
In H NMR, what dies the number of H atoms on neighbouring carbon atoms determine?
the number of peaks within a multiplet and can be determined using the n+1 rule, where n is the number of H atoms on the neighbouring carbon atom.
