analytical chem

• Infra-Red Spectroscopy is a technique used to identify compounds based on changes in vibrations of atoms when they absorb IR of certain frequencies.
• A spectrophotometer irradiates the sample with electromagnetic waves in the infrared region and then detects the intensity of the wavelength of IR radiation which goes through the sample.
• All organic molecules absorb IR radiation and depending on which energies of radiation are absorbed, bonds between atoms will vibrate by stretching, bending and twisting.
• The molecules will only vibrate at a specific frequency, which is known as the resonance frequency.
• The resonance frequency is the specific frequency at which the molecules will vibrate to stimulate larger vibrations.
• For each vibration, different wavelengths of IR radiation are absorbed, which are also known as the wavenumber (cm).
• Absorption range of bonds table -1
• Due to some absorption bands overlapping each other, other analytical techniques such as mass spectroscopy should be used alongside IR spectroscopy to identify an unknown compound.
• There will also be a line for the unfragmented Br ion, which will give 3 molecular ion peaks: Br ion containing the isotopes 79 + 79 = 158, Br containing the isotopes 79 + 81 = 160, and Br containing the isotopes 81 + 81 = 162.
• The mass spectrum of compounds containing one bromine atom will have a peak at 79, 79, and 81.
• The ratio of the peak heights is 1:2:1.
• Some of the bromine molecules will fragment to create Br+Br, resulting in three molecular ion peaks: Br+, Br=, and [M+2].
• When bromine is passed through the mass spectrometer, an electron is given off to create the molecular ion, Br2+.
• Bromine is a diatomic molecule, so there will be 5 peaks on the mass spectrum of bromine.
• The Br atom passes through the machine, and the Br ions will give lines at 79 and 81.
• A compound containing two bromine atoms will have three molecular ion peaks: Br+, Br=, and [M+2].
• The peak with the highest m/e value is the molecular ion (M) peak which gives information about the molecular mass of the compound.
• The height of the [M+1] peak for a particular ion depends on how many carbon atoms are present in that molecule; the more carbon atoms, the larger the [M+1] peak is.
• The relative atomic mass of an element can be calculated by using the relative abundance values.
• The [M+1] peak is a smaller peak which is due to the natural abundance of the isotope carbon-13.
• The relative abundance of an isotope is either given or can be read off the mass spectrum.
• The molecular ion is the entire molecule that has lost one electron when bombarded with a beam of electrons.
• Each peak in the mass spectrum corresponds to a certain fragment with a particular m/e value.
• For example, the height of the [M+1] peak for an hexane (containing six carbon atoms) ion will be greater than the height of the [M+1] peak of an ethane (containing two carbon atoms) ion.
• Atoms of the same elements can have different mass numbers, so the mass of an element is given as relative atomic mass (A) by using the average mass of the isotopes.
• In an IR spectrum, the presence of a strong, sharp absorption around 1710 cm corresponds to the characteristic C=O, carbonyl, group in a ketone.
• In an IR spectrum, the presence of a strong, broad absorption around 3200 − 3500 cm suggests that there is an alcohol group present, which corresponds to the -OH group in propan-2 − ol.
• Mass spectrometry is an analytical technique used to identify unknown compounds.
• The peak heights show the relative abundance of the boron isotopes: boron-10 has a relative abundance of 19.9% and boron-11 has a relative abundance of 80.1%.
• The base peak is the peak corresponding to the most abundant ion.
• In mass spectroscopy, these fragmentation ions are accelerated by an electric field based on their mass (m) to charge (e) ratio, and then separated by deflecting them into the detector.
• The relative abundance of an isotope is the proportion of one particular isotope in a mixture of isotopes found in nature.
• Mass spectroscopy can be used to find the relative abundance of the isotopes experimentally.
• An ion with mass 16 and charge 2+ will have a m/e value of 8.
• Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
• The molecular ion can further fragment to form new ions, molecules, and radicals.
• In mass spectrometry, the molecules in the small sample are bombarded with high energy electrons which can cause the molecule to lose an electron, resulting in the formation of a positively charged molecular ion with one unpaired electron.
• The smaller and more positively charged fragment ions will be detected first as they will get deflected the most and are more attracted to the negative pole of the magnet.
• Each fragment corresponds to a specific peak with a particular m/e value in the mass spectrum.
• A small m/e value corresponds to fragments that are either small or have a high positive charge or a combination of both.