Elise Austin - January 25, 2022 - Chemistry Worksheet
In the early days of the development of chemistry, the molecular mass of a compound was determined by measuring the vapor density or freezing point depression of the compound, while its molecular formula was determined by elemental analysis. This method takes a long time and is complicated, this technique also requires a large number of samples and with high purity. Currently, the molecular weight and molecular formula can be determined quickly and in small sample sizes using an instrument known as a mass spectrophotometer (MS).
One of the functions of mass spectroscopy is to identify the chemical structure of a molecule. Determination of the molecular structure of both organic and inorganic molecules is based on the fragmentation pattern of the ions formed when a molecule is ionized. Mass spectroscopy is an analytical technique based on the separation of traces of ions according to the mass-to-charge ratio and measurement of the intensity of the beam of these ions. In mass spectroscopy, molecules of organic compounds are bombarded with electron beams and converted into high-energy positive ions (molecular ions or parent ions), which can be broken down into smaller ions (molecular ions). fractional ions) or fragments. The loss of electrons from the molecule will produce cation radicals. The pattern of fragmentation of a molecule is very different from that of another molecule and the analysis results are reproducible.
In a mass spectrophotometer, the first reaction of a molecule is ionization the loss of an electron, resulting in a molecular ion. The peak for this ionic radical is usually the rightmost peak in the spectrum, the molecular weight of this compound can be determined. It is assumed that the electrons in the high-energy orbitals are the first to escape. If a molecule has n electrons lonesome, then one of them will be released. If there are non-electrons, a pi-electron is lost. If neither n nor pi electrons exist, a molecular ion will be formed by losing a sigma electron.
After the initial ionization, the molecular ion will undergo fragmentation, namely, the process of releasing free radicals or small neutral molecules are released from the molecular ion. A molecular ion does not break apart randomly but tends to form the most stable fragments. Let’s consider the mass spectrum of methanol. This spectrum consists of three main peaks at m/z = 29, 31, and 32. The structure of the fragments can often be inferred from their masses. Peak M+ methanol (at 32)
In the mass spectrum, it can be seen that there are broad peaks in the non-spherical masses such as m/e 60.2 and m/e 43.4 which are known as metastable ions, they have lower kinetic energy than the metastable ions. ions are normal and arise from the fragments that occur during the escape from the ionizing chamber. If a large number of M molecules are converted into M+ molecular ions, not all of these ions will have the same excitation energy, so some will have a longer life than others. The M+ ions with a shorter lifetime may decompose in the ionizing chamber into A+ ions and B radicals, the A+ ions will be detected by the collector normally.
Molecular ions released from the ion source will be accelerated by the accelerator voltage to have a translational power of eV. Some of these M ions can reach the collector and be detected. However, if the other M ions dissociate into A+ and B, as soon as they are accelerated, then the parent translational energy M (eV) will be shared between A+ and B in proportion to their masses. The translational energy of the A+ ion must be lower than that of the parent, and this A+ ion will reach a different collector than it should be (abnormal). The A+ ion with abnormal translation is known as a metastable ion.
These equations often give a mass unit result 0.1 to 0.4 lower than the observed mass. For example, the mass spectrum of toluene shows strong peaks at m/z 91 and m/z 65, together with broad and strong metastable peaks at m/e 46.5. By using the equation for calculating m*, we get 46.4, so we can interpret that the ion m/e 91 decomposes by releasing 26 mass units to become ion m/e 65, and several fragments from metastable ions.
Branching in a hydrogen chain results in fragmentation that occurs mainly in the branches, because the secondary ionic radicals and secondary carbocations are more stable than the primary form. Carbocation stability is a more important factor than free radical stability. For example, the molecular ion of methylpropane produces an isopropyl cation and a methyl radical.
Consider the spectrum of N-Ethylpropylamine shown in the figure below. The molecular ion has m/z 87. Fragmentation of this molecular ion occurs at the alpha position concerning the nitrogen atom and produces fragments with m/z 58 (loss of ethyl group) and m/z 72 (loss of methyl group). This type of fragmentation is called – defense and is common in both amines and ethers.
The contributing factor to this cleavage is that the cations formed in this reaction are resonance stabilized. Similar fragmentation occurs at a bond near a carbonyl group (or to oxygen). It is the resulting cation is also stabilized by resonance.
Stable small molecules such as H2O, CO2, CO, and C can be separated from within a molecular ion. For example, alcohol easily loses H2O and exhibits a peak that is 18 mass units smaller than the peak for the molecular ion. This peak is said to be the M-18 peak. In many alcohols, the elimination of H2O is so easy that the molecular ion peak is not even found in the spectrum. For example, the spectrum of 1-butanol, which can be seen in the figure below, is a typical mass spectrum of alcohol.
The Mc Lafferty rearrangement occurs when there is a hydrogen atom to a carbonyl group in the molecular ion. The sample to be analyzed is inserted into the ionizing chamber in the first part of the mass spectroscopic apparatus. The sample can be a gas, a solid, and a solution according to the shape of the sample and the ionization technique chosen.
In general, mass spectroscopy consists of three important parts, namely the ionization site of the sample, the separation of ions, and the detection of ions formed. The sample is inserted into the chamber, evaporated by increasing the chamber temperature, shot with high-energy electrons, the fragment ions formed are accelerated and separated in a magnetic field, then detected by a detector.
Along with the development of technology, each section has changed to improve the ease of use and the ability of the tool to analyze. Currently, mass spectroscopy is usually used independently in sample analysis or used in conjunction with other tools, such as High-Performance Liquid Chromatography (HPLC), Gas Chromatography (GC), Capillary Electrophoresis (CE) so that the term HPLC-MS, GC is known. -MS, and CE-MS. HPLC, GC, or CE serve to separate the sample mixture, which then each component that has been separated will be analyzed one by one in MS. The following is an example of an AP chemistry mass spectrometry worksheet.