When identifying compounds in a lab, you utilise different techniques to get an accurate data output for the compound in question. This part will focus two important parts of experimental chemistry (you may recognise some things from high school chemistry): elemental analysis using techniques such as combustion, atomic spectroscopy, and mass spectrometry, which utilises the ionization of the molecule.
Mass Spectrometry (MS): Separating ions
There are a variety of mass spectrometry techniques which all share one thing in common: separating ions from the compound, whether they are atomic or molecular, and separating them according to their mass-to-charge ratio (m/z). I'll only cover two classic techniques, such as Electron Ionization (EI-MS), and electrospray (ESI) techniques. Other techniques include: fast atom bombardment (FAB) and as well as matrix assisted laser desportion ionization time-of-flight (MALDI-TOF). These techniques are familiar to all chemists and biochemists as extensively useful tools in analytical chemistry,
Electron Ionization (EI) Mass Spectrometry
Electron Ionization or electron impact mass spectrometry would probably be the most familiar to you if you did high school chemistry. This technique is known as a 'hard technique' as it involves bombarding the analyte with high-energy electrons ( ≈
70 eV, electron volts), causing the molecule to fragment thus producing ions. It is widely used for analysing organic compounds, but becomes increasingly more limited as the molecular mass of a compound increases (Mr < 1500). This technique can't be used on ionic compounds, and the analyte must be stable when vaporized (if it isn't a gas at 298.15 K). As such this technique is limited to vaporized substances. In general, the molecular breakage can be represented generally as:
M(g) + e-(high energy) -------------> [M]+ (g) + 2e- (low energy)
Using a high-energy electron stream is essential because it is used to break the high covalent bond energies present in the analyte. Two ions are produced, the parent ion and the ion. The [M]+ (g) cation is a radical and is written as [M]●+ which are highly reactive. Fragmentation of molecules is always considered a 'hard technique.' After bombardment with electrons, they pass through a magnetic field, where the positive ions are deflected into a detector. Deflection is entirely dependent on the size of the m/z ratio: a larger m/z value leads to a greater radius of deflection. For ions with a value of z = 1, the m/z value is the same as the molecular mass; if z=2, the m/z ratio is half that of the molecular mass of the ion, and so on.
However, due to being restricted to molecules with relatively low molecular and low energy of vaporization, most ions have z=1. The mass spectrum is plotted so that m/z lies on the x-axis, and the y-axis is the relative intensity of the fragments, arbitrarily set on a scale of 0 to 100%. The final output depends on isotopes of elements that may be present in the molecule too, leading to an observation termed peak envelopes. The device can be summarised below:
Electrospray Ionization (ESI) Mass Spectrometry
This technique is widely used in molecules that have relatively high molecular weight (Mr ≤ 200 000). In contrast with EI mass spectrometry, this technique can be used with ionic substances, where singly and multiply charged ions can be observed in the resultant mass spectrum. This gives give it an advantage over the EI technique. It is also termed a 'soft' technique, as it involves the injection of a sample dissolved in a volatile (such as MeCN or MeOH which are easily vaporized) solvent, which is then sprayed (at 1 atm) into an applied electrical potential. The potential between the original point of injection to the counter electrodes is ~3000 V in positive ion mode. As the ions move toward the counter electrodes, the solvent evaporates and the gas-phase ions produced eventually hit the mass analyser. Peaks produced that are one mass unit apart reveal an ion is singly charged; on the other hand if they are half a mass unit apart, it is doubly charged, and so forth.
Like FAB and MALDI-TOF, neutral molecules are converted into positive ions with the help of H+ and Na+. As a result, an aggregate may be produced, generally they either result in a [2M+Na]+ and [M + MeCN + H]+.
Elemental and Compositional Analysis
Combustion
For a quantitative analysis of carbon, hydrogen and nitrogen containing compounds, it is possible to fully combust the compound and using the reaction stoichiometry, reach a conclusion for the composition of an analyte. It is done when a known mass of a substance (e.g. 2 to 5 mg) is sealed in an aluminium or tin capsule. This is placed in a fully automated analyser, where it is injected into a pyrolysis/combustion tube and heated to 900 ℃ in a pure oxygen environment. For compounds containing C, H, and N, they are oxidised into CO2, H2O and nitro-oxide gases, respectively.
These are then moved using a carrier gas (He) into a copper chamber, where nitro-oxides are reduced into N2 gas and excess O2 is removed. From here, the CO2 and H2O gas mixture is separated and then moved into and analysis chamber, where separated using a type of gas chromatography, which doesn't have a mobile phase (refer to the previous part of the series). The separated gases are then detected using a thermal conductivity detector, the detection process takes about five minutes. The accuracy the recorded amounts of C, H and N present in the compound is about <0.3%.
More modern machines can determine the amount of O and S present, where they are converted into SO2 and CO to CO2, respectively.
Atomic Absorption Spectroscopy (AAS)
This technique is used for determining the quantitative amount of metal but utilising the absorption and emission spectrum of elements. For example, the emission spectrum of hydrogen consists of very sharp lines, each of which corresponds to electronic transitions between high/low energy levels. On the other hand, the absorption spectrum of hydrogen occurs when it becomes irradiated, each element has its own absorption and emission spectrum. AAS is a common type of spectroscopy which uses a hollow cathode lamp calibrated to a given wavelength (specific to a transmission from one energy state to another), which irradiates the analyte.
Generally, the metal being analysed isn't present in its pure elemental form, so the analyte must be broken down (a step called digestion) in a series of standards (in liquid form). Standards are used to construct calibration curve. Each standard passes through a nebulizing chamber, where oxygen is injected in the liquid sample, resulting in very fine spray. The spray then enters an atomizer (generally a flame atomizer/graphite furnace/electrically heated), where the sample becomes atomized. The hollow cathode lamp irradiates the atomized sample, which passes through the monochromator. This is an optical component that transmits a beam of light with a very narrow range (basically a single colour), by reflecting away the unwanted wavelengths. The results are then amplified and the absorption spectrum is output to a computer display.
Modern AAS devices are computer-controlled, where the data is automatically recorded processed onto the computer. The AAS device is extremely sensitive - in the range of μg dm-3 !
_______________________________________
As always, thanks for reading!
Don't forget to check out my Patreon, if you like the content I'm putting out:
Links provided bring you to some of the info I used from the web. the first/second-year university textbooks:
- "Inorganic Chemistry," 4ED, by Housecroft & Sharpe, Pearson Ed. Ltd., 'Chapter 4 - Experimental techniques,' pgs 90-98. You can buy it here.
- "Fundamentals of Analytical Chemistry," 9ED, Skoog & West, Pearson Ed. Ltd., 'Chapter 28 - Atomic Spectroscopy,' pgs 774-775, 790-799. You can buy it here.
No comments:
Post a Comment