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Elemental analysis
Encyclopedia
Percent Composition is a process where a sample of some material (e.g., soil, waste or drinking water
, bodily fluids, minerals, chemical compound
s) is analyzed for its elemental
and sometimes isotopic
composition. Elemental analysis can be qualitative (determining what elements are present), and it can be quantitative (determining how much of each are present). Elemental analysis falls within the ambit of analytical chemistry
, the set of instruments involved in deciphering the chemical nature of our world.
For organic chemists, elemental analysis or "EA" almost always refers to CHNX analysis—the determination of the mass fractions of carbon, hydrogen, nitrogen, and heteroatoms (X) (halogens, sulfur) of a sample. This information is important to help determine the structure of an unknown compound, as well as to help ascertain the structure and purity of a synthesized compound.
. In this technique, a sample is burned in an excess of oxygen, and various traps collect the combustion products—carbon dioxide, water, and nitric oxide. The masses of these combustion products can be used to calculate the composition of the unknown sample.
that fits with those results. This process is useful as it helps determine if a sample sent is a desired compound and confirms the purity of a compound. The accepted deviation of elemental analysis results from the calculated is 0.4%. The method for working out the ratio of elements from the results is shown below:
This process is tedious to perform by hand
, and automated tools have been released to simplify with this process. Each of the tools is different in its working. CHN+ works under Windows and was designed primarily for discovering solvents occluded in in a compound. The Solvent Correction CHN Calculator] works in a similar manner, but requires an internet connection. The Chemical Composition Calculator] works without an internet connection, calculates elemental analysis on the fly in a user's web browser but predicts molecular ion peaks for use in Mass Spectra
. Iteration to discover occluded solvents, is left to the user.
Drinking water
Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...
, bodily fluids, minerals, chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s) is analyzed for its elemental
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
and sometimes isotopic
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
composition. Elemental analysis can be qualitative (determining what elements are present), and it can be quantitative (determining how much of each are present). Elemental analysis falls within the ambit of analytical chemistry
Analytical chemistry
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...
, the set of instruments involved in deciphering the chemical nature of our world.
For organic chemists, elemental analysis or "EA" almost always refers to CHNX analysis—the determination of the mass fractions of carbon, hydrogen, nitrogen, and heteroatoms (X) (halogens, sulfur) of a sample. This information is important to help determine the structure of an unknown compound, as well as to help ascertain the structure and purity of a synthesized compound.
Methods
The most common form of elemental analysis, CHN analysis, is accomplished by combustion analysisCombustion analysis
Combustion analysis is a method used in both organic chemistry and analytical chemistry to determine the elemental composition of a pure organic compound by combusting the sample under conditions where the resulting combustion products can be quantitatively analyzed...
. In this technique, a sample is burned in an excess of oxygen, and various traps collect the combustion products—carbon dioxide, water, and nitric oxide. The masses of these combustion products can be used to calculate the composition of the unknown sample.
Quantitative
Quantitative analysis is the determination of the mass of each element or compound present. Other quantitative methods include:- GravimetryGravimetryGravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of gravitational field or the properties of matter responsible for its creation are of interest...
, where the sample is dissolved and then the element of interest is precipitated and its mass measured or the element of interest is volatilized and the mass loss is measured. - Optical atomic spectroscopyAtomic spectroscopyAtomic spectroscopy is the determination of elemental composition by its electromagnetic or mass spectrum. Atomic spectroscopy is closely related to other forms of spectroscopy. It can be divided by atomization source or by the type of spectroscopy used. In the latter case, the main division is...
, such as flame atomic absorption, graphite furnace atomic absorptionGraphite furnace atomic absorptionGraphite furnace atomic absorption spectrometry is a type of spectrometry that uses a graphite-coated furnace to vaporize the sample...
, and inductively coupled plasma atomic emission, which probe the outer electronic structure of atoms.
Qualitative
To qualitatively determine which elements exist in a sample, the methods are:- Mass spectrometric atomic spectroscopyAtomic spectroscopyAtomic spectroscopy is the determination of elemental composition by its electromagnetic or mass spectrum. Atomic spectroscopy is closely related to other forms of spectroscopy. It can be divided by atomization source or by the type of spectroscopy used. In the latter case, the main division is...
, such as inductively coupled mass spectrometry, which probes the mass of atoms. - Other spectroscopy which probes the inner electronic structure of atoms such as X-ray fluorescenceX-ray fluorescenceX-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...
, particle-induced X-ray emission, X-ray photoelectron spectroscopyX-ray photoelectron spectroscopyX-ray photoelectron spectroscopy is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material...
, and Auger electron spectroscopyAuger electron spectroscopyAuger electron spectroscopy is a common analytical technique used specifically in the study of surfaces and, more generally, in the area of materials science...
. - Chemical methods
- Sodium fusion testSodium fusion testThe sodium fusion test is used in elemental analysis for the qualitative determination of elemental halogens, nitrogen and sulfur in a sample. It was developed by J. L. Lassaigne....
- Schöniger oxidationSchöniger oxidationIn chemistry, the Schöniger oxidation is a method of elemental analysis developed by Wolfgang Schöniger.The test is conducted in an Erlenmeyer flask, or in a separatory funnel...
- Sodium fusion test
Analysis of results
The analysis of results is performed by determining the ratio of elements from within the sample, and working out a chemical formulaChemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
that fits with those results. This process is useful as it helps determine if a sample sent is a desired compound and confirms the purity of a compound. The accepted deviation of elemental analysis results from the calculated is 0.4%. The method for working out the ratio of elements from the results is shown below:
- Take the percentage of each element found and divide by the element's mass. Do this for all the elements for which you have results
- Find the smallest value from step 1 and divide every value obtained in step 1 by this smallest value
- Multiply the results in step 2 by a factor to obtain reasonable values for either carbon or nitrogen and then compare to what was expected from a pure sample of the compound that was thought to be submitted
This process is tedious to perform by hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
, and automated tools have been released to simplify with this process. Each of the tools is different in its working. CHN+ works under Windows and was designed primarily for discovering solvents occluded in in a compound. The Solvent Correction CHN Calculator] works in a similar manner, but requires an internet connection. The Chemical Composition Calculator] works without an internet connection, calculates elemental analysis on the fly in a user's web browser but predicts molecular ion peaks for use in Mass Spectra
Mass spectrum
A mass spectrum is an intensity vs. m/z plot representing a chemical analysis. Hence, the mass spectrum of a sample is a pattern representing the distribution of ions by mass in a sample. It is a histogram usually acquired using an instrument called a mass spectrometer...
. Iteration to discover occluded solvents, is left to the user.