Discussion Examples for Elemental Analysis | How to Interpret Differences Between Theoretical and Experimental Values

What Is Elemental Analysis?

Elemental analysis is an analytical method used to determine the proportions of elements contained in a sample.
For organic compounds, CHN analysis, which measures the contents of carbon, hydrogen, and nitrogen, is commonly used.
Sulfur and oxygen may also be measured.
For inorganic compounds and metal complexes, the content of metal elements may also be confirmed by another analytical method.

Elemental analysis results are usually expressed as mass percentages of each element.
For example, they may be expressed as C 60.00%, H 5.00%, and N 10.00%.
By comparing these measured values with theoretical values calculated from the molecular formula, it is possible to determine whether the composition of the sample agrees with that of the expected compound.

Main Items to Include in the Results

In elemental analysis results, organize the sample name, estimated molecular formula, theoretical values, measured values, differences from the theoretical values, elements measured, sample-drying conditions, and the possible presence of impurities or solvents.
Arranging the theoretical and measured values in a table makes it easier to see which elements deviate and in which direction.

Main Items to Include in the Results

• Sample name
• Estimated compound name
• Estimated molecular formula
• Molecular weight
• Elements measured
• Theoretical values
• Measured values
• Differences from theoretical values
• Sample-drying conditions
• Presence or absence of crystal water or solvation
• Possibility of impurities
• Need for remeasurement
• Comparison with other analytical results
• Sources of error and points for improvement

Reference Experimental Values and Calculation Examples for Comparing Theoretical and Measured Values in Elemental Analysis

Here, reference experimental values are organized for comparing measured values of carbon (C), hydrogen (H), nitrogen (N), and other elements obtained by elemental analysis of organic compounds with theoretical values calculated from the chemical formula and for connecting the results to relative error and discussion.

Elemental analysis measures the mass percentages of elements contained in a sample.
If the obtained measured values are close to the theoretical values, the purity and composition of the sample can be considered close to those of the target compound.
On the other hand, if the values deviate greatly from the theoretical values, impurities, insufficient drying, residual solvent, incomplete combustion, or decomposition of the sample must be considered.

Reference Experimental Conditions

Example Calculation of Theoretical Values: Benzoic Acid

The molecular formula of benzoic acid is taken as C7H6O2.
The calculation uses atomic weights of C = 12.01, H = 1.008, and O = 16.00.

The molecular weight is 122.12.
The theoretical value for carbon is calculated by dividing the mass of carbon by the molecular weight and multiplying by 100.

Theoretical C value = 84.07 ÷ 122.12 × 100 = 68.84%

The theoretical value for hydrogen is as follows.

Theoretical H value = 6.048 ÷ 122.12 × 100 = 4.95%

Because benzoic acid does not contain nitrogen, the theoretical N value is 0.00%.

Elemental Analysis Results for Standard Samples

Reference examples are shown in which known compounds were measured and the theoretical and measured values were compared.

If the measured values are close to the theoretical values, the composition of the sample is considered close to that of the target compound and the analytical procedure was generally appropriate.
However, complete agreement is uncommon, and small differences are discussed as effects of measurement error or sample condition.

Example Calculation of Relative Error

Relative error is calculated to evaluate the difference between theoretical and measured values.

Relative error (%) = (Measured value − Theoretical value) ÷ Theoretical value × 100

For carbon in benzoic acid, when the theoretical value is 68.84% and the measured value is 68.62%,

Relative error = (68.62 − 68.84) ÷ 68.84 × 100 = −0.32%

The measured carbon value is 0.32% lower than the theoretical value, and because the difference is small, the result can be considered relatively good.

Example Summary of Relative Errors

In this reference example, the measured H values tend to be slightly high.
If moisture or residual solvent is present, the proportion of hydrogen may appear high.

Example Measurement of an Unknown Sample

A reference example is shown in which unknown sample X was subjected to elemental analysis and compared with the theoretical values of candidate compounds.

The measured C, H, and N values of unknown sample X are closest to the theoretical values of acetanilide.
From this result, unknown sample X is considered likely to be acetanilide or a compound having a composition close to it.

Differences Between Unknown Sample X and Candidate Compounds

In elemental analysis, candidates are narrowed down by examining the degree of agreement among multiple elements such as C, H, and N rather than judging from only one element.

Changes in Measured Values Caused by Insufficient Drying

If moisture remains in the sample, the proportions of H and O may become high, while the proportions of C and N may appear relatively low.
A reference example is shown in which an acetanilide sample was measured in an insufficiently dried condition.

With insufficient drying, contamination by moisture increases the measured hydrogen value, while the proportions of carbon and nitrogen become relatively low.
If H is higher than the theoretical value while C and N are low, moisture or residual solvent in the sample can be suspected.

Changes in Measured Values Caused by Impurities

If the sample contains impurities having a composition different from that of the target compound, the elemental-analysis values deviate from the theoretical values.
A reference example is shown for acetanilide containing inorganic salts or unreacted substances.

If impurities such as inorganic salts that contain almost no C, H, or N are mixed into the sample, the proportions of C, H, and N may all appear low.
On the other hand, if an unreacted substance containing nitrogen is present, the measured N value may become high.

Effects of Incomplete Combustion

In elemental analysis, C and H are detected as CO2 and H2O by completely combusting the sample.
If combustion is incomplete, the measured C and H values may become low.

If both C and H are lower than the theoretical values, incomplete combustion, sample-amount errors, or the condition of the instrument must be checked.

Variation Caused by Differences in Sample Amount

Because the amount of sample used in elemental analysis is very small, weighing errors and sample nonuniformity readily affect the results.

If the sample amount is too small, even slight weighing errors or bias in the sample have a large effect.
It is important to perform multiple measurements and confirm reproducibility.

Example of How to Write the Experimental Results

Elemental analysis of benzoic acid C7H6O2 gave measured values of 68.62% for C and 5.02% for H.
The theoretical values were 68.84% for C and 4.95% for H, giving relative errors of −0.32% for C and +1.41% for H.
The measured values were close to the theoretical values, and the sample was considered to approximately agree with the composition of the target compound.

The measured values of unknown sample X were C 71.02%, H 6.76%, and N 10.28%.
These values are close to the theoretical values of acetanilide C8H9NO, namely C 71.09%, H 6.71%, and N 10.36%.
Therefore, unknown sample X is considered likely to be acetanilide or a compound having a composition close to it.

In the insufficiently dried sample, the measured H value tended to be higher than the theoretical value, while the C and N values tended to be lower.
This was considered to be because moisture or residual solvent in the sample increased the proportion of hydrogen and relatively decreased the proportions of carbon and nitrogen originating from the target compound.

Points for Connecting the Results to the Discussion

In a discussion of elemental analysis, it is important not only to show the difference between theoretical and measured values but also to examine which elements are high or low and consider specific causes.

• Can the molecular weight and theoretical mass percentage of each element be calculated from the chemical formula?
• Can the difference or relative error between measured and theoretical values be organized?
• Can it be confirmed which of C, H, and N are high or low?
• If H is high, can the effects of moisture or residual solvent be discussed?
• If C and N are generally low, can contamination by inorganic salts or nonvolatile impurities be discussed?
• If C and H are low, can incomplete combustion or problems with sample amount be discussed?
• If N differs greatly from the theoretical value, can nitrogen-containing impurities or differences among candidate compounds be discussed?
• Can the need to judge the result together with other analytical results such as melting point, IR, and NMR be explained?

Example Discussion

In this experiment, the mass percentages of C, H, and N in the sample were determined by elemental analysis and compared with theoretical values calculated from the chemical formula.
The theoretical values for acetanilide were C 71.09%, H 6.71%, and N 10.36%, whereas the measured values were C 70.88%, H 6.80%, and N 10.22%.
All values were close to the theoretical values, and the sample was considered to approximately agree with the composition of acetanilide.

However, the measured H value was slightly higher than the theoretical value, while C and N were slightly lower.
Such a tendency readily occurs when moisture or residual solvent is present in the sample.
If water or solvent remains, the proportion of hydrogen increases and the proportions of carbon and nitrogen originating from the target compound decrease relatively.
Therefore, insufficient drying before measurement may have occurred.

For unknown sample X, the measured C, H, and N values were closest to the theoretical values of acetanilide.
Benzoic acid does not contain nitrogen, so it does not agree with unknown sample X, in which 10.28% N was detected.
Urea also has a very high theoretical N value and a low theoretical C value, so it differs greatly from the results for unknown sample X.
From this, unknown sample X can be judged to be a compound close to acetanilide.

Possible causes of deviations in elemental-analysis values from theoretical values include insufficient drying, contamination by impurities, incomplete combustion, and errors in sample amount.
If inorganic salts are mixed into the sample, the proportions of C, H, and N decrease overall, while incomplete combustion may cause C and H to be measured as low.
In addition, if the sample amount is too small, the effects of weighing errors and sample nonuniformity become large, so it is important to perform multiple measurements and confirm reproducibility.

Elemental analysis is a useful method for confirming the composition of a compound, but it does not directly determine the structure itself.
Because isomers having the same elemental composition cannot be distinguished by elemental-analysis values alone, the results must be comprehensively evaluated together with other analytical results such as melting-point measurement, infrared absorption spectra, and NMR.

Summary

In elemental analysis, theoretical values calculated from the chemical formula are compared with measured values of C, H, N, and other elements obtained experimentally.
If the measured values are close to the theoretical values, the composition of the sample is considered close to that of the target compound.

On the other hand, if H is high, moisture or residual solvent can be considered; if C or N is low, inorganic impurities can be considered; and if C or H is low, incomplete combustion can be considered.
In a report, it is useful to discuss the theoretical values, measured values, relative errors, directions of deviation, sample condition, and the possibility of impurities or insufficient drying in relation to one another.

What Are Theoretical Values?

The theoretical values in elemental analysis are the mass percentages of each element calculated from the assumed molecular formula.
For example, if the molecular formula contains many carbon atoms, the theoretical C value becomes high, while the theoretical N value is 0 for a compound containing no nitrogen atoms.
Theoretical values assume that the compound is completely pure and has exactly the assumed molecular formula.

When calculating theoretical values, it is important to determine whether crystal water, coordinated water, solvating molecules, salts, counterions, and similar components should be included in the molecular formula.
For metal complexes and hydrates, whether crystal water is included can greatly change the theoretical values.

Theoretical value of an element = Total mass of that element in the molecule / Molecular weight × 100

What Are Measured Values?

Measured values are the contents of each element actually measured with an elemental analyzer.
Measured values are affected not only by the composition of the sample itself but also by the drying condition, purity, measurement errors, combustion efficiency, impurities, moisture absorption, and residual solvents.
Therefore, measured values rarely agree perfectly with theoretical values, and some deviation occurs.

What is important is whether the deviation is within an acceptable range, which elements deviate and in which direction, and whether the cause of the deviation can be explained chemically.
For example, if only H is high, the effects of moisture or solvent can be considered; if C is low and H is high, moisture contamination can be considered; and if N is low, insufficient purity of the intended nitrogen-containing compound can be considered.

Meaning of Comparing Theoretical and Measured Values

In elemental analysis, the validity of the sample as the target compound is judged by checking the differences between theoretical and measured values.
If the target compound has been obtained with high purity, the measured values become close to the theoretical values.
Conversely, if unreacted substances, impurities, moisture, solvents, or decomposition products are present, the measured values deviate from the theoretical values.

However, elemental analysis alone cannot completely determine the structure.
Elemental analysis is strong for confirming composition, but structural isomers having the same elemental composition cannot be distinguished.
Therefore, the results are judged together with NMR, IR, mass spectrometry, XRD, and other analyses.

Discussion When the Deviation Is Small

When the difference between theoretical and measured values is small, the composition of the sample is considered close to the assumed molecular formula.
For organic compounds and complexes, C, H, N, and other values being close to the theoretical values provide one piece of evidence that the target substance was obtained.
However, the acceptable difference varies depending on the field and experimental criteria.

Even when the deviation is small, it is best to avoid concluding that the sample is completely pure.
Impurities that are difficult to detect by elemental analysis or by-products having the same elemental composition may be present.
It is important to confirm consistency with other analytical results.

Discussion When the Deviation Is Large

When the difference between theoretical and measured values is large, the sample may not have exactly the composition represented by the assumed molecular formula.
Possible causes include contamination by impurities, residual unreacted starting materials, by-products, insufficient drying, moisture absorption, residual solvents, crystal water, incomplete combustion, insufficient sample amount, and measurement errors.

When the deviation is large, it is important to separately consider which elements are high and which are low.
For example, low C and high H may indicate the effect of moisture, high C may indicate carbon-rich impurities, and low N may indicate insufficient purity of the intended nitrogen-containing compound.

Discussion When the C Value Is Low

If the measured carbon value is lower than the theoretical value, a component containing no carbon or having a low carbon content may be mixed into the sample.
Representative causes include moisture, inorganic salts, metal salts, residual inorganic reagents, ash, crystal water, and adsorbed water.
Incomplete combustion may also lower the C value if carbon is not sufficiently detected as carbon dioxide.

If the C value is low and the H value is high at the same time, contamination by moisture or solvent is suspected.
It is important to discuss changes in H and N together with the C value rather than considering the C value alone.

Discussion When the C Value Is High

If the measured carbon value is higher than the theoretical value, carbon-rich impurities may be mixed into the sample.
Examples include organic solvents, unreacted organic starting materials, by-products, and contamination by organic substances such as filter paper or resin.
In addition, if crystal water or solvating molecules have not been included in the assumed molecular formula, the theoretical values may not have been set appropriately.

If both C and H are high, residual organic solvent becomes a likely possibility.
If C is high and N is low, contamination by nitrogen-free organic impurities or insufficient purity of the target product is suspected.

Discussion When the H Value Is High

If the measured hydrogen value is higher than the theoretical value, moisture or residual solvent may be present in the sample.
Solvents such as water, alcohols, ethers, and acetone contain hydrogen, so even small amounts may affect the H value.
If the sample is hygroscopic, it may absorb moisture from the air before measurement and cause the H value to increase.

If H is high and C is low, the effect of moisture is relatively easy to consider.
If both H and C are high, residual organic solvent can be considered.
Even if only the H value deviates, the drying condition of the sample must be checked.

Discussion When the H Value Is Low

If the measured hydrogen value is lower than the theoretical value, the sample may have a composition with a lower hydrogen content than expected.
Possible causes include dehydration, oxidation, decomposition, carbonization, contamination by impurities, effects of combustion conditions, and an incorrect assumption about the theoretical molecular formula.
However, H is an element readily affected by measurement error, so it is important not to make excessive conclusions from a slight deviation alone.

If H is low and C is high, rather than the sample simply being overdried, it may contain impurities or decomposition products that are rich in carbon but have a low hydrogen ratio.
The result should be considered together with other analytical data.

Discussion When the N Value Is Low

If the measured nitrogen value is lower than the theoretical value, possible causes include contamination by nitrogen-free impurities, low purity of the intended nitrogen-containing compound, residual unreacted substances or solvents, and an incorrect molecular-formula assumption.
For metal complexes and organic synthesis products, the N value may also become low if ligands or nitrogen-containing functional groups are not present as expected.

The N value is important for confirming the composition of nitrogen-containing compounds.
If the N value deviates greatly, incomplete formation of the target product may also be considered.
However, if C and H also deviate at the same time, the overall effects of moisture, solvents, or impurities must be considered.

Discussion When the N Value Is High

If the measured nitrogen value is higher than the theoretical value, nitrogen-rich impurities or unreacted reagents may remain.
Possible causes include amines, ammonium salts, nitrates, nitrogen-containing solvents, and excessive residual nitrogen-containing ligands.
Deviations from the theoretical value may also occur if counterions or solvating molecules are not correctly included in the assumed molecular formula.

When the N value is high, the reagents used and purification process should be checked.
Particularly when nitrogen-containing reagents were used in excess or washing was insufficient, unreacted material may remain.

Effect of Moisture

Moisture is a common cause of deviations in elemental analysis.
If the sample is not sufficiently dried or is highly hygroscopic, it may contain moisture during measurement.
Moisture contains hydrogen and oxygen but no carbon or nitrogen, so it may relatively lower the proportions of C and N and increase the H value.

If the effect of moisture is suspected, vacuum drying, heat drying, storage in a desiccator, and weighing immediately before measurement are important.
However, excessive heating may decompose the sample or remove even crystal water, so drying conditions appropriate for the sample must be selected.

Effect of Residual Solvent

If solvents used in synthesis, recrystallization, extraction, or washing remain in the sample, elemental-analysis values may deviate from the theoretical values.
Organic solvents contain C and H and may therefore increase the C or H values.
On the other hand, if water or an inorganic solvent remains, the C or N values may decrease.

Residual solvents may be incorporated into gaps in the crystal, adsorbed on the surface, or included in the crystal structure as solvates.
In the case of a solvate, the solvent molecules must be included in the molecular formula when calculating theoretical values.
Residual solvent may also be confirmed by TG or NMR.

Effects of Crystal Water and Coordinated Water

In metal complexes and inorganic salts, crystal water and coordinated water greatly affect elemental-analysis values.
Crystal water consists of water molecules incorporated into a crystal, while coordinated water consists of water molecules coordinated to the metal center.
Whether either is included in the molecular formula changes the theoretical values of C, H, N, and other elements.

When crystal water or coordinated water is present, the proportion of H increases while the proportions of C and N become relatively low.
Even when elemental-analysis values do not agree with the theoretical values calculated for the anhydrous compound, they may agree when calculated as a hydrate.
Confirming mass loss caused by dehydration with TG measurement is useful for estimating the number of water molecules.

Effects of Impurities

One of the main causes of deviations between elemental-analysis and theoretical values is impurities.
If unreacted starting materials, by-products, insufficiently washed reagents, inorganic salts, catalysts, pieces of filter paper, drying agents, solvents, or decomposition products are mixed into the sample, the elemental proportions change.
Which elements become high or low depends on the elemental composition of the impurity.

For example, if an inorganic salt containing no carbon is mixed into the sample, the C value tends to become low, while if an unreacted reagent containing nitrogen remains, the N value tends to become high.
Which impurities may enter must be considered from the synthesis and purification procedures.

Effect of Incomplete Combustion

In CHN analysis, the sample is combusted and the resulting carbon dioxide, water, nitrogen oxides, and other products are measured to determine the amount of each element.
If the sample does not combust completely, carbon and hydrogen may not be fully detected, causing the C and H values to become low.
In flame-resistant samples, complexes containing large amounts of metal, and samples containing inorganic components, combustion conditions may affect the results.

If incomplete combustion is suspected, the instrument conditions, use of a combustion aid, sample amount, and sample homogeneity are checked.
However, in student experiments it is often not possible to directly change the instrument conditions, so this may be limited to discussion as a possible source of error.

Importance of Sample Drying

Sample drying is extremely important in elemental analysis.
If the sample is wet or contains solvent, large deviations occur between theoretical and measured values.
Even a small amount of moisture can particularly affect the H value and relatively lower the C and N values.

Samples are dried by vacuum drying, storage in a desiccator, heat drying, and similar methods.
However, for samples containing volatile components or crystal water and for heat-sensitive samples, excessively strong drying conditions may change the sample composition.
It is important to consider what should be removed by drying and what should remain.

Discussion of Elemental Analysis of Metal Complexes

In elemental analysis of metal complexes, the presence or absence of ligands, counterions, crystal water, coordinated water, and solvating molecules is important.
Complexes readily incorporate water and solvents into crystals, and their composition may change depending on the drying conditions.
In addition, because metal elements may not be directly measured by CHN analysis, the composition may be evaluated indirectly from the C, H, and N values.

If the ligand contains nitrogen, the N value is useful for confirming complex formation and the amount of ligand.
If C, H, and N are all close to the theoretical values, the ratio of ligands to counterions may be close to that expected.
However, combining the results with TG, ICP, XRF, and other analyses of metal content and moisture provides greater certainty.

Discussion of Elemental Analysis of Organic Compounds

In elemental analysis of organic compounds, the C, H, and N values are checked for agreement with the molecular formula.
For confirmation of new compounds and synthetic products, the structure is estimated by NMR, IR, and mass spectrometry, and elemental analysis is used to confirm the validity of the composition.
If C, H, and N are close to the theoretical values, this supports the conclusion that the target compound was obtained with relatively high purity.

In organic compounds, residual solvents, unreacted starting materials, and by-products are major causes of deviation.
Even after recrystallization or column purification, solvent may be incorporated into crystals.
Particularly when residual solvent is suspected from melting-point or NMR results, it is considered in relation to deviations in elemental analysis.

Discussion of Elemental Analysis of Polymers

In elemental analysis of polymers, the effects of monomer composition, copolymerization ratio, residual monomers, initiator residues, additives, and moisture absorption are considered.
Because polymers have molecular-weight distributions, they may be difficult to represent strictly with a single molecular formula as low-molecular-weight compounds are.
Therefore, theoretical values may be considered based on the repeating unit or copolymer composition.

If the polymer is hygroscopic, the H value may become high and the proportions of C and N may decrease.
In addition, if unreacted monomers, solvent, or additives remain, the elemental composition deviates from the theoretical values.
Combining the result with TG or NMR makes it easier to discuss the effects of residual components.

How to Think About the Acceptable Range of Experimental Values

In elemental analysis, the smaller the difference between theoretical and measured values, the easier it is to judge the result as good.
However, the degree of acceptable deviation differs depending on the field, purpose, measuring instrument, sample properties, and criteria specified by the instructor.
In student experiments, it is more important to be able to reasonably explain the cause of deviation than to seek perfect agreement.

If the deviation is slight, it can be considered within the range of measurement error or variation in sample preparation.
If the deviation is large, the sample may not be sufficiently pure as the target product or the assumed molecular formula may be incorrect.
If an acceptable range is specified, the instructions in the laboratory manual or from the instructor should be followed.

Common Mistakes in Calculating Theoretical Values

A surprisingly common mistake in discussions of elemental analysis is an error in calculating the theoretical values.
Theoretical values change because of miscounting the number of atoms in the molecular formula, molecular-weight calculation errors, omission of counterions, handling of crystal water, omission of solvating molecules, or differences in salt form.
If the theoretical values are incorrect, comparison with measured values becomes meaningless.

Particular care is required for metal complexes, salts, hydrochloride salts, hydrates, and solvates.
Before calculating, confirm whether the target product is a neutral molecule or a salt, whether it contains crystal water, and what the counterion is.

Combining Elemental Analysis With Other Analytical Results

Elemental analysis is effective for confirming composition, but it does not directly determine structure.
Isomers and mixtures having the same elemental composition may not be distinguishable by elemental analysis alone.
Therefore, the result is judged together with NMR, IR, mass spectrometry, XRD, TG, melting-point measurement, HPLC, and other analyses.

For example, if the structure is supported by NMR, the molecular weight agrees by mass spectrometry, and C, H, and N values from elemental analysis are close to the theoretical values, the evidence that the target compound was obtained becomes considerably stronger.
On the other hand, if only elemental analysis deviates, moisture, solvents, and impurities are considered in particular.

When Elemental Analysis Can Be Judged to Have Given Good Results

Elemental analysis can be judged to have given good results when the measured C, H, N, and other values are close to the theoretical values, the deviations are small, and the results do not contradict other analytical data.
In addition, if the deviations among the elements show a consistent pattern and can be explained by effects such as moisture or solvent, the results can also be discussed as reasonable.

However, measured values being close to theoretical values alone does not mean that the structure has been completely proven.
Elemental analysis should be treated as one method for confirming the composition of the target compound and judged comprehensively together with structural analysis and purity evaluation.

Example Discussion When the Experiment Did Not Go Well

When elemental analysis does not go well, the causes are considered from results such as large deviations between theoretical and measured values, only a particular element being considerably high or low, variation upon remeasurement, or inconsistency with other analytical results.
It becomes easier to discuss the causes by organizing them into sample drying, residual solvent, impurities, assumptions about the molecular formula, measurement errors, and incomplete combustion.

How to Write Points for Improvement

In a discussion of elemental analysis, including not only sources of error but also points for improvement makes the report easier to organize.
Improvements can be organized by dividing them into sample preparation, drying, purification, theoretical-value calculation, and comparison with other analyses.

Improvements to Sample Preparation

• Dry the sample sufficiently
• Avoid moisture absorption
• Store the sample in a desiccator
• Weigh immediately before measurement
• Make the sample homogeneous
• Remove impurities and unreacted substances
• Perform sufficient recrystallization and washing

Improvements to Theoretical-Value Calculation

• Confirm the molecular formula correctly
• Include counterions in the calculation
• Confirm whether crystal water should be included
• Consider the possibility of solvating molecules
• Confirm the salt form
• Confirm atomic weights and numbers of atoms

Improvements to Analysis

• Arrange theoretical and measured values in a table
• Examine the direction of deviation for each element
• Consider the effects of moisture, solvents, and impurities separately
• Confirm moisture and solvents by TG
• Confirm residual solvent by NMR
• Judge together with IR and mass spectrometry
• Remeasure when necessary

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of elemental analysis, simply writing that “the values were close to the theoretical values” or that “they deviated” results in a superficial discussion.
A good discussion explains which elements deviated and in which direction and how the deviations are related to moisture, solvents, impurities, and assumptions about the molecular formula.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of elemental analysis.
Adjust the necessary parts according to your own experimental results.

• Elemental-analysis values are indicators used to confirm whether the elemental composition of a sample agrees with the estimated molecular formula.
• Because the measured values were close to the theoretical values, the target compound was considered to have been obtained as the main component.
• Deviations between theoretical and measured values may be caused by moisture, residual solvent, impurities, or unreacted substances in the sample.
• If the H value is high, the effects of adsorbed water or residual solvent can be considered.
• If the C value is low, moisture or inorganic impurities containing no carbon may have been mixed into the sample.
• If the N value is low, contamination by nitrogen-free impurities or insufficient purity of the target product can be considered.
• Theoretical values change depending on whether crystal water or solvating molecules are included.
• For metal complexes, theoretical values must be calculated while considering the presence or absence of coordinated water, crystal water, and counterions.
• Because elemental analysis alone cannot completely determine structure, it must be judged together with NMR, IR, mass spectrometry, and other analyses.
• The drying and storage conditions before measurement greatly affect the reliability of elemental-analysis values.

Points to Check When Discussing Elemental Analysis

Checking the following points before writing the report makes the discussion easier to write.

• Is the estimated molecular formula clearly stated?
• Have the theoretical values been calculated correctly?
• Has it been confirmed whether counterions and crystal water should be included?
• Are the theoretical and measured values compared in a table?
• Has it been confirmed which elements are high and which are low?
• Are the effects of moisture and residual solvent considered?
• Is the possibility of unreacted substances and impurities considered?
• Are the sample-drying conditions stated?
• Are incomplete combustion and measurement errors considered?
• Has it been considered whether moisture and solvent can be confirmed by TG or NMR?
• Has it been confirmed that the results do not contradict other analytical results?
• Do the points for improvement correspond to the sources of error?

Summary

Elemental analysis is an analytical method used to measure the elemental contents of a sample and compare theoretical and measured values to confirm the composition, purity, and validity of the molecular formula of the sample.
C, H, and N values are commonly used for organic compounds, while for metal complexes the presence or absence of ligands, counterions, crystal water, and coordinated water is considered in the discussion.

When the theoretical and measured values are close, this supports the conclusion that the target compound was obtained as the main component.
On the other hand, when the deviation is large, moisture, residual solvents, impurities, unreacted substances, incorrect assumptions about the molecular formula, and incomplete combustion must be considered.
In particular, examining which elements are high and which are low makes it possible to discuss the cause of the deviation specifically.

In a report, rather than simply writing that “the values were close to the theoretical values,” organize and discuss the basis for calculation of the theoretical values, the differences from the measured values, the direction of deviation for C, H, and N, the effects of moisture and solvents, sample drying, and consistency with other analytical results.
Elemental analysis is a powerful method for confirming composition, but structure determination requires judgment in combination with NMR, IR, mass spectrometry, XRD, and other results.