Molecular Formula Difference in Mass Spectrometry

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August 5, 2025

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Mass spectrometry is an analytical technique widely used in the fields of chemistry, biology and materials science. It provides molecular structure information by measuring the mass-to-charge ratio (m/z) of molecular ions and their fragments in Sample. In mass spectrometry, molecular formula (molecule formula) is one of the key parameters to interpret spectra, but other concepts related to molecular formula, such as structural formula (structure formula) and experimental formula (empirical formula) are often confused. This article will delve into the differences between these concepts and help readers better understand how to analyze the composition and structure of compounds by mass spectrometry.

Difference between Molecular Formula and Structural Formula in 1. Mass Spectrometry

1. Definition and Meaning The molecular formula (molecule formula) is the atomic number ratio of each element in a substance molecule expressed in a chemical formula. For example, the molecular formula of water is H₂ O, which means that each water molecule consists of two hydrogen atoms and one oxygen atom. In mass spectrometry, the determination of the molecular formula is usually achieved by the peak position of the molecular ion (M), which corresponds to a single molecule in Sample.

Structural formula (structure formula), also known as molecular formula, is the same term as molecular formula, and the two are essentially the same. Therefore, the term structural formula does not have a special meaning in mass spectrometry and may be an error in some literature.

Other concepts not related to structural formula

in mass spectrometry, molecular formula and structural formula are often confused, but in fact there are significant differences between them:

structural formula usually refers to the molecular structure of a substance, including the arrangement of atoms and the type of chemical bonds. In mass spectrometry, the structural formula is usually not directly used as an analytical parameter, but is inferred from the analysis of molecular formulas and peaks.
Molecular formula it refers to the elements that make up a substance and their atomic number ratio, which is essentially the same concept as the structural formula.

Therefore, in mass spectrometry, the molecular formula is the core parameter for interpreting the composition of a compound, and the term structural formula does not apply to the quantitative analysis of mass spectrometry.

Relationship between Molecular Formula and Experimental Formula of 2. Mass Spectrometry

1. Definition and difference The experimental formula (empirical formula) is the simplest integer ratio of the mass ratio of the elements in the substance, which is used to represent the basic composition of the compound. For example, the molecular formula of ethanol is C₂ H≡O, while its experimental formula is CHI. The experimental formula does not reflect the arrangement of atoms in the molecule, so it cannot be directly used to determine the structural formula.

2. The relationship between experimental formula and molecular formula The molecular formula is an integer multiple of the experimental formula. For example, the molecular formula of glucose is C, H, O, and its experimental formula is CHO. In mass spectrometry, the determination of the molecular formula needs to combine the information of the experimental formula, and infer the atomic number ratio of each element in the molecule through the experimental data.

Analytical Misunderstanding and Optimization Suggestions for Molecular Formula of 3. Mass Spectrometry

how to correctly interpret the molecular formula
in mass spectrometry, the determination of the molecular formula requires a combination of the following factors:

peak position of molecular ion: Through the m/z value of the molecular ion, combined with the accurate mass data table, the relative atomic mass of the molecular formula can be determined.
isotopic abundance: The presence of isotopes will cause the splitting of molecular ion peaks. By analyzing the relative intensity and distribution of the peaks, the molecular formula can be more accurately determined.
Difference between electrospray mass spectrometry and quadrupole mass spectrometry different mass spectrometry techniques have different effects on molecular formula. EFI mass spectrometry is more sensitive to molecular formula analysis, while quadrupole mass spectrometry is more suitable for large-scale sample analysis.

Key Points to Avoid Misjudgment of Molecular Formula

avoid over-reliance on experimental formulas, which can only provide basic composition information of compounds and cannot be directly used to determine molecular formulas.
To ensure the accuracy of the experimental data, especially in the case of isotopes, it is necessary to confirm the molecular formula in combination with the accurate mass data table.
Don't confuse molecular formula and structural formula. Molecular formula only reflects the composition of compounds, while structural formula describes the arrangement of atoms in molecules.

4. Summary

mass spectrometry is one of the indispensable tools in the field of chemistry, and the determination of molecular formula is the core content of mass spectrometry. By comparing the differences between molecular formula, structural formula and experimental formula, we can better understand the principle and application of mass spectrometry. In practice, it is necessary to combine the peak analysis of molecular ions, accurate mass data tables and isotope abundance to determine the molecular formula of the compound more accurately and provide a reliable basis for subsequent structural analysis.

For professionals in the chemical industry, mastering the knowledge of mass spectrometry molecular formula can significantly improve the ability of molecular structure analysis, so as to provide strong technical support for product development, quality control and research and development.