Elemental Composition Determinations Using the Abundant Isotope

Amino Acid Composition Determination From the Fractional Mass of Peptides

A peptide's fractional mass is directly associated with its elemental composition and thus amino acid composition. Here it is demonstrated that a peptide's fractional mass alone can be a useful identifier or indicator of that composition for small to mid-sized peptides (5-7 amino acids) and can significantly reduce the number of viable amino acid compositions for larger peptides (> 8 residues) to include or exclude certain possibilities. Separate consideration of the integer portion of the peptide's mass helps to reduce the number of possibilities where many duplicate fractional mass values are found. Adoption of this fractional mass strategy should aid approaches that are presently employed for peptide identification, including in the use of mass map data to search protein databases for proteomics applications.

Automated Identification of Ions Observed in Mass Spectra of Inorganic Compounds Using Isotopic Distribution Brute Force: Methodology and Performance Measurements

This Article describes the method of isotopic distribution brute force, which can be used to identify ions registered in mass spectra of inorganic compounds in an automated manner when a library search cannot be conducted. A detailed description of the isotopic distribution brute force methodology is presented, including a discussion of computation-related difficulties. The ability of the proposed algorithm to identify various inorganic ions is tested on a small set of real-life low-resolution mass spectra of lead halides and copper halides. An evaluation of the isotopic distribution brute force performance is conducted using the low-resolution experimental mass spectra of natural rhenium sulfide and lead(II) chloride. Based on identification results and obtained performance measurements, we formulate the empirical restrictions on the input data, ensuring that the application of isotopic distribution brute force is feasible from the standpoints of search space reduction and identification time.

Monoisotopic Mass?

The current IUPAC-recommended definition of the term "monoisotopic mass" of a chemical species is based on the most abundant isotopes of the constituent elements. It has even been proposed to constrain the definition to be based only on the atomic masses of the most abundant stable isotopes. Such an approach is flawed because in this way several elements and their compounds, in addition to isotopically enriched species, would not merit to be assigned a monoisotopic mass. Furthermore, for large molecules, such as proteins, the monoisotopic mass as currently defined loses its significance. Therefore, we propose to eliminate using the current definition altogether. Instead, the term isotopologue mass should be applied uniformly to every species denoted by a specific chemical formula.

Kendrick Analysis and Complex Isotopic Patterns: A Case Study of the Compositional Analysis of Pristine and Heated Polybrominated Flame Retardants by High-Resolution MALDI Mass Spectrometry

The Kendrick analysis is used for the processing and visualization of mass spectra obtained from polymers containing C, H, O and/or Si with simple isotopic patterns (monoisotope=lightest isotope=most intense isotope for short chains). In the case of heteroatoms with complex isotopic patterns, the impact of the chosen isotope on point alignments in Kendrick plots has not been examined extensively. Rich isotopic patterns also make the evaluation of the mass and nature of the repeating unit and end-groups more difficult from the mass spectrum in the case of unknown samples due to the number of peaks and the absence of a monoisotopic peak. Using a polybrominated polycarbonate as running example, we report that horizontal point alignments can be obtained in a Kendrick plot using the mass of the most abundant isotope instead of the monoisotopic mass as is usually done. Rotating the plot (“reverse Kendrick analysis”) helps to accurately evaluate the mass of the most abundant isotope of the repeating unit, as well as the nature of the brominated neutral expelled upon gentle heating (debromination or dehydrobromination). The whole procedure is then applied to the characterization of an unknown polybrominated flame retardant in an industrial formulation before and after heating.