Skip to main content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.


The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Electron Ionization Library Component of the NIST/EPA/NIH Mass Spectral Library and NIST GC Retention Index Database


The National Institute of Standards and Technology (NIST) has developed and maintains an Electron Ionization (EI) Library as part of the NIST/EPA/NIH Mass Spectral Library, and the NIST GC Retention Index (RI) Database. These databases contain evaluated reference data used for the identification of chemicals.


Intended Impact

The speed and accuracy of chemical analyses are vital issues in many areas. Monitoring for explosives, drugs (both legal and illegal), chemical weapons, pollutants, petrochemical processing, food contamination, and health/disease status biomarkers is a challenge with a high reward in terms of quality of life and security for the nation.

The first step in chemical analysis is usually identifying the chemical or chemicals present and the single most general technique for identifying molecules is mass spectrometry. Although there are numerous methods for ionizing materials to generate a mass spectrum, one of the oldest and the best established is electron ionization (EI). Although a mass spectrum reflects the underlying properties of the molecules, the process is sufficiently complex that identifications often require the comparison of the spectrum of an unknown with a measured spectrum of the compound.

The purposes of both the Electron Ionization (EI) Library, which is part of the NIST/EPA/NIH Mass Spectral Library, and the Retention Index (RI) Database is to aid in the identification of chemicals. In a substantial number of cases the EI Library is part of the only feasible identification process and both make the process faster and more reliable.


The Standard Reference Data Act (Pub. L. 90-396, Sec. 1, July 11, 1968) states: "The Congress hereby finds and declares that reliable standardized scientific and technical reference data are of vital importance to the progress of the Nation's science and technology. It is therefore the policy of the Congress to make critically evaluated reference data readily available to scientists, engineers, and the general public. It is the purpose of this Act to strengthen and enhance this policy."

The objective of the EI Library is to provide the largest possible collection of reliable reference spectra with an emphasis on compounds that are environmentally or biologically relevant. The Retention Index (RI) Database provides a comprehensive collection of reported retention indices.


  • EI Library: The goal is to Improve the utility of a tool that is widely used to identify chemical compounds. This is accomplished by Increasing the number and quality of relevant spectra in the library and by providing easy to use and capable software.
  • RI Database: The goal is to make available in an easily accessible form as much published retention index data as possible to aid in the identification of compounds by gas chromatography. This was accomplished by collecting most of the previously widely scattered literature values, finding and removing errors and providing the software necessary to use the information.

Research Activities and Technical Approach

EI Library

The size and scope of the EI Library is increased by the purchase of specialized collections, by both solicited and spontaneous contributions, by adding spectra from the literature, and, when not otherwise available, by obtaining the spectra at NIST of compounds both purchased and prepared.

Quality control is essential since the reliability of identifications depends on the spectra being of the correct compounds and of high quality. Although some aspects of the quality control can be and are automated, much of it must be done by experts.

An essential element in the NIST/EPA/NIH Mass Spectral Library is the software necessary to use the various libraries effectively. Past developments include the addition of synonyms for chemical names, the addition of chemical structures and Chemical Abstracts registry numbers, the development and implementation of spectral matching algorithms, the development of a retention index database with values linked to the appropriate spectra, and the development of AMDIS, a program that extracts clean spectra from complex gas chromatography/mass spectrometry data for comparison to library spectra. This software is available on a wide variety of commercial mass spectrometry instrumentation. Planning is underway to incorporate IUPAC International Chemical Identifiers (InChI) (mainly developed at NIST) into the system.

RI Database

The RI Database is the largest available collection of retention indices. Most of the data was collected from the published literature although some was obtained at NIST at the same time as spectra for the EI Library. As with the NIST/EPA/NIH Mass Spectral Library the software provides chemical names, synonyms, Chemical Abstracts registry numbers, and structures.

Major Accomplishments

  • The current version of the EI library contains 306,622 EI spectra of 267,376 compounds.  Over the last decade about 10,000 spectra have been added each year.  Approximately 4500 copies of the NIST EI library are distributed each year for a fee as part of the NIST/EPA/NIH Mass Spectral Library. Most manufacturers of mass spectrometers offer the Database as an option and most instruments capable of generating EI spectra are delivered with the Database.
  • The GC Retention Index Database contains 404,045 retention index values for 99,400 compounds and will be released in the near future. The retention index values for which there is a corresponding mass spectrum are included in the NIST/EPA/NIH Mass Spectral Library.
  • Tandem (MS/MS) Library,  small molecules, contains  574,826 spectra of 118,082 precursor ions from 13,808 chemical compounds.
  • Tandem (MS/MS) Library,  biologically-active peptides, contains 90,244 spectra of 6,803 precursor ions from 1,904 peptides.
Created February 26, 2009, Updated September 23, 2019