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Persistent Identifiers (PIDs)

Persistent identifiers (PIDs) are unique, long-lasting references to identified sources, such as publications, datasets, institutions, persons or analytical instrumentation, and allow to reliably locate, identify, and verify the references.

The two main PIDs employed within the chemistry community are Digital Object Identifiers (DOIs) as PIDs for objects such as publications, datasets, or software as well as Open Research and Contributor iDs (ORCID iDs) for people. Additionally, the Research Organization Registry (ROR) is used to identify research institutions. (InChI) and (CAS are technical identifier for chemical compounds and substances. Current development also includes Persistent Identification of Instruments (PIDINST) as PIDs for the instrumentation used to obtain research data. The Persistent Identifiers for eResearch (ePIC) is specifically designated for digital research data storage.

The main benefits of PIDs are increased findability, visibility, and ease of access of their scientific work. In the context of research data, PIDs are especially useful to interconnect publications and datasets in repositories.

About the individual PIDs​

DOIs​

DOI logo

Digital Object Identifiers are PIDs for objects such as publications and datasets, but also physical objects. DOIs are resolved based on the Handle Systemto lead to the corresponding landing page and are assigned by members of the International DOI Foundation. Most well-recognised DOI registration agencies include CrossRef for publications and DataCite focusing on datasets.

A DOI consists of a character string divided into two parts by a slash. The prefix always starts with a 10. and identifies the registrant. Every registrant has its own unique prefix. The suffix identifies a specific object. DOIs are most frequently displayed as an URL such as https://doi.org/10.1000/182.

Crossref logoDataCite logo
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Most importantly, domain-independent metadata also including provenance information is attached to the DOI. This metadata follows the metadata schemes provided by registration agencies such as the CrossRef Metadata Schema and the DataCite Metadata Schema.

Registrants are liable to update the URL connected to the DOI if the object is moved.

ORCID iDs​

ORCiD logo

ORCID iDs are open and non-proprietary PIDs for authors that can be used by any author free of charge. They are provided by the non-profit organization ORCID Inc., providing application program interfaces (API) for their members to integrate ORCID services.

An ORCID iD is a 16-digit alphanumerical code to unambiguously identify authors, whille author names might not be unique, may change, or may have different ordering conventions depending on cultural differences. ORCID iDs can be depicted as an URL, e.g. https://orcid.org/0000-0000-0000-0000 and typically as an URN, e.g. ORCID:0000-0000-0000-0000. ORCID iDs are a subset of International Standards Name Identifiers (ISNI) provided by the International Organisation for Standardisation (ISO) as ISO 27729.

The ORCID record connected to ORCID iDs includes the bibliographic output and may also provide information on employment, education and qualification, invited positions and distinctions, memberships as well as service and funding.

In addition to non-proprietary ORCID iD, there are also proprietary author identifiers such as Thomson Reuters’ Web of Science ResearcherID and Elsevier’s Scopus Author ID.

Research Organisation Registry​

ROR logo CC BY 4.0

The Research Organisation Registry (ROR) provides persistent identifiers for research organisations, while names might not be unique, may change and organisations might be merged, split, shut down or re-emerge. ROR enables to connect research organisations to research outputs and their researchers by identifying their affiliations.

ROR should be provided in affiliations of researchers in scientific publications as well as for data publications.

ROR does support parent-child hierarchies as well as lateral relationships between organisations. An example might include a university having a child organisation such as a research institute [German: Fakultät] and related organisations such as a university hospital. However, ROR focuses on fundamental affiliations and is not focused on all subdivisions of an organisations such as departments [German: Institut] as high-level institutional information is more stable.

InChI (International Chemical Identifier)​

InChI Header logo

The InChI (International Chemical Identifier) is a standardised, text-based identification system for chemical compounds. It was developed under the auspices of (IUPAC) to represent chemical structures unambiguously and in a machine-readable format. The aim of the system is to facilitate the exchange of chemical information between databases, scientific publications and software applications.

An InChI describes the structure of a chemical compound using a defined character string, e.g. InChI=1S/CH4/h1H4 for CH4 (methane). Information on the molecular formula, atomic bonding, hydrogen atoms, charges, stereochemistry and isotopic compositions is encoded across several successive levels (‘layers’). This standardised representation enables identical chemical structures to be reliably recognised even when they are stored in different databases or programmes.

The InChIKey is a shortened form of the InChI. It is a compact hash code consisting of letters, generated from the InChI. The InChIKey is particularly suitable for internet searches and for the rapid indexing of large datasets.

The key advantages of InChI include its openness, its international standardisation and its high level of interoperability. Unlike proprietary identification systems, the standard can be used freely.

CAS-Number​

The CAS number (Chemical Abstracts Service Registry Number, (CAS RN®) is a globally recognised identification system for chemical substances. It is assigned by the Chemical Abstracts Service and is used to uniquely identify chemical compounds, polymers, alloys, biomolecules and other substances. The use of CAS numbers allows substances to be uniquely identified, regardless of their name or the language used.

A CAS number consists of three blocks of numbers separated by hyphens, for example 7732-18-5 for water. The last block of numbers contains a check digit. The number itself does not contain any information about the chemical structure or properties of a substance, but serves solely as a unique identifier.

The register comprises many millions of substances and is continually being expanded. CAS numbers are used in safety data sheets, scientific publications, patent documents, substance databases and regulatory frameworks. They facilitate the search for chemical substances and help to avoid confusion that may arise from different common names, trade names or systematic names.

Due to its uniqueness and widespread use, the CAS number is regarded as one of the most important identifiers for chemical substances. However, unlike structure-based identifiers such as InChI, the CAS number does not provide any information about the molecular structure of a compound.

ePICs​

ePIC Header logo

The Persistent Identifier for eResearch (ePICs) are based on the Handle.Net Registry (HNR) and are intended to be used for unpublished digital research objects. To this end, ePICs can be assigned at early stages of research to locate research data and may be used by various research data management or archiving systems. Upon publication of a dataset, the full research data can be linked via the ePIC, for example, in cases that include proprietary or sensitive information.

This type of PID will commonly lead to a landing page, the content of which is maintained by the PID owner and includes the dataset's metadata or a method of contacting those responsible for the dataset.

ARK​

ARK logo

The Archival Research Key (ARK) are open identifiers hich are assigned to physical and digital information objects, mostly in the cultural studies. ARK prefixes are centrally assigned on request by the ARK Alliance and can be resolved centrally on services like N2T. The single ARKs are assigned and managed by the single institutions on an own server. Other that DOIs, they are free of charge.
An ARK usually has the form ark:12148/btv1b8449691v/f29 and can be combined with different ARK URI prefixes leading to different resolving servers. Attaching a ? at the end of an ARK allows adding a structured description of the resource. Much software is available to mint, store, resolve, interpret ARKs.

Sources and further information​

Main author: ORCID:0000-0003-4480-8661