There's a free DOI lookup tool available at:
http://www.crossref.org/guestquery/
Alternately, if you need the DOI of a journal article that's indexed in PubMed, try using PubMed to find the needed number:
- Find the PubMed record for the article.
- Use the PubMed "Display setting" drop-down menu to select and apply the "MEDLINE" format.
- Look for the [AID] fields near the bottom of the PubMed record. The DOI if present, will be followed by [doi].
If you see the AID field followed by [pii], this is also a unique identifier, but it is assigned by the publisher. Articles that were written prior to 1998 will definitely not have a doi or pii assigned to it. Articles after 1998 may have a doi or pii, but it is dependent upon when the publisher began assigning doi or pii to articles.
