Publishers assign a DOI when an article is published electronically. This alphanumeric string enables users to access content through a unique and permanent internet url. DOIs must be added to citations in APA.
Finding a DOI.
If you don't see a DOI, check crossref.org and search by the name of the author and the journal article.
Karplus, M. (2010). Dynamical aspects of molecular recognition. Journal of Molecular Recognition, 23(2), 102-104. doi: 10.1002/jmr.1018
Articles without DOIs
Include the url of the journal home page in your citation. If you found the article in a database and it is only published in that database, then you can add the database name and accession number or the database URL
Berman, R. P. (2009). Fused voices: Narrated monologue in jane austen's emma. Nineteenth-Century Literature, 64(1), 1-15. Retrieved from http://ucpressjournals.com/journal.php?j=ncl
Bilger, A.(2009). Women Novelists Before Jane Austen: The Critics and Their Canons (review). Tulsa Studies in Women's Literature 28(1), 201-202. The University of Tulsa. Retrieved October 9, 2013, from Project MUSE database.