E-books and electronic journal articles can contain a number which identifies them, called a DOI 
(Digital Object Identifier). A DOI is a string of numbers, letters and symbols used to permanently identify an article or document and link to it on the web. A DOI will help your reader easily locate a document from your citation. It is like a PPS number for the article you’re citing — it will always refer to that article, and only that one (source: UIC). Picture source: Wiki
A DOI is not the same as an ISBN, but it does the same thing - it is a unique number assigned to a piece of work that allows you to find the work even if the work moves databases or websites, etc. The DOI is a hyperlink which can take you to the article in one click.
If you know the DOI it is best to include it in your references/bibliography so that you know you're linking directly to the correct article, and to make it easy for the reader to check the source.
Reference: Author(s) Last name, Initials. (Year). Article title. Journal title, volume number(Issue number), page numbers.
Example: Lum, J. A. G., & Bleses, D. (2012). Declarative and procedural memory in Danish speaking children with specific language impairment. Journal of Communication Disorders, 45(1), 46-58.
In-Text-Citation:
Example:
Note: For journal articles only include the issue number when each issue is individually paginated.
Articles with multiple authors follow the same pattern.
Reference: Author Last name, Initials. (Year) 'Article title', Journal Title, Volume(Issue), pp. page numbers.
Example: Tovey, H. (2002) 'Risk, morality, and the sociology of animals - reflections of the foot and mouth outbreak in Ireland', Irish Journal of Sociology, 11(1), pp. 23-42.
In-Text-Citation:
Example:
There are two styles of MHRA referencing - footnotes/bibliography style and author/date style. This guide covers notes/bibliography style. For more information on both types of styles please see information here from the Univeristy of St. Andrews, and here from the MHRA style guide online.
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Reference: First author(s) Last name, First name and next author(s) First name Last name. "Title of Article." Title of Journal, vol. Volume, no., Year, pp. page range.
Example: Mann, Susan. "Myths of Asian Womanhood." Journal of Asian Studies, vol. 59, 2000, pp. 835-62.
In-Text-Citation:
Example:
Please note that there are two style of Chicago referencing: author/date style and notes/bibliography style. This guide covers notes/bibliography style.
In-Text Citation: Use a superscript number (like this: ¹) in the text at the place where you are indicating that you are citing from a source.
Footnote: #. Author(s) First name Last name, “Title of Article,” Journal Title Volume, Issue no. (Year): Page.
Bibliography: Author(s) Last name, First name. “Title of Article.” Journal Title Volume, Issue no. (Year): pages.
Examples:
In-Text Citation: According to Lanscombe, the first Australian health minister to mention global warming in a speech was Peter Baume in 1981.¹
Footnote: 1. Marcus Barber, “Global Warming and the Political Ecology of Health: Emerging Crises and Systemic Solutions,” The Australian Journal of Anthropology 21, no. 3 (2010): 390.
NOTE: When a source is referenced more than once on the same page a shortened form of footnote is used after the first reference, as seen below.
Second footnote: 2. Barber, "Global Warming", 390.
Bibliography: Barber, Marcus. “Global Warming and the Political Ecology of Health: Emerging Crises and Systemic Solutions.” The Australian Journal of Anthropology 21, no. 3 (2010): 390 – 391.
Source: UCD Library