He worked in Monrovia, Liberia from 1980 to 1981 and in Vienna, Austria from 1988 to 1989 as a member of the U.S. conventional arms control delegation.

In the aftermath of 9/11, Uzbekistan had become an area of heightened importance to the US, owing to its border with Afghanistan, and a strategic ally in the War on Terror.

One of the most serious events that Purnell had to deal with was the May 2005 Andijan massacre, in which the Uzbek government killed hundreds, possibly thousands, of civilians.

Calls from Western governments, including the US, for an international investigation prompted a major shift in Uzbek foreign policy favoring closer relations with Asian nations instead of with the West.

In July of that year, the Uzbek government ordered the closing of the United States air base in Karshi-Khanabad, which was accomplished by November.