When I applied for my PhD, I used a chapter from my MA's thesis. The sample was about a relatively unknown memoir. It's fine because the committee is looking for a demonstration of your ability to do research and do rigorous analysis.
I recommend looking into Washington University in St. Louis (WashU). Within English, they have Faculty Research Areas in Transatlantic and Transnational, Irish Literature, African American Literature, and Postcolonial Literature.