We give criteria for subcategories of a compactly generated algebraic triangulated
category to be precovering or preenveloping. These criteria are formulated in terms of
closure conditions involving products, coproducts, directed homotopy colimits and
further conditions involving the notion of purity. In particular, we provide sufficient
closure conditions for a subcategory of a compactly generated algebraic triangulated
category to be a torsion class. Finally we explore applications of the previous results
to the theory of recollements.