Two words mentioned in the Qur'an are mukhlis (with I) and mukhlas (with A). They come from the same root word as "ikhlaas," which means sincerity.
A "mukhlis" (doer of the action) is the one who strives for sincerity (with intentions etc.) and is purely devoted to Allah.Surah Ghafir: Ajah 14:
فَٱدۡعُواْ ٱللَّهَ مُخۡلِصِينَ لَهُ ٱلدِّينَ وَلَوۡ كَرِهَ ٱلۡكَـٰفِرُونَ
Call ye, then upon Allah with sincere devotion to Him, even though the Unbelievers may detest it.
A "mukhlas" (actually Mukhlasan) who is chosen by Allah for his devotion and sincerity to Allah and is endowed with it by Allah. Usually "Mukhlasan" refers to the Prophets (Peace be upon all of them) in Qur'an. Surah As-Saffaat: Ayah 74
إِلَّا عِبَادَ ٱللَّهِ ٱلۡمُخۡلَصِينَ
Except the sincere (and devoted) servants of Allah.