During his first collegiate season in 2021, Purnell used the season to
redshirt. In week seven of the 2022 season, he entered the game for an injured
Khalid Duke, where he finished with three tackles and two pass deflections in a win over
Iowa State. Purnell finished the 2022 season, totaling 21 tackles, two pass deflections, and a fumble recovery. In the 2023 season, he made 13 starts, where he notched 52 tackles, an interception, three forced fumbles, two fumble recoveries, and a touchdown. Purnell finished the 2024 season playing in all 13 games with 12 starts, racking up 46 tackles with nine and a half being for a loss and four sacks. He entered the 2025 season as one of the Wildcats top players and leaders, as he was selected to attend the Big 12 media days. In week seven of the 2025 season, he tallied five tackles with two going for a loss, a sack, and two interceptions, one of which he returned 25 yards for a touchdown in a victory over
TCU. ==Professional career==