Had Alexander lost the fight, Purdy would not have claimed his belt as the Colchester fighter failed to make weight for a bout he accepted at four weeks' notice.
However under the rules the American's victory goes down a defence of the title he won from Randall Bailey last October.
Alexander was a class above Purdy throughout, landing slick right hooks and uppercuts from start to finish while the Englishman, staying close to his man with a heavy guard at all times, was only able to land the occasional decent shot.
Purdy's close friend and ranking middleweight boxer Darren Barker, running a corner for the first time, made the decision to withdraw his fighter before the start of the eighth round, much to the chagrin of 'Lights Out'.
It was the right call, however: Purdy was simply not offering enough offence after seven to convince anyone that he had a late rally up his sleeve after soaking up all that abuse.
Purdy nonetheless showed his toughness against a top competitor at short notice, while Alexander said after the fight he is now targeting a big bout, against someone such as Floyd Mayweather or Amir Khan, after cruising to victory despite hurting his left hand in round two.
There were some success stories to come from the Atlantic City card from a British standpoint, as professional greenhorns Anthony Ogogo and Haroon Khan, kid brother of Amir, each claimed their second victories.
Ogogo looked solid in a six-round shut-out over Puerto Rican Edgar Perez, while Khan - who by his own admission suffered a bit of stage fright in his winning debut in Sheffield last month - introduced some killer offense in wiping out no-hoper Vincente Medellin in just 57 seconds.
Ogogo had IBF light-heavyweight champion and the oldest-ever world title-holder Bernard Hopkins offering him advice both before and during the bout and admitted it was an honour to continue his progress under the eye of an idol of his.
"That was a dream fulfilled," he told Sky Sports. "To have Bernard coaching me from the outside was amazing.
"I don’t want to blow everyone away. [Promoter] Richard Schaefer could have got someone who would have taken part for a round or two but I needed someone durable.
"It was a massive learning curve. I could have done another six rounds, I enjoyed it so much."