A 22-year-old Columbia University graduate student distraught over breaking up with her boyfriend plunged to her death from his 16th-floor balcony at a luxury East Side building, police said yesterday.

Earlier on Sunday, Samantha Kwek had texted her boyfriend, Benjamin Small, saying that she had taken OxyContin and that she wanted to come over to his apartment at 100 United Nations Plaza, police sources said.

There was an argument and she said words to the effect that she couldn’t live without him, the sources said.

Then she went out to the balcony and jumped at around 11:40 p.m., the sources said.

The plunge was being treated as a suicide.

“There were text messages between she and her boyfriend about her intentions to kill herself,” a source said.

Yesterday, Small was seen walking outside his building, staring at the ground, his hands in his pockets.

Asked why such a beautiful woman would end her life, he said, “I really don’t know why. She was really, really sad, I guess.’’

When asked how he was dealing with the tragedy, Small replied, “I’m mortified.’’

The family of the beautiful young woman said she was planning to dump her beau — and they don’t believe she would have taken her own life.

“This is really a shock for all of us,’’ brother-in-law Vincenzo Campanella said. “It’s very hard for all of us to believe.’’

Campanella, 31, who is married to Kwek’s sister, said the family has a lot of unanswered questions.

Asked about the possibility of suicide, he said, “I know my sister-in-law. She’s not like that.’’

“To jump out 16 floors takes a lot — to take your life. It all seems a little too fishy,’’ said Campanella, who was mourning with other relatives at Kwek’s home in East Northport, LI. “She’s not one to be attached to a guy like that. She’s strong and smart and involved.’’

He said the family is originally from Singapore. Family members were celebrating the Chinese New Year Sunday.

“We went out for Chinese for dinner for New Year last night. But she got into a fight with him and they didn’t come.

“She said, ‘You know what? I’m done with him,’ ’’ Campanella said.

Asked if she was doing the dumping, Campanella said, “Yes.’

Detectives were to meet with Kwek’s family yesterday.