Lee John Wright, 41, of Mill Street, Farington, Leyland, was found guilty by magistrates in his absence, but was arrested and hauled before the Crown Court.
The court was told Wright had believed the woman’s partner was grooming a child he knew.
Prosecuting, Emma Keogh said: “ She and the defendant had known each other for about six months through others.
“The matter took place on November 17 last year. Sara Hesmondhalgh had gone to bed at her home on Mosley Street - an address she shares with her three children, who were also in bed.
“She was awoken by her dog barking and then heard shouting.
“Dressed in her nightclothes she put on her jacket and went out to look for her dog. She went down the back alley and as she bent down to grab her dog she became aware the defendant was present. She became immediately aware he had a syringe and noticed it contained a bright yellow liquid.
“Instantly she felt burning to her chest and face and she screamed out in pain. She describes herself being hysterical. She says she was struggling to breathe.
“Her boyfriend was present and assisted her and he contacted police.”
When Wright was arrested less than an hour later a credit card shaped knife was found in his wallet.
His defence lawyer said he had legitimate concerns about Hesmondhalgh’s partner.
Recorder Andrew Long jailed him for eight months.