An industrial gas supply firm and its chief executive officer failed to take necessary measures to ensure workplace safety, leading to a fatal blast that killed a chemist in 2015.

New mother Lim Siaw Chian, 30, was killed and at least seven others were injured after an explosion at Leeden National Oxygen's Specialty Gas Centre Quality Control Laboratory on Oct 12 that year.

Ms Lim had returned to work from maternity leave a week before the tragedy at the laboratory in Tanjong Kling Road near Pioneer Road.

The former Malaysian had received her Singapore citizenship just a month before she died. She left behind a daughter who was six months old at the time.

Leeden National Oxygen and its CEO Steven Tham Weng Cheong, now 69, each pleaded guilty yesterday to an offence under the Workplace Safety and Health Act.

Among other things, they had failed to take measures to ensure that unsafe modified regulatory valve assemblies were not used when testing combustible gases. They had also failed to ensure that there was a system in place for the accurate tracking of gas cylinders.

Ministry of Manpower prosecutor Erdiana Hazlina said that Ms Lim was carrying out gas analysis on a cylinder shortly before the explosion occurred. Ms Lim was last seen touching a regulatory valve assembly (RVA) connected to a cylinder, the court heard.

Ms Erdiana added: "The said RVA was found to have been modified with an unqualified welded joint. The failure of the welded joint... was found to have pre-existed but was not prudently checked before usage prior to the incident.

"The failure of the unqualified welded joint... resulted in a leak of flammable methane-oxygen-nitrogen mix from the RVA during the testing of the cylinder."

The court heard that the leaking gas mixture could have been ignited by the frictional heat generated due to the escaping gas mixture.

An explosion occurred and the ensuing blaze engulfed the ground-floor laboratory. Officers from the Singapore Civil Defence Force were then deployed to the scene.

Ms Lim's charred remains were later found on six occasions over a two-month period. They were identified using her daughter's DNA.

In 2016, then State Coroner Marvin Bay found that Ms Lim had died of blast injuries. He also found her death to be an industrial misadventure.

Both offenders are expected to be sentenced on Jan 12 next year.