The study examined the ink used to draw the Vinland Map, which belongs to Yale University. The map is valued at $20 million - if it is real and not a clever, modern-day forgery.
A study last summer said the ink on the parchment map was made in the 20th century.
But chemist Jacqueline Olin, a retired researcher with the Smithsonian Institution in Washington, said Tuesday her analysis shows the ink was made in medieval times.
"There is no evidence this is a forged titanium dioxide ink," said Olin, whose paper appears in the December issue of the journal Analytical Chemistry.
The authenticity of the map has been debated since the 1960s, when philanthropist Paul Mellon gave it to Yale. The university has not taken a position on its authenticity.
The map depicts the world, including the north Atlantic coast of North America. It includes text in medieval Latin and a legend that describes how "Leif Eiriksson," a Norseman, found the new land called Vinland around the year 1000.
Scholars have dated the map to around 1440. Some scholars have speculated that Columbus could have used the map to find the New World in 1492.
Last summer, Olin and other researchers announced that carbon-14 dating of the parchment showed it was made around 1434 - exactly the right time for the map to be genuine.
However, researchers from University College in London examined the ink on the map and announced last summer that it cannot be more than 500 years old.
Tests in the 1970s by Walter McCrone - who also had disputed the authenticity of the Shroud of Turin - found the ink contained anatase, a form of titanium dioxide that is common in inks made after 1920. Anatase is found in nature, but the crystals of anatase were too regular-shaped to have been natural, McCrone said.
Olin's study looked at various minerals found in the ink, including aluminum, copper and zinc. All these minerals, she said, would have been byproducts of the medieval ink manufacturing process.
Also, she said anatase also could have ended up in the ink because of the manufacturing process, and its crystal size and shape could have changed over time.
Research is continuing into the Latin writing on the map.
By Diane Scarponi