3He is three million times more rare than 4 He. An increase in the amount of 3He in the deep ocean compared to the ambient ocean water is due to the release of 3He from the mantle. It turns out that the earth's air also has a similar ratio of 3He to 4He to that of deep ocean water that hasn' t been enriched. The special copper tubing is capped at both ends to keep it very clean before it is filled with the samples of deep ocean water. In the filling process both caps are removed and sample water from the nisken bottle flows through the copper tube to flush it out.
Some air can cling to the inside of the tube walls as tiny bubbles during the initial collection.
To release these bubbles the tube is vigorously tapped up and down its' length. In the lab work area the tubes are clamped shut and when the tube is shaken it has a special metallic sound that lets the scientist know all the air has been removed from the tube. The tubes are sent back to the Newport, Oregon lab so the water samples can be analyzed using a mass spectrometer. How much the 3He to 4He ratio of this deep ocean sample deviates from the Earth's air 3He to 4He ratio is determined. He is an inert gas that has stable isotopes that are not affected by the living organisms in the ocean.