7
Nitrogen
氮
氣佔大氣層78％，是地球最豐富的
氣體。1772年，蘇格蘭醫師丹尼爾·盧瑟福首次分離
氮氣。氮是生存不可缺少的物質，然而地球的高等生
物，多數不能直接利用大氣中的氮，而是從食物中
的硝酸化合物攝取氮元素。固氮菌可以透過固
氮作用將氮氣轉化成氨氮、亞硝酸鹽和硝酸
鹽。這些微生物的存在，讓人思考空氣
中是否必然要有大量的氮氣。氮在大
氣中佔有如此分量，這一事實亦
引發了對其起源的探討。
曾經有學說認為氮可能是彗星留下來的，因為這
些星際天體含有大量的氨（NH
3
）。研究員分析同位
素的訊號或同位素組合，再與假設的源頭對比，就可
以追溯元素的來歷。例如，在大氣中，有7粒中子的
14
N是最豐富的氮同位素，含量達到99.6%；然而，具
有8粒中子的
15
N，也是一個穩定的氮同位素。同位素
訊號就是這兩個穩定同位素的比率。研究結果讓科
學家感到困惑，他們發現地球及太陽風的氮同位素
訊號並不相同，說明彗星不太可能是我們這個星球氮
元素的來源[1]。
近年，再有科學家嘗試破解謎團。德國耶拿市弗
裡德里希·席勒大學的丹尼斯·哈裡斯和他的團隊從
天然的時間囊發現線索。他們分析在1979年從南極
洲發掘的兩顆古老隕石，發現隕石包含一種氮化物晶
體，稱為氮鉻礦。哈裡斯的研究指出其中一顆晶體中
This article may be useful as supplementary reading for
biology classes, based on the DSE syllabus.
根據生物科文憑試課程綱要，本文或可
作為有用的補充讀物。
氮的歷史
By Thomas Lee
李浩賢
which bewildered scientists, it was found that
the isotopic signature of terrestrial nitrogen and
that of solar wind did not match, indicating
that comets are unlikely to be the source of our
planet’s nitrogen [1].
The myster y was not dug up again until
recently. Dennis Harries and his team at the
Friedrich-Schiller University Jena, Germany,
uncovered a cl ue wi th i n a natu ra l t ime
capsule. Their research studied two ancient
meteorites, recovered in 1979 in Antarctica,
and found that a type of nitride crystal called
carlsbergite was encapsulated within. It was
also found that the isotopic signature of one
crystal in particular, had a close value to that
of terrestrial nitrogen [2]. What this suggests is
that the formation of nitrogen on Earth and
that of the meteorite may share a common
origin – the protoplanetary disc, or the rotating
disc of dense gas around a newly formed star
[3].
The nitrogen in carlsbergite is mysterious in
itself. Nitrogen is rarely found in crystals, and is
more typically found as a gas. The team posits
that the formation of carlsbergite involved thin
gas surrounding “freely floating dust grains
covered by ice with the presence of ammonia”
[3] that was interrupted by a large body, which
created a gigantic shock wave. The energy
from this occurrence raised the temperature
