Because of its nuclear spin properties (spin +1/2, just like the hydrogen atom), this isotope responds to a resonant radiofrequency(RF) signal. The absorption and emission of the RF signal by the nuclei can be monitored and detected using NMR spectroscopy, a technique that gives information on the the identity and number of atoms adjacent to other atoms in said molecule = thereby giving clues to the structure of an organic molecule. C-12 has zero spin and does give a signal in NMR and since only 1% of the atoms in a molecule are C-13, it is unlikely that carbon-carbon coupling is seen and acquiring a carbon-13 NMR spectrum can take much time, often a couple of minutes or hours, because many scans have to be added together in order to have results distinguishable from background noise.
Detection by mass spectroscopy A mass spectrogram of an organic compound will usually contain a small peak of one mass unit greater than the apparent molecular ion peak (M). This is known as the M+1 peak and originates due to the presence of carbon-13 atoms. A molecule containing one carbon atom will be expected to have an M+1 peak of approximately 1.1% of the size of the M peak as 1.1% of the carbon atoms will be carbon-13 rather than carbon-12. Similarly a molecule containing two carbon atoms will be expected to have an M+1 peak of approximately 2.2% of the size of the M peak, as there is double the previous likelihood that a molecule will contain a carbon-13 atom.
In the above the mathematics and chemistry have been simplified, however it can be used effectively to give the number of carbon atoms for small to medium sized organic molecules. In the following formula the result should be rounded to the nearest integer:
C = number of C atoms X = amplitude of the M ion peak Y = amplitude of the M+1 ion peak
Carbon-13-enriched compounds are used in the research of metabolic processes by means of mass spectroscopy. Such compounds are safe because they are non-radioactive.en:Carbon-13 fi:Hiili-13
放射性同位素C-14的應用
自然界中碳元素有三種同位素,即穩定同位素12C、13C和放射性同位素14C,14C的半衰期為5730年,14C的應用主要有兩個方面：一是在考古學中測定生物死亡年代,即放射性測年法；二是以14C標記化合物為示蹤劑,探索化學和生命科學中的微觀運動.
一、14C測年法
自然界中的14C是宇宙射線與大氣中的氮透過核反應產生的.碳-14不僅存在於大氣中,隨著生物體的吸收代謝,經過食物鏈進入活的動物或人體等一切生物體中.由於碳-14一面在生成,一面又以一定的速率在衰變,致使碳-14在自然界中（包括一切生物體內）的含量與穩定同位素碳-12的含量的相對比值基本保持不變.
當生物體死亡後,新陳代謝停止,由於碳-14的不斷衰變減少,因此體內碳-14和碳-12含量的相對比值相應不斷減少.透過對生物體出土化石中碳-14和碳-12含量的測定,就可以準確算出生物體死亡（即生存）的年代.例如某一生物體出土化石,經測定含碳量為M克（或碳-12的質量）,按自然界碳的各種同位素含量的相對比值可計算出,生物體活著時,體內碳-14的質量應為 m克.但實際測得體內碳-14的質量內只有m克的八分之一,根據半衰期可知生物死亡已有了3個5730年了,即已死亡了一萬七千二百九十年了.美國放射化學家W．F．利比因發明了放射性測年代的方法,為考古學做出了傑出貢獻而榮獲1960年諾貝爾化學獎.
由於碳-14含量極低,而且半衰期很長,所以用碳-14只能準確測出5～6萬年以內的出土文物,對於年代更久遠的出土文物,如生活在五十萬年以前的周口店北京猿人,利用碳-14測年法是無法測定出來的.
二、碳-14標記化合物的應用
碳-14標記化合物是指用放射性14C取代化合物中它的穩定同位素碳-12,並以碳-14作為標記的放射性標記化合物.它與未標記的相應化合物具有相同的化學與生物學性質,不同的只是它們帶有放射性,可以利用放射性探測技術來追蹤.
自 20世紀 40年代,就開始了碳-14標記化合物的研製、生產和應用.由於碳是構成有機物三大重要元素之一,碳-14半衰期長,β期線能量較低,空氣中最大射程 22cm,屬於低毒核素,所以碳-14標記化合物產品應用範圍廣.至80年代,國際上以商品形式出售的碳-14標記化合物,包括了氨基酸、多肽、蛋白質、糖類、核酸類、類脂類、類固醇類及醫學研究用的神經藥物、受體、維生素和其他藥物等,品種已達近千種,約佔所有放射性標記化合物的一半.
以碳-14為主的標記化合物在醫學上還廣泛用於體內、體外的診斷和病理研究.用於體外診斷的競爭放射性分析是本世紀60年代發展起來的微量分析技術.應用這種技術只要取很少量的體液（血液或尿液）在化驗室分析後,即可進行疾病診斷.由於競爭放射性分析體外診斷的特異性強,靈敏度高,準確性和精密性好,許多疾病就可能在早期發現,為有效防治疾病提供了條件.
碳-14標記化合物作為靈敏的示蹤劑,具有非常廣泛的應用前景.
碳13是碳的穩定同位素之一,在地球自然界的碳中佔約1.109%.
Because of its nuclear spin properties (spin +1/2, just like the hydrogen atom), this isotope responds to a resonant radiofrequency(RF) signal. The absorption and emission of the RF signal by the nuclei can be monitored and detected using NMR spectroscopy, a technique that gives information on the the identity and number of atoms adjacent to other atoms in said molecule = thereby giving clues to the structure of an organic molecule. C-12 has zero spin and does give a signal in NMR and since only 1% of the atoms in a molecule are C-13, it is unlikely that carbon-carbon coupling is seen and acquiring a carbon-13 NMR spectrum can take much time, often a couple of minutes or hours, because many scans have to be added together in order to have results distinguishable from background noise.
Detection by mass spectroscopy A mass spectrogram of an organic compound will usually contain a small peak of one mass unit greater than the apparent molecular ion peak (M). This is known as the M+1 peak and originates due to the presence of carbon-13 atoms. A molecule containing one carbon atom will be expected to have an M+1 peak of approximately 1.1% of the size of the M peak as 1.1% of the carbon atoms will be carbon-13 rather than carbon-12. Similarly a molecule containing two carbon atoms will be expected to have an M+1 peak of approximately 2.2% of the size of the M peak, as there is double the previous likelihood that a molecule will contain a carbon-13 atom.
In the above the mathematics and chemistry have been simplified, however it can be used effectively to give the number of carbon atoms for small to medium sized organic molecules. In the following formula the result should be rounded to the nearest integer:
C = number of C atoms X = amplitude of the M ion peak Y = amplitude of the M+1 ion peak
Carbon-13-enriched compounds are used in the research of metabolic processes by means of mass spectroscopy. Such compounds are safe because they are non-radioactive.en:Carbon-13 fi:Hiili-13
取自"http://www.wiki.cn/wiki/%E7%A2%B313"
12C是質子數和中子都為6的碳原子,它是碳元素的一種同位素,在世界現存碳元素中丰度為98.89%,是最常見的碳同位素.
碳12原子被用來作為阿伏伽德羅常數的標準：12克碳12中所含原子的個數被定義為阿伏伽德羅常數6.022×1023.