元素分析Elemental analysis

样品在约 1000 °C 的纯氧中燃烧，C 转化为 CO₂、H 转化为 H₂O、N 转化为氮氧化物再经铜还原为 N₂、S 转化为 SO₂，产物经色谱柱分离后用热导检测器定量。氧含量通过在惰性气氛中高温裂解生成 CO 测定。根据各元素质量分数除以原子量得到原子数比，即可推出实验式。学术期刊通常要求新化合物实测值与计算值偏差在 ±0.4% 以内。

The sample burns in pure oxygen at about 1000 °C: carbon to CO₂, hydrogen to H₂O, nitrogen to oxides reduced over copper to N₂, and sulfur to SO₂; the gases are separated on a column and measured with a thermal conductivity detector. Oxygen is found by pyrolysis in inert gas to CO. Dividing each mass fraction by atomic weight gives atom ratios and hence the empirical formula. Journals usually require found values within ±0.4% of calculated ones for new compounds.

1831 年德国化学家李比希发明了用氧化铜燃烧有机物、以氯化钙管吸收水和以五球钾碱管（李比希钾球）吸收 CO₂ 的方法，使有机元素分析变得准确可靠，奠定了有机化学的定量基础。1833 年杜马发明定氮法；1883 年凯氏定氮法问世；1911 年普雷格尔发展微量分析（1923 年诺贝尔化学奖），所需样品从数百毫克降到几毫克。1960 年代起自动元素分析仪普及。

In 1831 the German chemist Justus von Liebig devised combustion over copper oxide, absorbing water in a calcium chloride tube and CO₂ in his five-bulb potash apparatus, making organic elemental analysis accurate and reliable and giving organic chemistry its quantitative footing. Dumas devised his nitrogen method in 1833 and Kjeldahl his in 1883. Pregl developed microanalysis from 1911 (Nobel Prize in Chemistry 1923), cutting sample sizes from hundreds of milligrams to a few. Automated analysers spread from the 1960s.

金属和微量元素常用原子吸收光谱、电感耦合等离子体发射光谱或质谱（ICP-OES、ICP-MS）、X 射线荧光光谱测定；卤素和硫可用氧瓶燃烧法后滴定或离子色谱测定；食品和饲料中的蛋白质含量常用凯氏定氮法（总氮 × 6.25）测定——2008 年三聚氰胺事件正是利用了这一方法只测氮不辨来源的缺陷。

Metals and trace elements are measured by atomic absorption, inductively coupled plasma emission or mass spectrometry (ICP-OES, ICP-MS) and X-ray fluorescence; halogens and sulfur by oxygen-flask combustion followed by titration or ion chromatography; and protein in food and feed by the Kjeldahl method (total nitrogen × 6.25), a flaw exploited in the 2008 melamine scandal because it measures nitrogen without regard to its source.