Elemental analysis - elementary analysis

A method to detect the constituent elements of organic substances and determine their content. Usually, when simply referring to elemental analysis, it often means quantitative elemental analysis, especially the analysis of carbon, hydrogen, and nitrogen. There are various detection methods, i.e. qualitative elemental analysis methods, but the principle is the same in all of them, which is to decompose organic compounds into simple inorganic substances and detect them according to the respective method. In most cases, carbon and hydrogen are contained, so detection of carbon and hydrogen is often not performed. Quantitative elemental analysis methods are divided into ordinary method (about 0.2 to 0.3 grams), semi-micro method (also called semi-micro method, about 1/10 of ordinary method), trace method (also called micro method, about 1/100 of ordinary method), and ultra-trace method (100 micrograms or less) depending on the amount of sample to be analyzed. However, with the remarkable advances in analytical equipment, balances, and purity of reagents, organic elemental analysis is almost always performed by the trace method except in special cases. A wide variety of quantitative methods have been reported, but most of them involve first quantitatively decomposing the sample without loss or contamination, converting it into substances that are easy to measure, and then quantifying each of them using an appropriate method. The conversion methods can be broadly divided into combustion methods in a carrier gas and wet methods in which a decomposition agent is added in a liquid. Quantitative analysis can be performed using various methods, including gravimetric methods in which the decomposition products are absorbed in an appropriate absorbent and the increase in the amount is measured, volumetric methods by titration, and others. For example, carbon and hydrogen are analyzed by completely burning the sample in the presence of oxygen, oxidizing them to carbon dioxide and water, respectively, and measuring their amounts by capturing them in an appropriate manner. Nitrogen is often determined by the Dumas method (combustion method), in which a sample is completely burned in a carbon dioxide gas stream with a combustion aid to convert it into nitrogen gas _N2 , which is collected in an azotometer (nitrogen meter) filled with an alkaline solution and its volume is measured, and the Kjeldahl method (wet method), in which a decomposition agent is added to the sample, the sample is converted to ammonia, and the ammonia is collected in a collection agent. There are many other methods for measuring the amount of halogen, such as the Carius method, which converts halogens into silver halide and measures their weight, and the method for measuring sulfur by converting it into barium sulfate and measuring its weight. Various analytical methods have also been devised for oxygen, phosphorus, arsenic, and other metals.

Since organic substances are composed of only a small number of elements, it is difficult to determine the type of unknown compound by elemental analysis alone. Instead, the percentages of the constituent elements are determined separately using the methods mentioned above to determine the composition formula, and then the molecular formula is derived from the molecular weight determined by another method, and the original compound is then identified.

In addition, in a broad sense, elemental analysis also includes the analysis of elements in unknown substances using physical methods, such as X-rays or other electromagnetic waves, or mass spectrometry, in parallel with chemical methods.

[Takada Takeo]

"Practical Analysis of Trace Elements" by C. Vandecasteele, translated by Hiroki Haraguchi et al. (1995, Maruzen)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

有機物質の構成元素を検出し、その含有量を定める方法。普通、単に元素分析というときは、定量元素分析、とくに炭素、水素、窒素の分析を行うことをいう場合が多い。検出法、すなわち定性元素分析法は種々あるが、その原理はいずれも同じで、有機化合物を分解して簡単な無機体とし、それをそれぞれの方法に従って検出する。ほとんどの場合、炭素と水素が含まれているので、炭素と水素の検出は行わないことが多い。定量元素分析法は、分析に供する試料の量により、常量法（0.2～0.3グラム程度）、半微量法（セミミクロ法ともいい、常量法の10分の1程度）、微量法（ミクロ法ともいい、常量法の100分の1程度）、超微量法（100マイクログラム以下）などに分けられるが、現在では分析装置、天秤(てんびん)、試薬の純度などが格段と進歩し、特別な場合を除いて有機元素分析はほとんど微量法で行われている。定量法には多岐多様な方法が報告されているが、そのほとんどは、まず試料を損失、汚染なく定量的に分解し、これを測定が容易な物質に変換したのちに、それぞれについて適当な方法で定量する方法がとられている。変換の方法を原理的に大別すると、キャリヤーガス中における燃焼法と、液体中で分解剤を加えて行う湿式法とになる。定量は、分解生成物を適当な吸収剤に吸収させて増加量を測定する重量法、滴定による容量法、その他いろいろな方法が行われている。たとえば、炭素、水素の分析は、酸素の存在下で完全燃焼させ、それぞれ二酸化炭素と水とに酸化し、適当な方法で捕捉(ほそく)してその量を測定することによって同時に決定する。窒素は、炭素や水素とは別に、試料を二酸化炭素気流中で燃焼補助剤とともに完全燃焼させて窒素ガスN₂に変え、これをアルカリ液を満たしたアゾトメーター（窒素計）に補集してその体積を測定するデュマ法（燃焼法）、試料に分解剤を加え、アンモニアに変換後、補集剤に補集するケルダール法（湿式法）などがよく行われる。ハロゲンは、ハロゲン化銀に変えてその重量を計るカリウス法、硫黄(いおう)は、硫酸バリウムとしてその重量から求める方法、そのほかにも数多くの方法がある。酸素、リン、ヒ素、その他の金属についても種々の分析法が考案されている。

　有機物質は少数の種類の元素から構成されるので、元素分析だけで未知化合物の種類までを決定するのは困難であり、構成成分元素の百分率を以上の諸方法でそれぞれ単独に求めて組成式を決め、さらに他の方法で求めた分子量から分子式を導き、そのうえで元の化合物を判別するという手続をとる。

　そのほか化学的方法と並行して、物理的方法、たとえばX線その他の電磁波や、質量分析などの方法を使って未知物質の元素の分析を行うことも広義には元素分析といえる。

［高田健夫］

『C. Vandecasteele著、原口紘気他訳『微量元素分析の実際』（1995・丸善）』

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