Name: mass accuracy in hrms analysis
Uploaded: 2024-04-04T10:51:34.000Z
Duration: 1 min 15 s
Channel: JoVE

12°C同位体を除いて、元素核の質量は整数に近いですが、正確ではありません。
ただし、マススペクトルは通常、粒子質量を最も近い整数に丸めたものを示し、これは公称原子質量または分子質量と呼ばれます。
この近似により、分子量が類似し、小数点のみが異なる分子を区別することが困難になります。
例えば、シクロペンタノンとシクロヘキサンは、最も近い整数に四捨五入すると、公称分子量が同じになります。このような分子は、高分解能質量分析法を用いて区別することができます。
HRMSは、狭い磁場ステージを使用して、0.0001統一原子質量単位の精度で粒子質量を検出します。高分解能質量分析によって決定された質量は、正確な分子量として知られています。
正確な分子量の詳細を使用して、同じ公称分子量を持つ分子を区別できます。これは、サンプルの分子式を特定するのに役立ちます。

高分解能質量分析 (hrms)

High-Resolution Mass Spectrometry (HRMS)

質量分析計の分解能は、異なるイオン質量を持つイオンを分離する効率に依存します。原子の質量は、陽子と中性子の質量が等しいとみなして、内部の陽子と中性子の質量の合計に近似されます。ただし、陽子 (1.6726 × 10−24 g) と中性子 (1.6749 × 10−24 g) の質量は完全に等しいわけではありません。最も単純な水素原子と比較した原子質量の表現には若干の誤差があります。たとえば、ヘリウムの質量 (陽子 2 個 + 中性子 2 個) は、水素の質量 (陽子 1 個) の正確に 4 倍ではありません。また、複数の核子が原子核内に閉じ込められている場合、位置エネルギーが増加し、質量が減少します（質量とエネルギーの相互変換により）。これが、陽子と中性子の総数がどちらの場合も同じであるにもかかわらず、6 つの重水素 (2H) 原子の質量に比べて 12C の質量が小さい理由です。原子質量は多くの場合、統一原子質量単位 (u) で表され、自由基底状態の 12C 原子の静止質量を 12 で割ったものとして定義されます。 (統一原子質量単位は、記号 Da のダルトンとしても知られています。記号「amu」を付けて「原子質量単位」と呼ばれることがよくありますが、これは酸素に基づく同様の単位の名前と記号であり、統一原子質量単位に置き換えられました。)

自由な基底状態の静止 12C を除くすべての原子核について、正確な原子質量は整数ではありません。その結果、分子量は必ずしも整数ではありません。通常、原子および分子の質量は、最も近い整数 (公称質量と呼ばれます) に近似されます。特定の同位体の質量は、同位体の質量を原子質量定数 (1 u に等しい) で割った相対質量としても表すことができます。Table 1 に、さまざまな同位体の相対質量、正確質量、および公称質量を示す。

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">アイソトープ</td>
			<td style="color: black; border: 1px solid black; text-align: center;">陽子の数</td>
			<td style="color: black; border: 1px solid black; text-align: center;">中性子の数</td>
			<td style="color: black; border: 1px solid black; text-align: center;">相対質量</td>
			<td style="color: black; border: 1px solid black; text-align: center;">正確な質量</td>
			<td style="color: black; border: 1px solid black; text-align: center;">公称質量</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">1H</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">0</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1.0078 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1 u</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">4He</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">4</td>
			<td style="color: black; border: 1px solid black; text-align: center;">4.0026 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">4 u</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">12C</td>
			<td style="color: black; border: 1px solid black; text-align: center;">6</td>
			<td style="color: black; border: 1px solid black; text-align: center;">6</td>
			<td style="color: black; border: 1px solid black; text-align: center;">12</td>
			<td style="color: black; border: 1px solid black; text-align: center;">12.0000 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">12 u</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">14N</td>
			<td style="color: black; border: 1px solid black; text-align: center;">7</td>
			<td style="color: black; border: 1px solid black; text-align: center;">7</td>
			<td style="color: black; border: 1px solid black; text-align: center;">14</td>
			<td style="color: black; border: 1px solid black; text-align: center;">14.0032 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">14 u</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">16O</td>
			<td style="color: black; border: 1px solid black; text-align: center;">8</td>
			<td style="color: black; border: 1px solid black; text-align: center;">8</td>
			<td style="color: black; border: 1px solid black; text-align: center;">16</td>
			<td style="color: black; border: 1px solid black; text-align: center;">15.9949 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">16 u</td>
		</tr>
	</tbody>
</table>

Table 1: 水素、ヘリウム、炭素、窒素、酸素の最も豊富な同位体の相対質量、正確質量、および公称質量。

通常の質量分析計は、少なくとも 1 u 異なるイオンを識別できます。このため、同じ公称質量値を持つ異なる分子を区別することが困難になります。シクロペンタノンとシクロヘキサンの例を考えてみましょう。両方とも 84 u という同じ公称分子量を持っています。ただし、正確な分子量、化学構造、原子数は異なります (シクロペンタノン: (CH2)4CO、84.0573 u、シクロヘキサン: C6H11、84.0936 u)。ただし、高分解能質量分析計は、0.0001 u しか違わないイオンも区別できます。高分解能質量分析計は、微細な磁場ステップを使用して、分析チャンバー内の荷電種をスキャンします。微細なステップは、正確な分子量に応じてイオンの経路を偏向させることにより、同様の分子量を持つイオンを効率的に分離します。

高分解能質量分析 (HRMS)

質量分析計の分解能は、イオン質量の異なるイオンを分離する効率に依存します。原子の質量は、...

Education

JoVE Core

Analytical Chemistry

分析化学

Unit 1

ユニット1

Principles of Mass Spectrometry

質量分析法の原理

KEY TERMS AND CONCEPTS

主要用語と概念

mass accuracy in hrms analysis

High-resolution mass spectrometry separates ions by very small differences in mass. This matters because atomic masses are not always exact whole numbers, even though they are often rounded for simple calculations.

Atomic mass is usually estimated by adding the masses of protons and neutrons. In reality, the proton and neutron do not have exactly the same mass. That creates small differences when atomic masses are compared to the simplest hydrogen atom. Helium, for example, is made of 2 protons and 2 neutrons, but its mass is not exactly four times the mass of hydrogen.

Mass can also change because of energy inside the nucleus. When many nucleons are packed together, potential energy increases and mass decreases through mass-energy interconversion. This is why 12C has a lower mass than six deuterium atoms, even though both contain the same total number of protons and neutrons.

Atomic masses are commonly given in unified atomic mass units, or u. This unit is defined as one-twelfth of the rest mass of a free, ground-state 12C atom. It is also called the dalton, or Da. The older term amu was used for a similar oxygen-based unit that has been replaced.

For most nuclei other than free, ground-state, at-rest 12C, the exact atomic mass is not a whole number. That means molecular mass is not always a whole number either. Scientists often round to the nearest whole number and call that the nominal mass. The mass of an isotope can also be written as a relative mass, which is the isotope mass divided by the atomic mass constant, equal to 1 u. Table 1 shows relative mass, exact mass, and nominal mass for common isotopes such as 1H, 4He, 12C, 14N, and 16O.

Normal mass spectrometers usually separate ions that differ by at least 1 u. That can make it hard to tell apart molecules with the same nominal mass. Cyclopentanone and cyclohexane are one example. Both have a nominal molecular mass of 84 u, but their exact masses, structures, and atom counts are different. Cyclopentanone is (CH2)4CO at 84.0573 u, while cyclohexane is C6H11 at 84.0936 u.

A high-resolution mass spectrometer can separate ions that differ by only 0.0001 u. It uses very small magnetic field steps in the analyzing chamber to scan charged species. These fine steps deflect ions according to exact molecular weight and help distinguish compounds with very similar masses.

Mass Accuracy in HRMS Analysis

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key terms and concepts

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Mass Accuracy in HRMS Analysis | JoVE K12

JoVE Core 分析化学 は、複雑な化学原理をアニメーションで視覚化することで、SI 単位から NMR 分光法を含むトピックをよりわかりやすくします。実験を撮影した実写動画では、これらの概念が世界中の研究所でどのように応用されているかを取り上げています。

analytical chemistry

Mass Accuracy in HRMS Analysis

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