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

À l’exception de l’isotope 12C, les masses des noyaux élémentaires sont proches, mais pas précisément, des nombres entiers.
Cependant, les spectres de masse montrent généralement la masse des particules arrondie au nombre entier le plus proche, appelé masse atomique ou moléculaire nominale.
Cette approximation rend difficile la distinction de molécules de masses moléculaires similaires qui ne diffèrent que par des décimales.
Par exemple, la cyclopentanone et le cyclohexane ont la même masse moléculaire nominale lorsqu’ils sont arrondis au nombre entier le plus proche. De telles molécules peuvent être distinguées à l’aide de la spectrométrie de masse à haute résolution.
Le HRMS utilise des étages de champ magnétique étroits pour détecter les masses des particules avec une précision de 0,0001 unité de masse atomique unifiée. La masse déterminée par spectrométrie de masse à haute résolution est connue sous le nom de masse moléculaire exacte.
Les détails de la masse moléculaire exacte peuvent être utilisés pour distinguer les molécules ayant la même masse moléculaire nominale. Ceci, à son tour, aide à identifier la formule moléculaire de l’échantillon.

spectrométrie de masse à haute résolution (hrms)

High-Resolution Mass Spectrometry (HRMS)

La résolution d'un spectromètre de masse dépend de l'efficacité de la séparation des ions de masses ioniques différentes. La masse d'un atome est approchée par la somme des masses de protons et de neutrons à l'intérieur, en considérant les masses de protons et de neutrons comme égales. Cependant, les masses du proton (1,6726 × 10−24 g) et du neutron (1,6749 × 10−24 g) ne sont pas vraiment égales. Il existe une erreur mineure dans l’expression des masses atomiques par rapport à l’atome d’hydrogène le plus simple. Par exemple, la masse de l’hélium (2 protons + 2 neutrons) n’est pas exactement quatre fois celle de l’hydrogène (1 proton). De plus, lorsque plusieurs nucléons sont confinés dans le noyau, cela provoque une augmentation de l'énergie potentielle et une diminution de la masse (due à l'interconversion de la masse et de l'énergie). C'est la raison pour laquelle la masse du 12C est inférieure à celle de six atomes de deutérium (2H), même si le nombre total de protons et de neutrons est le même dans les deux cas. Les masses atomiques sont souvent exprimées en unités de masse atomique unifiées (u), définies comme la masse au repos d'un atome 12C libre à l'état fondamental divisée par 12. (L'unité de masse atomique unifiée est également connue sous le nom de dalton avec le symbole Da. Elle est souvent appelée « unité de masse atomique » avec le symbole « amu », qui était le nom et le symbole d'une unité similaire basée sur l'oxygène qui a été remplacée par l'unité de masse atomique unifiée.)

Pour tous les noyaux atomiques, à l’exception du 12C libre, à l’état fondamental et au repos, la masse atomique exacte n’est pas un nombre entier. La masse moléculaire n’est donc pas nécessairement un nombre entier. Habituellement, les masses atomiques et moléculaires sont approximées au nombre entier le plus proche (appelé masse nominale). La masse d'un isotope donné peut également être exprimée en masse relative, qui est la masse de l'isotope divisée par la constante de masse atomique, qui est égale à 1 u. Table 1 répertorie la masse relative, la masse exacte et la masse nominale pour différents isotopes.

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Isotope</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nombre de protons</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nombre de neutrons</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Masse relative</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Masse exacte</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Masse nominale</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 : La masse relative, la masse exacte et la masse nominale des isotopes les plus abondants de l'hydrogène, de l'hélium, du carbone, de l'azote et de l'oxygène.

Les spectromètres de masse normaux peuvent distinguer les ions qui diffèrent d'au moins 1 u. Cela pose des difficultés pour distinguer différentes molécules ayant les mêmes valeurs de masse nominale. Prenons l'exemple de la cyclopentanone et du cyclohexane : tous deux ont la même masse moléculaire nominale de 84 u ; cependant, ils ont des masses moléculaires exactes, des structures chimiques et des nombres d'atomes différents (cyclopentanone : (CH2)4CO, 84,0573 u ; cyclohexane : C6H11, 84,0936 u). Cependant, un spectromètre de masse à haute résolution peut même différencier des ions qui ne diffèrent que de 0,0001 u. Un spectromètre de masse à haute résolution utilise de minuscules étapes de champ magnétique pour scanner les espèces chargées dans la chambre d'analyse. Les étapes infimes sont efficaces pour séparer les ions de poids moléculaire similaire en déviant les chemins des ions en fonction de leur poids moléculaire exact.

Spectrométrie de masse à haute résolution (HRMS)

La résolution d’un spectromètre de masse dépend de l’efficacité de la séparation des ions de différentes masses d’ions. La masse d’un atome est approchée de la ...

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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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Mass Accuracy in HRMS Analysis

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