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

Fatta eccezione per l'isotopo 12C, le masse dei nuclei elementari sono vicine, ma non precisamente, ai numeri interi.
Tuttavia, gli spettri di massa mostrano tipicamente la massa della particella arrotondata al numero intero più vicino, che è chiamato massa atomica o molecolare nominale.
Questa approssimazione rende difficile distinguere molecole con masse molecolari simili che differiscono solo per punti decimali.
Ad esempio, il ciclopentanone e il cicloesano hanno la stessa massa molecolare nominale se arrotondati al numero intero più vicino. Tali molecole possono essere distinte utilizzando la spettrometria di massa ad alta risoluzione.
HRMS utilizza stadi di campo magnetico stretti per rilevare le masse delle particelle con una precisione di 0,0001 unità di massa atomica unificata. La massa determinata dalla spettrometria di massa ad alta risoluzione è nota come massa molecolare esatta.
I dettagli dell'esatta massa molecolare possono essere utilizzati per distinguere molecole con la stessa massa molecolare nominale. Questo, a sua volta, aiuta a identificare la formula molecolare del campione.

spettrometria di massa ad alta risoluzione (hrms)

High-Resolution Mass Spectrometry (HRMS)

La risoluzione di uno spettrometro di massa dipende dall'efficienza di separazione degli ioni con masse ioniche diverse. La massa di un atomo si approssima alla somma delle masse dei protoni e dei neutroni al suo interno, considerando uguali le masse dei protoni e dei neutroni. Tuttavia, le masse del protone (1,6726 × 10−24 g) e del neutrone (1,6749 × 10−24 g) non sono veramente uguali. C'è un piccolo errore nell'espressione delle masse atomiche relative all'atomo più semplice di idrogeno. Ad esempio, la massa dell'elio (2 protoni + 2 neutroni) non è esattamente quattro volte la massa dell'idrogeno (1 protone). Inoltre, quando più nucleoni sono confinati nel nucleo, si verifica un aumento dell'energia potenziale e una diminuzione della massa (dall'interconversione di massa ed energia). Questo è il motivo della massa inferiore di 12C rispetto alla massa di sei atomi di deuterio (2H), anche se il numero totale di protoni e neutroni è lo stesso in entrambi i casi. Le masse atomiche sono spesso espresse in unità di massa atomica unificata (u), definite come la massa a riposo di un atomo 12C libero, allo stato fondamentale, divisa per 12. (L'unità di massa atomica unificata è anche nota come dalton con il simbolo Da. è spesso chiamata "unità di massa atomica" con il simbolo "amu", che era il nome e il simbolo di un'unità simile basata sull'ossigeno che fu sostituita dall'unità di massa atomica unificata.)

Per tutti i nuclei atomici eccetto il 12C libero, allo stato fondamentale e a riposo, la massa atomica esatta non è un numero intero. Di conseguenza, la massa molecolare non è necessariamente un numero intero. Di solito, le masse atomiche e molecolari vengono approssimate al numero intero più vicino (noto come massa nominale). La massa di un dato isotopo può anche essere espressa come massa relativa, che è la massa dell'isotopo divisa per la costante di massa atomica, che è uguale a 1 u. Table 1 elenca la massa relativa, la massa esatta e la massa nominale per diversi isotopi.


<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Isotopo</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Numero di protoni</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Numero di neutroni</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Massa relativa</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Massa esatta</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Massa 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: massa relativa, massa esatta e massa nominale degli isotopi più abbondanti di idrogeno, elio, carbonio, azoto e ossigeno.

I normali spettrometri di massa possono distinguere ioni che differiscono di almeno 1 u. Ciò pone difficoltà nel distinguere diverse molecole con gli stessi valori di massa nominale. Consideriamo l'esempio del ciclopentanone e del cicloesano: entrambi hanno la stessa massa molecolare nominale di 84 u; tuttavia, hanno masse molecolari esatte, strutture chimiche e numeri di atomi diversi (ciclopentanone: (CH2)4CO, 84,0573 u; cicloesano: C6H11, 84,0936 u). Tuttavia, uno spettrometro di massa ad alta risoluzione può anche differenziare ioni che differiscono solo di 0,0001 u. Uno spettrometro di massa ad alta risoluzione utilizza piccoli passi di campo magnetico per scansionare le specie cariche nella camera di analisi. I piccoli passi sono efficienti nel separare ioni con pesi molecolari simili deviando i percorsi degli ioni in base al loro esatto peso molecolare.

Spettrometria di massa ad alta risoluzione (HRMS)

La risoluzione di uno spettrometro di massa dipende dall'efficienza della separazione di ioni con masse ioniche diverse. La massa di un atomo è approssimata al ...

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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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