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

A excepción del isótopo 12C, las masas de los núcleos elementales están cerca, pero no precisamente, de los números enteros.
Sin embargo, los espectros de masas suelen mostrar la masa de la partícula redondeada al número entero más cercano, que se denomina masa atómica o molecular nominal.
Esta aproximación hace que sea difícil distinguir moléculas con masas moleculares similares que difieren solo en puntos decimales.
Por ejemplo, la ciclopentanona y el ciclohexano tienen la misma masa molecular nominal cuando se redondean al número entero más cercano. Estas moléculas pueden distinguirse mediante espectrometría de masas de alta resolución.
HRMS utiliza etapas de campo magnético estrechas para detectar masas de partículas con una precisión de 0,0001 unidades de masa atómica unificada. La masa determinada por espectrometría de masas de alta resolución se conoce como masa molecular exacta.
Los detalles de la masa molecular exacta se pueden utilizar para distinguir moléculas con la misma masa molecular nominal. Esto, a su vez, ayuda a identificar la fórmula molecular de la muestra.

espectrometría de masas de alta resolución (hrms)

High-Resolution Mass Spectrometry (HRMS)

La resolución de un espectrómetro de masas depende de la eficacia de separar iones con diferentes masas iónicas. La masa de un átomo se aproxima a la suma de las masas de los protones y neutrones de su interior, considerando iguales las masas de los protones y los neutrones. Sin embargo, las masas del protón (1,6726 × 10−24 g) y del neutrón (1,6749 × 10−24 g) no son realmente iguales. Hay un error menor en la expresión de masas atómicas en relación con el átomo de hidrógeno más simple. Por ejemplo, la masa del helio (2 protones + 2 neutrones) no es precisamente cuatro veces la masa del hidrógeno (1 protón). Además, cuando varios nucleones están confinados en el núcleo, se produce un aumento de la energía potencial y una disminución de la masa (debido a la interconversión de masa y energía). Esta es la razón de la menor masa del 12C en comparación con la masa de seis átomos de deuterio (2H), aunque el número total de protones y neutrones es el mismo en ambos casos. Las masas atómicas a menudo se expresan en unidades de masa atómica unificada (u), definidas como la masa en reposo de un átomo de 12C libre en estado fundamental dividida por 12. (La unidad de masa atómica unificada también se conoce como dalton con el símbolo Da, a menudo se le llama 'unidad de masa atómica' con el símbolo 'amu', que era el nombre y símbolo de una unidad similar basada en oxígeno que fue reemplazada por la unidad de masa atómica unificada).

Para todos los núcleos atómicos excepto el 12C libre, en estado fundamental y en reposo, la masa atómica exacta no es un número entero. Como resultado, la masa molecular no es necesariamente un número entero. Por lo general, las masas atómicas y moleculares se aproximan al número entero más cercano (conocido como masa nominal). La masa de un isótopo determinado también se puede expresar como masa relativa, que es la masa del isótopo dividida por la constante de masa atómica, que es igual a 1 u. Table 1 enumera la masa relativa, la masa exacta y la masa nominal de diferentes isótopos.

<table style="margin-left: auto; margin-right: auto;">
	<thead>
		<tr>
			<th style="color: black; border: 1px solid black; text-align: center;">Isótopo</th>
			<th style="color: black; border: 1px solid black; text-align: center;">No. de protones</th>
			<th style="color: black; border: 1px solid black; text-align: center;">No. de neutrones</th>
			<th style="color: black; border: 1px solid black; text-align: center;">Masa relativa</th>
			<th style="color: black; border: 1px solid black; text-align: center;">Masa exacta</th>
			<th style="color: black; border: 1px solid black; text-align: center;">Masa nominal</th>
		</tr>
	</thead>
	<tbody>
		<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>
	</tbody>
</table>

Table 1: Masa relativa, masa exacta y masa nominal de los isótopos más abundantes de hidrógeno, helio, carbono, nitrógeno y oxígeno.

Los espectrómetros de masas normales pueden distinguir iones que difieren en al menos 1 u. Esto plantea dificultades a la hora de distinguir diferentes moléculas con los mismos valores de masa nominal. Consideremos el ejemplo de la ciclopentanona y el ciclohexano: ambos tienen la misma masa molecular nominal de 84 u; sin embargo, tienen diferentes masas moleculares exactas, estructuras químicas y números de átomos (ciclopentanona: (CH2)4CO, 84,0573 u; ciclohexano: C6H11, 84,0936 u). Sin embargo, un espectrómetro de masas de alta resolución puede incluso diferenciar iones que difieren solo en 0,0001 u. Un espectrómetro de masas de alta resolución emplea diminutos pasos de campo magnético para escanear las especies cargadas en la cámara de análisis. Los pasos diminutos son eficaces para separar iones con pesos moleculares similares al desviar los caminos de los iones según su peso molecular exacto.

Espectrometría de masas de alta resolución (HRMS)

La resolución de un espectrómetro de masas depende de la eficiencia de la separación de iones con diferentes masas iónicas. La masa de un átomo se aproxima a la ...

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Analytical Chemistry

Química Analítica

Unit 1

Unidad 1

Principles of Mass Spectrometry

Principios de la espectrometría de masas

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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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