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

Exceto pelo isótopo 12C, as massas dos núcleos elementares estão próximas - mas não precisamente - de números inteiros.
No entanto, os espectros de massa normalmente mostram a massa da partícula arredondada para o número inteiro mais próximo, que é chamado de massa atômica ou molecular nominal.
Essa aproximação dificulta a distinção de moléculas com massas moleculares semelhantes que diferem apenas por pontos decimais.
Por exemplo, ciclopentanona e ciclohexano têm a mesma massa molecular nominal quando arredondados para o número inteiro mais próximo. Essas moléculas podem ser distinguidas usando espectrometria de massa de alta resolução.
O HRMS usa estágios de campo magnético estreitos para detectar massas de partículas com uma precisão de 0,0001 unidades de massa atômica unificadas. A massa determinada por espectrometria de massa de alta resolução é conhecida como massa molecular exata.
Os detalhes da massa molecular exata podem ser usados para distinguir moléculas com a mesma massa molecular nominal. Isso, por sua vez, ajuda a identificar a fórmula molecular da amostra.

espectrometria de massa de alta resolução (hrms)

High-Resolution Mass Spectrometry (HRMS)

A resolução de um espectrômetro de massa depende da eficiência de separação de íons com diferentes massas iônicas. A massa de um átomo é aproximadamente a soma das massas dos prótons e nêutrons em seu interior, considerando as massas dos prótons e nêutrons iguais. No entanto, as massas do próton (1,6726 × 10−24 g) e do nêutron (1,6749 × 10−24 g) não é verdadeiramente iguais. Há um pequeno erro na expressão das massas atômicas em relação ao átomo mais simples de hidrogênio. Por exemplo, a massa do hélio (2 prótons + 2 nêutrons) não é precisamente quatro vezes a massa do hidrogênio (1 próton). Além disso, quando múltiplos núcleons estão confinados no núcleo, provoca um aumento na energia potencial e uma diminuição na massa (da interconversão de massa e energia). Essa é a razão para uma massa menor de 12C em comparação com a massa de seis átomos de deutério (2H), embora os números totais de prótons e nêutrons sejam os mesmos em ambos os casos. As massas atômicas é frequentemente expressas em unidades de massa atômica unificada (u), definidas como a massa de repouso de um átomo 12C livre no estado fundamental, dividido por 12. (A unidade de massa atômica unificada também é conhecida como Dalton, com o símbolo Da. É é frequentemente chamada de 'unidade de massa atômica', com o símbolo 'amu', que era o nome e o símbolo de uma unidade similar baseada em oxigênio e foi substituída pela unidade de massa atômica unificada.)

Para todos os núcleos atômicos, exceto 12C livre, no estado fundamental e em repouso, a massa atômica exata não é um número inteiro. Como resultado, a massa molecular não é necessariamente um número inteiro. Normalmente, as massas atômicas e moleculares é aproximadas ao número inteiro mais próximo (conhecido como massa nominal). A massa de um determinado isótopo também pode ser expressa como massa relativa, que é a massa do isótopo dividida pela constante de massa atômica, que é igual a 1 u. A Table 1 lista a massa relativa, a massa exata e a massa nominal para diferentes isótopos.

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Isótopo</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nº de prótons</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nº de nêutrons</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 exata</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Massa nominal</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 exata e massa nominal dos isótopos mais abundantes de hidrogênio, hélio, carbono, nitrogênio e oxigênio.

Os espectrômetros de massa normais podem distinguir íons que diferem em pelo menos 1 u. Isso representa dificuldade em distinguir diferentes moléculas com os mesmos valores de massas nominais. Considere o exemplo da ciclopentanona e do ciclo-hexano: ambos têm a mesma massa molecular nominal de 84 u; no entanto, eles têm massas moleculares exatas, estruturas químicas e números de átomos diferentes (ciclopentanona: (CH2)4CO, 84.0573 u; ciclo-hexano: C6H11, 84.0936 u). No entanto, um espectrômetro de massa de alta resolução pode diferenciar íons que diferem apenas em 0,0001 u. Um espectrômetro de massa de alta resolução emprega passos minúsculos de campo magnético para escanear as espécies carregadas na câmara de análise. As etapas minúsculas é eficientes na separação de íons com pesos moleculares semelhantes, desviando os caminhos dos íons de acordo com o peso molecular exato deles.

Espectrometria de Massa de Alta Resolução (HRMS)

A resolução de um espectrômetro de massa depende da eficiência de separar íons com diferentes massas de íons. A massa de um átomo é aproximada ao ...

Education

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidade 1

Principles of Mass Spectrometry

Princípios da Espectrometria de Massas

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

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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