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

Except for the 12C isotope, the masses of elemental nuclei are close to&#8212;but not precisely&#8212;whole numbers.
However, mass spectra typically show the particle mass rounded to the nearest whole number, which is called the nominal atomic or molecular mass.
This approximation makes it difficult to distinguish molecules with similar molecular masses that differ by only decimal points.
For example, cyclopentanone and cyclohexane have the same nominal molecular mass when rounded to the nearest whole number. Such molecules can be distinguished using high-resolution mass spectrometry.
HRMS uses narrow magnetic field stages to detect particle masses with an accuracy of 0.0001 unified atomic mass units. The mass determined by high-resolution mass spectrometry is known as the exact molecular mass.
The details of the exact molecular mass can be used to distinguish molecules with the same nominal molecular mass. This, in turn, helps identify the molecular formula of the sample.

massaspectrometrie met hoge resolutie (hrms)

High-Resolution Mass Spectrometry (HRMS)

De resolutie van een massaspectrometer hangt af van de efficiëntie van het scheiden van ionen met verschillende ionenmassa's. De massa van een atoom wordt benaderd als de som van de massa's van protonen en neutronen binnenin, waarbij de massa's van protonen en neutronen als gelijk worden beschouwd. De massa's van het proton (1,6726 x 10−24g) en het neutron (1,6749 x 10−24 g) zijn echter niet echt gelijk. Er is een kleine fout in de uitdrukking van atoommassa's ten opzichte van het eenvoudigste waterstofatoom. De massa van helium (2 protonen + 2 neutronen) is bijvoorbeeld niet precies vier keer de massa van waterstof (1 proton). Wanneer meerdere nucleonen in de kern zijn opgesloten, veroorzaakt dit bovendien een toename van de potentiële energie en een afname van de massa (door de onderlinge omzetting van massa en energie). Dit is de reden voor een lagere massa van 12C vergeleken met de massa van zes deuterium (2H) atomen, ook al zijn de totale aantallen protonen en neutronen in beide gevallen hetzelfde. Atoommassa's worden vaak uitgedrukt in verenigde atomaire massa-eenheden (u), gedefinieerd als de rustmassa van een vrij 12C-atoom in de grondtoestand gedeeld door 12. (De verenigde atomaire massa-eenheid staat ook bekend als de dalton met het symbool Da. Het wordt vaak de 'atomaire massa-eenheid' genoemd met het symbool 'amu', wat de naam en het symbool was van een soortgelijke eenheid gebaseerd op zuurstof die werd vervangen door de verenigde atomaire massa-eenheid.)

Voor alle atoomkernen behalve vrije, grondtoestand, in rust 12C, is de exacte atoommassa geen geheel getal. Als gevolg hiervan is de molecuulmassa niet noodzakelijkerwijs een geheel getal. Gewoonlijk worden de atoom- en molecuulmassa's benaderd tot het dichtstbijzijnde gehele getal (ook wel nominale massa genoemd). De massa van een bepaalde isotoop kan ook worden uitgedrukt als relatieve massa, dat wil zeggen de massa van de isotoop gedeeld door de atomaire massaconstante, die gelijk is aan 1 u. Table 1 geeft een overzicht van de relatieve massa, exacte massa en nominale massa voor verschillende isotopen.

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<th style="color: black; border: 1px solid black; text-align: center;">Isotoop</th>
			<th style="color: black; border: 1px solid black; text-align: center;">Aantal protonen</th>
			<th style="color: black; border: 1px solid black; text-align: center;">Aantal neutronen</th>
			<th style="color: black; border: 1px solid black; text-align: center;">Relatieve massa</th>
			<th style="color: black; border: 1px solid black; text-align: center;">Exacte massa</th>
			<th style="color: black; border: 1px solid black; text-align: center;">Nominale massa</th>
		</tr>
	</tbody>
	<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>
		<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: De relatieve massa, exacte massa en nominale massa van de meest voorkomende isotopen van waterstof, helium, koolstof, stikstof en zuurstof.

Normale massaspectrometers kunnen ionen onderscheiden die minimaal 1 u verschillen. Dit levert problemen op bij het onderscheiden van verschillende moleculen met dezelfde nominale massawaarden. Beschouw het voorbeeld van cyclopentanon en cyclohexaan: beide hebben dezelfde nominale molecuulmassa van 84 u, ze hebben echter verschillende molecuulmassa's, chemische structuren en aantallen atomen (cyclopentanon: (CH2)4CO, 84,0573 u, cyclohexaan: C6H11a, 84,0936 u). Een massaspectrometer met hoge resolutie kan echter zelfs ionen onderscheiden die slechts 0,0001 u verschillen. Een massaspectrometer met hoge resolutie maakt gebruik van kleine magnetische veldstappen om de geladen soorten in de analysekamer te scannen. De minuscule stappen zijn efficiënt bij het scheiden van ionen met vergelijkbare molecuulgewichten door de paden van ionen af ​​te buigen op basis van hun exacte molecuulgewicht.

massaspectrometrie met hoge resolutie (HRMS)

The resolution of a mass spectrometer depends on the efficiency of separating ions with different ion masses. The mass of an atom is approximated to the ...

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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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