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.

high-resolution mass spectrometry (hrms)

High-Resolution Mass Spectrometry (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 sum of the masses of protons and neutrons inside, considering the masses of protons and neutrons as equal. However, the masses of the proton (1.6726 &#215; 10&#8722;24 g) and neutron (1.6749 &#215; 10&#8722;24 g) are not truly equal. There is a minor error in the expression of atomic masses relative to the simplest atom of hydrogen. For example, the mass of helium (2 protons + 2 neutrons) is not precisely four times the mass of hydrogen (1 proton). Also, when multiple nucleons are confined in the nucleus, it causes an increase in the potential energy and a decrease in the mass (from the interconversion of mass and energy). This is the reason for a lower mass of 12C compared to the mass of six deuterium (2H) atoms, even though the total numbers of protons and neutrons are the same in both cases. Atomic masses are often expressed in unified atomic mass units (u), defined as the rest mass of a free, ground-state 12C atom divided by 12. (The unified atomic mass unit is also known as the dalton with the symbol Da. It is often called the &#39;atomic mass unit&#39; with the symbol &#39;amu&#39;, which were the name and symbol of a similar unit based on oxygen that was replaced by the unified atomic mass unit.)
For all atomic nuclei except free, ground-state, at-rest 12C, the exact atomic mass is not a whole number. As a result, the molecular mass is not necessarily a whole number. Usually, the atomic and molecular masses are approximated to the nearest whole number (known as nominal mass). The mass of a given isotope can also be expressed as relative mass, which is the mass of the isotope divided by the atomic mass constant, which is equal to 1 u. Table 1 lists the relative mass, exact mass, and nominal mass for different 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;">No. of protons</td>
			<td style="color: black; border: 1px solid black; text-align: center;">No. of neutrons</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Relative mass</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Exact mass</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nominal mass</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: The relative mass, exact mass, and nominal mass of the most abundant isotopes of hydrogen, helium, carbon, nitrogen and oxygen.
Normal mass spectrometers can distinguish ions that differ by at least 1 u. This poses difficulty in distinguishing different molecules with the same nominal mass values. Consider the example of cyclopentanone and cyclohexane: both have the same nominal molecular mass of 84 u; however, they have different exact molecular masses, chemical structures, and numbers of atoms (cyclopentanone: (CH2)4CO, 84.0573 u; cyclohexane: C6H11, 84.0936 u). However, a high-resolution mass spectrometer can even differentiate ions that differ only by 0.0001 u. A high-resolution mass spectrometer employs minute magnetic field steps to scan the charged species in the analyzing chamber. The minute steps are efficient in separating ions with similar molecular weights by deflecting the paths of ions according to their exact molecular weight.

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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Principles of Mass Spectrometry

KEY TERMS AND CONCEPTS

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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