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

Z wyjątkiem izotopu 12C, masy jąder pierwiastków są bliskie – ale nie dokładnie – liczbom całkowitym.
Jednak widma masowe zazwyczaj pokazują masę cząstek zaokrągloną do najbliższej liczby całkowitej, która nazywana jest nominalną masą atomową lub cząsteczkową.
To przybliżenie utrudnia rozróżnienie cząsteczek o podobnych masach cząsteczkowych, które różnią się tylko o przecinki dziesiętne.
Na przykład cyklopentanon i cykloheksan mają tę samą nominalną masę cząsteczkową po zaokrągleniu do najbliższej liczby całkowitej. Takie cząsteczki można odróżnić za pomocą spektrometrii mas o wysokiej rozdzielczości.
HRMS wykorzystuje stopnie wąskiego pola magnetycznego do wykrywania mas cząstek z dokładnością do 0,0001 zunifikowanych jednostek masy atomowej. Masa wyznaczona za pomocą spektrometrii mas o wysokiej rozdzielczości jest znana jako dokładna masa cząsteczkowa.
Szczegóły dotyczące dokładnej masy cząsteczkowej można wykorzystać do rozróżnienia cząsteczek o tej samej nominalnej masie cząsteczkowej. To z kolei pomaga zidentyfikować wzór cząsteczkowy próbki.

spektrometria mas o wysokiej rozdzielczości (hrms)

High-Resolution Mass Spectrometry (HRMS)

Rozdzielczość spektrometru mas zależy od skuteczności rozdzielania jonów o różnych masach jonów. Masę atomu przybliża się do sumy mas protonów i neutronów znajdujących się w środku, przyjmując, że masy protonów i neutronów są równe. Jednak masy protonu (1,6726 × 10-24 g) i neutronu (1,6749 × 10-24 g) nie są tak naprawdę równe. Występuje niewielki błąd w wyrażeniu mas atomowych w stosunku do najprostszego atomu wodoru. Na przykład masa helu (2 protony + 2 neutrony) nie jest dokładnie czterokrotnością masy wodoru (1 proton). Ponadto, gdy wiele nukleonów jest zamkniętych w jądrze, powoduje to wzrost energii potencjalnej i spadek masy (w wyniku wzajemnej konwersji masy i energii). Jest to przyczyną mniejszej masy 12C w porównaniu z masą sześciu atomów deuteru (2H), mimo że całkowita liczba protonów i neutronów jest w obu przypadkach taka sama. Masy atomowe często wyraża się w ujednoliconych jednostkach masy atomowej (u), definiowanych jako masa spoczynkowa wolnego atomu 12C w stanie podstawowym podzielona przez 12. (Ujednolicona jednostka masy atomowej jest również znana jako dalton z symbolem Da. Jest to jest często nazywana „jednostką masy atomowej” z symbolem „amu”, który był nazwą i symbolem podobnej jednostki opartej na tlenie, która została zastąpiona ujednoliconą jednostką masy atomowej.)

Dla wszystkich jąder atomowych z wyjątkiem wolnych, w stanie podstawowym, w stanie spoczynku 12C, dokładna masa atomowa nie jest liczbą całkowitą. W rezultacie masa cząsteczkowa niekoniecznie jest liczbą całkowitą. Zwykle masy atomowe i cząsteczkowe są przybliżane do najbliższej liczby całkowitej (zwanej masą nominalną). Masę danego izotopu można również wyrazić jako masę względną, która jest masą izotopu podzieloną przez stałą masy atomowej, która jest równa 1 u. Table 1 przedstawia masę względną, masę dokładną i masę nominalną dla różnych izotopów.

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Izotop</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Liczba protonów</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Liczba neutronów</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Masa względna</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Dokładna masa</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Masa nominalna</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: Masa względna, dokładna masa i masa nominalna najpowszechniej występujących izotopów wodoru, helu, węgla, azotu i tlenu.

Normalne spektrometry masowe potrafią rozróżnić jony różniące się co najmniej o 1 u. Stwarza to trudności w rozróżnieniu różnych cząsteczek o tych samych wartościach mas nominalnych. Rozważmy przykład cyklopentanonu i cykloheksanu: oba mają tę samą nominalną masę cząsteczkową 84 u; mają jednak różne dokładne masy cząsteczkowe, struktury chemiczne i liczbę atomów (cyklopentanon: (CH2)4CO, 84,0573 u; cykloheksan: C6H11, 84,0936 u). Jednak spektrometr mas o wysokiej rozdzielczości może nawet rozróżnić jony, które różnią się tylko o 0,0001 u. Spektrometr mas o wysokiej rozdzielczości wykorzystuje minutowe kroki pola magnetycznego do skanowania naładowanych cząstek w komorze analitycznej. Minutowe etapy skutecznie oddzielają jonów o podobnych masach cząsteczkowych poprzez odchylanie ścieżek jonów zgodnie z ich dokładną masą cząsteczkową.

Spektrometria mas o wysokiej rozdzielczości (HRMS)

Rozdzielczość spektrometru mas zależy od skuteczności rozdzielania jonów o różnych masach jonów. Masa atomu jest przybliżona do ...

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Analytical Chemistry

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Principles of Mass Spectrometry

Zasady spektrometrii mas

KEY TERMS AND CONCEPTS

KLUCZOWE TERMINY I POJĘCIA

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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