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

12C izotopu dışında, element çekirdeğinin kütleleri tam sayılara yakındır, ancak tam olarak değildir.
Bununla birlikte, kütle spektrumları tipik olarak, nominal atomik veya moleküler kütle olarak adlandırılan en yakın tam sayıya yuvarlanmış parçacık kütlesini gösterir.
Bu yaklaşım, yalnızca ondalık noktalarla farklılık gösteren benzer moleküler kütlelere sahip molekülleri ayırt etmeyi zorlaştırır.
Örneğin, siklopentanon ve sikloheksan en yakın tam sayıya yuvarlandığında aynı nominal moleküler kütleye sahiptir. Bu tür moleküller, yüksek çözünürlüklü kütle spektrometresi kullanılarak ayırt edilebilir.
HRMS, parçacık kütlelerini 0.0001 birleşik atomik kütle birimi hassasiyetiyle tespit etmek için dar manyetik alan aşamalarını kullanır. Yüksek çözünürlüklü kütle spektrometresi ile belirlenen kütle, tam moleküler kütle olarak bilinir.
Tam moleküler kütlenin detayları, aynı nominal moleküler kütleye sahip molekülleri ayırt etmek için kullanılabilir. Bu da numunenin moleküler formülünün tanımlanmasına yardımcı olur.

yüksek Çözünürlüklü kütle spektrometrisi (hrms)

High-Resolution Mass Spectrometry (HRMS)

Bir kütle spektrometresinin çözünürlüğü, farklı iyon kütlelerine sahip iyonları ayırma verimliliğine bağlıdır. Bir atomun kütlesi, proton ve nötronların kütleleri eşit kabul edilerek içindeki proton ve nötronların kütlelerinin toplamına yaklaşılır. Ancak protonun (1,6726 × 10−24 g) ve nötronun (1,6749 × 10−24 g) kütleleri gerçekte eşit değildir. En basit hidrojen atomuna göre atom kütlelerinin ifadesinde küçük bir hata vardır. Örneğin helyumun kütlesi (2 proton + 2 nötron), hidrojenin (1 proton) kütlesinin tam olarak dört katı değildir. Ayrıca, birden fazla nükleon çekirdekte hapsedildiğinde, potansiyel enerjide bir artışa ve kütlede bir azalmaya (kütle ve enerjinin birbirine dönüşmesinden) neden olur. Her iki durumda da toplam proton ve nötron sayıları aynı olmasına rağmen, altı döteryum (2H) atomunun kütlesiyle karşılaştırıldığında 12C kütlesinin daha düşük olmasının nedeni budur. Atomik kütleler genellikle, serbest, temel durumdaki 12C atomunun geri kalan kütlesinin 12'ye bölümü olarak tanımlanan birleşik atomik kütle birimleri (u) cinsinden ifade edilir. (Birleşik atomik kütle birimi aynı zamanda Da sembolüyle dalton olarak da bilinir. Genellikle 'amu' sembolüyle 'atomik kütle birimi' olarak anılır; bu birim, oksijene dayalı benzer bir birimin adı ve sembolü olup yerini birleşik atomik kütle birimi almıştır.)

Serbest, temel durum, dinlenme 12C dışındaki tüm atom çekirdekleri için tam atom kütlesi bir tam sayı değildir. Sonuç olarak moleküler kütle mutlaka bir tam sayı olmak zorunda değildir. Genellikle atomik ve moleküler kütleler en yakın tam sayıya (nominal kütle olarak bilinir) yaklaştırılır. Belirli bir izotopun kütlesi, izotopun kütlesinin 1 u'ya eşit olan atomik kütle sabitine bölünmesiyle elde edilen bağıl kütle olarak da ifade edilebilir. Table 1'de farklı izotoplar için bağıl kütle, tam kütle ve nominal kütle listelenmektedir.

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">İzotop</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Proton (1H) sayısı</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nötron sayısı</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Bağıl kütle</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Tam kütle</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nominal kütle</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: Hidrojen, helyum, karbon, nitrojen ve oksijenin en bol izotoplarının bağıl kütlesi, tam kütlesi ve nominal kütlesi.

Normal kütle spektrometreleri en az 1 u farklılık gösteren iyonları ayırt edebilir. Bu, aynı nominal kütle değerlerine sahip farklı molekülleri ayırt etmede zorluk yaratır. Siklopentanon ve sikloheksan örneğini düşünün: her ikisi de 84 u'luk aynı nominal moleküler kütleye sahiptir; ancak tam moleküler kütleleri, kimyasal yapıları ve atom sayıları farklıdır (siklopentanon: (CH2)4CO, 84.0573 u; sikloheksan: C6H11, 84.0936 u). Bununla birlikte, yüksek çözünürlüklü bir kütle spektrometresi, yalnızca 0,0001 u kadar farklılık gösteren iyonları bile ayırt edebilir. Yüksek çözünürlüklü bir kütle spektrometresi, analiz odasındaki yüklü türleri taramak için çok küçük manyetik alan adımları kullanır. Dakika adımları, iyonların yollarını tam moleküler ağırlıklarına göre saptırarak benzer moleküler ağırlığa sahip iyonları ayırmada etkilidir.

Yüksek Çözünürlüklü Kütle Spektrometrisi (HRMS)

Bir kütle spektrometresinin çözünürlüğü, farklı iyon kütlelerine sahip iyonları ayırma verimliliğine bağlıdır. Bir atomun kütlesi şuna yaklaşıktır ...

Education

JoVE Core

Analytical Chemistry

Analitik Kimya

Unit 1

Ünite 1

Principles of Mass Spectrometry

Kütle Spektrometrisi İlkeleri

KEY TERMS AND CONCEPTS

TEMEL TERİMLER VE KAVRAMLAR

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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analytical chemistry

Mass Accuracy in HRMS Analysis

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