6.4
Espectrômetro de Massa
Esta lição detalha a instrumentação de um espectrômetro de massa – um instrumento físico para realizar espectrometria de massa em moléculas de analito…
O espectrômetro de massa ioniza moléculas e produz um espectro de massa a partir da distribuição do fragmento.
Ele desempenha três funções: ionização de moléculas ou átomos, separação dos fragmentos de íons e detecção de íons.
Na ionização de elétrons, um método comum de ionização, uma amostra vaporizada na câmara de ionização bombardeada com um feixe de elétrons de alta energia produz cátions radicais. O cátion radical se fragmenta rapidamente em moléculas neutras, radicais e cátions.
Os cátions são então acelerados em uma câmara de análise por uma série de placas aceleradoras carregadas negativamente.
Muitos analisadores comuns aplicam um campo magnético ou elétrico à câmara de análise, que dobra as trajetórias dos íons com base em sua massa e carga.
Apenas íons de valores específicos de m / z têm a trajetória exata para passar pela fenda estreita colocada na frente do detector de íons para um determinado campo magnético ou elétrico.
Ao alterar o campo, o analisador de massa varre uma variedade de massas de íons, com suas abundâncias registradas no espectro de massa.
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Q1: What are the three main functions of a mass spectrometer?
A mass spectrometer performs three essential functions: ionization of molecules or atoms to create charged species, separation of ion fragments based on their mass-to-charge ratio using magnetic or electrical fields, and detection of ions to record their relative abundance. Together, these functions produce a mass spectrum showing the fragment distribution of the analyzed sample.
Q2: How does electron ionization create ions in a mass spectrometer?
In electron ionization, a vaporized sample in the ionization chamber is bombarded with a high-energy electron beam, typically around 70 eV. These electrons strip electrons from analyte molecules, producing radical cations. The radical cation quickly fragments into neutral molecules, radicals, and cations, which are then directed toward the analyzing chamber for separation and detection.
Q3: Why do mass spectrometers only detect charged ions and not neutral molecules?
Mass spectrometers record only charged species because the electric or magnetic fields in the analyzing chamber control and manipulate ions based on their charge and mass. Neutral molecules lack charge and cannot be deflected by these fields, making them impossible to separate or detect. Only charged fragments reach the detector and contribute to the mass spectrum.
Q4: How does a magnetic field separate ions by mass in a mass spectrometer?
A magnetic field applied in the analyzing chamber bends the trajectories of accelerated ions based on their mass-to-charge ratio. At a constant magnetic field strength, ions with different masses follow different curved paths with varying radii of curvature. A narrow slit positioned before the detector allows only ions of a specific mass to pass through, enabling selective detection of individual ion masses.
Q5: What role do accelerator plates play in a mass spectrometer?
Accelerator plates, positioned after the ionization chamber, accelerate the generated ions toward the analyzing chamber using a series of negatively charged plates. This acceleration gives the ions kinetic energy, which is essential for their subsequent separation by magnetic or electrical fields. The accelerated ions then enter the mass analyzer where they are separated based on mass-to-charge ratio.
Q6: How does scanning the magnetic field produce a complete mass spectrum?
By varying the magnetic field strength, the mass spectrometer can selectively allow ions of different masses to pass through the detector slit one at a time. As the field is scanned through a range of values, each mass-to-charge ratio reaches the detector sequentially. The relative abundance of each ion type is recorded, creating a complete mass spectrum showing all charged species present in the sample.
Q7: What happens to a molecular ion after it forms in the ionization chamber?
After formation, the molecular ion is unstable and quickly undergoes mass spectrometry molecular fragmentation, breaking into smaller charged fragments, neutral molecules, and radicals. Only the charged fragments are accelerated by the accelerator plates and separated in the analyzing chamber. This fragmentation pattern is characteristic of each molecule and provides structural information in the resulting mass spectrum.