1.2 mass number and isotopes Flashcards
Which letter is used to represent the atomic (proton) number of an atom?
Z
What does the atomic number tell us about an element?
The number of protons in an atom.
What letter represents mass number?
A
How is the mass number calculated?
Mass number = number of protons + number of neutrons
Define relative atomic mass (Ar).
The average mass of an atom of an element relative to 1/12 of the mass of a carbon-12 atom.
Define relative isotopic mass.
The mass of an atom of an isotope of an element relative to 1/12 of the mass of a carbon-12 atom.
What is an isotope?
Atoms of the same element that contain the same number of protons and electrons but a different number of neutrons, resulting in a different mass number.
The different mass number in different isotopes means…
that they have different physical properties.
What is an ion?
An ion is an atom with more or less electrons than protons so therefore have either a positive or negative charge.
How is an ion formed?
Ions are formed when an atom loses or gains an electron/s.
What is mass spectrometry?
An analytical technique used to determine all the isotopes present in a sample and to therefore identify elements
What is time of flight mass spectrometry?
This form of mass spectrometry records the time it takes for ions of each isotope to reach a detector. Spectra (a graph) can be produced showing each isotope present.
What happens at stage 1 ionisation (electrospray) in TOF mass spectrometry?
- sample is dissolved in a volatile and polar solvent
- injected at high pressures through a fine hollow needle
- tip of the needle has high voltage
- at tip of the needle the sample molecule (X) gains a proton (H+) from the solvent, forming XH+
- solvent evaporates into the vacuum
- XH+ ions move towards a negative plate
What is the equation of electrospray ionisation?
X(g) + H+ → XH+(g)
When do we use electrospray ionisation?
When analysing large organic molecules.
- This method prevents fragmentation
What happens at stage 1 ionisation (electron impact) in TOF mass spectrometry?
- sample is vapourised and injected at low pressures
- a heated coil “electron gun” fires high energy electrons at the sample
- knocks out an outer electron
- forms positive ions with different charges
What is the equation of electron impact ionisation?
X(g) → X+(g) + e-
When do we use electron impact ionisation?
When analysing elements and substances with low formula mass.
- This technique CANNOT be used for larger molecules.
What happens at stage 2 acceleration in TOF mass spectrometry?
- positive ions are accelerated by an electric field to a constant kinetic energy (KE) towards a negatively charged plate
- lighter ions will travel faster than the heavier ions reaching the negative plate quicker
What happens at stage 3 ion drift in TOF mass spectrometry?
- ions enter the flight tube (length d)
- ions with different masses (m) have a different time of flight
What happens at stage 4 ion detection in TOF mass spectrometry?
- detector is a negatively charged plate
- a current is produced when ions hit the plate
- more ions more current
What happens at stage 5 data analysis in TOF mass spectrometry?
- current produced and the flight tube times are taken
- produces a mass spectrum
- y axis shows the no of particles (abundance) of each mass that hit detector
- x axis shows mass/charge ratio
What two equations are needed for TOF mass spectrometry?
KE = 1/2mv2
KE = kinetic energy / J - joules
m = mass / kg - kilograms
v = velocity (speed) / ms-1 - metres per second
t = d/v
t = time of flight / s - seconds
d = distance / m - metres
v = velocity (speed) / ms-1 - metres per second
