Mass Number and Isotopes Flashcards
What is the atomic (proton) number?
The atomic number of an element is the number of protons in the nucleus
What is the mass number of an element?
The total number of protons and neutrons in the nucleus
What are isotopes? What is example using chlorine?
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons
E.g: 35Cl and 37Cl
What is mass spectrometry?
Mass spectrometry is a very powerful instrumental technique which is used to find the relative atomic mass of elements and compounds
What is a real life example of using mass spectrometry?
Forensic scientists use mass spectrometers to help identify substances such as illegal drugs
How do mass spectrometers work?
Mass spectrometers work by forming ions from a sample and then separating the ions according to the mass/charge ratio of the atoms
How many stages is mass spectrometry split into? What is each stage named?
Mass spectrometry is split into four stages. In order: ionisation stage, acceleration stage, drift stage and lastly detection stage
The first method of mass spectrometry is ionisation. There are two methods of ionisation. What are they both called?
Electron impact ionisation and electrospray ionisation
What occurs during electron impact ionisation?
During electron impact ionisation, the sample is vaporised, high energy electrons are fired towards it, knocking off an electron from each particle to form +1 ions
What occurs during electrospray ionisation?
The sample is dissolved into a volatile solvent and forced through a fine, hollow needle. A very high voltage is applied to the end of the needle, which makes the particles gain a proton and become ions as a fine mist. The solvent evaporates leaving +1 ions
What is the word equation for electron impact ionisation? What is the word equation for electrospray ionisation?
Electrospray ionisation:
X(g) + H+ (g) = XH+ (g)
Electron impact ionisation:
X(g) = X+ (g) + e-
What do you need to remember when calculating TOF after using electrospray ionisation?
When calculating TOF after using electrospray ionisation, at the end of the test, one electron must be taken away from the total
The second stage of mass spectrometry is the acceleration stage. What occurs in this stage?
The positive ions are attracted towards a negatively charged plate and accelerate towards it. This is done through an electric field so that all the ions have the same kinetic energy.
What type of ions will achieve higher speeds?
Lighter ions and more highly charged ions will achieve higher speeds
The third stage of mass spectrometry (drift stage) is the separation of the charged ions. What occurs in this stage?
Ions enter the flight tube. Ions with different masses have different time of flights. Therefore the lighter ions travel faster and take less time to reach the detector
The fourth and last stage (detection stage) is detection of ions. What occurs in this stage?
The ions hit the detector (a negatively charged plate) and produce a current. The more ions that hit the detector, the bigger the current produced.
How is the signal that is produced by the ions recieved and interpreted?
The signal from the detector is passed to a computer which generates a mass spectrum
Outline the full process of mass spectrometry:
a. discuss both types of ionisation
- Electrospray ionisation - the sample is dissolved into a volatile solvent and injected into the vacuum through a fine, hollow needle. The end of the needle is charged with a very high voltage, allowing the particles to gain a proton and making ions. The solvent dissolves leaving the +1 ions
Electron impact - High energy electrons are fired at the sample which knock off one electron from each atom to form a +1 ion
- The ions are then attracted to a negatively charged plate and accelerate towards it. This is done through an electric field so the ions achieve the same kinetic energy. However light ions or more highly charged ions achieve higher speeds.
- The ions enter the flight tube. They have different masses which lead to different time of flights. As a result lighter ions travel faster and take less time to reach the detector.
- The ions hit the detector (a negatively charged plate) and produce a current. The more ions that hit the detector, the bigger the current will be
- The signal from the detector is passed to a computer which generates a mass spectrum
Isotopes of an element have the same chemical properties. Why is this?
Isotopes of the same elements have the same chemical properties because they have the same electronic configurations
What is the definition of the term “relative atomic mass”?
The term relative atomic mass refer to the average mass of an atom relative to 1/12 of a carbon 12 atom
How do you calculate the mass of an atom, in grams?
relative atomic mass divided by avogadro’s number
How do you calculate the mass of an atom, in kgs? (normally)
relative atomic mass divided by (avogadro’s number multiplied by 1000
State, in terms of the numbers of fundamental particles, one similarity and one difference between the atoms 50Cr and 53Cr
Similarity: Same number of protons
Difference: Different number of neutrons
Why would there be peaks with much lower m/z ratios?
When using electron impact ionisation there may also be peaks at lower m/z values due to fragments caused by the breakup of molecular ions
