Topic 7 - Modern Analytical Techniques 1 Flashcards
What is Mass Spectrometry?
Mass Spectrometry is an analytical technique that is used to identify the m/z ratio of the molecules present in a sample.
What is Mass Spectrometry used for when an element is out through it?
Mass Spectrometry is used for measuring the mass to charge ratio of atoms of the element. From your mass spectrum you can calculate the Relative Atomic Mass of your element/sample.
What is Mass Spectrometry used for when a molecule is put through it?
Mass Spectrometry is used to measure the mass to charge ratio of the fragments of a Molecule. These Fragment ion peaks can help to deduce the structure of the molecule e.g its functional group.
What is the formula for Relative Atomic Mass?
Relative Atomic Mass = Σ Isotopic Mass x Relative Abundance/ Total Abundance (not always 100)
What happens in the ionisation chamber of the Mass Spectrometer?
The sample is bombarded with high energy electrons causing it to lose an electron and become an ion.
What is used to deflect the ions moving through the mass spectrometer?
- Magnet
- Magnetic Field
- An Electromagnet
Why is there a vacuum in a mass spectrometer?
- As otherwise the air particles would interfere with the movement of/ collide with / deflect the ions
- The presence of air would cause additional/incorrect peaks to be detected
Write an equation for what happens when the ethane molecule undergoes ionisation in the mass spectrometer?
CH3CH3 + e- → CH3CH3+ + 2e-
OR
CH3CH3 → CH3CH3+ + e-
Describe the process by which neutral species are produced in a mass spectrometer.
The Molecular ion is bombarded with high energy electrons which cause the molecule to lose an electron and become a molecular ion.
Then, this molecular ion may undergo fragmentation to form a positive fragment ion and a neutral species.
This neutral species cannot be detected by the Mass Spectrometer.
What is the molecular ion peak/parent ion peak equivalent to in Mass Spectrometry?
The molecular mass/ molar mass
Why is there often a very small peak one unit higher than the molecular ion peak?
Due to the presence of the carbon-13 isotope in the organic compounds
How would you distinguish propanal and propanone from their mass spectra?
Both propanone and propanol have molecular ion peaks at m/z 58. However, propanone has fragment ion leaks at m/z 15 and 43, whereas propanal has its peaks at m/z 57 and 29
Write equations showing the formation of the two fragments at m/z 15 and m/z 43 from the propanone molecular ion.
C2H5O+ → CH3CO+ + CH3
What is Infrared Spectrscopy?
Infrared Spectroscopy is an analytical technique used to identify the functional groups present in a molecule due to the change in vibration of molecules when they absorb IR radiation.
What happens when a substance is irradiated by Infrared Absorption?
The molecules present in the substance will absorb some of this infrared radiation causing the bonds of the molecule to vibrate. The different functional groups will absorb the infrared radiation at a characteristic frequency. When talking about absorption refer to the BOND absorbing the IR radiation.
What are the 3 types of vibration?
- symmetric stretch
- asymmetric stretch
- bending
What does the characteristic frequency that the bond vibrates at depend on?
The functional group present
What are stretching vibrations?
This is when the atoms move closer together and further away from each other
What are bending vibrations?
This is when the atoms remain the same distance apart from each other but move positions relative to each other
What can Infra red Spectroscopy be used to determine?
The functional groups present in a sample
What is responsible for Infra red Absorptions?
The bond e.g O-H, C-H etc.
What is the Fingerprint region? What is it range?
The fingerprint region is the part of the spectrum which is completely unique to each molecule and can be used to help identify an unknown molecule. It’s range is from 0cm -1 → 1500 cm-1.
When describing an Infrared Spectra, what must you do?
- Draw out the display formula of the molecule you have been given
-Mark off where the fingerprint region is on your spectrum - Mark the peaks present on the spectrum
- Identify the functional group that corresponds to the peak on the spectrum
- Use the exact wave numbers and describe the peak as sharp or broad
Why can’t you just use Infrared Spectroscopy when analysing a compound?
As some absorption bands overlap each other so therefore other analytical techniques such as mass spectrometry must be used
Why does O2 not produce an infrared spectroscopy?
Only molecules that undergo a net change in the dipole moment during vibrational and rotational motions can absorb IR radiation. Therefore diatomic molecules do not produce an Infrared Spectroscopy as there is no change in the dipole moment so no IR radiation is absorbed.
