Mass spec Flashcards
MS requires generation of analyte ions in order to measure the relative mass of molecules. True or false?
True
What is on the x and y axis?
Y is abundance of each ion
X is m/z ratio
What are the four major stages of MS?
sample vaporisation
ion generation
ion separation
ion detection
What are the three ways in which sample volatilisation can be achieved?
Heat
Placing sample in a vacuum
Using fast atom bombardment
How can sample ionisation be achieved?
Bombarding the volatilised molecules with electrons from an electron gun. This will knock an electron out of the molecule, leaving it with a positive charge. Charged plates then accelerate the ionised molecules into the deflection chamber
In electron impact ionisation, the sample is vaporised by heat. True or false?
True
In electron impact ionisation, how is ionisation achieved?
By bombarding the volatilised molecules with an electron beam
When an analyte molecule loses an electron, does its mass change?
No
In electron impact ionisation, what are the fragments analysed based on?
mass to charge ratio
How does chemical ionisation work?
Uses a stream of electrons to ionise a reagent gas (ammonia or methane). The ionisation of the reagent gas produces a strong acid. Volatilised analyte molecules are ionised by strong acid via protonation
Electron ionisation and chemical ionisation are gentle methods and produce very little fragmentation. True or false?
False - not gentle methods and produce a lot of fragments
Electron ionisation and chemical ionisation are suitable for small volatile molecules with molecular weight more than 1000 Da. True or false?
False - less than 1000 Da
What atom does fast atom bombardment involve?
Xe
What is the role of the matrix in fast atom bombardment?
Protect the analyte from fragmentation
In FAB, do analyte molecules need to be evaporated beforehand?
No - bombardment by Xe atoms has dual action:
desorption of analyte molecules into gas phase and
ionisation
FAB only generates positively charged ions. True or false?
False - generates both positive and negative ions
Does the molecular weight of molecule change in FAB?
Yes - loss/gain of proton will change mass as protons have a weight
Is FAB a gentle technique? Does it produce fragments?
Yes, a gentle technique with very little fragmentation
FAB is suitable for the analysis of molecules up to 6000 Da. True or false?
True
In FAB, energy transfer takes place from the matrix to Xe atoms. True or false?
False - from Xe to matrix
What is the main difference between FAB and MALDI?
In MALDI, energy transferred from laser beam to matrix, whereas in FAB, energy is transferred from Xe to matrci
FAB is a more expensive technique than MALDI, True or false?
False
MALDI is suitable for biomolecules of up 500,000 Da. True or false?
True
What is the role of the pH modifier in electrospray ionisation?
to produce ions - this will determine whether ionisation proceeds via protonation or deprotonation
In which type of ionisation is the analyte dissolved in a mixture of organic solvent and water?
Electrospray ionisation
In electrospray ionisation, a high voltage is applied to the capillary. Why?
To disperse the droplets into the air
The droplets formed in electrospray ionisation are big, how are they diminished in size and why is this done?
a warm gas is applied (nitrogen) - creates smaller ions which are easier to ionise
Electrospray ionisation is used for non-polar analytes. True or false?
False - used for polar analytes
Atmospheric pressure chemical ionisation is similar to electrospray ionisation in that an organic solvent and a pH modifier is used. True or false?
True
APCI is more effective at analysing less polar species than electrospray ionisation. True or false?
True
The advantage of APCI is that the molecular mass range is typically less than 2000 Da. True or false?
False - this is a disadvantage
Out of all the ionisation techniques, which one creates the most fragmentation?
Electron impact ionisation, followed by chemical ionisation
Why does fragmentation take place?
Because the molecular ion produced by ionisation is in an excited state. This corresponds to excess vibrational energy leading to fragmentation
Neutral species can be seen in MS. True or false?
False - only ionised species can be seen
What does the process of ion separation involve?
once ionised, the analyte ions are separated by their interaction with an electrical or magnetic field in high vacuum
How can the mass-to-charge ratio of fragments be changed?
By varying the strength of the magnetic field
How are ions detected?
the positive ions and molecular fragments produced in the ionisation chamber are accelerated into an analysing tube. The path of the charged molecules is bent by an applied magnetic field
In ion detection, high momentum ions are deflected the most and collide with the walls of the analyser. True or false?
False - deflected the least and collide with the walls. Low momentum ions are deflected the most, but too collide with the walls.
Ions have to have the proper mass-to-charge ratio to collide with the collector and generate an electrical current. True or false?
True
Analysis of MS involves re-assembling of fragments, after identifying their molecular structure, and working backwards to generate the original molecule. True or false?
True
