forensic toxicology - analytical techniques - mass spectrometry

1. Explain the concept of the mass-to-charge ratio (m/z). 2. Explain the key differences between the electron and electrospray ionization sources. 3. Explain the terms ‘resolution’ and ‘accuracy’ as related to mass analysers. 4. Briefly explain how quadrupole, triple quadrupole and time of flight mass analysers function. 5. What are the two types of data generated by MS analysis? How does this differ between hard and soft ionization techniques?

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Last updated: August 23, 2023

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First submitted	August 23, 2023
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what state of matter does the sample have to be in for mass spectrometry?	what is the process of mass spectrometry?		what is the mass-to-charge ratio (m/z)?
gas	the sample must be ionised in an ion source		the mass of an ion/the charge of an ion
	the ions are then separated in an electromagnetic field known as a mass analyser
	the ions must then be detected by a detector
what are the three most common ionisation sources used in mass spectrometry?	how do they work?	are they a soft or hard technique?	what pressure can this take place in?
electron ionisation (EI)	gas phase molecules interact with electrons to produce ions	hard (fragments molecules)	must be in a vacuum
	accelerated electrons are produced by a filament
	molecules collide with the electrons and are ionised and fragmented
electrospray ionisation (ESI)	applies a high voltage to a liquid to create an aerosol containing ions	soft (does not fragment molecules)	at atmospheric pressure
	the solvent in evaporated (desolvation) using hot nitrogen gas to produce free gas-phase ions
matrix-assisted laser desorption/ionisation (MALDI)	the sample is mixed with a matrix plate	soft (does not fragment molecules)	at atmospheric pressure
	a laser irradiated the sample, causing desorption
	the analytes are then ionised in the plume of desorbed gases
what are the key characteristics of mass analysers used in mass spectrometry?
resolution: the ability to distinguish two peaks of similar m/z
accuracy: the ratio of the measurement error to the true m/z
mass range: the range of m/z that can be analysed
linear dynamic range: the concentration range over which signal is linear
what does a quadrupole mass analyser contain?	how does a quadrupole mass analyser function?	what does this allow?	what does varying the voltage applied to the quadrupole allow?
four rods that are electrically connected	oscillating electric fields are applied to the rods	specific masses to be transmitted	the entire mass range to be scanned continuously
what does a triple quadrupole (QQQ) mass analyser contain?	what is each used for?		what is multiple reaction monitoring (MRM)?
three quadrupoles connected in series for tandem MS analysis	the first quadrupole is used to select the mass(es) of interest (m/z selection)		hundreds of pairs of ions being scanned
	the second quadrupole is a collision cell, where ions are fragmented by collision with a gas (e.g. argon) (fragmentation)
	the third quadrupole is used to scan for specific product masses of the mass of interest (m/z selection of fragments)
what is a time-of-flight (TOF) mass analyser?	how does it work?		how does mass relate to time of flight?
an instrument that accelerates ions in an electric field	ions move through the mass analyser at a velocity defined by the m/z		smaller ions have a faster time of flight
what is the most common type of detector used in mass spectrometry?		how does it work?
the electron multiplier (EM)		it amplifies the signal of the ions
		one electron strikes the plate and produces up to 3 more electrons and so on
what are the two types of data generated by mass spectrometry analysis?		what is the x-axis for each?	what is the y-axis for each?
mass chromatogram		time	signal intensity
mass spectrum		m/z	signal intensity/abundance
what is a total ion chromatogram (TIC)?	what is an extracted ion chromatogram (XIC)?	how do hard ionisation techniques affect data analysis?	how do soft ionisation techniques affect data analysis?
the intensity of all masses being detected at every point in the analysis	the chromatogram for one or more specific m/z values	because they fragment the molecule, it will lower the molecular weight reading	as they add one proton to the molecule, the m/z will increase by 1