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Core 8 > Advanced Mass Spectrometry > Flashcards

Advanced Mass Spectrometry Flashcards

1
Q

What does mass spectrometry rely on?

A

MS depends on ionisation, transfer to the gas phae then separation of the charged species according to their mass/charge (m/z) ratio.

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2
Q

Describe the charge of the positive rods in a quadrupole.

A

Positive direct current with superimposed alternating current - when combined this makes the rods positively charged most of the time with small negative excursions.

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3
Q

How do the positive rods of a quadrupole work?

A

During the positive charge, positively charged ions are repelled and focused in the centre of the quadrupole (which is most of the time). During small negative excursions, light positive ions respond quickly to the attraction making them unstable/lost and eventually filtered out. Heavy positive ions don’t have the momentum to react/move before the rods become positively charged again so they remain on a stable trajectory.

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4
Q

Describe the charge of the negative rods of a quadrupole.

A

They are the exact opposite of the positive rods. Negative direct currect with a superimposed alternating current, making the rods negatively charged most of the time with small positive excursions.

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5
Q

How do the negative rods of a quadrupole work?

A

The heavy positive ions are attracted to the negative charges and don’t have enough time to turn around/be ‘saved’ by the brief positive charge so they’re filtered out. The light positive ions are repelled during the brief positive charge and so remain stable on their trajectory.

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6
Q

How can quadrupoles be used as mass analysers?

A

If the DC and AC voltages are chosen correctly, only one m/z value will be stable and make it all the way through the quadrupole. Changing the DC/AC voltages means it’s possible to trasmit one m/z value after another to form a mass spectrum.

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7
Q

Describe triple quadrupole tandem mass spectrometry.

A

Quadrupoles are often used here. If soft ionisation is used in the ion source then no fragmentation will be produced. External energy can be added to promote fragmentation via collision induced dissociation (CID)

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8
Q

How does collision induced dissociation (CID) occur?

A
  • accelerate the ions and collide them with a neutral gas, e.g. N2 or He
  • kinetic energy is converted to internal energy and fragmentation occurs
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9
Q

Describe a tandem mass spectrometer.

A

CID is usually carried out on a tandem mass spectrometer. A tandem mass spectrometer allows two or more sequential stages of mass spectrometric analysis to be carried out with CID in between.

MS1 and MS2 can both be the same type of analysis (e.g. both quadrupoles) or they can be different (e.g. MS1 = quadrupole, MS2 = TOF).

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10
Q

What are the four major tandem MS experiments?

A
  1. Product ion experiment
  2. Precursor ion experiment
  3. Constant neutral loss
  4. Selected/multiple reaction monitoring (SRM/MRM)
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11
Q

Describe product ion scanning.

A

This analyses what product ions are created by fragmentation of a selected precursor ion.

  • MS1 fixed to one m/z
  • CID
  • MS2 scans full spectrum of product ions
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12
Q

Describe precursor ion scanning.

A

This analyses what precursor ions fragment to produce a selected product ion.

  • MS1 scans all m/z
  • CID
  • MS2 fixed to one product ion m/z

This discovers which ions generate a particular charged fragment, which can be diagnostic of a compound class or structural feature.

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13
Q

Describe constant neutral loss scanning.

A

This analyses which precursor ions fragment to lose a specific neutral fragment (which doesn’t show due to no charge).

  • MS1 scans all m/z
  • CID
  • MS2 scans at contant mass offset from MS1 (= neutral molecule)

A signal reaches the detector when a selected precursor loses a neutral fragment equal to the MS2 offset.

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14
Q

Describe selected/multiple reaction monitoring (SRM/MRM).

A

This analyses if there’s any specific ions that fragment to produce a specific product ion.

  • MS1 fixed to one precursor m/z
  • CID
  • MS2 fixed to one product m/z
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15
Q

Describe the quantitation of ‘real’ samples.

A

Any external or internal calibration approaches assume that the signal from the standard is the same as the signal from the same amount of analyte in the real sample. This works if the real sample is just pure solvent. However, if the real sample contains lots of other components, then they can alter the signal that’s recorded.

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16
Q

What is the matrix effect?

A

Where something else in the rest of the sample affects the response of the technique to the analyte, causing either an increase or decrease in the signal.

  • increase - e.g. the solvent amplifies the signal
  • decrease - the signal of the analyte is surpressed by sample matrix components

Matrix effects can be compensated for by used the standard addition method.

17
Q

What is the standard addition method?

A
  • take multiple aliquots of the same volume of sample
  • add increasing increments of standard to each
  • make up each aliquot to the mark
  • plot a graph of concentration of standard added vs. response to give a straight line
  • extrapolate backwards to x-intercept = amount of analyte in sample
  • take in dilutions
18
Q

What are the pros and cons of the standard additions method?

A

Pros:

  • corrects for matrix effects, even if you dont know what they are

Cons:

  • uses more of the real sample than other quantitative approaches
  • assumes linearity - relies on extrapolation instead of intrapolation
19
Q

What is ICP-MS?

A

Elemental MS uses a technique called ICP-MS. This technique allows for the determination of elements with atomic range from 7 to 250 Da. The ion source is very high energy and converts analytes to their atomic ions which are then separated based on their m/z values.

20
Q

Describe the main points of ICP-MS.

A

The sample is generally introduced as a liquid and the technique can be coupled to LC and GC. The mass analysers can be either a low-resolution mass analyser or a high resolution one.

The heart of ICP-MS is the argon plasma. Ar has one of the highest ionisation potentials therefore it can ionise almost all elements.

21
Q

Describe the Ar plasma in ICP-MS.

A

It is a gas-like state of matter in which some of the particles are ionised. Ionisation can be caused by heating, a strong electromagnetic field, etc. Because particles in plasma are charged, plasma responds to electromagnetic fields.

  • a spark ionises Ar to Ar+ and e-
  • electrons are accelerated by an e.m. field
  • electrons collide with other Ar atoms in a torch, transferring energy, and maintaining the high T in a chain reaction
  • analyte atoms in the plasma are ionised (collisions with Ar+ or energetic Ar).
22
Q

What physiochemical processes occur in Ar plasma?

A
  1. Solvent evaporation
  2. Solid evaporation
  3. Atomisation
  4. Ionisation
23
Q

What are the advantages of ICP-MS?

A
  1. multielement analysis capabilities
  2. low limits of detection
  3. measures isotope ratios
  4. simple mass spectra
  5. linear range/response over 8 orders of magnitude
24
Q

What are the disadvantages of ICP-MS?

A
  1. isobaric interferences - isotopes of different elements having the same mass
  2. polyatomic ion interferences - many Ar examples from plasma gases
  3. oxide and hydroxide interferences
25
Q

What is isotope ratio mass spectrometry?

A

It measures the relative intensities of isotopes of different elements. Isotope ratio MS specifically measures stable isotopes (non-radioactive). Results are expressed in terms of variation from a ‘normal’, where the diffreences tend to be very small.

26
Q

Where do the isotopic differences in IR-MS originate from?

A

The average isotope ratio is fixed from when the earth was formed, but localised variation can occur with selective enhancement/depletion of heavier isotopes due to fractionation.

It depends on the fact that although isotopes are different forms of the same element, they do not behave in the same way. Lighter isotopes react faster/need less activation energy while heavier isotopes from stronger bonds.

27
Q

Why is IR-MS needed to measure isotopic ratios?

A

Isotopic ratios have very small differences and neither ICP-MS or ‘regular’ mass analysers can be used. ICP-MS is not stable enough and ‘regular’ mass analysers are not sensitive or precise enough to detect such subtle differences.

28
Q

What does the instrumentation of IR-MS consist?

A
  1. Sample introduction system
  2. Electron ionisation (EI) source
  3. Magnetic sector mass analyser
  4. Detector array
  5. Computer
29
Q

What are two current applications of IR-MS?

A

IR-MS is used a lot in forensics.

  1. Analysis of controlled drugs, e.g. GHB.
    • differentiating between endogenous and exogenous sources to detect spiking
  2. Victim tracking/identification
    • can perform isotope analysis to discover where the victim is from
30
Q

What accelerator mass spectrometry?

A

It is an isotope ratio method detecting naturally occurring, long-lived radioactive isotopes. It uses a particle accelerator alongside ion sources, large magnets and detectors to separate out interferences and count single atoms.

The data is a simple isotope ratio expressed as x-moles of isotope 2 vs y-moles of isotope 2.

31
Q

What does AMS instrumentation consist of?

A
  1. Ion source
  2. Injector magnet
  3. Tandem accelerator
  4. Analysing magnet
  5. Electrostatic analyser
  6. Gas ionisation detector
32
Q

What are the pros of AMS?

A
  1. Extremely sensitive
  2. Uses a smaller sample
  3. Faster analysis time
  4. Removes background interferences surpresses molecular isobars and can separate atomic isobars
33
Q

What are the cons of AMS?

A
  1. Very expensive to buy and run
  2. Samples need careful and time-consuming preparation
  3. Contamination issues
  4. Very specialised applications
34
Q

What are the applications of AMS to studies of human metabolism?

A

Studying the fate, rate, and routes of excretion of drugs in the body:

  1. Radiactive doses - AMS allows for smaller and safer doses of radioactive drug to be adminsitered in studies
  2. Fate of an anti-infective - AMS allows for analysis of very small samples for radioactive isotopes to test where the drug ended up and how long it stayed there

Core 8 (7 decks)

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