Advanced Mass Spectrometry

Question 1

How are quadrupoles used as mass filters?

Answer 1

The positively charged pair of rods spend more time being positively charged than negatively charged because the AC is superimposed on the positive DC. The positive ions are repelled into the centre, but during the negative excursions, the positive ions are attracted to the rods. The light ions respond quickly to the negative charge and become unstable. Therefore, the light m/z ions are filtered out and the high m/z ions remain on stable trajectories.

The negatively charged pair of rods spend more time being negatively charged. The heavy ions are attracted to the negative rods and have too much momentum to be turned around during the positive excursions, so they are filtered out.

Question 2

How are quadrupoles used as mass analysers?

Answer 2

If the DC and AC voltages are correctly chosen, only one m/z value will be stable and make it all the way through the quadrupole. Changing the DC and AC voltages means that you can transmit one m/z value after another.

Question 3

How is CID used for fragmentation?

Answer 3

Collision induced dissociation (CID) occurs when you accelerate ions and they collide wiith molecules of a neutral gas. This converts some of the kinetic energy into internal energy, giving fragmentation.

Question 4

What is a tandem mass spectrometer?

Answer 4

“A tandem mass spectrometer allows two or more sequential stages of mass spectrometric analysis to be carried out” - IUPAC.

MS1 and MS2 can be the same type of mass spectrometer or they can be different.

Question 5

What are the four major MS/MS experiments?

Answer 5

Product ion, precursor ion, constant neutral loss and selected (multiple) reaction monitoring.

The last three are usually done with LC.

Question 6

How does product ion scanning work?

Answer 6

MS1 is fixed to transmit one selected precursor m/z ion. CID occurs in the collision cell. MS2 scans the full spectrum of the product ions created from the precursor.

It is used to get information about chemical structure and to identify compounds from their characteristic CID spectra.

Question 7

How does precursor ion scanning work?

Answer 7

MS1 scans through all m/z values sequentially. CID occurs in the collision cell. MS2 is fixed to record only a single product m/z ion.

It is used to discover which compounds generate a particular, charged fragment. The fragment can be diagnostic of a class of compounds or the presence of a structural feature.

Question 8

How does constant neutral loss scanning work?

Answer 8

MS1 scans through all m/z values sequentially. CID occurs in the collision cell. MS2 scans at a constant mass offset from MS1.

The signal reaches the detector when a selected precursor loses a neutral fragment that is equal in mass to the offset.

It is used to detect compounds with similar chemical groups or features.

Question 9

How does selected (multiple) reaction monitoring work?

Answer 9

MS1 is fixed to transmit only one selected precursor m/z. CID occurs in the collision cell. MS2 is fixed to transmit only one selected product m/z.

MRM is looking at many transitions together.

It provides sensitive and specific detection of known analytes.

Question 10

What is the key assumption for external and internal calibration approaches?

Answer 10

External and 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 (i.e. there is no effect from other components in the sample).

If the real sample is in pure solvent, this is a fair assumption. However, if the sample contains lots of other components, they can alter the signal. This is the matrix effect.

Question 11

What is standard addition in terms of calibration?

Answer 11

We analyse the standard in the same matrix as the sample. The concentration of the standard should match the range for the samples.

Increments of standard solution are added to the same sized aliquots of test solution. The aliquots are made up to a constant volume and analysed. You need a linear relationship.

Question 12

How do you find the concentration of an unknown from standard addition calibration?

Answer 12

The concentration of the unknown is given by the x-intercept of the signal vs concentration of standard plot. However, you need to take into account dilutions.

Question 13

What are the advantages and disadvantages of standard addition calibration?

Answer 13

Advantage: corrects for matrix effects, even if you don’t know what these are.

Disadvantages: it uses more real sample than other quantitative approaches and it relies on extrapolation rather than interpolation, assuming linearity.

Question 14

What does ICP-MS allow the determination of?

Answer 14

ICP-MS allows the determination of elements with atomic mass in the range 7 to 250 Da. It is capable of the determination of a range of metals and several non-metals at concentrations below one part in 10¹², so it is highly sensitive.

Question 15

How are samples introduced to the ICP-MS?

Answer 15

The sample is generally introduced as a liquid. The ICP-MS can be coupled to LC or GC. Solids can be analysed using a probe.

Question 16

Which mass analysers can be used in ICP-MS?

Answer 16

The mass analysers can be low resolution (e.g. quadrupole) or high resolution (e.g. magnetic sector).

Question 17

How are samples ionised in ICP-MS?

Answer 17

The ion source is an inductively coupled plasma (Argon).

A spark ionises Ar to give Ar^+• + e^-.

The electron is accelerated by an electromagnetic field due to RF in the induction coils.

The electrons collide with other Ar atoms in the torach, transferring the energy and maintaining the very high gas temperature in a chain reaction (coolant gas needed).

The plasma is sustained by the RF field.

The analyte atoms in the plasma are ionised by collisions with Ar^+•, electrons or energetic Ar atoms.

Question 18

Why is Ar plasma used in ICP-MS?

Answer 18

Ar is an inert gas. Ar plasma is Ar gas in which a large percentage of its Ar atoms are ionised. The ionisation potential of Ar is higher than almost all other elements (15.76 eV). It has a high gas temperature (5000-10,000 K). The Ar atoms are ionised by heating or using a strong electromagnetic field. Because the particles in plasma are charged, the plasma responds to electromagnetic fields.

Question 19

What physiochemical processes occur in plasma?

Answer 19

Solvent evaporation, solid evaporation, atomisation and ionisation.

Question 20

What are the advantages and disadvantages of ICP-MS?

Answer 20

Advantages:

Capable of multielemental analysis. Extremely low limits of detection. Measures isotope ratios. Extremely simple mass spectra. Linear range over 8 orders of magnitude.

Disadvantages:

Isobaric (same mass), polyatomic, oxide and hydroxide interferences. Matrix effects. Expensive kit and consumables (Ar).

Question 21

What is IR-MS used for?

Answer 21

IR-MS measures the relative intensities of stable isotopes of different elements. The results are expressed in terms of the variation from a “normal” ratio, and these differences are usually very small.

Question 22

How is the δ value calculated?

Answer 22

R_sample = abundance ratio of minor heavier isotope to the major lighter one, e.g. ¹³C/¹²C. Most sample are depleted in the heavy isotope so you get negative δ values. The materials used to generate R_standard values are usually enriched in the heavy isotope.

Question 23

What is fractionation?

Answer 23

Fractionation is the selective enrichment/depletion of heavier isotopes. It depends on the fact that although isotopes are different forms of the same element, they do not behave in exactly the same way. Lighter isotopes react faster and need less activation energy but heavier isotopes form stronger bonds.

Question 24

What are the ¹³Cδ values of C₃ and C₄ plants?

Answer 24

C₃ plants use the Calvin-Benson photosynthetic cycle and have ¹³Cδ values of -22 to -35 ‰.

C₄ plants use the Hatch-Slack photosynthetic cycle and have ¹³Cδ values of -8 to -20 ‰.

Certain plants can do either, depending on the conditions.

Question 25

How are samples introduced to the IR-MS?

Answer 25

The sample is weighed into a capsule and burnt in a combustion furnace in O₂. The C in converted into CO₂, and H₂O, NO_x and SO₂ can also be formed. The NO_x is converted into N₂ and O₂, then the H₂O and O₂ is removed. Finally, the N₂ and CO₂ are separated.

Pre-separation of the analytes can be achieved by LC or GC.

Question 26

How does the ion source work in AMS?

Answer 26

The ion source produces a beam of ions from a few milligrams of solid material.

The element is first chemically extracted from the sample and loaded into a copper holder, which is inserted into the ion source through a vacuum lock. Caesium ions bombard the sample surface, generating negativey charged ions (C^-). The negative ions are extracted from the ion source and are sent down the evacuated beamline towards the first magnet.

Question 27

How does the injector magnet work in AMS?

Answer 27

The injector magnet bends the negative ion beam by 90° to select the mass of the relevant isotope and reject the much more intense neighbouring stable isotope. Several vacuum pumps remove all of the air from the beamline so the ions have a free path.

Question 28

How does the tandem accelerator work in AMS?

Answer 28

The tandem accelerator consists of two accelerating gaps with a large positive voltage in the middle (the terminal). The negative ions are accelerated towards the terminal. At the terminal they pass through an electron stripper, either a gas or a very thin carbol foil, and emerge as positive ions. The ions are repelled, accelerating again to ground potential as the far end. The final velocity is a few percent of the speed of light.

Question 29

How do the analysing and switching magnets work in AMS?

Answer 29

The analysing and switching magnets select the mass of the target radionuclide, further reducing the intensity of neighbouring stable isotopes. The magnets eliminate molecules by only selecting the highly charged atomic ions (highly charged molecules are unstable). Isotope ratios are measured by alternately selecting the stable and radioisotopes with the injector and analysing magnets.

Question 30

How does the electrostatic analyser work in AMS?

Answer 30

The electrostatic analyser is a pair of metal plates at high voltage that deflects the beam to the left by 20°. This selects particles based on their energy so it removes the ions that happen to recieve the wrong energy from the accelerator.

Question 31

How does the gas ionisation detector work in AMS?

Answer 31

The gas ionisation detector counts ions one at a time as they come down the beamline. The ions are slowed down and come to rest in propane gas, knocking electrons of the gas atoms. These electrons are collected on metal plates, amplified and read into the computer. For each atom, the computer determines the rate of energy loss and deduces the nuclear charge to distinguish interfering isobars.

Question 32

What are the advantages and disadvantages of AMS?

Answer 32

Advantages:

Extremely sensitive. Allows the use of a smaller sample. Faster analysis time and greater selectivity than other techniques. Removes background interferences.

Disadvantages:

Very expensive to buy and run. Samples need careful and time consuming preparation. Contamination issues. Very specialised applications.