CHEM 2510 / Topic 7: MS

What are the three universal functions of MS?

> Prod ions.

> Sep ions.

> Det ions.

(Mass spectrum shows abundance vs. mass/charge of ions)

• What is m/z?

• What is abundance?

> mass of a given ion divided by # of elem charge

> signal intensity of every m/z valu.

Define the ff.:

• Base peak

• Product ions.

• Molecular ion.

• Precursor ion.

• base peak: p w/ greatest intensity (may/may not be molecular ion).

• product ions: ions prod from precursor dissoc (aka fragment/daughter ions).

• molecular ion: ion prod by removal from (positive ions) or addition of (negative ions) a molecule of one/more e^- w/o frag of mol struc.

• precursor ion: ion that dissoc to form prod ions (may/may not be molecular ion).

Define the ff.:

• atomic mass

• molecular mass

• nominal mass

• atomic mass: weighted avg of masses of isotopes of an element (use Da = 1/12 mass of single ¹²C).

• molecular mass: sum of unified atomic mass unites of a molecule or ion (use u = unified atomic mass unit).

• nominal mass: integer mass of most abundant isotope of each element in an ion/molecule.

What are the ways that analytes are usually introduced into the first stage of MS?

• chromatog.

• direct insertion probe (when MM of one X is required).

In GC, ionization of gaseous X is at low pressures. In HPLC and CE,

What are the five ionization modes?

• Electron ionization.

• Chemical ionization.

• Field ionization.

• Field desorption.

• Plasma ionization.

How does EI work?

• e^-’s are emitted from a heated filament w/ energy of 70 eV.

• Neutral X molecules enter the ion source orthogonal to beam of e^-.

• Molecules are bombarded w/ the e^- beam → ionized.

(Less than 0.1% of neutral molecules are ionized).

• What drives ionization in EI?

• Why does EI use 70 eV?

• Which electrons ionize first?

• Why does EI cause fragmentation?

• ionization energy (min energy to remove e^- from gaseous mol).

• 70 eV >> typical molecular IE (e^- loss is easy).

• lowest IE e^- (nonbonding < π < σ).

• xs from 70 eV stays in ion = int E → bonds vibrate → mol fragments.

• Why M^+• often weak/absent in EI?

• Why is EI a “hard” ionization technique?

• xs E → rapid fragmentation.

• high-E e^- beam → extensive fragmentation (softer ionization tech (CI) can be employed).

How does CI work?

• Reagent gas ionized.

• Reagent ions form.

• X ionized.

(Lower int E transferred w/ little frag)

What are other ionization modes?

• Electrospray ionization.

• Atmospheric pressure chemical ionization.

• Matrix-assisted laser desorption.

How does ESI ionize?

> high V sprays liquid → charged droplets.

> solv evaporates → gas-phase ions (often [M+H]⁺ / [M−H]⁻, multicharged).

How does APCI ionize?

> nebulized + heated liquid → gas.

> Reagent ions prod.

> proton transfer to X (usually single-charge ions).

How does MALDI ionize?

> Laser hits M + X.

> M absorbs E.

> X desorbs & ionizes.

> big biomolecules, minimal frag.

What are mass analyzers?

part of MS-mer separating ions by m/z.

For the second stage, why do mass analyzers operate at very low pressure?

Maximize mean free path → fewer ion-molecule collisions → accurate m/z separation (mean free path ≈ 1/P).

What are six types of mass analyzers?

• quadrupole.

• ion trap.

• magnetic sector.

• time of flight.

• orbitraps.

• ion cyclotron resonance.

Define the ff.:

• Mass range.

• Resolution.

• Mass accuracy.

• Scan rate.

• Resolving power.

• mass range: range over which the MS-meter analyzer can operate.

• mass accuracy: how close the value determined is to the true value.

• scan rate: speed at which an analyzer can scan (typically reported in decades/sec; decade = m/z 100 to m/z 1000).

• resolution (∆m): how well a mass analyzer separates ions of different mass / smallest difference in m/z values that can be detected as separate peaks.

• resolving power (R): how good the instrument is at separating close peaks.

(Good = big R and small ∆m.)

What are the two common approaches to defining R?

• Valley approach: Measure ∆(m/z) b/w the centres of two neighbouring peaks, ensuring valley b/w them = 10% of peak height.

• Full-width at half-maximum: Measure width of one peak at half max height (smaller w = better res).

What are the equations for determining R for valley approach and FWHM?

R_{valley}=\frac{\left(\frac{m}{z}\right)}{\Delta\left(\frac{m}{z}\right)}

R_{FWHM}=\frac{\left(\frac{m}{z}\right)}{\left(\frac{m}{z}\right)_{\frac12}}

Why does scan speed matter?

• Chromatographic peaks are short-lived.

• You need multiple spectra per peak to define it (more scans → better peak shape + quant).

• GC peaks ~6–10 s → need fast scans. LC peaks ~10–20 s → can use slower scans. Goal: ~7–10 scans per chromatographic peak.

How does a full scan work?

• Choose m/z range.

• Instrument scans that range over and over (each sweep = one MS-rum).

How does a selected ion monitoring work?

• pre-select specific m/z values / not scan everything → only small # of ions (m/z values characteristic of X) = usually strong/high intensity.

• better DL b/c focused on smaller range.

What’s the advantage for SIM when compounds co-elute?

same t_r but different m/z → sepped by mass resolution, not chromatog.

How do QqQ and QTOF differ?

• QqQ: Q1 selects precursor → Q2 fragments → Q3 selects product (best for targeted quant, high sens).

• QTOF: quadrupole selects ion → TOF measures exact m/z (best for ID + accurate mass, high resolution).

What is product/daughter ion scan used for? How does it work?

• Used for structure elucidation.

• Q1: Selects ONE parent (precursor) ion

• Q2: Fragments the selected parent ion by CID.

• Q3: Scans ALL daughter (product) ions

• Many peaks = many daughters from one parent

What is precursor/parent ion scan used for? How does it work?

• Used to link specific fragment ions w/ parent ion & det if peak present from desired cpd / interfering cpd.

• Q1: Scans ALL parent (precursor) ions

• Q2: Fragments each parent ion by CID.

• Q3: Selects ONE daughter (product) ion.

• Many peaks = many parents that produce the same daughter.

What is multiple reaction monitoring used for? How does it work?

• Used for highly selective and sensitive quantitative analysis of known target analytes.

• Q1: Selects ONE parent (precursor) ion.

• Q2: Fragments the selected parent ion by CID.

• Q3: Selects ONE daughter (product) ion.

• One signal per parent → daughter transition.

(In MRM, the mass spectrometer rapidly cycles through predefined precursor–product ion transitions to monitor specific fragmentation reactions.)

What are the two types of MS detectors?

• Faraday cup.

• electron multiplier.

What are the six desirable features of a detector?

• fast response time.

• low electronic noise.

• high collection efficiency.

• same response for all m/z values.

• large dynamic range.

• long-term stability.