Mass Spectrometry Questions and Answers

Answer: A

Since the axis is mass divided by charge, doubling the charge halves the apparent position, so a molecule of mass two hundred appears at a hundred. It is certainly detected, and often shows a half-unit isotope spacing that gives it away. Recognising a peak at half the expected mass as a charge-state artefact rather than a fragment is what prevents an incorrect molecular weight being reported. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

The isotope spacing is one mass unit divided by the charge, so a doubly charged ion's isotopes sit half a unit apart - a direct read-out of the charge state given enough resolution. Height, width and position carry no such information. That the isotope cluster encodes the charge is what makes electrospray deconvolution of a protein possible at all.

Answer: C

About one carbon atom in a hundred is carbon-13, so a molecule with ten carbons shows an M plus one peak roughly eleven per cent of the molecular ion. Carbon-14 is vanishingly rare, deuterium contributes about a hundredth of that, and oxygen-17 less still. Counting carbons from the M plus one height was a standard technique before high resolution made exact mass available, and it still serves as a check. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

Dividing by carbon-13's natural abundance:

\[ n \approx \frac{11}{1.1} = 10 \]

giving about ten carbons, which with a mass of 128 suggests naphthalene. The method is approximate and degrades for larger molecules where the peaks overlap, but as a first constraint on a formula it costs nothing beyond reading two peak heights.

Answer: A

With many carbons the chance of two heavy isotopes rises, so M plus two and beyond become significant and the simple proportion breaks down. Abundance is a constant of nature, and molecular ion strength and resolution are separate issues. An approximation valid when a probability is small must fail when it is not, and here the failure sets in around thirty or forty carbons.

Answer: B

Distinguishing candidate formulae of the same nominal mass often turns on differences in the third or fourth decimal, so anything coarser leaves several possibilities open. Instruments report what they can, abundances are a separate matter, and convention follows the requirement rather than causing it. Quoting a measurement to the precision the decision actually needs, and no further, is what makes a number meaningful. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: C

Multiplying the relative figure by the mass:

five parts per million of five hundred is about 0.0025 mass units, which is usually enough to select a single formula for a small molecule. Quoting accuracy relatively rather than absolutely makes it comparable across the mass range, and the absolute figure worsens in proportion as the mass rises.

Answer: D

The number of possible combinations of carbon, hydrogen, nitrogen and oxygen grows rapidly with mass, so a given accuracy admits more candidates the higher one goes. Resolution and isotope patterns do degrade somewhat, and fragmentation is a separate matter. Combining exact mass with the isotope pattern and with chemical sense is what narrows the list, and no one of the three suffices alone. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: A

Computing the degrees of unsaturation a formula implies rejects any candidate giving a negative or fractional value, and an implausibly high one is suspect too. Peak intensities, total current and scan speed carry no such information. Applying a chemical constraint to a list generated by physics is what turns a set of arithmetically possible formulae into a short list worth considering.

Answer: B

The sample must survive vaporisation and pass through a heated column, so the technique covers solvents, pesticides, drugs of abuse and environmental pollutants. Proteins, salts and thermally labile compounds all need liquid chromatography with a soft ionisation source. Matching a hyphenated technique to a sample means asking first whether the chromatographic half can handle it at all.