Mass Spectrometry Questions and Answers

Answer: C

Applying the definition of resolution:

\[ \Delta m = \frac{m}{R} = \frac{500}{1000} = 0.5 \]

giving 0.5 mass units, which is enough to separate adjacent integers but nowhere near enough to distinguish two formulae of the same nominal mass. The resolvable difference grows with mass, so an instrument adequate at low mass may be inadequate higher up.

Answer: D

Because each element's exact mass differs slightly from an integer, carbon monoxide at 27.9949 and nitrogen at 28.0062 are distinguishable given enough resolution, though both are nominally twenty-eight. Isotope abundance, charge state and calibration are separate matters. Turning a mass measurement into a formula assignment is what high resolution buys, and it is often more decisive than any amount of fragmentation data. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

Dividing the mass by the difference to be resolved:

\[ R = \frac{28}{0.0113} \approx 2500 \]

giving about 2500, well beyond a quadrupole's unit resolution but easy for a sector or a modern time-of-flight instrument. That a difference in the third decimal place demands thousands of resolving power is why high-resolution instruments are built and maintained.

Answer: B

The range of masses with stable trajectories is set by the ratio of the direct and radio-frequency voltages, and narrowing it to improve resolution rejects most of the ions and destroys the sensitivity. Machining, scan speed and the detector are secondary. That resolution and transmission trade directly against each other in this analyser is why a quadrupole is chosen for robustness and speed rather than for resolving power. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

In this context mass defect means how far an element's exact mass departs from the nearest integer - hydrogen is 1.00783 and oxygen 15.9949, one above and one below. The nuclear binding energy interpretation is the physicist's usage of the same term. Because hydrogen-rich compounds have positive defects and oxygen-rich ones negative, the decimal part of a measured mass itself constrains the formula. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: D

Each hydrogen adds 0.00783 above its nominal value, so a molecule with many of them sits well above the integer, while oxygen at 15.9949 pulls the other way. Carbon is exactly twelve by definition, and oxygen's defect is negative rather than absent. Reading the decimal part of a high-resolution mass as evidence about composition is a shortcut that narrows candidate formulae before any calculation is done. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Rescaling the mass axis so that a methylene group weighs exactly fourteen makes every member of a homologous series fall on a horizontal line, which reveals families in a complex mixture at a glance. Charge state, resolution and isotope patterns are read other ways. Choosing a coordinate system in which the structure of interest becomes a straight line is the same idea that makes Bode plots useful. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

An ion must travel from source to detector without colliding, and at atmospheric pressure its mean free path is a fraction of a micrometre against the metre or so it must cross. Chemical reaction and detector operation are secondary consequences, and air does not distort a magnetic field. The whole vacuum system exists to make a straight, uninterrupted flight possible, and every design constraint that follows traces back to it. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: C

The analyser runs at around ten to the minus six millibar, where the mean free path is tens of metres and collisions are rare. A thousandth of a millibar is roughly the source pressure in chemical ionisation, and the higher figures would scatter every ion. Two or three pumping stages in series are needed to reach it from atmosphere, which is much of the instrument's bulk and cost.

Answer: D

A mixture gives a superposed spectrum that is difficult to interpret, so separating the components in time and taking a spectrum of each as it elutes solves both problems at once. The spectrometer does nothing for the separation, supplies no vacuum, and the combination takes as long as the chromatography does. Two techniques each supplying what the other lacks is the definition of a useful hyphenation. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.