Atomic Absorption Spectroscopy Questions and Answers

Answer: C

At nearly three thousand degrees a substantial fraction of easily ionised metals is converted to ions, which do not absorb at the atomic line and so are lost from the measurement. The burner is narrower rather than wider, background emission is greater, and uptake is essentially unchanged. That the remedy for one interference introduces another is the recurring shape of this topic, and it is why an ionisation suppressant so often accompanies the hot flame.

Answer: D

Adding a large excess of caesium or potassium produces so many free electrons that the analyte's own ionisation equilibrium is pushed back towards neutral atoms. It does not bind the analyte, cool the flame or remove anything. Shifting an equilibrium by flooding the system with one of its products is a general chemical technique, and here the product is the electron.

Answer: A

Where phosphate or aluminium ties calcium into a compound the flame cannot break, adding lanthanum or strontium preferentially binds the interferent and releases the analyte to atomise. Nebulisation, ionisation and dissolved oxygen are other matters. Adding a competitor that binds the troublemaker more strongly than the analyte does is the standard answer to a chemical interference, and it costs only a reagent in every standard and sample. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

A small graphite tube is heated electrically in stages, drying, ashing and then atomising a few microlitres of sample placed directly inside it. There is no flame and no plasma, and nothing is nebulised. Holding the entire sample in the light path for a second or more, rather than sweeping most of it away as a flame does, is what gives the technique its far greater sensitivity.

Answer: C

In a flame most of the aspirated sample is lost to waste and the rest passes through the beam in an instant, whereas a furnace holds the entire few microlitres in the light path for around a second. Temperature is comparable, the lamp is the same, and only absorption is measured. Increasing the time an analyte spends in the measurement, rather than the amount of it, is what buys two or three orders of magnitude in detection limit. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

The tube is ramped through a drying step to remove solvent, an ashing step to burn off the matrix, a fast atomisation step where the reading is taken, and a hot clean-out. Nebulisation belongs to the flame, ionisation and emission to other techniques, and the third sequence describes chromatography. That the matrix can be destroyed before the analyte is measured is the furnace's second advantage after sensitivity. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: A

Ashing must remove as much matrix as possible without volatilising the analyte itself, and for a volatile element such as cadmium or lead that window is narrow. The tube is purged with argon so oxidation is not the constraint, and the lamp and detector are unaffected. A step bounded above by losing what you want and below by keeping what you do not is exactly the kind that a matrix modifier is introduced to widen. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

A modifier such as palladium or magnesium nitrate either stabilises a volatile analyte so it survives a hotter ash, or makes the matrix more volatile so it leaves sooner. Volume, cooling and background correction are separate matters. Widening the window between two competing requirements, rather than compromising within it, is what a modifier does, and it is why furnace methods so often specify one. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

A spectral interference arises when something else absorbs or scatters at the wavelength being measured, so the reading includes a contribution the analyte did not make. Refractory compounds are a chemical interference, ionisation an ionisation interference, and viscosity a physical one. Sorting interferences into four kinds, each with its own remedy, is what makes an unexpected result diagnosable. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: D

A chemical interference is the analyte tied up in a thermally stable compound, so fewer free atoms form and the reading is low - calcium with phosphate being the classic case. The other options describe spectral, background and ionisation interferences respectively. Each has its own remedy, and a releasing agent addresses this one.