Atomic Absorption Spectroscopy Questions and Answers

Answer: C

For a laboratory determining two or three metals routinely, an absorption instrument is a fraction of the cost and far simpler to run. Simultaneous multi-element work belongs to the plasma, non-metals are outside both techniques' usual reach, and solids need dissolution for either. A technique displaced from the general case can remain the right answer in the specific one, and cost is usually why. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

Mercury is the only metal with an appreciable vapour pressure at room temperature, so reducing it to the element and sweeping it into a cell needs no flame at all. Arsenic and selenium use the related hydride generation technique, refractory elements need the hot flame, and alkali metals atomise easily. A technique built around one element's unique property will only ever serve that element. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: A

Reacting the sample with a borohydride produces a gaseous hydride that is swept into a heated cell, separating the analyte from its matrix and delivering it all at once. It does not produce free metal, precipitate the matrix or heat the flame. Both the separation from the matrix and the concentration in time contribute to a detection limit far below what direct aspiration achieves.

Answer: B

Arsenic, selenium, antimony, bismuth and tin form volatile hydrides readily, which is what the technique depends on. Alkali metals, refractory elements and common transition metals do not. That an entire analytical approach is available for one chemical family and useless for the rest is worth knowing before a method is sought for an unfamiliar element.

Answer: C

Comparing the volumes consumed for one reading:

\[ \frac{5 \ \mathrm{mL}}{20 \ \mathrm{\mu L}} = 250 \]

giving about 250, and the furnace is nonetheless the more sensitive of the two because none of its sample is wasted. That a technique can use far less material and still detect far less of the analyte is what makes the furnace indispensable when sample is scarce.

Answer: D

Drying, ashing, atomising and cleaning take a couple of minutes per determination against a few seconds for a flame aspiration. Warm-up, lamp stabilisation and tube life are secondary. Trading throughput for sensitivity is the choice between the two atomisers, and which way to take it depends on whether the samples are few and difficult or many and straightforward.

Answer: A

Adding known amounts of analyte to portions of the sample means the standards experience exactly the same viscosity, ionisation and chemical environment as the sample does, so a matrix effect applies equally to both and cancels. It takes longer, still needs a blank, and does not extend linearity. Calibrating inside the thing being measured is the general answer wherever the surroundings affect the measurement. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

A spectral overlap adds a constant absorbance whatever the analyte concentration, so it shifts the whole line upward and the extrapolated intercept is wrong. The other three all scale with the analyte and are handled correctly. Knowing which interferences a technique corrects and which it merely carries along is what prevents it being trusted beyond its reach.

Answer: C

Light from the lamp that the atoms cannot absorb - nearby filler lines and the wings of the emitted line - passes through regardless and sets a ceiling on the measurable absorbance. Flame temperature, ionisation and nebulisation do not produce this curvature. It is the same stray light mechanism that limits molecular photometry, arising here from the source's own spectral impurity rather than from the monochromator. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: D

Standards in water and samples in a real matrix differ in viscosity and chemistry, and both suppress the signal, so the sample reads low against a calibration that never saw the matrix. A lamp, monochromator or detector fault would affect standards and samples alike. That a systematic error affecting only one of two otherwise identical measurements points at what differs between them is the first step in any such diagnosis. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.