Atomic Absorption Spectroscopy Questions and Answers

Answer: A

A two electron volt gap leaves a measurable fraction of sodium atoms thermally excited, while zinc's six electron volt gap leaves essentially none. Concentration, oxide formation and wavelength region are secondary. The Boltzmann factor is exponential in the gap, so a threefold difference in energy is a difference of many orders of magnitude in excited population - which is what divides the elements between the two techniques. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: B

The linear range runs from the lowest concentration distinguishable from noise to the point where stray light and line broadening bend the curve, and it is typically two orders of magnitude - narrower than in molecular photometry. Wavelength range, burner height and the two limits are separate specifications. A narrow linear range means dilution is needed more often, which is a real practical cost of the technique. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: C

The hollow cathode lamp emits filler gas lines and the wings of its own line, none of which the atoms can absorb, and that unabsorbable fraction behaves exactly like stray light. Absorption strength, path length and detector saturation are not the cause. A source that is not perfectly monochromatic imposes the same ceiling as an imperfect monochromator, which is why both techniques curve at high absorbance. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: D

A signal three times the standard deviation of the blank is the usual criterion for saying an element is present, with ten times required before a number is quoted. The other options describe anecdote, an arbitrary absorbance and the calibration's extent. Two thresholds for two different claims - that something is there, and how much - reflects that the second is much the stronger claim. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

Characteristic concentration is the concentration producing 0.0044 absorbance, that is one per cent absorption, and it is quoted as a measure of an instrument's sensitivity for each element. It is not a range limit, a standard, or an ionisation threshold. Because it is defined at a fixed small signal it compares instruments and conditions directly, in a way a detection limit - which depends on noise - cannot. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: B

Characteristic concentration measures how much signal a given concentration produces, while the detection limit also depends on how noisy that signal is. Two instruments with identical sensitivity can have quite different detection limits if one is noisier. Both apply to flame work, and the furnace equivalent is quoted as a characteristic mass. Separating sensitivity from noise is what lets an improvement be attributed to the right part of the instrument.

Answer: C

Characteristic concentration is defined at exactly that absorbance:

\[ c_{char} = 0.02 \ \mathrm{ppm} \]

so the answer is 0.02 ppm. The figure 0.0044 is the absorbance the definition uses, not a concentration. A quantity defined at a fixed response makes instruments and days directly comparable, which is exactly what a specification should do.

Answer: D

Residual solution in the nebuliser and spray chamber would contribute to the next reading, so water or dilute acid is aspirated until the signal returns to baseline. Cooling, wavelength and lamp stability are not addressed. Carry-over is worst when a high concentration precedes a low one, which is why samples are often run in ascending order where their levels are known roughly.

Answer: A

Running from low to high means any carry-over is from a weaker sample into a stronger one, where it matters proportionally less. Flame stability, linearity and volume are not affected by the order. Arranging a sequence so that its unavoidable errors fall where they do least harm is a cheap improvement available in many analytical procedures, and it costs only a moment's planning.

Answer: B

The first two are correct and the third is false: both methods correct scattering as readily as molecular absorption, since both are broad attenuation that the correcting measurement sees. Scattering is in fact one of the principal things they are there to remove, particularly in furnace work where a smoky matrix is common. A method's coverage is worth checking against what it was designed to measure rather than assumed.