Spectrophotometry Fundamentals Questions and Answers

Answer: C

The law holds for a single wavelength, and if the band admitted contains wavelengths of differing absorptivity, the less absorbed ones dominate the transmitted light and the curve bends downward. Detector response and scattering are separate effects. The remedy is a narrower bandwidth, which is why the slit width and the position of the analytical wavelength on a flat absorption maximum both matter for linearity. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

At a peak the absorptivity is largest, giving the best sensitivity, and the curve is flat, so a small wavelength error changes the reading very little. Stray light, source output and detector response depend on the instrument rather than the analyte. Choosing an operating point where a quantity is both large and insensitive to error is a general principle, and here it buys sensitivity and reproducibility together. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: A

The chain runs from a source of light, through a device that selects a wavelength, through the sample, to a detector that measures what emerges. Amplifiers, recorders and choppers are real components but they support these four rather than replacing them. Reading an instrument as a chain of four functions, each of which can fail in its own way, is what makes a fault traceable rather than mysterious.

Answer: B

A tungsten filament gives a smooth continuum from about 320 nm well into the infrared, which covers the visible region completely. Deuterium is the ultraviolet source, a hollow cathode lamp belongs to atomic absorption, and a Nernst glower is an infrared source. Since no single lamp covers the whole range usefully, an instrument spanning ultraviolet and visible switches between two and the changeover wavelength is one of its specifications. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

A deuterium discharge gives a useful continuum through the ultraviolet where tungsten's output has collapsed. A globar is an infrared source, and a mercury lamp gives lines rather than a continuum, which suits wavelength calibration rather than absorption measurement. That a spectrophotometer needs a continuum, not lines, is what rules out several otherwise bright sources.

Answer: D

A monochromator disperses the source's light with a prism or a grating and passes only a narrow band through its exit slit. Beam splitting is the chopper's or the beam splitter's job, conversion is the detector's, and stray light rejection is a matter of baffles and gratings rather than the primary function. Since the Beer-Lambert law assumes a single wavelength, the narrowness of that band is what decides how well the law holds. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: A

The slit width sets how wide a band of wavelengths reaches the sample, so a narrow slit gives better resolution and a wide one gives more light. It does not change the source, the path length or the centre wavelength. That resolution and signal-to-noise pull in opposite directions through one adjustment is the central compromise of the instrument, and the right setting depends on how sharp the absorption band being measured actually is. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

A narrow slit admits a smaller band and hence less energy, so the detector's signal falls against a noise floor that is largely unchanged. The detector's noise does not depend on the slit, and the source's output is what it is. Stray light does become relatively more important, which is a real secondary effect. Trading light for resolution is unavoidable in a dispersive instrument, and the only escape is a brighter source or a longer measurement time. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: C

Stray light is any radiation at wavelengths other than the nominal one that reaches the detector, arising from imperfect dispersion, scattering inside the monochromator and reflections off internal surfaces. Room light and cell reflections contribute but the term covers the whole of it. Because stray light is not absorbed by the sample, it sets a floor on how little light can appear to get through and therefore an upper limit on measurable absorbance. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

Even with the sample absorbing everything, the stray light still reaches the detector:

\[ A_{max} = -\log_{10}(0.001) = 3 \]

giving about 3, and readings approaching it are increasingly compressed and non-linear. Reducing stray light by a factor of ten buys one more unit of absorbance, which is why high-absorbance work needs a double monochromator rather than a better detector.