Question 1

Which molecular property is primarily responsible for a compound's ability to absorb light in the UV-VIS range?

Accepted Answer

Presence of conjugated double bonds

Question 2

In reversed-phase HPLC, which type of compound will have the longest retention time on a C18 column?

Accepted Answer

A highly nonpolar compound

Question 3

Why is 254 nm commonly used as the detection wavelength in UV-VIS detectors for HPLC?

Accepted Answer

Many aromatic compounds absorb strongly at 254 nm

Question 4

What is the main advantage of a Diode Array Detector (DAD) compared to a single-wavelength UV-VIS detector in HPLC?

Accepted Answer

DAD can record absorbance across a range of wavelengths simultaneously

Question 5

Which of the following compounds is least likely to be retained on a C18 reversed-phase column?

Accepted Answer

A highly polar compound such as glucose

Question 6

Given the following equation for absorbance in UV-VIS spectroscopy: $$A = \epsilon c l$$, what does $$\epsilon$$ represent?

Accepted Answer

Molar absorptivity

Question 7

A mixture of four parabens is separated by HPLC. Which factor most strongly influences their order of elution on a C18 column?

Accepted Answer

Hydrophobicity of the paraben molecules

Question 8

In the chromatogram shown, what is the term for the time at which a compound elutes from the column?

Accepted Answer

Retention time

Question 9

Which statement best describes the difference between isocratic and gradient elution in HPLC?

Accepted Answer

Isocratic elution uses a constant mobile phase composition, while gradient elution changes the composition over time

Question 10

If you want to quantify ibuprofen in human serum using HPLC-UVVIS, which sample preparation step is most critical?

Accepted Answer

Protein precipitation or extraction to remove interfering substances

Question 11

Which of the following compounds would show strong absorbance in the visible region (400–800 nm) of the UV-VIS spectrum?

Accepted Answer

A compound with an extended conjugated system, such as β-carotene

Question 12

Why do compounds with more conjugated double bonds generally absorb at longer wavelengths in UV-VIS spectroscopy?

Accepted Answer

Conjugation lowers the energy gap between HOMO and LUMO, shifting absorption to longer wavelengths

Question 13

A chromatogram shows four peaks at 5, 10, 15, and 20 minutes. If the compounds are methylparaben, ethylparaben, propylparaben, and butylparaben, which is most likely to elute last?

Accepted Answer

Butylparaben

Question 14

Which property of the mobile phase is most important when choosing it for reversed-phase HPLC?

Accepted Answer

Polarity

Question 15

In HPLC, what is the effect of increasing the proportion of organic solvent in the mobile phase during gradient elution?

Accepted Answer

Nonpolar compounds elute faster

Question 16

Which equation describes the relationship between absorbance and concentration in UV-VIS spectroscopy?

Accepted Answer

$$A = \epsilon c l$$

Question 17

A sample contains both a colored and a colorless compound. Which detector would best allow you to distinguish between them in HPLC?

Accepted Answer

Diode Array Detector (DAD)

Question 18

Which of the following is a reason why the choice of stationary phase and mobile phase must be compatible in HPLC?

Accepted Answer

To ensure proper separation and avoid phase collapse

Question 19

If a compound does not absorb at 254 nm, what is the best strategy to detect it using HPLC?

Accepted Answer

Use a DAD to scan for absorbance at other wavelengths

Question 20

Which of the following statements about UV-VIS detection in HPLC is correct?

Accepted Answer

Compounds must have a chromophore to be detected

Instrumental Analysis: UV-Vis Spectroscopy and HPLC Study Guide

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