Question 1

Which molecular property is most critical for a compound to absorb light in the UV-VIS range?

Accepted Answer

Conjugated double bonds and aromatic rings

Question 2

A compound with more than 6-8 conjugated double bonds is likely to:

Accepted Answer

Absorb in the UV-VIS range

Question 3

Which of the following compounds would be expected to show strong absorption at 254 nm in a UV-VIS spectrum?

Accepted Answer

A molecule with an aromatic ring

Question 4

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

Accepted Answer

Non-polar, hydrophobic compounds

Question 5

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

Accepted Answer

It corresponds to the absorption of many aromatic compounds, giving a high signal-to-noise ratio

Question 6

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

Accepted Answer

DAD can record spectra at multiple wavelengths simultaneously, allowing compound identification and impurity detection

Question 7

Which equation best describes the relationship between absorbance ($$A$$), molar absorptivity ($$\epsilon$$), path length ($$l$$), and concentration ($$c) in UV-$$VIS spectroscopy?

Accepted Answer

$$A = \epsilon l c$$

Question 8

A student analyzes ibuprofen in PBS buffer using UV-VIS at 254 nm. Why is this matrix suitable for direct measurement?

Accepted Answer

PBS buffer is a simple, clean matrix with minimal interference

Question 9

In reversed phase HPLC, what effect does increasing the alkyl chain length of a paraben have on its retention time?

Accepted Answer

Retention time increases due to stronger hydrophobic interactions

Question 10

Which separation principle is dominant in reversed phase HPLC?

Accepted Answer

Partitioning based on hydrophobicity

Question 11

A compound with a carboxylic acid group and several aromatic rings is injected into a reversed phase HPLC. What is its expected behavior?

Accepted Answer

It will have moderate retention due to both polar and non-polar regions

Question 12

Which of the following best explains why the choice of mobile phase and stationary phase in HPLC are interdependent?

Accepted Answer

The stationary phase sets the separation mechanism, while the mobile phase controls analyte elution

Question 13

What is the difference between isocratic and gradient elution in HPLC?

Accepted Answer

Isocratic elution uses a fixed mobile phase composition; gradient elution changes composition over time

Question 14

A UV-VIS spectrum of a compound shows a strong peak at 300 nm. What does this indicate about the compound?

Accepted Answer

It has a conjugated system or aromatic ring

Question 15

Which property is used to separate compounds in reversed phase HPLC?

Accepted Answer

Hydrophobicity

Question 16

A sample containing proteins and small molecules is analyzed for ibuprofen using UV-VIS. What must be done before measurement?

Accepted Answer

Sample preparation is needed to remove proteins and interfering substances

Question 17

Which of the following is a plausible reason for a compound to have no retention in reversed phase HPLC?

Accepted Answer

It is highly polar and hydrophilic

Question 18

If a compound absorbs strongly at 254 nm, what structural feature is most likely present?

Accepted Answer

Aromatic ring

Question 19

Which statement best describes the function of a DAD spectrum in HPLC analysis?

Accepted Answer

It records absorbance across a range of wavelengths, allowing peak identification and impurity detection

Question 20

A mixture of methylparaben, ethylparaben, propylparaben, and butylparaben is separated by reversed phase HPLC. Which will elute first?

Accepted Answer

Methylparaben

Instrumental Analysis: UV-Vis Spectroscopy and Reversed Phase HPLC Study Notes

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