Questions Set-3_Forensic Instrument based questions

 

Q1. Which of the following is a technique used to correct for baseline drift in spectrophotometric measurements?

a.      Dilution

b.      Blank subtraction

c.       Heating

d.      Refraction

ANSWER:

b. Blank Subtraction

EXPLANATION:

The Blank subtraction involves measuring the absorbance of a blank (solvent without analyte) and subtracting it from the absorbance of the sample to correct for baseline drift and solvent absorbance.

Q2. Which one of the following is an advantage of using a double-beam spectrophotometer over a single-beam spectrophotometer?

a.      Higher sensitivity

b.      Simpler operation

c.       Reduced baseline drift

d.      Faster measurement times

ANSWER:

  c.  Reduced base line drift

EXPLANATION:

Spectrophotometers often employ double-beam operation where two separate light paths are used—one for the sample and one for the reference (typically a blank). This helps in continuously monitoring and correcting for any baseline drift that affects both paths equally.

Q3. Given below are two statements one is labelled as Assertion A and the other is labelled as Reason R

Assertion A: Infrared spectrophotometers are used for analyzing organic compounds based on their absorption patterns in the infrared region.

Reason R: Infrared radiation causes molecular vibrations and rotations in organic molecules, leading to characteristic absorption bands.

In the light of the above statements, choose the most appropriate answer from the options given below

a.      Both A and R are true and R is the correct explanation of A.

b.      Both A and R are true but R is not the correct explanation of A.

c.       A is true but R is false.

d.      A is false but R is true.

ANSWER:

a.      Both A and R are true and R is the correct explanation of A.

EXPLANATION:

Infrared spectrophotometers analyze organic compounds by detecting the absorption of infrared radiation, which corresponds to molecular vibrations and rotations. These absorption bands are characteristic and can be used to identify and analyze organic compounds, validating the reasoning provided.

Q4. In NMR spectroscopy, what does the coupling constant (J) between two nuclei indicate?

a.      Chemical shift difference

b.      Spin-spin coupling strength

c.       Relaxation rate

d.      Integration value

ANSWER:

b.      Spin-spin coupling strength

EXPLANATION:

Spin-spin coupling occurs when two or more nuclei in a molecule interact with each other through their magnetic fields. This interaction results in the splitting of NMR signals into multiplets (doublet, triplet, quartet, etc.), depending on the number of equivalent neighboring nuclei.

The coupling constant (J) is a measure of the energy difference between the different spin states of the coupled nuclei. It is expressed in Hertz (Hz) and represents the distance between the peaks in a multiplet.

Larger coupling constants (e.g., 7 Hz, 8 Hz) indicate stronger coupling interactions, whereas smaller coupling constants (e.g., 1 Hz, 2 Hz) indicate weaker interactions.

Q5. Which parameter is primarily responsible for the broadening of NMR signals in solid-state NMR spectroscopy?

a.      Chemical shift anisotropy

b. Line broadening due to magnetic field inhomogeneity

c.       Dipolar coupling

d.      Spin-spin relaxation

ANSWER:

c. dipolar coupling

EXPLANATION:

Dipolar coupling in NMR spectroscopy refers to the magnetic interaction between two nuclei that have a magnetic moment due to their nuclear spin. This interaction arises from the magnetic dipole-dipole interaction between the nuclei, which can influence the NMR spectra in both liquid-state and solid-state NMR experiments.

Q6. Which solvent is commonly used in NMR spectroscopy to avoid interference in the spectrum?

A.      Acetonitrile-d3

B.      Chloroform-d

C.      Deuterium oxide

D.     Dimethyl sulfoxide

ANSWER:

b. Chloroform-d

EXPLANATION:

Chloroform-d is a deuterated form of chloroform (CHCl3), where most of the hydrogen atoms are replaced by deuterium (^2H) atoms. It is a colorless liquid with a boiling point of 61.2°C and is relatively stable.

Chloroform-d is commonly used as a solvent in NMR spectroscopy because its ^1H NMR signal is a single sharp peak that does not interfere with the signals of most organic compounds of interest

Q7. Which type of detector is sensitive to changes in refractive index and is commonly used in HPLC for detecting non-UV-absorbing compounds?

a.      UV-Vis detector

b.      Refractive index detector

c.       Fluorescence detector

d.      Mass spectrometer

ANSWER:

c.       Refractive index detector

EXPLANATION

RID in HPLC detects changes in refractive index caused by solute concentration changes, making it suitable for compounds that do not absorb UV-visible light.

Q8. Which type of HPLC column packing material provides enhanced separation efficiency due to its uniform particle size and shape?

a.      Silica gel

b.      Polymer based stationary phase

c.       Monolithic column

d.      Porous graphitic column

ANSWER:

c. Monolithic column

EXPLANATION:

A monolithic column in HPLC refers to a type of stationary phase where the packing material is a single continuous piece. These columns have a unique macro-porous structure that allows the mobile phase to flow through the column with minimal resistance, leading to low backpressure.

The continuous structure of monolithic columns eliminates the interstitial spaces and voids between particles found in traditional packed columns, resulting in enhanced separation efficiency and reduced band broadening.

Q9. In gas chromatography (GC), which factor primarily determines the resolution of closely eluting peaks?

a.       Flow rate of the carrier gas

b.      Temperature programming

c.       Stationary phase polarity

d.      Column length

ANSWER:

b. Temperature programming

EXPLANATION:

In gas chromatography, temperature programming is a technique used to improve the resolution of closely eluting peaks. By gradually increasing the temperature of the column during the analysis, compounds with slightly different volatilities can be separated more effectively. This technique alters the retention times of compounds, allowing for better peak separation and resolution.

Q10. How does the wavelength of incident light affect the absorbance of a sample in UV-Visible spectroscopy?

a.       Higher wavelength increases absorbance

b.      Lower wavelength increases absorbance

c.       Absorbance is independent of wavelength

d.      Absorbance decreases with increasing wavelength

ANSWER:

a.      Higher wavelength increases absorbance.

EXPLANATION:

Beer-Lambert's Law states that absorbance (A) is directly proportional to the concentration (c) of the absorbing species, the path length (b), and the molar absorptivity (ε) at a given wavelength (λ).

Content writer:- Bhawana Sharma