Forensic Instrumentation and Spectroscopy Questions and Answers

Q: Method for fragmenting molecules in tandem mass spectroscopy is known as :

The method is collision induced dissociation (CID).In tandem mass spectrometry (MS/MS) the first analyser selects a precursor ion. That ion is accelerated into a collision cell containing an inert gas such as argon, nitrogen or helium, where collisions convert part of its kinetic energy into internal energy and break it into product ions, which the second analyser then separates. The product ion pattern identifies the molecule with high specificity, which is why MS/MS is used to confirm drugs and toxins.The other names are not the standard technique. Fragmentation is caused by collision with gas atoms, not by mass, charge or a generic 'impact'.

Q: The value of induced dipole moment divided by strength of the field is better known as :

The quantity is polarizability.An electric field distorts the electron cloud of a molecule and induces a dipole moment. For moderate fields the induced dipole is proportional to the field, and the constant of proportionality is the polarizability (alpha):\[ \alpha = \frac{\mu_{\text{ind}}}{E} \]A change in polarizability during a vibration is what makes it Raman active. The total dipole moment includes any permanent dipole, 'unit dipole moment' is not a defined quantity, and depolarization (as in the Raman depolarization ratio) describes scattered light, not this ratio.

Q: Which of the following compounds is infra-red inactive ?

The key's answer is ethylene.A vibration absorbs infrared radiation only if it changes the molecule's dipole moment. Hydrochloric acid and carbon monoxide are polar diatomic molecules whose single stretching vibration changes the dipole, so both absorb strongly in the IR, and nitric acid is a polar molecule with many IR bands.Ethylene is a non-polar, centrosymmetric molecule. Its characteristic C=C stretching vibration is symmetric, causes no change in dipole moment, and is therefore IR inactive (it is seen in the Raman spectrum instead). Ethylene does have some other IR-active vibrations, but of the four it is the one whose key vibration is IR inactive, which is the reading the key takes.

Q: Precolumn used for the conditioning of mobile phase is known as :

The precolumn is a scavenger column.In HPLC a scavenger (or saturator) column is placed between the pump and the injector. It is packed with silica, which dissolves slightly into the mobile phase, so the mobile phase is conditioned (pre-saturated) before it reaches the analytical column and does not dissolve that column's packing.A guard column sits after the injector, just before the analytical column, and protects it by trapping particles and strongly retained sample components; it does not condition the mobile phase. 'Conditioning column' and 'mobilizing column' are not standard terms.

Q: Time-of-flight mass spectrometry operates in :

Time-of-flight mass spectrometry operates in pulse mode.In a TOF analyser, ions are accelerated by the same voltage so that they all gain the same kinetic energy, and their mass is found from the time they take to cross a field-free flight tube: lighter ions arrive first. For the timing to be meaningful all ions must start together, so they are produced or extracted in short pulses, for example by a pulsed laser in MALDI or by orthogonal pulsed extraction.A continuous beam would give no start time, and 'direct' and 'syntactic' are not operating modes of a TOF instrument.

Q: Which of the following is the correct sequence of the components of a monochromator in Emission Spectrometry ?

The correct sequence is entrance slit, collimating lens, prism, focusing lens, exit slit.Light enters a monochromator through the entrance slit. A collimating lens (or mirror) makes the rays parallel, the prism (or grating) disperses them into their wavelengths, the focusing lens forms an image of the slit for each wavelength, and the exit slit lets through only a narrow band of wavelengths.Placing the focusing lens before collimation, or the prism before the collimating lens, would disperse non-parallel light, and the third option runs the light path backwards.

Q: In HPLC, LOD and LOQ are obtained using :

LOD and LOQ are obtained from the signal to noise ratio.The limit of detection is the smallest amount of analyte that gives a peak clearly distinguishable from baseline noise, conventionally a signal-to-noise ratio of about 3:1. The limit of quantitation is the smallest amount that can be measured with acceptable precision and accuracy, conventionally about 10:1.Retention time, column efficiency, peak asymmetry and dead time describe chromatographic separation and peak shape, not the sensitivity limits of a method.

Q: Which of the following is used as a source of radiation in XRE Spectrometer ?

The source is the Coolidge tube, spelled 'Collidge' in the paper.An X-ray fluorescence (XRF) spectrometer, which the paper calls 'XRE', excites a sample with primary X-rays and measures the secondary (fluorescent) X-rays it emits to identify its elements. The primary X-rays usually come from a Coolidge-type X-ray tube, in which electrons from a heated filament are accelerated onto a metal anode such as rhodium or tungsten.A Crookes tube was the early cold-cathode discharge tube, a hollow cathode lamp is the line source used in atomic absorption spectroscopy, and an incandescent bulb gives visible light, not X-rays.

Q: Analysis of a drug by UHPLC generates a pressure of 14000 psi. A column with which of the following particle size is most suitable for analysis :

A column with 2 µm particles is the most suitable.Ultra-high-performance liquid chromatography uses columns packed with very small particles, typically below 2 micrometres, which give much higher efficiency and faster separations. Back-pressure rises steeply as particle size falls, so these columns need pumps that work at around 15,000 psi or more; a pressure of 14,000 psi is exactly what a sub-2 micrometre column generates.Particles of 5 and 10 micrometres are the sizes of conventional HPLC, run at a few thousand psi, and 20 micrometres is used for preparative work. None of them needs UHPLC pressure.

Q: ....................... is used to generate X-ray.

X-rays are generated with a Coolidge tube.The Coolidge tube, introduced by William Coolidge in 1913, is the standard X-ray tube. A heated tungsten filament (cathode) emits electrons, which are accelerated through a high vacuum by a high voltage and strike a metal target (anode); their sudden deceleration and the excitation of the target atoms produce X-rays.The Anger camera is a gamma camera for detecting radiation in nuclear medicine, not a source of X-rays, and 'West tube' and 'Mayer tube' are not X-ray tubes.

21.

Method for fragmenting molecules in tandem mass spectroscopy is known as :

A.Mass Induced Dissociation
B.Collision Induced Dissociation
C.Charge Induced Dissociation
D.Impact Induced Dissociation

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

The method is collision induced dissociation (CID).

In tandem mass spectrometry (MS/MS) the first analyser selects a precursor ion. That ion is accelerated into a collision cell containing an inert gas such as argon, nitrogen or helium, where collisions convert part of its kinetic energy into internal energy and break it into product ions, which the second analyser then separates. The product ion pattern identifies the molecule with high specificity, which is why MS/MS is used to confirm drugs and toxins.

The other names are not the standard technique. Fragmentation is caused by collision with gas atoms, not by mass, charge or a generic 'impact'.

Discuss in Forum

22.

The value of induced dipole moment divided by strength of the field is better known as :

A.Polarizability
B.Total dipole moment
C.Unit dipole moment
D.Depolarization

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

The quantity is polarizability.

An electric field distorts the electron cloud of a molecule and induces a dipole moment. For moderate fields the induced dipole is proportional to the field, and the constant of proportionality is the polarizability (alpha):

\[ \alpha = \frac{\mu_{\text{ind}}}{E} \]

A change in polarizability during a vibration is what makes it Raman active. The total dipole moment includes any permanent dipole, 'unit dipole moment' is not a defined quantity, and depolarization (as in the Raman depolarization ratio) describes scattered light, not this ratio.

Discuss in Forum

23.

Which of the following compounds is infra-red inactive ?

A.Ethylene
B.Hydrochloric acid
C.Carbon monoxide
D.Nitric acid

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

The key's answer is ethylene.

A vibration absorbs infrared radiation only if it changes the molecule's dipole moment. Hydrochloric acid and carbon monoxide are polar diatomic molecules whose single stretching vibration changes the dipole, so both absorb strongly in the IR, and nitric acid is a polar molecule with many IR bands.

Ethylene is a non-polar, centrosymmetric molecule. Its characteristic C=C stretching vibration is symmetric, causes no change in dipole moment, and is therefore IR inactive (it is seen in the Raman spectrum instead). Ethylene does have some other IR-active vibrations, but of the four it is the one whose key vibration is IR inactive, which is the reading the key takes.

Discuss in Forum

24.

Precolumn used for the conditioning of mobile phase is known as :

A.Guard column
B.Scavenger column
C.Conditioning column
D.Mobilizing column

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

The precolumn is a scavenger column.

In HPLC a scavenger (or saturator) column is placed between the pump and the injector. It is packed with silica, which dissolves slightly into the mobile phase, so the mobile phase is conditioned (pre-saturated) before it reaches the analytical column and does not dissolve that column's packing.

A guard column sits after the injector, just before the analytical column, and protects it by trapping particles and strongly retained sample components; it does not condition the mobile phase. 'Conditioning column' and 'mobilizing column' are not standard terms.

Discuss in Forum

25.

Time-of-flight mass spectrometry operates in :

A.Continuous mode
B.Pulse mode
C.Direct mode
D.Syntactic mode

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

Time-of-flight mass spectrometry operates in pulse mode.

In a TOF analyser, ions are accelerated by the same voltage so that they all gain the same kinetic energy, and their mass is found from the time they take to cross a field-free flight tube: lighter ions arrive first. For the timing to be meaningful all ions must start together, so they are produced or extracted in short pulses, for example by a pulsed laser in MALDI or by orthogonal pulsed extraction.

A continuous beam would give no start time, and 'direct' and 'syntactic' are not operating modes of a TOF instrument.

Discuss in Forum

26.

Which of the following is the correct sequence of the components of a monochromator in Emission Spectrometry ?

A.Entrance slit, focusing lens, collimating lens, prism, exit slit
B.Entrance slit, collimating lens, prism, focusing lens, exit slit
C.Exit slit, prism, collimating lens, focusing lens, entrance slit
D.Entrance slit, prism, collimating lens, focusing lens, exit slit

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

The correct sequence is entrance slit, collimating lens, prism, focusing lens, exit slit.

Light enters a monochromator through the entrance slit. A collimating lens (or mirror) makes the rays parallel, the prism (or grating) disperses them into their wavelengths, the focusing lens forms an image of the slit for each wavelength, and the exit slit lets through only a narrow band of wavelengths.

Placing the focusing lens before collimation, or the prism before the collimating lens, would disperse non-parallel light, and the third option runs the light path backwards.

Discuss in Forum

27.

In HPLC, LOD and LOQ are obtained using :

A.Retention time to Signal ratio
B.Efficiency to Asymmetry ratio
C.Signal to Noise ratio
D.Retention time to Dead time ratio

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

LOD and LOQ are obtained from the signal to noise ratio.

The limit of detection is the smallest amount of analyte that gives a peak clearly distinguishable from baseline noise, conventionally a signal-to-noise ratio of about 3:1. The limit of quantitation is the smallest amount that can be measured with acceptable precision and accuracy, conventionally about 10:1.

Retention time, column efficiency, peak asymmetry and dead time describe chromatographic separation and peak shape, not the sensitivity limits of a method.

Discuss in Forum

28.

Which of the following is used as a source of radiation in XRE Spectrometer ?

A.Collidge Tube
B.Crookes Tube
C.Hollow Cathode Tube
D.Incandescent light bulb

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

The source is the Coolidge tube, spelled 'Collidge' in the paper.

An X-ray fluorescence (XRF) spectrometer, which the paper calls 'XRE', excites a sample with primary X-rays and measures the secondary (fluorescent) X-rays it emits to identify its elements. The primary X-rays usually come from a Coolidge-type X-ray tube, in which electrons from a heated filament are accelerated onto a metal anode such as rhodium or tungsten.

A Crookes tube was the early cold-cathode discharge tube, a hollow cathode lamp is the line source used in atomic absorption spectroscopy, and an incandescent bulb gives visible light, not X-rays.

Discuss in Forum

29.

Analysis of a drug by UHPLC generates a pressure of 14000 psi. A column with which of the following particle size is most suitable for analysis :

A.10 µm
B.2 µm
C.5 µm
D.20 µm

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

A column with 2 µm particles is the most suitable.

Ultra-high-performance liquid chromatography uses columns packed with very small particles, typically below 2 micrometres, which give much higher efficiency and faster separations. Back-pressure rises steeply as particle size falls, so these columns need pumps that work at around 15,000 psi or more; a pressure of 14,000 psi is exactly what a sub-2 micrometre column generates.

Particles of 5 and 10 micrometres are the sizes of conventional HPLC, run at a few thousand psi, and 20 micrometres is used for preparative work. None of them needs UHPLC pressure.

Discuss in Forum

30.

....................... is used to generate X-ray.

A.Anger tube
B.West tube
C.Mayer tube
D.Coolidge tube

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: D

X-rays are generated with a Coolidge tube.

The Coolidge tube, introduced by William Coolidge in 1913, is the standard X-ray tube. A heated tungsten filament (cathode) emits electrons, which are accelerated through a high vacuum by a high voltage and strike a metal target (anode); their sudden deceleration and the excitation of the target atoms produce X-rays.

The Anger camera is a gamma camera for detecting radiation in nuclear medicine, not a source of X-rays, and 'West tube' and 'Mayer tube' are not X-ray tubes.

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