Chemistry #expch001 Important Questions
Ch. 7. Chromatography #expch_007
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- Chromatography is a physical method that is used to separate and analyse __
a) Simple mixtures
b) Complex mixtures
c) Viscous mixtures
d)none of these
Answer: B
Explanation
Chromatography is a physical method that is used to separate complex mixtures. The mixture of different components is flushed through the system at different rates.
Chromatography can function as a separation technique because different components in a mixture have different attractions to the stationary and the mobile phase. The different attractions are due to the different properties of the components in the mixture.
As the complex mixture passes through the column, how fast the individual components of the mixture elute from the column will depend upon two features:
1) The extent to which each compound ‘holds on to’ the stationary phase. This depends upon interactions between the individual components and the silica gel.
2) How soluble the individual components are in the mobile phase (the eluent). This depends on the interactions between the components and the solvent system.
- In chromatography, which of the following can the mobile phase be made of?
a) Solid or liquid
b) Liquid only
c) Gas only
d) Liquid or gas
Answer: d
Explanation
What is Mobile Phase
The mobile phase is the phase, which flows through the chromatographic medium. Generally, the mixture to be separated is dissolved in the mobile phase.
Also, it passes through the structure of the chromatography, holding the material called the stationary phase.
Significantly, differential migration is the principle of chromatography. Therefore, different components of the mixture travel at different speeds based on the differential interactions of these components towards the mobile phase and stationary phase.
In chromatography, the mobile phase can be composed of liquid or gas. It cannot be a solid material.
The mobile phase refers to the liquid or gas, which flows through a chromatography system, moving the materials to be separated at different rates over the stationary phase while stationary phase refers to the solid or liquid phase of a chromatography system on which the materials are to be separated or selectively adsorbed.
- Which of the following cannot be used as an adsorbent in Column adsorption chromatography?
a) Potassium permanganate
b) Silica gel
c) Activated alumina
d) Magnesium oxide
Answer: a
Explanation
The given options are all examples of adsorbents in Column adsorption except potassium permanganate. Some other adsorbents are starch and chromatographic purified siliceous earth.
- In which of the following type of paper, chromatography does the mobile phase move horizontally over a circular sheet of paper?
a) Ascending paper chromatography
b) Descending paper chromatography
c) Radial paper chromatography
d) Ascending – descending chromatography
Answer: c - Which of the following is not an advantage of Syringe type pumps used in High pressure liquid chromatography?
a) Independent of viscosity
b) Pulse-less flow
c) High pressure capability
d) Unlimited solvent capacity
Answer: d - Syringe pumps used in High pressure liquid chromatography are most suitable for which of the following columns?
a) Capillary columns
b) Guard columns
c) Short-fast columns
d) Small bore columns
Answer: d - Which of the following is a special adsorbent used in gas-solid chromatography?
a) Molecular sieves
b) Silica gel
c) Alumina
d) None of these
Answer: a
Explanation
Molecular sieves are a special adsorbent used in gas-solid chromatography. It is used in open tubular columns. Molecular sieves have the ability to distinguish materials on the basis of their size. This property can be used in separating molecules of linear structure from bulky ones.
A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be absorbed, while smaller molecules can. As a mixture of molecules migrate through the stationary bed of porous, semi-solid substance referred to as a sieve (or matrix).
- Which of the following is the disadvantage of hydrogen, which can be used as carrier gas in gas chromatography?
a) low density
b) Expensive
c) Reduced sensitivity
d) None of these
Answer: d
Explanation
In gas chromatography the carrier gas has an important role transporting the sample through the column and into the detector. The choice of carrier gas depends on the type of gas chromatography detector that is used and the components that are to be determined. Beside this hydrogen is combustible and concentrations of >4% in air bear a high risk of explosion since hydrogen has a very low ignition energy.
In Gas Chromatography there are three gases that are commonly used as a carrier gas: nitrogen, helium and hydrogen. The importance of carrier gas selection has been a discussion point amongst the users of Gas Chromatography for many years. When selecting the right carrier gas the user has to consider different parameters such as; price, performance, speed, analytical compatibility or just availability.
- in which of the following methods are liquid samples injected into the column in gas chromatography?
a) Gas tight syringe
b) Micro-syringe
c) Rotary sample valve
d) None of these
Answer: b
Explanation
For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a “plug” of vapour – slow injection of large samples causes band broadening and loss of resolution.
The most common injection method is where a microsyringe is used to inject sample through a rubber septum into a flash vapouriser port at the head of the column. The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample.
For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. Capillary columns, on the other hand, need much less sample, typically around 10-3 mL. For capillary GC, split/splitless injection is used.
The injector can be used in one of two modes; split or splitless. The injector contains a heated chamber containing a glass liner into which the sample is injected through the septum. The carrier gas enters the chamber and can leave by different routes.
- Capillary columns are open tubular columns constructed from which of the following materials?
a) Glass
b) Metal
c) Stainless steel
d) None of these
Answer: d - Which of the following is not a desirable feature of the ovens used in gas chromatography?
a) It must have a fast rate of heating
b) Power consumption should be kept low
c) It must have maximum thermal gradients
d) None of these
Answer: c
Explanation
This equilibrium has a direct impact on temperatures in gas chromatography. If the oven temperature is too cool, a compound will spend most of its time condensed in the stationary phase. Only a very small amount that can evaporate will transfer down the column.
Conversely, if the temperature is very high then the equilibrium will shift in the opposite direction. The compound will spend all of its time in the vapor phase and not condense into the stationary phase.
Gas chromatography works when a compound can transition freely both into and out of the stationary (condensed) phase. Only when the compound is condensed can it interact with the stationary phase. Only when the compound is in the vapor phase can the mobile phase push the compound along the column to the detector.
- Which of the following steps takes place after injection of feed in Column chromatography?
a) Detection of components
b) Separation in the column
c) Elution from the column
d) Collection of eluted component
Answer: b
Explanation
The operation that occurs after the injection of feed is a separation in the column. After that, elution from the column and detection of components takes place.
When the mobile phase along with the mixture that needs to be separated is introduced from the top of the column, the movement of the individual components of the mixture is at different rates.
The components with lower adsorption and affinity to the stationary phase travel faster when compared to the greater adsorption and affinity with the stationary phase. The components that move fast are removed first whereas the components that move slowly are eluted out last.
- What happens during the ‘elution from the column’ phase in chromatography?
a) Components with greatest affinity elute first
b) Components with least affinity elute first
c) Components elute in a random manner
d) Components elute according to their concentration in the mixture
Answer: b - In chromatogram, the position of peaks on the time axis can be used to determine which of the following?
a) Components of the sample
b) Column resolution
c) Column efficiency
d) none of these
Answer: a
Explanation
Chromatogram is a detector that responds to concentration solute and is placed at the end of the column. The position of peaks on the time axis can be used to determine components of the sample.
Each peak represents a component present in the sample. Retention time is time interval between sample injection and the maximum of the peak. It is characteristic of the identity of the component under the operating conditions.
The chromatogram is a two-dimensional plot with the ordinate axis giving concentration in terms of the detector response, and the abscissa represents the time.
The detector gives a response as a peak whose height should be ideally dependent on the concentration of the particular component.
- Affinity chromatography separation is based on
a) specific interaction between the analyte and the ligand
b) the flow-through time of the analyte
c) the duration of the analyte
d) molecular weights
Answer: a - Which of the following is not the application of gel filtration?
a) Purification
b) Relative molecular mass determination
c) Protein concentration
d) None of these
Answer: d - Which solvent is usually used for SEC in our lab?
a) tetrahydrofuran
b) toluene
c) water
d) None of these
Answer: a
Explanation
Tetrahydrofuran (THF) is one of the most commonly used solvents for non-aqueous size-exclusion chromatography (SEC) because it is a good solvent for a wide range of polymer types.
Because a solvent’s ability to dissolve a given polymer is temperature dependent, the experimental conditions over which polymer solubility varies are often referred to as good, poor, theta, and non-solvent.
A good solvent dissolves the polymer in any proportion at most temperatures.
- Jet Separator is used to introduce the———–into MS than carrier gas.
a) More analyte
b) Blank sample
c) Gross sample
d) none of these
Answer a - Charge transfer is commonly known as:
(a) Cationization
(b) Desorption
(c) Electron Ejection
(d) none of these
Answer b
Explanation
Laser desorption mass spectrometry uses a laser to ionize the analyte. In some cases, the laser can be applied directly to the sample. It will remove molecules from the surface of the material and then ionize them. Very little decomposition of the parent ion takes place so molecular ions are always prominent.
- Which of the following species will be deflected to the greatest extent?
(a) 37Na+
(b) 35Na+
(c) 37Na
(d) 35Na2+
Answer d - Which of the following is not used for mass spectrometry?
(a) calculating the isotopic abundance in elements
(b) investigating the elemental composition
(c) confirming the presence of O-H and C=O in organic compounds
(d) None of these
Answer. c - The correct order for the basic features of a mass spectrometer is…
(a) acceleration, deflection, detection, ionization
(b) ionization, acceleration, deflection, detection
(c) acceleration, ionization, deflection, detection
(d) None of these
Answer b
Explanation
The correct order for the basic features of a mass spectrometer is ionization, acceleration, deflection, and defection.
Mass spectrometry is the method in which the chemical substances are identified by sorting the gaseous ions in electric and magnetic fields which occurs during the mass to charge ratios.
- Separations in ions in mass spectrometer take place based on which of the following?
(a) Charge to mass ratio
(b) Molecular weight
(c) Mass to charge ratio
(d) none
Answer c - In a mass spectrometer, the ion currents are measured using one of the following?
(a) Scintillation counter
(b) ion counter
(c) Electric fields
(d) Electrometer tube
Answer d - Which type of ionic species are allowed to pass through the slit and reach the collecting plate?
(a) Negative ions of all masses
(b) positive ions of the specific mass
(c) Positive ions of all masses
(d) none of these
Answer b
Explanation
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.
A mass spectrum is a type of plot of the ion signal as a function of the mass-to-charge ratio. These spectra are used to determine the elemental or isotopic signature of a sample, the masses of particles and of molecules, and to elucidate the chemical identity or structure of molecules and other chemical compounds.
In a typical MS procedure, a sample, which may be solid, liquid, or gaseous, is ionized, for example by bombarding it with a beam of electrons. This may cause some of the sample’s molecules to break up into positively charged fragments or simply become positively charged without fragmenting. These ions (fragments) are then separated according to their mass-to-charge ratio
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