QUALITY CONTROL INTERVIEW QUESTIONS - WET INSTRUMENTATION

1)      WHAT IS THE PRINCIPLE OF UV-VIS SPECTROSCOPY?

       In UV-VIS, by passing the UV light through the analyte solution, Based on changes in electronic transition (electronic energy levels) of analyte we can determine conjugation present in the sample.

2) WHAT ARE BEER-LAMBERTSLAW? EXPLAIN?

 Beers law: Beer's law states that for a parallel beam of monochromatic radiation passing through homogeneous solutions of equal path length,

 The absorbance is proportional to the concentration. (A œ C)

Lamberts law: Lambert's law states that for a parallel beam of monochromatic radiation passing through homogeneous solutions of equal concentration,

The absorbance is proportional to the path length.  (A œ t)

By combining both laws

                         A œ Ct => a = ε ct (This is known as Beer lamberts law)

A is Absorbance, C is concentration, t is thickness and ε is molar extinction coefficient.

Beer lamberts law states that for a parallel beam of monochromatic radiation passing through homogeneous solutions then the absorbance is proportional to concentration and path length of the solution.

3) WHAT IS LIMIT OF STARY LIGHT? HOW TO MEASURE LIMIT OF STRAY LIGHT?

1.      Stray light is defined as detected light of any wavelength that is outside the    bandwidth of the selected wavelength.

2. To measure stray light, some kind of filter is required that absorbs all light of the wavelength at which the measurement is to be made and transmits higher and lower wavelengths.
3.  In practice such filters do not exist, so “cut-off” filters which transmit all light above or below a certain wavelength and block all light in the wavelength range.

Measure of Stray light:

# Stray light can measure by following material.

           Material                                Cut off                            Concentration

 NaNO2-Sodium Nitrite -----------------390 nm--------------------------   5% aqueous

 KI-Potassium Iodide -------------------- 260 nm -------------------------   1% aqueous

NaI-Sodium Iodide------------------------ 260 nm--------------------------   1% aqueous

Li2CO3-Lithium Carbonate -------------227 nm---------------------------   Saturated aqueous

NaCl-Sodium Chloride --------------------205 nm---------------------------   1% aqueous

KCl-Potassium Chloride ------------------200 nm---------------------------   1.2% aqueous 

☼Stray light is usually checked with a 1.2% potassium chloride solution, where the absorbance for 1 cm path length should exceed 2.0 at 200 nm against a water reference. (Absorbance more than 2 means % of Transmittance less than 1)

☼1.2% potassium chloride solution UV Cut off is 200 nm.It means that it should absorb all the light below 200 nm and transmit all the light above 200 nm.

a)      If we get absorbance of 1.2% KCl solution more than 2 (Transmittance less than 1.0%) then there is no stray light.

b)     b) If we get absorbance of 1.2% KCl solution less than 2 (Transmittance more than 1.0%) then that transmitted light is known as Stray light.

A = log₁₀ 100 / %T (or) A = 2 - log₁₀ %T 

4) WHICH PARAMETERS DO WE CHECK IN UV-VIS CALIBRATION?

1. Control of wavelength by HOLMIUM PERCHLORATE SOLUTON (Holmium oxide in perchloric acid)

2. Control of absorbance by POTASSIUM DICHROMATE (In 0.005M sulphuric acid)

3. Limit of stray light by POTASSIUM CHLORIDE SOLUTION (In water)

4. Wavelength accuracy

5. Resolution by TOLUENE IN n-HEXANE.

6. Resolution power by TOLUENE IN METHANOL.

7. Absorption of corvettes (Cuvettes).

Why:

Why do we use holmium per chlorate in control of wavelength test?

Holmium per chlorate solution can give very sharp and accurate peaks at constant places throughout the UV-VIS region(At 241.15nm,287.15nm,361.5nm,536.3nm) and it is suggested by NIST (National Institute of Standards and Technology) and Pharmacopeia.

Why do we use potassium dichromate in control of absorbance test?

Potassium dichromate in sulphuric acid can give more maxima in the UV-VIS region (At 235nm, 257nm, 313nm, 350nm, 430nm) so that we can check the whole UV-VIS region with a single run and it is suggested by NIST and Pharmacopeia.

Potassium dichromate itself is stable and available in high purity. In dilute perchloric acid solution, it has a linear response with temperature.

Why do we use potassium chloride in limit of stray light test?

The UV cut off for potassium chloride is 200 nm so it is neutral for UV-VIS region. Means It cannot absorb in UV-VIS region.

Why we use Toluene in hexane in resolution test?

Toluene can give one maxima at 269nm and minima at 266.These two are very close to each other if any small change in the resolution we can easily identify the absorbance differences. Hexane cannot absorb uv light and it is miscible with Toluene.

5) WHAT IS THE PRINCIPLE OF IR SPECTROSCOPY?

                In IR, by passing the IR light through the analyte, based on changes in vibration & rotational movements of the atoms present in the sample we can determine functional groups present in the sample.

6)      WHY WE USE KBR FOR PELLET MAKING IN IR?

1. It does not absorb IR radiation in whole region (400-4000 cm-1). (RX does not absorb IR radiation).
2. Due to its high mechanical strength. (With very small amount of KBr we can make a very transparent pellet).
3. It does not react with the sample due to its neutral properties.
              NaCl (mostly for liquids), NaI, KCl can also use for IR pellet making.

7) WHAT IS THE THICKNESS OF POLYSTYRENE FILM USED FOR IR?

Thickness:

Generally we use 38 micron (0.038mm=0.04mm) thickness polystyrene film for FTIR calibration. There are some other micron thickness films also available (like 76 micron).

8)   WHAT IS THE UNITS OF WAVE NUMBER IN FTIR?

Units:

The units of wave number in FTIR are cm-1

9)      WHY WE USE POLYSTYRENE FILM FOR CALIBRATION OF FT-IR?

Polystyrene film for calibration:

☼In polystyrene film all bands executed which covers whole IR region. And the bands are at constant places with sharp intensities.

☼polystyrene film is made by the polymer of styrene. So it’s having highly durability and stabled at any temperature.

10) WHAT ARE THE ADAVNTAGES OF FTIR OVER IR?

Below are some of major advantages of FT-IR over dispersive IR.

1. Speed: All the measurements are made simultaneously most measurement are made by FTIR are made in a matter of seconds rather than in minutes as in dispersive.

2. Sensitivity: Sensitivity is dramatically improved in FTIR for many reasons. Detectors employed are much more sensitive. Sensitivity directly prepositional to number of scans, If number of scans increases sensitivity increases. Noise is low.

3. Mechanical Simplicity: The moving mirror in the interferometer is only continuously moving part in the instrument. Then there is very little possibility of mechanical breakdown.

4. Internally Calibrated: This instrument employs a helium-neon laser light as an internal wavelength calibration standard. FTIR instruments are self calibrating and never need to be calibration check by user.          

 These advantages along with several other make measurements made by FTIR extremely accurate and reproducible. Thus it is very reliable technique for positive identification of virtually any sample.The sensitivity benfits enables identification of even the smallest of contaminants.

11)  WHAT IS THE PRINCIPLE OF POLARIMETER?

             By passing plane polarized light (moves in a single direction) through the sample, Based on the change in the direction of plane polarized light we can identify the optical isomers present in the sample. That means whether it is dextro or levo. If light rotates toward right we confirm it as Dextro and if to left we can confirm it as Levo.

     12)  WHAT IS THE DIFFERENCE BETWEEN OPTICAL ROTATIONS & SPECIFIC                         ROTATION (or) SPECIFIC OPTICAL ROTATION?

      Optical Rotation (OR): α

             It is the angle of rotation of a plane of polarized light when passed through an optically active substance.

·      It is measured by the instrument Polari meter and is denoted by α. 

      Specific optical Rotation (SOR): [α]             

           The specific rotation of a chemical compound [α] is defined as the observed optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 milliliter.

·      It is measured from the optical rotation and is denoted by  [α].

                                                  [α] = 100α /cl

                               c = Concentration of sample in Gram/100 ml

                                                       l = Length of cell in dm (decimeter) (1dm=10 cm=100 mm)

     13) ON WHICH FACTORS OPTICAL ROTATIONS & SPECIFIC OPTICAL ROTATION                DEPENDS?

   

       1.      The optical rotation depends upon: 

·      The type of sample (example: sugar solution).

·       Concentration of the optically active component(s).

·      The length of the sample tube.

·       The wavelength of the light source.

·       Temperature of the sample.

·      Solvent used for dilution.

       2. The specific optical rotation depends on

·         Type of sample

·          Wavelength of light source and

·          Temperature

But it does depend on, concentration of sample and cell length. That means
                  if we analyze a sample with different concentrations or different cell lengths under same conditions the OR valves change but SOR values are same.

     14)  WHY WE USE SUCROSE FOR POLARIMETER CALIBRATION? EXPLAIN MUTAROTATION?

1. It is a non mutarotating compound. (It has no isomeric forms to get mutarotation)
2. It is easily soluble in water up to 30% and soluble up to 50%.

Mutarotation:
                     The change in the specific rotation between isomeric forms of a compound is known as Mutarotation.

Explanation:
     D (+)-Glucose exists in two isomeric forms which undergo mutarotation.

a) Crystals of ordinary D(+)-Glucose of melting point 146°C are dissolved in water the specific rotation gradually drops from an initial +112 to +52.7.
b) Crystals of ordinary D(+)-Glucose of melting point 150°C (obtained by crystallization at temp 98°C) are dissolved in water the specific rotation gradually rises from an initial +19 to +52.7.

                      The form with higher positive rotation is called Alpha D (+)-Glucose. And with lower positive rotation is called Beta D (+)-Glucose .The change in the specific rotation of each of these is known as Mutarotation.

         15)     WHAT IS THE PRINCIPLE OF KARL FISCHER TITRATION?

         In Karl Fischer titration one mole of iodine reacts with one mole water present in the sample and forms two moles of HI. Based on this reaction we can determine water present in the sample.
                          I2 + H2O ---> 2HI

ROH + SO₂ +R’N à[R’NH] SO₃R + H2O + I₂+ 2R’N à 2[R’NH] I + [R’NH] SO₄R

Mechanism:

Step 1:          3R’N+ SO2 + I2 + H2O à [R’N] SO3 + 2[R’NH] I

Step 2:              [R’N] SO3 + ROH à [R’NH] SO4R {R’N = Pyridine}

       16)     WHY WE USE DST FOR KF REAGENT STANDARDIZATION & KF APPARATUS CALIBRATION?

Standardization of KF reagent:

             Standardization of KF reagent means checking of concentration of KF reagent. As per Karl Fischer 5mg of water is neutralized by 1 ml of KF reagent (If the KF reagent is good).That’s why we get the KF factor around 5 mg of water/ml.
So we need a standard which contains following properties.
1. It must be a primary standard 
2. It should Contains a known & constant percent of water in it at room temperature.
3. Should dissolve in methanol easily.

Generally we use 
                                      Water - 100% of water   (For volumetric)
 Disodium Tartrate Dihydrate - 15.66% of water (For volumetric)
            Lactose Mono hydrate - 5.0% of water    (For Coulometric)

KF Apparatus Calibration:

               We use KF Apparatus for determination of water present in the sample. Calibration means to check the instrument whether it works properly or not. So any sample which contain constant amount of water in it can use for KF Apparatus calibration. Generally we use above mentioned samples (Mostly DST)

      17) WHY WE ARE USING EXCESS METHANOL IN KF TITRATION?

The reaction in KF titration is

3 Im +I2 + SO2 + H2O ------> 2 Im. HI + Im.SO3
Im.SO3 + CH3OH      --------> Im.(H)SO4CH3.
(Stochiometri of I2 & H2O is 1:1)

In the 2nd step if methanol is not present the reaction is like below.

 3 Im +I2 + SO2 + H2O ------> 2 Im. HI +  Im.SO3  (Im= Imidazole)
 Im.SO3 + H2O      -------->  Im.NH + SO4H
(Stochiometri of I2 & H2O is change from1:1 to 2:1)
During the titration, If Excess methanol is not present  Im.SO3 can react with H2O which varies the Stochiometri of H2O & I2 from 1:1 to 2:1.

Note:
1.The above mentioned is the most suitable reason. And some other benefits also there for methanol like it can dissolve most of the organic solvents, low cost & availability.

      18) WHY WE ARE USING IPA or PYRIDINE IN PLACE OF METHANOL IN THE DETERMINATION OF WATER CONTENT PRESENT IN CARBONYL COMPOUNDS BY KARLFISCHER TITRATION?

    If we use methanol for carbonyl compounds, Methanol (Primary alcohol - Very reactive) reacts with carbonyl compounds and forms acetals, ketals and water.

    This means side reaction takes place.

  

  ☻If we use any secondary alcohol (less reactive than Primary alcohol) like IPA in place of Methanol we can easily stop the side reaction (Acetal or ketals with water formation) means IPA only reacts with water but not with carbonyl compounds.

  ☻We use this reaction for determination of water. But in above case water is produced in the reaction which leads to a wrong assumption.

  ☻ Pyridine also used in place of methanol.

    Note:

    1. We have the only choices of Alcohol (Primary, secondary & tertiary) and Basic buffers (like pyridine & Imidazole).Because karl fischer reagent is miscible with these two.

    2. Tertiary alcohol is very less reactive so it is not suitable. So the remaining choices are Primary & secondary alcohols and pyridine & Imidazole.

    19) WHY WE ARE USING SILICONE OIL IN MR APPARATUS?

Silicone oil is chosen for its ecofriendlyness by having below mention properties.

·         It cannot transfer electricity.

·         Non flammable and fire resistant.

·         Thermally stable in both cold and hot extremes·

·         No toxicity

• No odor and No taste