A Two question Exam of Organic Analysis (Very helpful for the final Year Undergraduate Students in BSc Chemistry at University and Job Applicants for Chemistry profession)

Question 1:

In laboratory analysis, the calculation of relative uncertainty, allows us to trust our results once the validated method, standards and calibrated instruments were used.

However, in reporting laboratory results, the accuracy and precision are required.

1.1 Differentiate the following terms

a)    Precision from accuracy

b)    Rugged method from robust method

c)    Standard from blank

d)    Method and procedure

e)    Sensitivity and selectivity

f)   Limit of detection and limit of quantification for Atomic Absorption Spectroscopy

1.2 During the determination of the viscosity of an organic sample, which of the following statements is not correct?

a)  The viscosity will depend on the relative density of the sample at a given temperature

b)    The viscosity is directly proportional to the density of the sample at a given temperature

  c)  The viscosity is inverse proportional to the retention time of the sample in Oswald Viscometer washed using acetone.

d)  The viscosity increases with temperature

e)  The viscosity increases with concentration

Question 2:

What is the utility of the following laboratory instruments?

2.1 Refractometer 2.2 Pychnometer 2.3 Atomic absorption spectrophotometer

2.4 Polarimeter     2.5 Laminar flux    2.6 Oven     2.7 UV light 2.8 Incubator 2.9 Gas chromatography                                 2.10 Rotavapor


Question 1 

1.1 The following terms are differentiated as follow:

                     (a)   Precision is an indication of the reproducibility of a measurement or result in a given experiment while accuracy is a measure of the agreement between an experimental result and its expected value( provided in literature, theoretical value).                                                                     1mark

(b)   Rugged method is a method that is relatively insensitive to changes in experimental conditions while robust method is a method that is free from chemical interferences, therefore it can be applied to the determination of analytes in a wide variety of sample matrices               1mark

(c)    Standard (solution) is a solution containing a known amount of analyte prepared separately from samples containing analyte that is used to determine the concentration of the specific analyte in a given sample while a blank(reagent/solution) is a solution that corrects the measured signal for signal due to reagent/solution other than the sample that are used in the analysis.           1mark

(d)   Method is a means for performing a given laboratory task( analyzing a sample for a specific analyte in a specific matrice) while procedure is a written directions(steps) outlining how to perform a given task( laboratory experiment).                                                                                   1mark

(e)    Sensitivity is a measure of a method’s ability to distinguish between two samples, reported as the change in signal per unit change in the amount of analyte while selectivity is a measure of a method’s freedom from interferences (as defined by the method’s selectivity coefficient).   1mark

(f)    The limit of detection(LOD) is the smallest concentration or absolute amount of analyte that can produce a signal significantly larger than the signal arising from a reagent blank while limit of quantification(LOQ) is the smallest concentration or absolute amount of analyte that can be reliably determined.

 1.2 C and D.                                                                                                        4mark

Question 2: Utility of the following instruments

         Refractometer: measuring the refractive index             1mark

              Pychnometer: determination of relative density             1mark

          Atomic absorption spectrophotometer: determination of concentration of metals in analyte. 1mark

              Polarimeter: measuring the rotation angle of an optically active sample. 1mark

Laminar flux: is the instrument used in controlling the smooth layered flow in a liquid or a gas in which neighboring layers do not mix but flow at different velocities.                              1mark

   Oven: is an instrument made of a compartment warmed by a heat source used for drying materials.    1mark

   UV-light: for determination of the molecular absorption (absorbance) that is proportional to the concentration of the analyte in a sample.                                                           1mark

Or UV-light is used in thin layer chromatography analysis when the spot are too small and obscure in order to be well observed and also when spots do not absorb light.

Or, UV-light is used in ultrasterilization with the purpose of killing bacteria by irradiation.

Incubator(laboratory): is used to control the growth of micro-organism such bacteria or fungi at fixed temperature and humidity during their culture without contaminating the laboratory environment.         1mark

Gas Chromatograph: is an instrument used for separating the volatile constituent of a matrix sample by means of a stationary gas phase in order to analyze it. It is used in quantitative analysis of most organic samples with strong bias to organic pollutant, food analysis, drugs, …       1mark

Rotavapor: for separating solvent and the extract by steam evaporation.             1mark