A gas chromatogram for separation of toluene and three isomers of xylene is show

Question

A gas chromatogram for separation of toluene and three isomers of xylene is shown.  The table gives the retention times and full width at baseline of the peaks.

Compound

retention time, s

  

A             B      C      D

A   toluene

B   p-xylene

C   o-xylene

D   m-xylene       

mobile phase

tm = 12.2

1  toluene

tR = 77.3

0.6

2  p-xylene

       81.5

0.8

3  o-xylene

       83.8

0.9

4  m-xylene

       84.1

0.9

                                                                                                                                                                          

                                                                                                      

                                                                                    Time, seconds   (not necessarily to scale)

(a) What is N of this column, for p-xylene?

(b)  The column has length 30 m (typical for GC columns). What is H for this column, for p-xylene?

(c) Calculate the capacity factors, k' for each component.

            toluene k' =

            p-xylene k' =

            o-xylene k' =

            m-xylene k' =

(d)  What could be done to improve the separation of the o- and m- isomers of xylene?

  

Compound

     

retention time, s

     

  

                  

A             B      C      D

                                           

A   toluene

      

B   p-xylene

      

C   o-xylene

      

D   m-xylene       

                     peak width at base, s      

mobile phase

     

tm = 12.2

     

     

1  toluene

     

tR = 77.3

     

0.6

     

2  p-xylene

     

       81.5

     

0.8

     

3  o-xylene

     

       83.8

     

0.9

     

4  m-xylene

     

       84.1

     

0.9

   A gas chromatogram for separation of toluene and three isomers of xylene is shown. The table gives the retention times and full width at baseline of the peaks. Time, seconds (not necessarily to scale) (a) What is N of this column, for p-xylene? (b) The column has length 30 m (typical for GC columns). What is H for this column, for p-xylene? (c) Calculate the capacity factors, k' for each component. toluene k' = p-xylene k' = o-xylene k' = m-xylene k' = (d) What could be done to improve the separation of the o- and m- isomers of xylene?

Explanation / Answer

a) we know that

theoretical plates

N = 16 ( tr/ W)^2

N = 16 ( 81.5 / 0.8) ^2

N = 166056

no of theoritical plates is 166056

b) Height = Length / N

Height H = 30000/ 166956

Height = 0.180 mm

hieght is 0.180 mm

c) capacity factor = ( tr- tm) / tm

capacity factor or toulene = ( 77.3 - 12.2 ) / 12.2 = 5.336

capactiy factor of p xylene = ( 81.5 - 12.2 ) / 12.2 = 5.68

capacity factor of o xylen = ( 83.8 - 12.2) / 12.2 = 5.8688

capacity factor of m xylene = ( 84.1 - 12.2 ) 12.2 = 5.893

d) it can be increased by using fused silica capillary column

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