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raquelwoods14 raquelwoods14
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4 years ago
You are given four additional polymeric materials (A, B, C, D) to examine for potential as a
  vascular graft material, and are told to use size-exclusion chromatography to determine the
  approximate molecular weights of the unknown polymers. You obtain the following
  information for the first three samples (A, B, C).
 
  a. Assuming monodisperse samples, what is the molecular weight of each?
  Standard 1: Molecular weight 50,000 g/mol
  Standard 2: Molecular weight 40,000 g/mol
  Standard 3: Molecular weight 35,000 g/mol
  Standard 4: Molecular weight 20,000 g/mol
  Standard 5: Molecular weight 10,000 g/mol
  Unknown A: Peak amount polymer eluted at 7.4 min
  Unknown B: Peak amount polymer eluted at 8.4 min
  Unknown C: Peak amount polymer eluted at 9.6 min
 
 

  b. A problem commonly encountered in SEC is overloading the chromatography column by
  injection of a highly concentrated sample, which results in broadening of the peak on the
  chromatogram. You injected a highly concentrated sample of unknown D into the
  chromatography column. Which of the following curves would result? Pick X or Y and
  describe why, based on molecular interactions of the sample with the column.
 

 
 
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qdoletqdolet
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4 years ago
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a. See Figure 2.57 in the book.

Std 1: Molecular wt. 50,000 g/mol approx. elution time: 7.0 min.
Std 2: Molecular wt. 40,000 g/mol approx. elution time: 7.7 min.
Std 3: Molecular wt. 35,000 g/mol approx. elution time: 8.0 min.
Std 4: Molecular wt. 20,000 g/mol approx. elution time: 9.0 min.
Std 5: Molecular wt. 10,000 g/mol approx. elution time: 9.8 min.
Now create a standard curve:



Use the equation from the standard curve to estimate the molecular weight.

Unknown A: Peak amount polymer eluted at 7.4 min MW  44,000 g/mol
Unknown B: Peak amount polymer eluted at 8.4 min MW  29,500 g/mol
Unknown C: Peak amount polymer eluted at 9.6 min MW  12,200 g/mol



b. Either X or Y is acceptable, as long as a reasonable, molecular-based reason was
given. In reality, Y occurs. This is because, if you add a sample that is highly concentrated, there will
be more of both the larger and smaller molecular weight fractions of the sample. The smaller
MW fractions will find their way into the pores of the packing material. Therefore, the smaller
MW polymers that are in too high of a concentration in the pores act to trap each other in the
pores and prolong their transition. A larger number of molecules take longer to be eluted than
normal; the shoulder is found on the right of the peak.

However, the scenario depicted in X could be explained by the presence of aggregates which
would act as high molecular weight (large size) polymers and would be eluted earlier from the
column.
This verified answer contains over 6150 words.

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