Problem Set 3: Internal Standard, Detection Limit, and Injection Variability

1.  Internal Standard Problem

A mixture of Benzene and Toluene in solution gave the following chromatography results:

Compound	Concentration (µg/mL in mixture)	Peak area (cm^2)
Benzene	236	4.42
Toluene	337	5.52

A solution was prepared by mixing 1.23 mg of Toluene in 5.00 mL, with 10.00 mL of a solution containing only an unknown concentration of Benzene, and then diluting the mixture to 25.00 mL. Peak areas of 3.33 and 2.22 cm² were observed for Benzene and Toluene respectively. Find the concentration of (µg/mL) in the unknown Benzene solution.

2.  For the cadmium data set, calculate the detection limit.

3. Injection Variability:

The purpose of this exercise is to characterize the variability associated with GC injection techniques.  In Room Sims 310 is a GC syringe (in a box), two small beakers and two analytical balances.  Working with a lab partner, each person should make 10 separate 5 microliter syringe injections into the tared beaker.  Prepare a spreadsheet that determines the following:

• The volume of each injection (remember to measure the temperature)
• The average volume injected for each lab partner
• The average volume for the overall group
• The standard deviation for each person
• The standard deviation for the overall group
• The Coefficient of Variation (CV) for each person and the overall CV.  CV is the standard deviation divided by the average, represented in %.
• Internal standard quantification is designed to minimize injection errors.  Generally, IS techniques are found to be quantitative within 1%.  Compare this 1% standard with the CV's you each obtained for your 10 injections.