Lab B6: Separation of 4-nitrotoluene and 4-nitroaniline by GC-MS
Introduction:
4-nitrotoluene (4-NT) and 4-nitroaniline (4-NA) are two of the more important breakdown products of nitrobenzene explosives, such as
trinitrotoluene (TNT).The relative and absolute abundance of these substances in soils or waters contaminated with explosives residues
(e.g. at abandoned army facilities) gives important information about the rates and mechanisms of explosive degradation reactions, so
it is essential that there are accurate analytical methods for their determination. Due to the fact that there are many different
breakdown products for explosives, gas chromatography with mass spectrometric detection (GC-MS) is a very popular method for their
determination. In order to quantify each compound accurately, it is important to achieve good chromatographic separation between the
individual compounds. In this lab, you will characterize the separation of 4-NT and 4-NA by GC-MS, and study parameters that influence
this separation.
Prelab:
1) Describe (briefly!; target: one paragraph) how the chromatographic separation approach (ignore the detection) you’re using in
this experiment works (in principle). Include the general nature of stationary and mobile phase, the general properties of the
analytes, as well an explanation of how and why the analytes are separated. Then, name the (one) most important experimental parameter
that influences the separation, and indicate qualitatively what effects a variation of this parameter has on the separation. Since this
material has not been covered in the lectures yet, consult the corresponding chapters in the textbook, or the overview provided in the
seminar.
2) Predict the elution order of your two analytes in this separation, and explain, based on the properties/structure of the
analytes, and the underlying principle of the separation, how you determined the predicted elution order. Information relevant to
answering this question can also be found in the same sources mentioned under 1).
3) Read the instructions for the samples that you need to prepare in week 1 of this experiment. Calculate the amounts of the neat
chemicals required to make the first two stock solutions. For the subsequent dilution, assume for your calculations that you have
prepared stock solutions containing exactly the target concentration. You will then modify these calculations in the lab during week 1,
once you know the actual concentrations of your stock solutions. Write out step-by-step recipes for the preparation of all solutions.
Your TA will check your calculations, and correct them with you, if necessary.
For tutorials on making standard solution, refer back to lab A. For instructions on using analytical balances, refer to the lab manual,
the textbook, or the following video:
4) Read the instructions for the measurements you will conduct in week 2, as well as the tasks you are asked to complete for the
lab report. Make a detailed list of all data you need to take home at the end of week 2 of this experiment. Your TA will check this
list for completeness, and discuss with you, if necessary.
Week 1:
You will be given the two substances to be separated, 4-NT and 4-NA, as pure compounds. These substances are solids under standard
conditions. From these neat substances, make twoseparate10 mL stock solutions in the appropriate solvent (which will be given to you),
each containing a target concentration of 1,000 mg/L of the respective substance. For this purpose, you will be using an analytical
balance, 10 mL volumetric flasks, and spatulas. It is OK to prepare solutions that are not exactly 1,000 mg/L (nominal concentration),
but close to it (actual concentration). Once you have prepared these solutions, calculate their actual concentrations, and use those in
the next step.
From these two stock solutions, prepare one mixed standard solution containing exactly 20 mg/L of each analyte, using 10 mL volumetric
flasks and a variable micropipetter with an appropriate working range (either 20 – 200 or 100 – 1,000 µL) for this dilution.
You will also be given an unknown sample, containing both 4-NT and 4-NA in concentrations around 20 mg/L, as well as other chemicals.
This will be your “sample” for testing the effect of the sample “matrix” on the separation.
Label all of your samples properly (name, date, content of container), and store them in the refrigerator until the day of analysis.
Week 2:
Analyze your samples by GC-MSunder two different chromatographic conditions (“normal” and “altered”) in the following order:
standard under normal conditions, replicate 1
standard under normal conditions, replicate 2
standard under normal conditions, replicate 3
unknown sample under normal conditions
standard under altered conditions, replicate 1
standard under altered conditions, replicate 2
standard under altered conditions, replicate 3
unknown sample under altered conditions
Make sure to bring a memory stick, and obtain electronic copies of all relevant chromatograms, including the data you will need to
complete the tasks listed in the following for the lab report. Also, take note of the exact separation conditions used for the “normal”
and the “altered” separation, as you will need these to complete the lab report.
Report tasks
1) determine elution time windows for each analyte
Determine the elution times for each analyte under “normal” conditions for each of the three replicate analyses. Then, determine the
average elution time and its variability (expressed as a standard deviation) for each analyte.
2) characterize peak shapes
Using the definition presented in the lectures, calculate the peak asymmetry factor for both peaks in each of the three replicate
chromatograms of the standard solution under “normal” conditions. In order to complete prelab question 4), you may need to look up the
definition of peak asymmetry in the textbook in advance. Additionally, calculate the average asymmetry factor and its standard
deviation from the three replicate analyses. Interpret all(8) determined asymmetry factors using “tailing/fronting” terminology.
3) quantify separation
Using the definition presented in the lectures, calculate the separation between the two analytes in each of the three replicate
samples. From these three results, calculate the average separation and its standard deviation under “normal” conditions.
4) describe influence of matrix on separation
Describe any differences you can visually detect in the chromatogram of the unknown sample vs. those of the standard. From the analysis
of the unknown sample under “normal” conditions, determine the elution time for the two analytes, the asymmetry factors for their
peaks, and the separation between the two peaks, in analogy to steps 1) – 3) above. Is either of these parameters different in the
unknown sample than in the standards? If so, describe the effect of the matrix on the separation.
5) characterize separation under “altered” conditions
In analogy to steps 1) – 3) above, determine elution windows, characterize peak shapes and quantify separation of the two analytes
under the “altered” chromatographic conditions. Are there any substantial differences between the two sets of separation conditions
(“normal” vs. “altered”) with respect to either of these parameters? If so, explain why these changes are observed, on the basis of the
fundamental mechanism of the used separation methodology, and/or the general aspects of chromatographic theory explained in the
lectures.
6) determine effect of “altered” separation conditions on matrix tolerance
Compare the chromatogram for the unknown sample obtained under the “altered” separation conditions to a) the chromatogram obtained for
the unknown sample under “normal” conditions, and b) the chromatograms obtained for the replicate analyses of the standard solution
under “altered” conditions (in analogy to task 4) above). Describe (incl. quantitative considerations) the effects of the sample
“matrix” on the separation under “altered” conditions, and compare them to such effects observed under “normal” conditions. How have
the “altered” chromatographic conditions changed the analysis of the two compounds of concern, and how can you explain this based on
the differences between the “normal” and “altered” separation conditions?
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