[/quote]
What was the lab you carried out? Standardisations or similar?
[/quote]
It was a hplc lab practical, here is a part of the schedule we where provided with
3. You are provided with a mixture of nitrazepam and diazepam in solution in methanol (each
30.0 mg dm-3) - solution marked A. Load the loop of the injection valve with solution A using
the HPLC syringe provided. In order to completely fill the loop you will need to inject ~50μl
of sample solution. Ensure that the loop is loading correctly by monitoring the injection loop
waste tube. Twist the handle to 'inject' and start the recording device.
4. Whilst the chromatogram is running, return the injection loop to the starting position and
rinse with ~100μl pure methanol – again checking the waste tube.
5. The chromatogram should take 5-10 minutes to run and should show a small solvent peak
and two analyte peaks. Check that the chromatographic performance is satisfactory before
continuing.
6. You are provided with separate reference solutions of the two analytes in methanol (each
15.0 mg dm-3) – B and C. Inject these and obtain the chromatograms. B is the diazepam, C is
nitrazepam.
7. Solutions D, E, F, G, contain varying amounts of the two analytes in mixtures. The
concentration of each analyte is as in the table below. Inject each in turn and obtain
chromatograms. Identify the analytes from the previous results and enter the peak heights (or
areas if a computing integrator is used) in the table
Tablet
Weigh the tablet provided accurately (4 d.p. balance). Carefully powder the tablet in the
mortar provided. Quantitatively transfer the powder to a 50 cm3 conical flask and add about 30
cm3 of the methanol. Use the funnel to help in the transfer. Place the flask in the ultrasonic bath
for 10 minutes. Filter the mixture through the sintered crucible and wash the residue with
methanol. Transfer quantitatively to a 100 cm3 volumetric flask and make up to the mark with
methanol. Label this solution H. Care in making this solution is critical to the accuracy of your
result. You must not lose any solid or solution at any stage.
11. Inject H and obtain a chromatogram. Repeat twice more if possible.
treatment of results
Treatment of Results
1. By comparing the chromatogram of H to those of B and C decide which benzodiazepine is
present in the tablet.
2. Using Excel (or similar software) plot a calibration graph of peak area/machine units
(height/mm if an integrator is not used) against concentration for the two analytes. Plot a line
of best fit through the data, using the method of least squares, for each of the two analytes, and
obtain equations describing the two lines.
3. Using the equation of the line of best fit, calculate the w/w% benzodiazepine in the tablet
and, the uncertainty in this value (See Miller and Miller Chapter 5, and the Semester 1 lecture
notes on calibration). Express the uncertainty as both an absolute (w/w%) and a relative (%)
value. Remember to allow for dilution steps in the sample preparation process in calculating
both the w/w% and its associated uncertainty.
4. Label the scan of a single a representative chromatogram, with tR, tm and tR’ , wb and w½ in
minutes for each analyte. Cut and paste this into your report together with a table of values in
minutes for the five parameters.
5. By using the values from 4 above calculate n and k for each analyte.
6. Calculate α and Rs for this pair of analytes under these conditions.
so its number 3 I'm having difficult with
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Topic: Calculating w/w% and uncertainty (Read 6258 times)