10 Calibration
Standard solutions in oil are required to calibrate analyses of extracts obtained from method A.
Standard solutions either in oil or in chromatographic eluent may be used with extracts prepared in accordance with method B.
10.1 Standard solutions in oil
10.1.1 Preparation of stock solution
- dissolve 0.025g of each of the five compounds in 25ml toluene (concentration 1000mg/l);
- stock solutions shall be stored in a brown glass bottle and kept in the dark. They should be replaced at intervals of not more than three months.
10.1.2 Preparation of standard solutions
- dilute an aliquot of the toluene stock solution in a weighed amount of unused mineral insulating oil to prepare a standard solution of the desired concentration (i.e. 0.5, 1.0, 5 or 10mg/kg);
- store in brown glass bottles and keep in the dark.
Note 1 Select an appropriate mineral insulating oil which is free from species likely to interfere with the analytes.
Note 2 Standard solutions deteriorate rapidly if they are not properly stored. Freshly-prepared solutions are preferred.
10.2 Standard solution in the chromatographic eluent
If method B is used, an alternative easier and faster calibration procedure may be used:
- weigh 0.015g of each of the five compounds;
- dissolve them in 100ml of the chromatographic eluent to obtain a stock solution of about 150mg/l;
- dilute the stock solution to obtain a standard solution of the desired concentration (i.e. 2.5, 5, 25 and 50mg/l).
Note Due to the concentration factor introduced by this method, a standard solution of, for example, 5mg/l in methanol/water is equivalent to a concentraion of 1mg/kg in the oil.
10.3 Procedure
Extract and analyse the standard solutions prepared in oil (10.1) following the precedures described in clause 9.
Alternatively, analyse the standard solutions prepared in chromatographic eluent (10.2) following the procedure described in 9.2.
Four standard solutions in the concentration range 0.5 to 10mg/kg are recommended to obtain a calibration line which should normally be a straight line through the origin. Experience has indicated that for everyday routine work, a single point calibration at 1mg/kg in oil is satisfactory.
Note See note 2 of 10.1.2.
Identify each peak in the chromatogram by its retention time and UV spectrum. For guidance, typical UV spectra are shown in Figure 3 but their appearance may change slightly depending on the particular instruments.
Calculate the response factor Fi of each analyte as follows:
Fi = Cis/Ris for standard solutions in oil
Fi = Cis/5Ris for standard solution in chromatographic eluent
where Cis is the concentration of analyte i in the sandard solution mg/kg (oil) s or mg/l (chromatographic eluent).
Ris is the response of the detector (peak area or peak height) of analyte i.
11 Calculations
Identify each peak in the sample chromatogram from its retention time and UV spectrum.
Measure the detector response (peak area or peak height) of each identified peak and calculate the concentration in oil as follows:
Ci = Fi x Ri
where
Ci is the concentration of analyte i in the oil (mg/kg oil)
Fi is the response factor of analyte i
Ri is the detector response of the peak relative to analyte i
12 Report
The report shall include the following items:
- number of this standard;
- method of extraction and tyep of calibration used;
- identification of the sample;
- concentration of each analyte expressed in mg/kg to the nearest 0.01mg/kg.