14 Calculating concentrations

Now that you have identified the pesticides present at each of your sample locations, your next task will be to complete a quantitative analysis of the samples.

Remember that the concentration of a particular pesticide can be determined from its peak area. This would be done by first comparing with standards of known concentration of that pesticide using a calibration graph.

We can use the following equation to calculate the concentration of each pesticide:

Described image
Show description|Hide description

Concentration of pesticide in sample = (peak area of sample multiplied by response factor) divided by (15 multiplied by percentage recovery).”

Click on 'reveal answer' if you’d like to know what the ‘response factor’ and ‘percentage recovery’ mean.

Answer

DETAILS OF TERMS USED IN THE EQUATION

The response factor for each pesticide is the ratio of pesticide concentration to peak area. This can be determined from the gradient of a calibration graph that plots peak area against concentration for calibration standards.

The percentage recovery (‘recovery %’) is the percentage of the pesticide compound that can be recovered during the solvent extraction phase. It is determined using a standard where a known mass has been introduced into a sample before the extraction and then measured after the extraction process to see how much of that mass is recovered. Generally, the extraction process is controlled so that a percentage recovery of approximately 90% is achieved.

The water samples taken had a volume of 1500 ml, which were concentrated during the preparation step to 1 ml. This leads to the factor of 15 in the equation.

Table 4 lists the response factors for the range of pesticides analysed, as well as their percentage recovery.

Table 4  Response factor and percentage recovery for some OCPs
Pesticide/metabolite   Response factor/‌μg l-1  Recovery/%
Aldrin0.007875.3
p,p’-DDD Footnotes   a0.037993.1
p,p’-DDE Footnotes   a0.008890.0
o,p’-DDT Footnotes   b0.015879.1
p,p’-DDT0.017588.2
Dieldrin0.012394.4
Endosulfan0.010989.9
Endrin0.015288.9
α-HCH Footnotes   c0.0064102.1 Footnotes   d
β–HCH Footnotes   c0.0159101.6 Footnotes   d
γ–HCH (lindane)0.0078110.7 Footnotes   d
δ–HCH Footnotes   c0.0087102.9 Footnotes   d
Heptachlor0.009772.1
Methoxychlor0.009897.0
PCNB0.008979.5

Footnotes  

Footnotes   a metabolite resulting from the breakdown of p,p’-DDT in the environment. The p, p’ and o is notation given to different isomers.

Footnotes  

Footnotes   b minor isomer present in commercial DD

Footnotes  

Footnotes   c present in commercial HCH (also with γ-HCH)

Footnotes  

Footnotes   d recoveries of greater than 100% can result from uncertainties in determination of the analyte concentration

Try the following example before you analyse your own results:

The peak area of the pesticide aldrin is 600 using GC. What is the concentration of aldrin in the sample?

Answer

Described image
Show description|Hide description

concentration of pesticide in sample equals (600 times 0.0078 micrograms per litre) all divided by (15 x 75.3) equals 0.004143 micrograms per litre equals 4.1 nanograms per litre (to two significant figures)

  • Calculate the average concentration for each pesticide at each of the sampling sites you selected.
  • Calculate the total pesticide concentration at each site.

13 Analysing samples from the bay

15 What do your results tell you?