**2.3 Dating of sediments**

398 Pesticides in the Modern World - Risks and Benefits

Column: HP-5 fused silica capillary column (30m×0.25mm×0.25um i.d.), constant flow1.5;

The GC-MS was calibrated with standard compounds of known concentrations at eight

ppm. The calibration curves of 16 PAHs based on the average response factors were used for concentration determination. Target compounds were qualified using the internal standard technique. Their concentrations were reported as nanogram/gram (µg/g) based on dry

The lack of standard compounds seems to bring difficulty to the qualitative and quantitative analysis of aliphatic composition, but the equally-spaced retention time makes it possible to determine the carbon number of aliphatic compositions properly once one of a series of samples was subject to GC-MS analysis. Because the response factors of aliphatic compositions with different carbon number are almost the same, the quantitative analysis

A strict Quality Control/Quality Assurance (QC/QA) measurement was followed

The recovery of the analytic procedure was determined by measuring the amounts of surrogate compounds added prior to sample extraction. The recoveries range from 44.2% to 118.3% (except Nap-d8 volatile even at room temperature), within the restrictive range of US EPA 610 method Therefore, except Nap whose values can only serve as reference, other data satisfy the demand for quantitative analysis. All data have not been revised and all concentration values are according to the instrumental recoveries. All the PAH

The solvent blanks were checked and none of the investigated substances were found in the

**2.2.2.2 The qualitative and quantitative analysis of aliphatic compositions** 

was accomplished in accordance to the response factor of internal standard C20d42.

concentrations were reported on a dry weight sediment basis.

A method blank experiment was carried out for every 12 samples.

The relative standard derivation for duplicate samples ranged from 0 to 15%.

, 0.4 ppm, 0.8 ppm, 1 ppm, 2 ppm, 4 ppm, 8 ppm, and 10

**2.2.1 Gas chromatography and gas chromatography /mass spectrometry** 

**2.2 Instrumental analyses** 

GC: HP 6890 GC/FID (flame ionization detector). GC-MS: HP5890II GC equipped with HP5972 MSD.

Oven: initial 60ºC keep 1 min, 6ºC /m to 290ºC keeps 20 min; Detector: flame ionization detector (FID), temperature 300ºC.

**2.2.2 Qualitative analysis and quantitative analysis 2.2.2.1 The qualitative and quantitative analysis of PAH** 

**2.2.1.1 Instrument** 

**2.2.1.2 GC parameters** 

Inlet: splitless, temperature 290ºC

different concentrations: 0.2 ppm<sup>1</sup>

throughout the process of analysis.

sediment weight.

**2.2.3 QC/QA** 

blanks.

1 ppm (parts per million)

The dating of the samples was carried out at the State Key Lab of Organic Geochemistry (SKLOG), Guangzhou Institute of Geochemistry, and Chinese Academy of Sciences.

Lead isotope 210 (210Pb) was used for dating the samples and the results obtained showed that at a depth of 18 cm from the surface of the lake's bed, the age of the samples was a little over 100 years, (precisely 101 years). A graph of depth age was plotted and indication showed that the age of the lake sediment at any depth could be found by simple extrapolation from the graph.

A regression graph was also plotted to indicate the accuracy of the technique of analysis. Both graphs are presented below.

Fig. 3a. Graph of Dating ( years) verses depth (cm)

Fig. 3b. Regression Diagram of accuracy of Results of Dating by (210Pb)
