*2.5.1. Soil sample extraction*

A solvent mix of pesticide residue (PR) grade acetone and methylene chloride (50:50) was prepared and 50 ml of the solvent mix was added to about 10 g of the sample, spiked with 1 ml of surrogate standard [78,79]. The sample was placed in the ultrasonic bath (Grant Instru‐ ments) and sonicated for about 10-15 minutes at 70o C. Then 10 g of anhydrous sodium sulphate (Na2SO4) was added to the sample to remove the water present initially in the sample and shaken gently until a clear extract was developed. The procedure was repeated once more with an additional 50 ml of solvent mix. The solvent was then concentrated on a rotary evaporator (Buchi equipment) and exchanged with 5 ml of n-hexane then re-concentrated to 1 ml. The concentration of the sample extract was necessary to remove the acetone and methylene chloride used in the extraction. In the course of the concentration of the solvent on the evaporator, the acetone and methylene chloride was removed leaving n-hexane with the extract. This is necessary to prepare the extract for clean-up and fractionation in a column containing n-hexane-mixed silica slurry. The extracted sample was then fractionated into the aliphatic and aromatic fractions (PAHs) using silica gel column [80]. The same procedure was repeated for the extraction of organochlorine pesticide in the sample using acetone and dichloromethane (DCM) solvent mixture (50:50).

#### *2.5.2. Column packing for fractionation and clean-up of extract*

Glass column was packed with 10 g of 100-200 mesh silica gel preconditioned (baked) at 105o C overnight. The silica was mixed with n-hexane to form slurry. The column was then eluted with about 15 ml of PR grade n-hexane and the solvent was collected to waste. Caution was taken not to allow the column to dry up. Using 1 ml pipette, 1 ml of the extract in n-hexane was added onto the column and then eluted with 60 ml of n-hexane. The aliphatic fraction was collected and the column was then eluted with 40 ml of PR grade dichloromethane (DCM) and another fraction was collected, which is the Polycyclic Aromatic Hydrocarbons (PAHs) fraction. Each of these fractions was concentrated to 1 ml using a rotary evaporator. After concentrating the PAH fraction to 1 ml, 5 ml of PR grade n-hexane was added and was further concentrated to remove the DCM using the rotary evaporator. The concentrates were later transferred into 2 ml sample vials using the graduated 2 ml pipettes. The final volumes of the extracts were noted and the sample extracts were subjected to GC-MS analysis. The same cleanup procedure was carried out on the organochlorine pesticide extract by eluting the column with PR grade n-hexane.

#### *2.5.3. Instrumental analysis*

extractant such as concentrated nitric acid, aqua regia (a mixture of concentrated HCl and concentrated HNO3 in ratio 3:1) and dilute form of the acids are used to determine total metals in contaminated soils [72]. The acids dissolve almost all elements that could become environ‐

However, heavy metals bound to silicate structures are not normally dissolved since they are not usually mobile in the environment [74] but the extractant normally give a reliable measure of metals added to soils as non-silicates from industrial sources that has potential for natural leaching and biological processes. The use of perchloric acid and hydrofluoric acid is generally fading away since complete dissolution of the soil is no longer required for total metal analysis

Most current environmental analysis work that involved total metal analysis has employed the use of 2 M HNO3 [3,76,77]. The levels of heavy metals extracted with 2 M HNO3 have been reported to represent maximum contents of potentially available metals for plants [77]. In this study, approximately 1.0 g each of the composite soil samples were digested with 20 ml of 2

20 minutes. The vessel was allowed to cool to room temperature. The digestate was filtered and made up to mark with distilled water. Two replicate samples were digested in the same way together with a reagent blank and all the samples were analyzed using Buck 200A Atomic

A solvent mix of pesticide residue (PR) grade acetone and methylene chloride (50:50) was prepared and 50 ml of the solvent mix was added to about 10 g of the sample, spiked with 1 ml of surrogate standard [78,79]. The sample was placed in the ultrasonic bath (Grant Instru‐

(Na2SO4) was added to the sample to remove the water present initially in the sample and shaken gently until a clear extract was developed. The procedure was repeated once more with an additional 50 ml of solvent mix. The solvent was then concentrated on a rotary evaporator (Buchi equipment) and exchanged with 5 ml of n-hexane then re-concentrated to 1 ml. The concentration of the sample extract was necessary to remove the acetone and methylene chloride used in the extraction. In the course of the concentration of the solvent on the evaporator, the acetone and methylene chloride was removed leaving n-hexane with the extract. This is necessary to prepare the extract for clean-up and fractionation in a column containing n-hexane-mixed silica slurry. The extracted sample was then fractionated into the aliphatic and aromatic fractions (PAHs) using silica gel column [80]. The same procedure was repeated for the extraction of organochlorine pesticide in the sample using acetone and

Glass column was packed with 10 g of 100-200 mesh silica gel preconditioned (baked) at 105o

overnight. The silica was mixed with n-hexane to form slurry. The column was then eluted

C for 2 hours with shaking every

C. Then 10 g of anhydrous sodium sulphate

C

mentally available especially metal oxides and carbonates [73].

M HNO3 by heating the vessel in a water bath between 90-100o

Absorption Spectrophotometer using air-acetylene flame.

**2.5. Organochlorine pesticides and PAHs analysis**

ments) and sonicated for about 10-15 minutes at 70o

dichloromethane (DCM) solvent mixture (50:50).

*2.5.2. Column packing for fractionation and clean-up of extract*

in environmental work [75].

238 Environmental Risk Assessment of Soil Contamination

*2.5.1. Soil sample extraction*

The extracts were analysed for PAHs and organochlorine pesticides using a GC equipped with MS detector. Separation was achieved by injecting about 1 µL of each extract into the GC system (in a splitless mode) through a capillary column (3.0 m length, 0.25 mm internal diameter, 0.25 µm film thickness). Helium gas was used as the carrier gas. Table 1 summarize the GC/MS condition. The quantification limit of the PAHs in the standard and the samples was 0.001 ppm, while that of the organochlorine pesticide residue was 0.002 ppm. The average response factor for the weight ranges were calculated and used for sample quantification.


**Table 1.** Operational conditions of GC/MS for PAHs and organochlorine pesticide

The concentration of each analyte was determined by calculating the amount of analyte or hydrocarbon range injected from the peak response in area ratio. The contribution from the solvent front and the surrogate compound were excluded from the total area of the sample.

$$C\_f = \frac{A(p) \ge R\_f \ge V\_f \ge D\_f \ge 1000}{W\_i}$$

Where,

Cf = Final Sample concentration (µg/L)

A(p) = Measured area of peak (peaks)

Wi = Initial weight extracted (g dry weight)

Vf = Final extract volume (ml).

Df = Dilution factor of sample or extract if diluted.

Rf = Response factor from the calibration standard calculation

*Rf* <sup>=</sup> *Concentration* (*P*) *Area* (*P*)

Concentration (p) = Total concentration of range
