Analysis of Persistent Organic Pollutants in Soil
Objectives
The students get to sample and analyze non-degradable organic compounds in
soil samples from their own residential area. In this activity, the students will learn:
– the persistence of organic pollutants in their own backyard
– sampling technique
– concentration (ppb, ppm) and dilution
– instrumental analysis of organic compounds using Gas Chromatography
/mass spectroscopy (GC/MS)
– calibration curve and analytical methods
Introduction
Persistent Organic Pollutants (POPs) such as polycyclic aromatic hydrocarbons
(PAHs), dichlorodiphenyltrichloroethane (DDT), and polychlorinated biphenyls (PCBs)
are a group of chemicals, which are characterized by their persistence in the
environment, toxicity, bioaccumulation, and long-range environmental transport.1
The major sources of the POPs emission to the environment involve
anthropogenic activities: (1) direct application of pesticide in the environment (e.g. the
use of DDT and chlordane for pest control), (2) industrial wastes disposal (e.g. release
of PCBs from electrical capacitor and transformer), and (3) unintentional byproducts of
industrial processes (e.g dioxin from industrial combustion).1 Once introduced, POPs
are transported over short and long distance from their sources. This movement occurs
principally through the atmospheric pathway, either on suspended particles or through a
process of global distillation and cold condensation,2 in which processes POPs can be
translocated to regions far away from emission sources.2,3 As a result, POPs have
been found in every part of the world, even in remote area like the open ocean and the
Artic, where no substantial pollution sources exist.
Soil is the primary environmental reservoir for semi-volatile organic compounds
like POPs.3,4 Because of their persistence and hydrophobicity (lack of affinity to water),
POPs accumulate in soils where they can be retained for many years after their
application.5, 6 Through the soil-(air)-plant-animal-human pathway,5 human can be
exposed to POPs which may have impact on health.
Sampling method
A 500-gram composite soil sample is collected from the surface (10 - 15 cm
deep) with a stainless steal shovel, and stored in a Ziploc bag. The location and
description of each sampling site is recorded in details. A diagram of composite
sampling is shown in Figure 1. After sampling, the soil samples are air-dried, sieved at
1mm to eliminate organic fragment and aggregates, and transferred into clean glassamber container and stored at ambient temperature prior to analysis.
Figure 1 Diagram of composite sampling.
Sample Preparation
10 grams of soil are accurately weighted in a 40mL vial and 30 mL of methanol
(extractant) is added. The sample is sonicated using an ultrasonic bath for 30 minutes
to promote the diffusion of analytes from the sample to the extractant. After sonication
and settling of the soil, 2.4 ml of the extract is diluted to 10 ml ultra-pure water spiked
with 100 ppb surrogate solution (dibutylchlorendate) in a 20 mL head space vial. The
analytes were extracted with a stir bar (10 mm length and 0.5 mm in film thickness,
Gerstel Twister) for 4 hours at a stirring speed of 1000 rpm under ambient temperature.
After 4 hours, the stir bar is removed with magnetic tweezers, rinsed with ultra-pure
water, dried with a lint free tissue, and placed in a empty glass thermal desorption tube.
The stir bar is then thermally dissolved by the thermal desorption unit (TDU) system
inline coupled with a GC-MS.
Calibration Curve
Seven-point calibration curves are conducted ranging from 0.01 to 1000 µg/kg
(ppb) of analytes. Each calibration standard solution is spiked with 100 l of 10 ppm
dibutylchlorendate solution. A laboratory blank sample is also analyzed.
TDU-GC-MS analysis
Capillary –TDU-GC-MS analysis are performed on an Agilent 6890 series gas
chromatography coupled with an Agilent 5973N mass selective detector (Agilent
Technology). The stir bars are thermally desorbed in the TDU under splitless mode.
The desorption process is programmed as following: initial temperature of 40°C with
ramp at 60° C/min to 300° C (held for 5 minutes); transfer line temperature set at 320°.
The desorbed compounds are cry-focused in the Gerstel CIS 4 at -40°C with liquid
nitrogen prior to injection. For injection the CIS 4 is raised from -40°C to 320°C at 12 °
C/s for 10 minutes to inject the trapped compounds in the glass liner on to the GC. The
separations were performed on a HP-5MS capillary column (0.25mm*30m*0.25 um,
Agilent Technology). The oven is programmed from 50°C (held for 2 minute) with 25°
C/min ramp to 150° C, 3° C/min to 230°C, 8 °C/min to 300°C and held for 10 minutes.
The carrier gas used is ultra pure helium at a constant flow of 1.0mL/min. The GC
transfer line is 280 ° C. The MSD is set for scan mode with a mass range from 40 to
500u. Identification of compounds is carried out using the NIST library. Data
quantification is performed by MSD Chemstation Software (Agilent technology).
Reference:
1 Environmental Protection Agency, The Foundation of Global Action on Persistent
Organic Pollutants: A United State Perspective (2002). www.epa.gov
2 Malanichev V. Shatalov, N. Vulykh, B. Strukov, Modelling of POP hemisphere
Transport. MSC-E Technical Report 2002. www.msceast.org
3 Ian T. Cousins, angus J. Beck, Kevin C. Jones, A review of the processes involved in
the exchange of semivolatile organic compounds (SVOC) across the air-soil
interface. The Science of the Total Environment, 228 (1999) 5-24.
4 K.C. Jones , P. de Voogt Persistent organic pollutants (POPs): state of the
science, Environmental Pollution 100 (1999) 209-221.
5 G. Vassilyeva, V.Shatalov Behavior of Persistent Organic pollutants in soil. PART
1.POP distribution between soil components:The role of dissolved organic matter in
POP migration down to soil profile MSC-E Technical Note 1/2002. www.msceast.org
6 Jiamo Fu, Bixian Mai , Guoying Shenga, Gan Zhang ,Xinming Wang ,Ping_an Peng
, Xianming Xiao , Rong Ran ,Fanzhong Cheng , Xianzhi Peng , Zhishi Wang U Wa
Tang, Persistent organic pollutants in environment of the Pearl River Delta, China:
an overview, Chemosphere 52 (2003) 1411– 1422.