Determination of lead concentration in Soil Samples by
Inductively Coupled Plasma Coupled with Optical
Emission Spectroscopy
ABSTRACT: Lead poisoning is an important health concern,
because of its ability to cause life-long adverse health ef-
fects1. Lead poisoning typically is a result of indigestion of
contaminated food and water, but also may occur from accidental exposure to contaminated soil, dust or lead based
paints. 1 The objective of this present study was to quantify
lead in soil using a modified digestion procedure and inductive coupled plasma (ICP) in tandem with optical emission spectroscopy (OES) for analysis of the analyte. ICP
read at the wavelengths of 217nm, 220.35nm, 261.425nm.
The lead concentrations for each wavelength was, 260.668
ppb, 242.411 ppb, and 272.251 ppb, respectively
INTRODUCTION
Lead is a major source of heavy metal poisoning and one
of the biggest environmental threats to young children2.
Ingestion or inhalation of high lead levels inhibits the production hemoglobin, which carries oxygen throughout the
body, ending in death2. Low levels of exposure to lead
cause neurological problems such as, ataxia, stupor, convulsions, and hyper-irritability2. The Environmental Protection Agency (EPA) has two current standards for the allowable amount of lead in soil, 400ppm in play-areas and
1,200ppm for other areas3. Soil that is not contaminated by
lead has lead concentration level lower than 50ppm3. Lead
oxide (PbO) and lead (Pb) metal are the most common
form of lead found in soil3. Lead oxide is insoluble in water, whereas lead nitrate is soluble in water3. 52.25 grams of
lead nitrate is soluble in 100 grams of water at 20◦C3. The
solubility of lead nitrate allows for the extraction of PbO
from soil samples. Converting PbO to Pb (NO3)2 by treating
it with nitric acid (NO3) allows for the lead to dissolve into
the soil. The lead can then be extracted by acid digestion
and then analyzed using ICP analysis. ICP analysis techniques allow of the detection of trace elements in a small
amount of analyte4.
EXPERIMENTAL
All glass wear was soaked in a 9:1 acid bath of distilled water to
nitric acid to prevent contamination
Digestion Method. A digestion was done for the soil sample
and the method blank. The method blank was composed of
a 1:1 mixture of nitric acid and distilled water. The method
blank was heated in a sand bath to 95 ◦C. The solution was
then cooled. 5mL of nitric acid was then added to the
method blank. The method blank was then refluxed for
30min while covered with a watch glass. After reflux the
method blank was filtered using gravity filtration and diluted with distilled water to 100 mL. The method blank was
used to offset any interference peaks that would disturb
data collection. The method blanks exact volume of the
filtrate was not recorded; however, it was estimated to be
the same volume of the soil’s filtrate. For the soil sample
digestion a 50 mL beaker weighing 28.1665 g was weigh
was recorded. Then a gram of soil was weighed out on an
analytical balance and the weight was recorded (28.1665g).
The soil was then dried in an oven for a week at about
115◦C. The dried soil was then weighed and the weight
was recorded (27.5388g). The moisture content was then
calculated using the following equation,
𝑀𝑤−𝑀𝑑
* 100
𝑀𝑤
(1)
where Mw is the mass of the wet soil, Md is the mass of the
dried sample. After the moisture content was determined
0.6012 g of the dried soil was grind in a mortar and pestle
for 10 minutes to increase the surface area. The soil was
then transferred to a 50mL breaker. An unwashed graduated cylinder was used to measure out 10 mL of 1:1 HNO3
acid solution. The solution was then added to the 25mL
beaker containing the soil. The mixture was then mixed and
covered with a watched glass and heated to 97◦C without
boiling. The mixture was then allowed to cool and 5mL on
concentrated nitric acid was then added to the mixture. The
mixture was then refluxed for thirty minutes where it was
reduced to 5 mL. The digested mixture was then filtered by
gravity filtration. The volume of the filtrate was 2.4mL.
The filtrate was then diluted to 100.8 mL with distilled
water.
ICP Analysis. Calibration standards prepared by another lab
group were used to help determine the concentration of
lead within the soil sample. The calibration standards used
were 50ppb, 100 ppb, 250ppb, 500ppb, and 1000ppb.
These standards were passed through the ICP from lowest
to highest concentration to give the calibration curve,
shown in figure 3.
Figure 1 Sample Key. ICP spectra key
Figure 3 ICP spectra of soil sample at 220.345nm
Figure 2. ICP spectra of soil sample taken at 216.999nm
Figure 4 Calibration curve of standards
RESULTS AND DICUSSION
The key for the spectra is found in figure 2. Figures one
and two are the ICP spectra of the soil sample. The spectra
were read at wavelengths of 217 nm and 220 nm, because
at these wavelengths the iron peak was less pronounced.
At 217nm, 220.35nm, and 261.425 nm the concentration of
lead was found to be 260.688ppb, 242.4119ppb, and
272.251ppb, respectively. The amount of lead present within the soil sample at each concentration was then determined by converting ppb to grams. At 260.688 ppb,
242.4119 ppb, and 272.251 ppb the amount of lead present
was .000026 g, .0000242 g, and .00002722g, respectively.
The significance of the data was determined by using the
Grubbs test. The average amount of lead found in the soil
was 0.0000257g, this amount is 0.3999743 lower than the
amount allowed by the EPA. Thus, the presence of lead in
the soil is not present in high enough levels to cause lead
poisoning.
The amount lead found in the soil sample may have been
less than the actual detected amount. Untreated glass wear
was used to measure out the 1:1 nitric acid solution that
was used in the digestion step.
Acknowledgements. I would like to acknowledge the Mer-
cer University Chemistry Department for providing the
recourses for this experiment to be successful.
REFERENCES
(1) Environmental Protection Agency. Lead Identification
of Dangerous Levels of Lead ; Final Rule. Article 40CFR
Part 745 1211
(2) Hlousek, D.A.; Dustin, K.; Phillips, T.A.; Lowe, D.F.
Leaching Chemistry. Remediation of Firing Range Impact
Berms. CRC Press, LLC. 2002, 14.
(3) Brown Jean Mary, Meyer A. Pamela, Falk Henry. Mutation Research/ Review in Mutation Research. Elsevier.
Mutation Research 659 (2008)166-175
(4) Thomas, R. A Practical Guide to ICP-MS: 2nd edition.
CRC Press. 2008, 7-10.