STEM Ed Institute Saturday Seminars
How much arsenic do we eat?
Julian Tyson
Department of Chemistry
Orpiment As2S3
Arsenic trioxide As2O3
Arsenic As
Realgar As4S4
Arsenic compounds commonly encountered in
environmental materials and in our food.
H3AsO3
arsenous acid
H3AsO4
arsenic acid
CH3AsO3H2 monomethylarsonic acid
(CH3)2AsO2H dimethylarsinic acid aka cacodylic acid
(CH3)3AsO
trimethylarsine oxide
(CH3)4As+
tetramethylarsonium
(CH3)3As+CH2CO2arsenobetaine
(CH3)3As+CH2CH2OH arsenocholine
Background to “The Arsenic Project”
Mandal and Suzuki, “Arsenic around the world” Talanta,
2002, 58, 201-235. (example of review article).
As, element 33, a mono-isotopic (75) metalloid, named
for the yellow pigment orpiment (Latin arsenicum),
probably isolated in 1250 by Albert Magnus.
It is widely distributed in all environments. “20th
most abundant in earth’s crust, 14th in seawater and,
12th in the body” (oops! this must be wrong).
Concentrations. In rocks: 2 mg kg-1, slightly higher in
soils. In sea water: 8 µg L-1. But there is a huge range.
Background to “The Arsenic Project”
Mandal and Suzuki, “Arsenic around the world” Talanta,
2002, 58, 201-235.
Uses (past and present): insecticide, herbicide, wood
preservation, embalming, poison, chemical warfare
agent, bullets, desiccant (cotton production), feed
additive, fireworks, medicine, electronics, glass,
paints, wallpapers, and ceramics.
The US no longer produces arsenic, but imports all it
needs. In 2005, US imported 812 tons of elemental
arsenic and 8300 tons of arsenic trioxide.
Intake of 70 to 300 mg of arsenic trioxide may be fatal.
Death typically occurs between 12 to 48 hours but can
occur within one hour. Those who survive arsenic trioxide
poisoning may develop encephalopathy* or severe
peripheral neuropathies.
Symptoms of acute poisoning usually occur within one
hour of ingestion but may be delayed for up to 12 hours,
particularly in the presence of food.
The principle toxic effects are hemorrhagic gastroenteritis, profound dehydration, cardiac arrhythmias,
convulsions, muscle cramps, shock and death.
*any diffuse disease of the brain that alters brain
function or structure
 Hot spots exist in:
Argentina, Australia, Bangladesh, Chile, China,
Hungary, India, Mexico, Mongolia, Peru, Thailand
and the United States of America
 Adverse health effects documented in:
Bangladesh, China, India (West Bengal), Taiwan,
and the United States of America, and 10 other
countries (because of water),
 Arsenic in drinking-water in Bangladesh
will cause 200,000 – 270,000 deaths per year
from cancer.
Arsenic contaminated water revealed in 1993.
4.5 million tube wells.
Arsenic contamination in 20% of those tested.
Many foods and drinks contain arsenic above the
detection capabilities of modern chemical analysis
instruments.
How much do we eat? How much is safe?
There’s a complication (or two):
Seafood contains the highest concentration of arsenic up
to 2400 mg kg-1.
The arsenic compounds vary from one food to another.
The arsenic compounds vary in their toxicity.
The compounds in seafood are “non-toxic.” The arsenic
compounds in everything else are more or less toxic.
The world’s rice supply is more or less contaminated with
the more or less toxic forms of arsenic.
Arsenic compounds commonly encountered in
environmental materials and in our food.
H3AsO3
arsenous acid
H3AsO4
arsenic acid
CH3AsO3H2 monomethylarsonic acid
(CH3)2AsO2H dimethylarsinic acid aka cacodylic acid
(CH3)3AsO
trimethylarsine oxide
(CH3)4As+
tetramethylarsonium
(CH3)3As+CH2CO2arsenobetaine
(CH3)3As+CH2CH2OH arsenocholine
Where did the background info come
from?
• second bulleted point
• third bulleted point
November 2012
Where did the background info come
from?
• A. A. Meharg and F-J Zhao
• Published 2012
• Mainly for agronomists
Where did the background info come
from?
• A. A. Meharg and F-J Zhao
EFSA Journal, 2009, 7, 1351
• second bulleted point
• third bulleted point
Intakes of higher amounts of
arsenic on a chronic basis may
cause hyperkeratosis, especially
of the palms and soles, skin
pigmentation, eczematous or
follicular dermatitis,
Leading eventually to
various cancers, such
as basal cell carcinoma
and bladder, liver and
lung cancers.
Environmental Health Perspectives, 2005, 113, A379
Ryker, S.J., Mapping arsenic in groundwater: Geotimes, 2001, 46, 34-36.
Locations of wells and concentrations of arsenic in water from bedrock
aquifer wells in New England.
The concentration data are shown with circles sized by concentration
ranges.
No. of samples = 2470
J. D. Ayotte et al., Arsenic in Groundwater in Eastern New England: Occurrence, Controls, and
Human Health Implications. Environmental Science & Technology, 2003, 37, 2075-2083
Recent studies estimate that 2-100 children per million exposed to PTW during
early childhood may develop lung or bladder cancer later in life as a result of
this exposure
Consumer Product Safety Commission (2003)
How arsenic gets into rice.
How does arsenic get into our food? Plants take up arsenic
compounds from the soil. Legacy of previous applications.
Concentration.
A student in my lab determined the amount of arsenic in a
brown rice sample, then removed the husk and
determined the amount in the “polished” white rice.
What are the concentrations?
Sample
Mass g
Brown
White
450
250
Arsenic content Concentration
µg
of arsenic in
ppb
390
?
140
?
Eat 45 g of the Lot 3 Rice Select material: how much
total arsenic did you eat? How much inorganic arsenic?
Arsenic compounds commonly encountered in
environmental materials and in our food.
H3AsO3
arsenous acid
H3AsO4
arsenic acid
CH3AsO3H2 monomethylarsonic acid
(CH3)2AsO2H dimethylarsinic acid aka cacodylic acid
(CH3)3AsO
trimethylarsine oxide
(CH3)4As+
tetramethylarsonium
(CH3)3As+CH2CO2arsenobetaine
(CH3)3As+CH2CH2OH arsenocholine
Concentrations in ppb: the quick route.
Strategy: Choose mass units that make the
multiplication by one billion easy.
Mass of arsenic = m g
Mass of rice = M g
Concentration = (m/M) x 109 = m x 109/M
If m were expressed in ng
Concentration = (m x 10-9/M) x 109 = m/M
i.e. 1 ppb = 1 ng in 1 g = 1 ng g-1
Concentrations in ppb: the quick route.
Look at another possibilities.
Mass of arsenic = m µg
Mass of rice = M kg
Convert masses to g
Concentration = (m x 10-6 /M x 103 ) x 109 = m/M
i.e. 1 ppb = 1 ng in 1 g = 1 ng g-1
= 1 µg in 1 kg = 1 µg kg-1
Concentration.
Several different ways of expressing this
ratio. A common one is to convert the
masses to the same units and express the
resulting fraction as parts per hundred
(pph), parts per million
(ppm), or even
parts per billion
(ppb).
The second “parts” is omitted from the
expression.
Concentration.
917 ppb is 917 g per 109 g (109 is giga, G)
917 ppb is 917 mg per 106 g (106 is mega, M)
917 ppb is 917 µg per 103 g (103 is kilo, k)
917 ppb is 917 ng per g
m, milli, is 10-3
µ, micro, is 10-6
n, nano, 10-9
Eat 45 g of rice containing 917 ppb arsenic, i.e.
917 ng per g.
Mass arsenic eaten = 917 x 45 ng = 41,265 ng
= 41,265/1000 µg = 41 µg (round to 2 digits).
Concentration.
Total arsenic content is 917 ppb
Mass arsenic eaten = 45 x 917 ng = 41,265 ng
= 41,265/1000 µg = 41 µg.
Inorganic content is 102 ppb. So in 45 g of the rice
there is ??? µg of inorganic arsenic.
“Inorganic” means H3AsO3 (arsenous acid) and
H3AsO4 (arsenic acid).
“Organic” means compounds of carbon, oxygen,
hydrogen, and other elements.
Calculate the “percent of inorganic arsenic.”
Note (a) this does NOT mean the percent of
inorganic arsenic in the rice
(214 ppb = 214 x 10-7 pph = 2.14 x 10-5 %)
(b) that the information given is the
concentration of the total arsenic and the
concentration of the inorganic arsenic.
How to proceed?
Percen
t of
inorga
nic
arseni
c?
Calculate the “percent of inorganic arsenic.”
i.e. the percentage of the total arsenic that
is inorganic arsenic.
Take 1 kg of rice. Mass of total arsenic is 398
µg and mass of inorganic arsenic is 214 µg.
Mass fraction inorganic arsenic = 214/398
pph = 100 x 214/398 =53.7688442 = 53.8%
Percen
t of
inorga
nic
arseni
c?
Why do the numbers not agree?
Percen
t of
inorga
nic
My guess is that it is the result of rounding. If the numbers
were 398.2 and 213.7 the answer would be 53.6664992
pph.
arseni
i.e. 53.7 %.
What is the concentration of arsenic in rice?
Quarter cup of rice and two cups of water. Boil to
extract all the arsenic. Measure the arsenic
concentration in the solution (50.0 ppb).
Need to know the mass of arsenic and the mass of
the rice (then, fix the units and calculate).
What is the concentration of arsenic in rice?
Quarter cup of rice and two cups of water. Boil to extract all the
arsenic. Measure the arsenic concentration in the solution.
Concentration of arsenic in solution = 50.0 ppb
= 50.0 ng mL-1
Volume of water added was 2 cups = 473.2 mL.
What is the concentration of arsenic in rice?
Quarter cup of rice and two cups of water. Boil to extract all the
arsenic. Measure the arsenic concentration in the solution.
Concentration of arsenic in solution = 50.0 ppb
= 50.0 ng g-1
Mass of arsenic is (50 x 473.2) ng = 23,660 ng
Mass of rice? Know the volume (0.25 cups = 59.15
mL) % of volume that is rice 55.5%.
Therefore volume of rice is 32.5 mL.
What is the concentration of arsenic in rice?
Quarter cup of rice and two cups of water. Boil to extract all the
arsenic. Measure the arsenic concentration in the solution (100
ppb).
Mass of arsenic is (50 x 473.2) ng = 23,660 ng
Mass of rice? Know the volume (0.25 cups = 59.15
mL). % of volume that is rice (about 55.5%).
Therefore volume of rice is 32.5 mL.
Density of rice is 1.45 g mL-1
Mass of rice = 47.2 g
Conc of arsenic = 23,660/47.2 = 502 ng g-1
Analytical chemistry
Some basic concepts:
Qualitative analysis: perform some diagnostic
chemistry and compare what happens with result for
authentic material (may be in memory).
E.g. the detection of iron by reaction with thiocyante.
Detect inorganic arsenic by formation of colored
product with mercuric bromide exposed to arsine
vapor formed by reaction of inorganic arsenic with
zinc in acid solution.
Gutzeit – Marsh test.
Analytical chemistry
Some basic concepts:
Quantitative analysis: (a) perform some chemistry
and measure the extent of the reaction.
(b) perform some chemistry and compare the extent
of the reaction with that of authentic material of
known concentration of analyte(s).
Authentic material of known concentration of
analyte(s) is a standard. A standard with no analyte is
a blank.
Analytical chemistry
(b) perform some chemistry and compare the extent
of the reaction with that of authentic material of
known concentration of analyte(s).
This is usually done with an instrument (not a
machine). Instrumental analysis involves measuring
the response to standards, establishing the underlying
relationship between instrument response and
concentration and applying this to the response of
the sample(s). The process is called calibration.
Is usually performed at the time of analysis.
Analytical chemistry
The process is called calibration.Is usually performed
at the time of analysis.
Instrument
response
Rx
Cx
Concentration of analyte
Analytical chemistry
Instrument
response
Rx
0
0
Cx
Concentration of analyte
The underlying relationship is often a straight line.
But sometimes a curve is a better fit.
Often does not pass through 0,0. That is there is a
measurable response to the blank.
Reagents are contaminated with analyte.
Analytical chemistry
Some basic concepts:
Detection Limit: the lowest concentration that can be
measured with a given confidence (usually about 95%
percent). Abbr. LOD or DL.
Analytical chemists still debate/argue over how to
measure LODs. Often done by measuring the
response to a standard of low concentration (maybe
the blank) and calculating the concentration that
gives a signal equal to three times the standard
deviation of that signal.
Analytical chemistry
Some basic concepts:
Limit of quantification: the lowest concentration that
can be measured with a reasonable uncertainty.
Abbr. LOQ.
Usually defined as concentration giving an instrument
response 10-times the standard deviation of the
response to the blank.
Dose-response relationships
Toxicology is the study of the adverse effects of chemicals
on living organisms. It involves the study of symptoms,
mechanisms, treatments and detection of poisoning,
especially of people.
Animal response
Excess lifetime risk
of getting a cancer
Dose Mass of chemical per mass of
animal per time.
Dose-response relationships. Toxicology
Inorganic arsenic is a non-threshold carcinogen
Animal response
Excess lifetime
risk of getting a
cancer. Usually
expressed as a
decimal fraction:
1 in 10,000 i.e.
0.0001
Dose. mass of chemical per mass of
animal per time. Usual units (for arsenic)
are: µg per kg body weight per day
Dose-response relationships. Toxicology
Inorganic arsenic is a non-threshold carcinogen
0.0001
Response. Excess
lifetime risk of
getting a cancer.
Slope = 3.67 (mg/kg-d)-1
D
Dose. µg per kg body weight per day
Calculate D. 0.0001/D = 3.67 (mg/kg-d)-1
D = 0.0001/3.67 = 2.725 x 10-5 x 1000 µg/kg-d = 0.027
Dose-response relationships. Toxicology
Dose. µg per kg body weight per day
D = 0.027 µg per kg body weight per day
If you weigh 175 lb (80 kg, as 1 kg = 2.2 lb)
“Safe” to consume 0.027 x 80 = 2.2 µg of inorganic arsenic
per day
I L of water at the USEPA maximum of 10 µg L-1 delivers a
dose of 10 µg i.e. about 5-times as much as the safe limit,
and so corresponds to a risk of about 5 in 10,000.
Eat 45 g of the Lot 3 Rice Select material: how much
total arsenic did you eat? How much inorganic arsenic?
Dose-response relationships. Toxicology
Dose. µg per kg body weight per day
D = 0.027 µg per kg body weight per day If you weigh 80
kg, it is “safe” to consume 0.027 x 80 = 2.2 µg of inorganic
arsenic per day.
A 45-g serving of rice containing 100 ppb of inorganic
arsenic delivers (100 x 45)/1000 µg = 4.5 µg. If you have 2
servings per week and you weigh 80 kg your dose is
(4.5 x 2)/ (7 x 80) = 0.016 µg per kg body weight per day.
i.e. within the safe limit
Consumer Reports November 2012
Dose-response relationships. Toxicology
Dose. µg per kg body weight per day
D = 0.027 µg per kg body weight per day If you weigh 80
it is “safe” to consume 0.027 x 80 = 2.2 µg of inorganic
arsenic per day.
A 45-g serving of rice containing 100 ppb of inorganic
arsenic delivers (100 x 45)/1000 µg = 4.5 g. If you have 2
servings per week and you weigh 80 kg your dose is
(4.5 x 2)/ (7 x 80) = 0.016 µg per kg body weight per day.
i.e. within the safe limit
Consumer Reports November 2012
Dose-response relationships. Toxicology
Dose. µg per kg body weight per day
D = 0.027 µg per kg body weight per day If you weigh 80
it is “safe” to consume 0.027 x 80 = 2.2 µg of inorganic
arsenic per day.
A 45-g serving of rice containing 100 ppb of inorganic
arsenic delivers (100 x 45)/1000 µg = 4.5 g. If you have 2
servings per week and you weigh 80 kg your dose is
(4.5 x 2)/ (7 x 80) = 0.016 µg per kg body weight per day.
i.e. within the safe limit