ALCOHOL
(The information below is based mainly on the text “Introduction to Forensic Chemistry,” by Kelly M. Elkins (2019) CRC Press)
After oral ingestion of ethanol, the blood alcohol concentration (BAC) rises, first upon absorption through the
stomach (20%) then continued absorption through the small intestine (80%). Blood transportation through the
body eventually leads it to the lungs where there exchange with the air presence of alcohol in the breath.
Maximum BAC upon alcohol consumption may occur from 30 minutes to several hours after consumption.
Various factors affect the rate of increase such as body weight, tolerance, and genetic predisposition as well as
type of drink, rate of consumption and whether food was consumed before or with the alcohol.
Metabolism of the alcohol upon arrival to the liver primarily through alcohol dehydrogenase (ADH) action in
the mitochondria shown as the first step in the pathway below. The body has several ADH enzymes with
varying catalytic efficiency. The acetaldehyde that is produced is considered toxic and is further metabolized by
the body with the help of acetaldehyde dehydrogenase (ALDH) to create acetate which is shuttled into the
Kreb’s cycle. There also various forms of ALDH, the mitochondrial form (ALDH2) and has a much higher
activity than cytosolic form (ALDH1). If the acetaldehyde accumulates due to inefficient metabolism, it
promotes vasodilation which in turn causes flushing, headache, malaise and chills. Alcohol deterrent drugs like
AntabuseR (disulfuram) block the action of ALDH2 promoting these symptoms.
Chronic alcohol consumers upregulate microsomal P450 monooxygenase activity to independently increase the
rate of ethanol oxidation. This pathway is not linked to respiration, hence energy generated from the reaction is
released as heat and can lead to increase in body temperature, flushing and sweating.
The ADH/ALDH pathway metabolizes about 80-90% of the alcohol while the cyt P450 pathway metabolizes
between 10-20%. A third, more minor pathway uses catalase. It is more active when a person is in the fasted
state. Janux Chemistry of Beer – Unit 2 – Chemical Concepts: Alcohol Metabolism summarizes these
pathways, available at https://www.youtube.com/watch?v=A4G261doYp4
Alcohol and its metabolites are through the skin, urine and breath. The average rate of elimination rate is
0.015g/100mL/hr, although there can be a 30% variation in this value. A BAC (%w/v) of as low as 0.02%–
0.03% will cause noticeable cognitive changes. At 0.10%–0.15%, a person will appear intoxicated and may
exhibit signs of delirium. Levels of 0.24%–0.36% typically lead to loss of consciousness and a concentration of
0.48% has been linked to severe coma and even death.
The BAC has been found to be proportional to the brain-alcohol concentration. In the brain, alcohol affects
- the forebrain, involved in thought, reasoning, behavior, and memory;
- the rear brain, responsible for speech, emotion, and understanding abstract concepts;
- the medulla, which regulates respiration and heart activity.
A summary of the impacts can be seen the video by Healthy McGill (Jul 29, 2013) Larning Module: Blood
Alcohol Concentration: Alcohol’s Story accessible at: https://www.youtube.com/watch?v=ck7B_Q0FBjU
To try to control accidents linked to alcohol consumption, various countries have set legal limits to the BAC
tolerated when operating a motor vehicle. The current BAC legal limit in Belize and the U.S. is 0.08% (w/v), in
France, Germany, Ireland, and Japan it is 0.05%, and in Sweden 0.02%.
The concentration of alcohol in the blood can be determined directly by sampling the blood or indirectly by
sampling the breath. Research has shown the concentration of alcohol exhaled by the breath to be directly
proportional to the BAC. According to Henry’s Law, when a volatile chemical is dissolved in a liquid—such as
blood—and brought to equilibrium with air, the ratio between the gas pressure (pgas) and the gas concentration
in the liquid (caq) is a constant (k).
k = pgas/caq
BAC can be estimated mathematically as Cp =D/[Vd ∙W]
Where Cp is the BAC in units of g/L or %w/v, D is the dose of alcohol in grams, Vd is the volume of
distribution (estimated as 0.70 for men and 0.60 for women; but may change based on factors such as having
had a meal before), and W is the weight in kilograms. In a case in which a 150-pound female consumes 3.5 1.5ounce drinks consisting of 80 proof alcohol, the computed BAC will be 0.1126% w/v.
You can estimate your BAC level by visiting the American Addiction Center’s BAC Calculator website at:
https://www.alcohol.org/bac-calculator/
Analysis of BAC can be done directly based on blood analysis or indirectly looking at alcohol in the breath.
Breath tests are rapid, convenient, performed onsite, clearly report the BAC, require no invasive sampling
measures, and are reliable. Three breath tests include the Breathalyzer, an infrared test, and a fuel cell test.
Breath Analysers
A short intro int to history of breathalyzers can be seen in the video by Beer By The Numbers (2017) – The
Science and History of Breathalyzers accessible at https://www.youtube.com/watch?v=oRPnd2apxvM
Chemical Breathalyzer tests normally involve a strong oxidizing agent like potassium dichromate. The color change
due to reaction with alcohol is read spectrophotometrically at a fixed wavelength resulting in an estimated concentration.
A 52.5 mL sample of breath is collected and heated to 50oC in the instrument before the reading is done. The
concentration is multiplied by 40 (it is estimated that the amount of alcohol in 2.1L of breath is the same as in 1 mL of
blood).
2K2Cr2O7+ 3C2H5OH + 8H2SO4 ➔ 4Cr3+ + 8SO42- + 2K+ + 3CH3COOH + 11H2O
A description of the process can be seen in the video by Forensic Science Hub (Jan 24, 2021): Breath Alcohol
analyzer/How Breathalyzer work?/ Chemistry of Breathanalyzers available at:
https://www.youtube.com/watch?v=qWk7M7GLRvY&list=PLv4JjDlKItAu5Cke926Hjxu-aUqKb92Uz&index=9
Newer breath testers, such as the
Intoxilyzer, determine the BAC using
the IR absorbance of the captured
breath. The instrument has two beams
so that a negative control sample can
be tested simultaneously to check for
interferences within the instrument. An
alcohol free breath is used as
background. Below is an example of an
attenuated total reflectance Fourier
transform-infrared (ATR FT-IR)
spectrum of a sample of ethanol. The
IR spectrum of ethanol contains peaks
at 3391, 2981, and 1055 cm−1,
correlated to the O–H stretch, C–H
stretch, and C–O stretch respectively.
Another breath test employs a fuel cell. The fuel cell breath test determines the quantity of alcohol in the blood
using the alcohol as a fuel that is ignited and burned in the presence of oxygen, creating energy that is converted
into an electrical current.
Blood tests
To directly determine the BAC, blood needs to be drawn. The time between when the blood is drawn and when
the alcohol was consumed can greatly influence the test results. Initially after consumption and up to ~30
minutes thereafter, the BAC can be up to 41 times higher in arteries than veins. As time passes, the
concentration in the two equilibrate. Later, concentration declines as the alcohol is metabolized. To avoid
contamination, a non-alcoholic sterilant must be used at the draw site. The blood must be stored in a sealed
container in the presence of an anticoagulant (e.g., EDTA) and a preservative (e.g., NaF) at 4°C. GC-MS is then
used to determine the BAC.
A dramatization of collecting blood sample for analysis is available via the following video The New Detectives
(Mar 18, 2020) How Scientists Test Blood Alcohol Levels/The New Detectives available at:
https://www.youtube.com/watch?v=l-vWiEbY-c4
The Image below shows sample GC-MS and GC-FID spectra for mixed alcohol samples.
Chromatograms with compounds labelled for FID and MS.
Your Turn
Forensics evidence is often used in criminal examinations. Criminal lawyers will often argue against the
validity of the readings from these different types of tests. Briefly discuss on a chemical/instrumental bases 1
argument that can be used against:
1) A breath test
2) A blood test using one of the GC methods.
Discussion should be 1-2 pages in length (400-600 words) with relevant images/ reactions.