Experiment #5
Forensic Science - Determination of Arsenate in Beer
(2005 04 18)
Purpose
The purpose of this experiment is to determine the concentration of a poisonous inorganic
species in beer using ion chromatography.
Introduction
As water is a primary ingredient within beer, its quality and type is a fundamental factor
in establishing many of the distinctive beers from different regions and countries around
the world. The majority of anions found in beer are a consequence of impurities derived
from the water used during the brewing process. In Canada,
Following sample
preparation, the anions present can be quantified using the technique of ion
chromatography with conductivity detection. The analysis of a U.K. beer produced a
result of the order 200 g/mL for chloride, phosphate and sulphate, and ~20 g/mL for
nitrate [1]. Sulphate, for instance, gives a sharp dry edge to well-hopped beers.
When “arsenic” comes up in mystery stories, the victim has usually been poisoned with
white arsenic  arsenic oxide. Just how toxic an arsenic-containing substance is
depends on the exact structure of the compound [2]. In this forensic science experiment,
a glass of beer is suspected to be contaminated with sodium arsenate that supplies
poisonous As(V). You are going to determine the concentration of arsenate by ion
chromatography (IC). For comparison, the safety limit of arsenate is 50 ppb in drinking
water [3].
You might even be able to identify the origin of the beer based on its IC
fingerprint of fluoride, chloride, nitrate, sulphate and phosphate anion peaks.
Theory of ion chromatography
Chromatography is a method for separating mixtures of analytes using two phases, one of
which is stationary, the other mobile moving in a particular direction. Chromatographic
techniques are divided according to the physical states of the two participating phases.
Ion chromatography (IC) is a subdivision of high-performance liquid chromatography
(HPLC). It includes all rapid liquid chromatography separations of ions in columns
coupled online with a flow-through detector. A sample solution of negatively charged
anions is injected on the column. An ion exchange reaction occurs between ions in a
solution and a solid substance carrying functional groups that can attract ions because of
electrostatic forces. In anion chromatography, these are quaternary ammonium groups.
In theory, ions with the same charge can be exchanged completely and reversibly
between the two phases. The process of ion exchange leads to a condition of equilibrium,
the side to which the equilibrium lies depends on the affinity of the participating ions to
the functional groups of the stationary phase. These anions will be delayed in a varying
degree. Individual anions are eluted as peaks at different retention times, and their peak
areas are measured separately with a conductivity detector. Qualitative identification by IC
is achieved by a comparison of the retention time for the unknown anion to those retention
times obtained for a series of common anions under identical conditions (i.e., eluent
composition and flow rate). Quantitative determination involves calibration of the
conductivity detector response using standard solutions. Compared with wet chemical
methods, IC offers rapid and reliable analysis for multiple anions.
Experimental
Instrument
Consult your Teaching Assistant with respect to operating instructions for the Dionex 4000i
IC instrument. The He pressure must be set up properly (at 100 psi) on the gas tank by the
Teaching Assistant or you.
AVOID INTRODUCTION OF ORGANICS TO THE DIONEX AS4A-SC COLUMN.
Analysis of beer
A total of 20 ml contaminated beer is poured into a clean beaker. The beer is then
decanted to another clean beaker to facilitate the removal of carbon dioxide. The process
is repeated 20 times before sonication for 15 min.
The beer is then diluted 1:20 with deionized water in a (100-mL) volumetric flask. Load
10 mL of the diluted beer sample in a syringe, attach a filter to the syringe, and inject ~0.1
mL of sample into the Dionex 4000i ion chromatograph. Avoid introducing any air
bubbles. Leave the syringe in place until the following step is complete.
Inject 20 l of the diluted beer directly into the ion chromatograph by pressing Rst (for
reset) on the Dionex 4000i and Inject A on the integrator at the same time.
Using a mobile phase eluent of 1.7 mM NaHCO3 and 1.8 mM NaCO3 (at a flow rate of 1.0
ml/min) with the Dionex analytical column, the response for the peaks is recorded on a
Varian 4270 integrator. Wait for a complete chromatogram in 10 minutes. Repeat the
analysis twice more.
Identify the arsenate peak in the beer chromatogram by running a 10 g/mL standard
solution of arsenate separately. By comparing the peak areas in the standard versus the beer
chromatograms, choose at least three appropriate standard concentrations of arsenate to give
a calibration graph. Ideally, there should be points above and below the unknown arsenate
concentration in beer. Analyze each standard three times.
Determine the linear working range of the IC system. Make sure that the beer analysis
result falls within this linear range. If unfortunately the result falls outside the linear range,
adjust the dilution ration of 1:20 and repeat the beer analysis. Report the concentration
(mean and analytical error values) for arsenate in the original beer.
Run another sample of the beer after spiking it with the same arsenate concentration as you
have determined above by the method of standard calibration. Compare the two results, and
calculate the % recovery (= arsenate concentration found / arsenate concentration spiked).
Estimate the detection limit as the concentration of arsenate that gives a signal (i.e., peak
height) three times larger than the baseline peak-to-peak noise. Prepare an OTA solution of
this estimated concentration, and run it through the IC system. If the arsenate peak is too
small and thus not detected by the integrator, decrease the attenuation by pressing Attn on
the Dionex 4000i, the new attenuation factor (= 2n), and Enter. Also, do a new evaluation of
the peak threshold by pressing PT Eval (and make sure that the mobile phase is running).
This evaluation takes 30-60 seconds to complete, at the end of which a new PT value is
printed out on the chart paper. Now you are ready to run the OTA solution again through
the IC system.
Run a third sample of the beer after spiking it with sulphate to a concentration of 10 ppm.
Does sulphate cause any interference with the determination of arsenate? [Hint: The OTA
peak has a retention time of 5.30.1 min, compared with 4.90.1 min for the sulphate
peak.]
Report
In the Result and Discussion section, report the concentrations and analytical error values
for arsenate in the original beer. If the recovery of arsenate in your beer analysis turns out
to be not 100%, comment on the suitability of solid phase extraction, sample preconcentration, and/or analyte derivatization for potential improvements.
Reference
[1]
J. Bruce, Journal of Automated Methods & Management in Chemistry, 24 (2002) 127–130.
[2]
Canadian Chemical News, May 2004, p. 25.
[3]