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 mg/mL for chloride, phosphate and sulphate, and ~20 mg/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 ¾ 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 ml 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 mg/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 pre-concentration, 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][]