Application of the photoionization detector for the determination of ...

Application of the photoionization detector for the determination of ...

Fresenius J Anal Chem (1995) 353 : 206–210 Fresenius’ Journal of © Springer-Verlag 1995 Application of the photoionization detector for the determination of ethanol in aqueous solutions and human breath Iris Reimann, Steffen Mergemeier, Ingo Ebner, Fritz Scholz Institut für Angewandte Analytik und Umweltchemie, Humboldt-Universität, Hessische Strasse 1–2, 10115 Berlin, Germany Received: 10 August 1994 / Revised: 19 December 1994 / Accepted: 22 December 1994 Abstract. A determination of ethanol is described, which is based on a purging system in conjunction with a photoionization detector. With that system a fast and reliable determination of ethanol in aqueous solutions is possible. The system has been used for the analysis of wine. The 3δ-detection limit has been 0.005% ethanol, the relative standard deviation 4.8 to 6.0% and the time constant of the entire analytical system 20 s. The photoionization detector has been also applied to the analysis of artificial and genuine human breath. A comparison with gas-chromatography and non-dispersive IR-detection has been proven the reliability of results. Introduction Ethanol is generated in living matter, in small concentrations, in higher concentrations from carbohydrates by microbiological activity and on a large industrial scale by yeast for alcohol-containing foodstuffs like beer and wine. A large number of methods for the determination of ethanol is described. The oldest method for high concentrations of ethanol is the measurement of the specific density of a distillate of the sample [1]. For the determination of small concentrations of ethanol in different matrices numerous gas-chromatographic methods have been developed [2, 3]. Gas-chromatography serves as a reference method in forensic chemistry. In recent years, several enzymatic methods have been appeared for the determination of ethanol [4] which can be advantageously used for automatic applications, where many determinations have to be performed. The Highway Codes provide limits for the ethanol concentration in blood. The check of the blood ethanol content in traffic is realized via the breath ethanol content with non-dispersive IR-methods, or with test tubes based on the oxidation of ethanol [5]. Previously the application of a photoionization detector for the determination of hydride forming elements is Correspondence to: F. Scholz described [6]. Here, we report the use of the photoionization detector (PID) for the determination of ethanol in aqueous solutions and breath. From liquids the ethanol is purged out of the solution with nitrogen or air, and then the gas stream is introduced into the PID. The breath was directly introduced into the PID via a gas injection system. Experimental Equipment All measurements in aqueous solutions were performed with a selfmade purging system in conjunction with a photoionization detector PI-02A (HNU systems, Germering, Germany). The gases used were nitrogen, hydrogen and air. To maximize the analytical sensitivity and precision of the system, three different purging devices were investigated in conjunction with the PID. The first purging device consisted of a porous glass fritt bottom on which the sample solution was placed. The gas stream was led through the bottom and the solution to take up the ethanol from the solution. This system showed pronounced memory effects when the samples were changed. An impregnation with Agar-Agar Fig.1. Schematic diagram of the purging device in conjunction with the photoionization detector

207 solution prevented the memory effect, but was accompanied by very slow response characteristics. The third device consisted of a flask with a conical standard ground joint NS 14 on the top and a side connection to the PID. The gas was fed into the solution by a bundle of glass capillaries coming from top of the flask which showed the best performance characteristics. The samples were introduced by a pipette. Figure 1 shows a diagram of the mainfold with the purging device. To test the applicability of the PID for the determination of ethanol in simulated human breath, a gas washing bottle with an aqueous solution of ethanol was used which was purged with nitrogen gas. The ethanol content of the nitrogen is a function of the ethanol content in the solution according to Henry’s law. Both, simulated breath and also the genuine breath, were fed into the photoionization detector via a 6-port gas injection system. The gas sample volumes were 4 ml for genuine breath and 8 ml for artificial breath. Nitrogen and air were used as carrier gases for the analysis of breath. The calibration was achieved by injection of gas samples prepared by evaporation of known amounts (0.1 to 1 µl) of pure ethanol in a 2 l-flask filled with air. The gas volume of the 2 l-flask was stirred while evaporating the ethanol. The photoionization detector houses two electrodes and an ultraviolet lamp. The dc-voltage between the two electrodes is 70 V. The ethanol ionization potential of 10.62 eV requires to use a 11.7 eV lamp (argon filling). The UV-lamps are low pressure gas-discharge sources emitting a line spectrum [7]. The 11.7 eV lamp emits three lines with the following relative output: line 1: 104.9 nm (11.82 eV) with 26.2% of the total output, line 2: 106.6 nm (11.62 eV) with 71.8% of the total output, line 3: 121.6 nm (10.20 eV) with 2.0% of the total output. The gas-chromatographic measurements were made with a gas-chromatograph GCHF 18.3 (Chromatron, Berlin, Germany) with a Porapak Q column (2 m × 3 mm) and thermoconductivity or flame ionization detector. The determination of the ethanol content in breath was compared with a commercial non-dispersive IR-instrument Alcotest (Dräger, Lübeck, Germany) and with gas-chromatography. Chemicals The chemicals used for the preparation of reagents and standards were of analytical reagent grade. All solutions were made by direct dilution of pure ethanol in deionized water. All samples for the study of interferences were produced by direct dilution of methanol (Fluka), i-propanol (Merck), i-butanol (Merck), acetic acid (Fluka), Na 2 CO 3 (Fluka), MgSO 4·7H 2 O (Fluka), KCl (Merck), MnSO 4·H 2 O (Merck), CaCl 2 (Merck) and KH 2 PO 4 (Fluka) in deionized water. Results and discussion Calibration, detection limit and time constant Figure 2 shows the response-time curve of a measurement. Even for the lowest concentrations, one observes a plateau on top of the signal because the amount of ethanol transferred in the gas phase remains constant within a short period of time. With increasing ethanol concentrations the peak height increases and the signal broadens up to a certain concentration of ethanol. Above this concentration, the peak height remains constant and the signals only broaden further. This effect can be understood as response saturation of the photoionization detector [8]. Figure 3 depicts the dependence of the peak height of a 0.5% ethanol solution on the flow rate of the carrier gas stream for nitrogen and hydrogen. The figure illustrates Fig.2. Response versus time curve for measurements with a photoionization detector Fig.3. Dependence of the peak height of a 0.5% ethanol solution on the flow rate of nitrogen and hydrogen gas that the peak height is more than twice as high for nitrogen compared to hydrogen. Figure 4 shows the dependence of the time constants for the determination of a 0.5% ethanol solution on the flow rate of the carrier gas stream. The time constant T K was determined with concentration pulse experiments. The signal response of a concentration pulse is: signal = 1 a 0 ⎡ 1–e ⎣⎢ ⎛ – t–T m⎞ ⎜ ⎟ ⎝ Tk ⎠ ⎤ ⎦⎥ (1/a 0 – gain of the system, T m – capacity delay, T k – time constant) The capacity delay T m was negligible compared to T K . The time constant T K results from the behaviour of the purging system and the gas-flow through the detector. The time constant decreases with increasing gas flow rate. For nitrogen a time constant of 18 s is reached for a gas flow rate greater than 50 ml/min. For hydrogen the time constant is generally much higher. Therefore, we selected a gas flow rate of 50 ml/min of nitrogen for the analysis. Figure 5 depicts the dependence of the peak height on the concentration of ethanol in the aqueous solution in the concentration range of 0.005–0.1%, using a gas flow rate of 50 ml/min. A linear relation between concentration and response exists up to 0.5% ethanol. Above 0.5% the curve

208 Fig. 4. Dependence of the time constant of a 0.5% ethanol solution on the flow rate of nitrogen and hydrogen gas Fig.6. Peak height of an 1% aqueous solutions of methanol, i- propanol, i-butanol, ethanol: Solution (A): 1% ethanol, 0.02% methanol and 0.004% i-butanol and solution (B). 1% ethanol, 0.1% methanol, 0.01% i-propanol and 0.01% i-butanol Fig. 5. Dependence of the peak height on the ethanol concentration in aqueous solution is bent toward the concentration axis. Above about 3% the signal becomes independent of ethanol concentration. The reason is a saturation of the response of the PID. The relative standard deviation (rsd) for 0.01% ethanol in water is 6.0% (n = 6), and 4.8% at 0.5% ethanol in water (n = 6). The 3δ-detection limit is 0.005%. The time constant of the determination is approximately 20 s. Interferences Potentially interfering compounds for the determination of ethanol in foodstuffs, like wine and brandy, can shift the equilibrium between gas and liquid phase or volatile compounds may give an additional response of the PID. Therefore, the effect of volatile alcohols, volatile acids and some mineral salts were studied by the addition of known amounts of the interfering compounds in concentrations higher than those usually found in wine. Figure 6 shows the peak height of 1% solutions of methanol, ethanol, i-propanol, i-butanol and two mixtures of alcohols. The mixture A consists of an aqueous solution of 1% ethanol, 0.02% methanol and 0.004% i-butanol, and the mixture B of 1% ethanol, 0.1% methanol, 0.01% i-propanol and 0.01% i-butanol. The response of the alcohols increases in the homologous series with exception of ethanol. The response of mixture A, with a content of non-ethanolic alcohols more than five times higher than the expected maximal content in wine, is 3.6% higher than in the pure 1% solution. The mixture B, with a very high content of non-ethanolic alcohols, gives an 11.0% higher response. The conclusion is that in systems with higher contents of methanol, propanol or butanol significant interferences appear. However, in wine and beer the interferences are negligible. As an example of volatile acids we used acetic acid. The responses of the ethanolic solutions with different contents of acetic acid were independent of the concentration of acetic acid. The responses only fluctuated in the range of the standard deviation, for the determination of ethanol in wine there is no influence. The dependence of the peak height of 1%-alcoholic solutions on the content of mineral salts was studied with aqueous solutions containing 1% ethanol and additionally different amounts of mineral salts: (A) 640 mg/l KH 2 PO 4 , (B) 122 mg/l Na 2 CO 3 and 1192 mg/l MgSO 4 × 7 H 2 O, (C) 958 mg/l KCl, 1152 mg/l CaCl 2 and 24 mg/l MnSO 4 × H 2 O and (D) 176 mg/l Na 2 CO 3 , 798 mg/l KCl, 652 mg/l KH 2 PO 4 , 1080 mg/l MgSO 4 × 7 H 2 O and 1082 mg/l CaCl 2 . These salts have been chosen because they are usually present in wine. The concentrations chosen are higher than expected in wines. The responses of the solutions were (A) 1.3%, (B) 5.3%, (C) 7.1% and (D) 4.7% higher than the response of the pure solution. The rsd of the solutions are in the same sequence 5.0%, 1.9%, 3.9% and 1.5%. The rsd of the determinations decreases generally with increasing salt content. From these results it can be concluded that the content of mineral salts in wine and beer cannot significantly influence the ethanol determination. Ethanol determination in wine For the determination of the ethanol content in wines the wine samples were diluted 1 :100 with distilled water. All samples were determined by direct photoionization mea-

209 Table 1. Comparison of the ethanol content in wine obtained by different methods (mean values of 6 determinations, confidence intervall P = 0.95) Method of analysis White wine 1 White wine 2 Red wine 3 Red wine 4 PID (9.1 ± 0.5)% (12.3 ± 0.5)% (12.4 ± 0.4)% (11.1 ± 0.3)% Manufacturer 9.0% 12.0% 12.0% 10.5% GC/TCD (9.6 ± 0.3)% (12.4 ± 0.6)% (12.5 ± 0.6)% (10.5 ± 1.5)% surement, and for comparison, by gas-chromatography with thermo-conductivity detection. Table 1 gives the results of the determinations with confidence intervalls (n = 6, P = 0.95) and for comparison also the values given on the bottle label. Both the PID analysis and GC/TCD analysis gave higher values than given by the manufacturers. However, only in one case this difference is significant. The determination of the ethanol content in wine by the direct determination with a PID is easy and fast, which is advantageous to gas-chromatography. Ethanol in human breath Because ethanol was determined in the gas phase, the technique appeared promising for the determination of ethanol in the breath of healthy people. As described in the experimental part, the ethanol content of the gas phase was adjusted by purging an ethanolic solution with a gas stream. For the evaluation of the method a comparison was made between the PID, the commercial Alcotest device (non-dispersive IR) and GC/FID. Figures 7 a, b show the correlation between the responses of the three methods. A residual analysis gave a normal distribution of the residuals and proved the linearity of the correlation. To test the PID for ethanol determinations in real breath samples, experiments with two volunteers were performed. The volunteers drank 200 g wodka (wodka Wyborowa, Poland) within 2 min. Volunteer A rinsed the mouth with water after drinking while volunteer B did not. Figures 8 a, b show a comparison of PID and GC determinations. The blood ethanol concentrations are calculated according to the excepted blood/breath ethanol relation, i.e. that 2.1 l of expired breath contains approximately the same amount of ethanol as 1 ml of blood [9]. The blood-ethanol values are only given for orientation. From Figs. 8 a, b follows that the PID and the GC results coincide well after an initial time period. The general shape of the time dependence of the breath ethanol concentration is in a good agreement with reported data [10, 11]. Within the initial period, the GC results decrease much faster than the PID results. From independent measurements it is well known that ethanol a a b Fig.7a, b. Comparison of the responses of the photoionization detector a with the non-dispersive IR-device (Alcotest), b with the GC/FID-measurement b Fig.8a, b. Dependence of breath ethanol concentration of a volunteer A and b volunteer B on time after drinking

210 measurements of breath are not reliable in the first 10–20 min after drinking [12]. In the course of time after the initial 20 min, GC and PID results do not deviate systematically. The time constant of the determination is approximately 4 s. These experiments show the principally applicability of a PID for screening people with respect to ethanol. It is note worthy that ethanol determinations in breath do not posses any legal significance. Due to the broad variations between the blood ethanol content and the measured breath ethanol content, no indirect measurement is accepted in forensic practice. Thus a screening of people’s breath is only relevant for deciding upon performing a blood analysis. For this purpose the PID is obviously applicable. The precision of the determination of the ethanol content in breath is comparable to the usual methods. However, the determination by using the photoionization detector is very easy and fast. The improvement of the apparatus to a hand-held device is possible. Conclusions This study shows the possibility of ethanol determinations in alcoholic beverages and human breath with a photoionization detector. For solution analysis a purging system is used to transfer the ethanol into the gas phase, in which the detection with the photoionization detector takes place. Analysis of human breath with respect to its ethanol content is very simple, fast and sensitive enough to become competitive with conventional techniques. The development of a prototype of a breath analyzer is scheduled for the future. Acknowledgement. We wish to thank Dr. G. Michael for his helpful cooperation in the gas-chromatographic measurements. We thank Doz. Dr. F. Pragst for providing the Alcotest device and for helpful advice. Support by Fonds der Chemischen Industrie is gratefully acknowledged. References 1. Bames E, Bleyer B, Büttner G, Diemair W, Holthöfer H, Reichard O, Vogt E (1938) Alkoholische Genussmittel. Handbuch der Lebensmittelchemie. Springer, Berlin Heidelberg New York, pp 290–293 2.Leibnitz E, Struppe G (1984) Handbuch der Gaschromatogaphie. Akademische Verlagsgesellschaft Geest & Portig, Leipzig, pp 270–272 3.Matissek R, Schnepel FM, Steiner G (1989) Lebensmittelanalytik. Springer, Berlin Heidelberg New York London Paris Tokyo, pp 173–175 4.Bergmeyer (1986) Methods of Enzymatic Analysis. VCH, Weinheim, pp 598–606 5.Püschel K, Kernbach G, Brinkmann B (1986) Zur Korrelation zwischen BAK und AAK-Ergebnisse aus einer Feldstudie. Beiträge zur gerichtlichen Medizin. Franz Deuticke, Wien, pp 23–27 6. Mergemeier S, Scholz F (1994) Fresenius J Anal Chem 350:659 7.Davenport JN, Adlard ER (1984) J Chromatogr 290:13 8.Senum GI (1981) J Chromatogr 205:413 9.Dubowski KM (1975) Z Rechtsmedizin 76:93 10.Zink P, Reinhard G (1980) Blutalkohol 17:400 11.Clasing D, Brackmeyer U, Bohn H (1981) Blutalkohol 18:98 12.Heifer U (1982) Blutalkohol 19:29

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