STUDIES CONCERNING THE TRACEABILITY OF BREATH ALCOHOL CONCENTRATIONS

Transcription

1 U.P.B. Sci. Bll., Series B, Vol. 70, No. 3, 008 ISSN STUDIES CONCERNING THE TRACEABILITY OF BREATH ALCOHOL CONCENTRATIONS Mirela Adelaida ANGHEL 1 Metoda folosită de către specialiştii din cadrl Instittli National de Metrologie în scopl asigrării trasabilităţii măsrărilor de alcool etilic din aerl alveolar expirat în România este prezentată în acest articol. Etaloanele de alcool etilic preparate şi rezltatele obţinte din măsrările efectate acoperă întregl domeni de concentraţie de alcool din aerl alveolar expirat. Bgetele de incertitdine a fost calclate folosind abordarea ISO [1,]. Măsrările de alcool din aerl expirat snt prezentate în acest articol împrenă c incertitdinea extinsă, U, tilizând n factor de acoperire k=, pentr n nivel de încredere de 95 %. Parametrii de calitate cheie snt incertitdinile asociate valorillor certificate şi încrederea în incertitdinea estimată. The method sed by specialists from National Institte of Metrology in order to assre traceability of breath alcohol measrement in Romania is presented here. The prepared ethanol standards and reslts obtained from measrements cover the entire range of concentrations of breath alcohol analyzers. Uncertainty bdgets have been calclated sing the ISO approach [1,]. Breath alcohol measrements are presented in this paper together with the expanded ncertainty, U, sing a coverage factor k= which gives a level of confidence of approximately 95%.The key qality parameters are the ncertainties associated with the certified vales and the reliability of the ncertainty estimate. Keywords: traceability, reference materials, breath alcohol concentration, metrology. 1. Introdction It s well known that the alcohol concentration from hman body is measred sing air from the lngs or venos blood. Breath testing has become over the years, a widely sed method for qalitative and qantitative determination of the level of person sspected of driving while nder the inflence of alcohol. After recognition of the need for qantitative assessment of intoxications, blood alcohol concentration was considered the most important variable. However, concern abot the invasiveness reqirements of drawing a 1 Scientific Researcher III, Physico-Chemical Laboratory, National Institte of Metrology Bcharest, ROMANIA

2 86 Mirela Adelaida Anghel blood sample led to the development of the breath test as non-invasive means of assessing level of intoxication. Scientists all over the world have started to look for new non-invasive principles to determine the alcohol concentration from hman body. Depending on accracy, specifity to alcohol, cross sensivity, long term stability etc. there are few measring principles sed for breath alcohol determination: chemical, biochemical (system based on oral flids), physical (semicondctor cell - srface reaction), electrochemical (fel cell), infrared spectroscopy, gas chromatography. The breath-alcohol testing methods have changed over the years from chemical oxidation and calorimetric procedres to physico-chemical techniqes sch as gas-chromatography, electrochemical oxidation and mltiple wave length infrared spectrophotometers. The breath alcohol measrements are based on Henry s law: When an aqeos mixtre of a volatile sbstance reaches eqilibrim with air, there will be a fixed ratio between the concentration of the sbstance in the air and its concentration in the soltion. It is well known that the liqid water and alcohol can be mixed in any ratio, reslting homogeneos mixtres. Both liqids have a tendency to evaporate from the liqid in the form of a gas. Alcohol has a greater tendency to do this. If an alcohol-water mixtre of this type is kept in a partly filled and sealed system, the concentration of gaseos alcohol in the air above the liqid will increase ntil a certain concentration is reached. At this stage, there is a defined ratio between the alcohol concentration in the liqid and that in the air. Scientists all over the world accepted the vale of this ratio in the range between 000:1 to 300:1. The concentration of alcohol in vapor phase above liqid-water mixtre depends on jst two factors: the temperatre of the mixtre and the alcohol concentration in the liqid. B x t ρ air = A x ρ Eth x e (1) where: t is soltion temperatre, 0 C; The following experimental coefficients A and B were established on several stdies on partition coefficient air/ethanol soltion: - A = [mg/l / g/l]; - B = [1 / 0 C]; In the case where t is C, the eqation becomes: 3 ρ air = 0, x 10 ρ Eth () Henry s law applies to the exchange processes in the hman body, especially in the lngs. The balance between the alcohol in the blood and in the

3 Stdies concerning the traceability of breath alcohol concentrations 87 breath is created in the lngs in the same way as described for alcohol in aqeos soltion and air in semi closed system. In accordance with this law, diffsion processes, which are also what cases oxygen to be taken p in the lngs, achieve a balance between the alcohol concentration in the blood in the lngs and the alcohol concentration in the air in the lngs. Ths, the breath alcohol measrement involves directly determining this concentration. Evidential breath analyzers are instrments that atomatically measre the mass concentration of alcohol in exhaled breath that originates from the alveoli of the lngs. Althogh the relationship between the breath and blood alcohol concentration is still ncertain, the evidential breath-alcohol instrments are sed in different contries for determination of alcohol concentration level for forensic prposes. National athorities may reqire specific conversion device that converts the measrement reslt obtained in terms of ethanol content and can approve evidential breath analyzers for law enforcement prposes with threshold limit of breath-alcohol concentration alongside the existing stattory blood-alcohol concentration limits. Qality assrance has become an indispensable accompaniment to forensic breath-alcohol analysis. Gas Concentration Laboratory from National Institte of Metrology is prepared to provide the following control procedres: pattern approval, initial verification of new evidential breath analyzers, periodic verification, performance test and calibrations, and focses on development, implementation and se of sch Qality Assrance programs for breath-alcohol testing. Evidential breath analyzer is an instrment which measres accrately the concentration of alcohol in end-expiratory air to provide a reslt which can be sed as evidence in drinking and driving offences. End-expiratory air is a breath sample containing air from the end of a forced expiration from lngs. The evidential breath analyzer in conjnction with the type approval and the independent official verification, ensres that measrement reslts achieve the extremely high level of reliability that eropean and national standards demand. The Gide to the Expression of Uncertainty in Measrement (GUM) provides general rles for evalating and expressing ncertainty in measrement that are intended to be applicable to a wide range of measrements and for se within standardization, calibration, laboratory accreditation and measrement services. Traceability of breath alcohol concentration is a new field of interest in Romania. Abot 1700 of breath alcohol analyzers were prchased by Ministry of Interior Police Department few years ago following an Eropean project of

4 88 Mirela Adelaida Anghel endowment of East Eropean police departments. Since than, the traceability of measrement performed with sch instrments was a priority in order to assre accracy measrements and acceptance in cort. Measrements made at different times or in different places are directly related to a common reference. Applying the concept of traceability to breath alcohol measrements is not easy, bt it has to provide qalitative reslts and analytical techniqes sed in calibration laboratories. Specialists from National Institte of Metrology start to prepare the basis necessary to transmit the specific measring nit from high level standards (Reference Materials) to the working level measrements. The measrements and tests were performed sing the following eqipment, in order to deliver test gases having ethanol concentrations analogos to those calclated theoretically and to those which evolve dring a real exhalation. - Evidential breath alcohol analyzers Alcotest 7110 MK III, manfactred by Dräger Safety AG & CO, KGaA, Germany, serial nmbers: ARNC-0145, with Calibration Certificate no. 0347/005 and ARND-0145, with Calibration Certificate no. 040/005, issed by EDN (Eichdirection Nord), Germany and traceability to PTB s standards (Physicalisch Technische Bndesanstalt) Germany; - Ethanol prity 99.8 %, manfactred by Merck, code K , batch ; - Wet bath simlator for testing and calibrating breath alcohol analyzers, type Mark II, serial no. DDSE P 0003 and DDSE P 0006, manfactred by Dräger Safety AG, Germany [8,9]; - Wet bath simlator for testing and calibrating breath alcohol analyzers, manfactred by ICIA Clj, Romania; - Pippete, (10.0 ± 0.) ml; - Analytical balance, type XS 05 manfactred by Mettler Toledo; - Distilled water; The Evidential breath alcohol analyzers Alcotest 7110 MK III satisfies the reqirements of DIN VDE 0405 and OIML R 16 and has been approved by Germany s national metrology institte, the PTB (Physikalisch Technische Bndesanstalt), and also by Romanian s National Metrology Institte, following a series of tests according to mentioned standards. This kind of measring system can be sed for breath alcohol concentration measrement either in Germany as in Romania [4-7]. In this paper where sed the following terminology according to International Vocablary of Basic and General Terms in Metrology [0] was sed:

5 Stdies concerning the traceability of breath alcohol concentrations 89 - Uncertainty of measrement is a parameter, associated with the reslt of a measrement, that characterizes the dispersion of the vales that cold reasonably be attribted to the measrand. - Experimental standard deviation for a series of n measrements of the same measrand, is the qantity that characterizes the dispersion of the reslts and is given by the formla: n ( xi x) i= 1 s = n 1 where x i being the reslt of the i th measrement and x being the arithmetic mean of the n reslts considered. In the same time, the breath alcohol measrements se specific measring nits. To determine the blood alcohol concentration (BAC) the following measrement nits are sed: - Per mille, (thosandth; one gram alcohol in 1 L of blood); - Per cent, % (hndredth); The measrement nits sed to determine the alcohol concentration in breathing air (BrAC) are: - Milligram per liter, mg / L (one milligram alcohol in one liter of breathing air); - Microgram per liter, μg / 100 ml; The following relations express the conversion between blood alcohol concentration (BAC) and breath alcohol concentration (BrAC): 1 g mg μg mg = ; 1 = 0.1 % = = 476 = 476 (3) L L L m. Preparation of Calibration Standards Traceability is the property of the reslt of a measrement or the vale of a standard whereby it can be related to stated references, sally national or internatioanl standards, throgh an nbroken chain of comparisons all having stated ncertainties [3]. It is known that traceability reqires an nbroken chain of comparisons between a measrement and the stated references. First step in this project was to prepare standard mixtres. Table 1 presents alcohol concentrations, expressed in (promile) and mg/l (milligram alcohol in a liter of air) obtained by mixing certain qantities of pre alcohol (ethanol) in distilled water.

6 90 Mirela Adelaida Anghel No. Breath alcohol concentration Volme of ethanol Qantity of ethanol in in standard distilled water, m mixtre, V CH5-OH, HO, g ml Breath alcohol concentration ρ air, Table 1 Breath alcohol concentration ρ air, mg/l Qantifying the Uncertainty Components In order to estimate the associated ncertainty for each prepared concentration all sorce of ncertainties were taken into consideration. The inflence qantities that can affect the measrement reslt are generated by the following devices sed: pipette, recipient, prity of ethanol, temperatre established by the simlator s thermostat. 3.1 Uncertainty de to pippete One important sorce of ncertainty is related to the pippete, which has the nominal range between (0...10) ml. In order to establish this contribtion to the final bdget of ncertainty 10 weightings of 4.60 g H O with a Mettler Toledo precision balance were performed; reslts are presented in Table, and the associated ncertainty was estimated according to ISO Gide [1,] and the associated ncertainty was estimated according to ISO Gide [1,]. Table Means vale and standard deviation for 10 weightings Conventional tre vale of distilled water, m 0, g Average vale of 10 weightings, m m, g The combined standard ncertainty is: Standard experimental deviation, s, g

7 Stdies concerning the traceability of breath alcohol concentrations 91 C_pippete = s + s1 + s = = ml where: s - standard deviation for 10 measrements of weight of 4.60 ml H O distilled water; 0.0 ml s1 = = ml 6 is the standard ncertainty calclated assming a trianglar distribtion, as stated by the pippete s manfactrer; C s = 10 ml = mL is the standard ncertainty calclated assming of a rectanglar distribtion for a temperatre variation and the coefficient of the volme expansion. The expanded ncertainty is obtained by mltiplying the combined standard ncertainty with a coverage factor of, giving U C_pippete = 0.00 ml x = ml; Ths, the volme of pipete is: V _pippete = (4.60 ± 0.04) ml 3. Uncertainty de to recipient Standard ncertainty specified by manfactrer in recipient s Calibration Certificate, calclated assming a rectanglar distribtion is: 1 V1 = = 0, ml 3 Standard ncertainty calclated assming of a rectanglar distribtion for the coefficient of the volme expansion has the vale: C V = ml = ml The combined standard ncertainty is: C_Volm = V 1 + V = = ml leading to the following vale for the volme of recipient sed for preparing the standard mixtre: V_ Volm = ( ± 0.59) ml

8 9 Mirela Adelaida Anghel 3.3 Uncertainty de to prity of ethanol The ethanol sed for preparation of different standards was 99.8 % by volme. Standard ncertainty has the vale: etalon = ( ) = Uncertainty de to temperatre established by the simlator s thermostat The simlator s thermostat was set to la (34.0 ± 0.1) 0 C dring the experiments. Standard ncertainty calclated assming of a rectanglar distribtion for the variation of thermostat s temperatre has the vale: termostat = ( ) = C 3 4. Example of total ncertainty bdget calclation for a concentration of 1.0 corresponding to a concentration of mg/l (mg alcohol in a liter of air) The reslt of measrement and ncertainty bget calclation are presented in Table 3. Table 3 Spreadsheet calclation of ncertainty: ethanol, g hanol rity lme ethanol/l) ρ et P Vo ρ 0( p V (ethanol/l) ρ i(ethanol/l) ρ 0 -ρ i (ρ 0 -ρ i ) (ρ 0 -ρ i )

9 Stdies concerning the traceability of breath alcohol concentrations 93 (ethanol/l) = ( ρ0 ρi ) = ml = g/l Uncertainty of alcohol concentration in air de to another qantity inpt: temperatre of soltion, t = C, calclated according to Dbowsky formla (1); spreadsheet calclation of ncertainty is presented in Table 4. Table 4 Spreadsheet calclation of ncertainty, for a temperatre vale t = 34 0 C ρ 0(ethanol/L) soltion temperatre, 0 C 34 t ρ air 0 (ethanol/l air) ethanol/l air Index, % 1.31 U (k=) ρ i(ethanol/l air) ρ 0 -ρ i (ρ 0 -ρ i ) (ρ 0 -ρ i ) Index, % (ethanol/l air) = ( ρ0 ρi) = mg/l = mg/l The alcohol concentration expressed as milligram in a liter of air is then: ρ air = ( ± ) mg/l taking in accont the nmber of digits available on the breath alcohol analyzers: ρ air = (0.474 ± 0.006) mg/l, presented graphically in Fig. 1. Fig. 1

10 94 Mirela Adelaida Anghel Alcohol concentration prepared accordingly to Dbowsky formla and the associated ncertainties calclated according to the latest gide to the expression of ncertainty in measrement [1,] are presented in Table 5 and Fig.. Table 5 Associated ncertainty of breath alcohol concentration Breath alcohol concentration, ρ air, Breath alcohol concentration, ρ air calc, mg/l Associated ncertainty, c, mg/l Index, % Fig.. The difference between the desired and the prepared concentration are very small; this means that the from theoretical and practical point of view the laboratory is prepared to assre traceability existing measring analyzers of breath alcohol concentration. Using the eqipment existent in Gas Concentration Laboratory from Romanian National Institte of Metrology several ethanol concentrations were prepared, covering a sal range of a breath alcohol analyzer. The reslts are presented in Table 6.

11 Stdies concerning the traceability of breath alcohol concentrations 95 Breath alcohol concentration related to conventional tre vale Conventional tre Alcohol Prepared alcohol vale of alcohol concentration, concentration, concentration, ρ air ρ air i, 0, mg/l mg/l Table 6 The Fig. 3 presents the final alcohol concentration prepared in laboratory (ρ i, mg/l) against theoretical concentrations (ρ 0, mg/l) calclated according to the Dbowsky formla. Fig. 3 The differences between the desired and the prepared concentrations are very small; this means that from theoretical and practical point of view the laboratory is prepared to assre traceability of existing measring analyzers of breath alcohol concentration.

12 96 Mirela Adelaida Anghel 5. Preparation of Ethanol in Air Concentrations by Variation of Temperatre According to Dbowski s eqation (1), the alcohol concentration in vapor phase above liqid-water mixtre depends on two factors: the temperatre of the mixtre and the alcohol concentration in the liqid. So, ethanol in air standards can be prepared by varying mixtre s temperatre while alcohol concentration in liqid is maintained constant, at g/l. Using this method the following standards were prepared in laboratory and each of them has been measred 40 times, in order to calclate the metrological characteristics. For each 10 th series of measrement the calclated mean vales, corresponding standard deviations and relative standard deviations are presented in Table 7. Relative standard deviation for a no. of measrement Table 7 Nmber of measrement, n Temperatre of the soltion (mixtre of alcohol in the liqid) t, o C Theoretical alcohol concentration according Dbowski s eqation, ρ air 0, mg/l Average vale of the soltion, ρ air m, mg/l 0 Standard deviation, σ, mg/l Relative standard deviation σ rel, % ρ m, mg/l σ, mg/l σ rel, % ρ m, mg/l σ, mg/l σ rel, % The average vales of 0 measrements performed with alcohol concentrations obtained by variation of temperatre (at 3 0 C, 33 0 C, 34 0 C, 35

13 Stdies concerning the traceability of breath alcohol concentrations 97 0 C, 36 0 C) against theoretical alcohol concentrations calclated according to Dbowski s eqation are presented graphicallyin Fig. 4: Fig Stability of Prepared Soltions Dring Repeated Measrements In order to evalate long term stability of soltions prepared in laboratory two different concentrations were sed, 0.39 and 0.80 exhaled air alcohol concentration. All data obtained from reprodctibility experiment nder reprodctibility conditions (different time, varios instrments, same sample) were calclated according to standard ISO 858:1991, Shewhart control chart [11]. The following parameters were calclated for each series of 30 measrements performed with alcohol concentrations: ρ air m R = ; ρair 0 ρ ρ air m air m ρ ρ air corr = ; En R s + air 0 = ; ( ρ air ) ρ air 0 c = s ( ρair ) + sr ( ρair ) (3) where: ρ air i measred concentration vale, mg/l; R prepared soltion recovery, %; ρ air 0 conventional tre vale of reference material, mg/l; ρ air corr corrected vale, mg/l; ρ air m mean vale of measred alcohol concentrations, mg/l; ε i intrinsic error, mg/l; ε t maximm permissible error, mg/l; E n accracy score; ncertainty of reference material, mg/l; ρair 0

14 98 Mirela Adelaida Anghel s(ρ air ) experimental standard deviation, mg/l; s(ρ air )rel relative standard deviation, mg/l; s R standard deviation of recovery, mg/l; RSU relative standard deviation, % The stability for 0.39 (0.184 mg/l) and 0.80% (0.38 mg/l) alcohol concentration and soltion recovery at 34 0 C, are presented in Table 8. Table 8 Stability for 0.39 (0.184 mg/l) and 0.80 (0.38 mg/l)alcohol concentration and soltion recovery at 34 0 C Theoretical alcohol concentration according Dbowski s eqation, ρ air 0, (mg/l) Nmber of measrements, n Range of vales of concentration measred, (mg/l) Average vales, (mg/l) Range of recovery of the soltion, R Average recovery of soltion, R m Range of intrinsic error, ε i, (mg/l) Maximm permissible error, ε t, (mg/l) ± ± The final reslt of breath alcohol concentration with corresponding combined standard ncertainty [1,] is: ρ air = (0.184 ± 0.006) mg/l and ρ air = (0.38 ± 0.006) mg/l The standard deviation, spreadsheet with ncertainty calclation and corrected concentration with it s associated ncertainty are presented in Tables 9,10 and 11. Tabel 9 Standard deviation ρ air m s(ρ air ) s(ρ air ) /n R m s R (ρ air )

15 Stdies concerning the traceability of breath alcohol concentrations 99 Table 10 Spreadsheet showing the ncertainty calclation: ρ air m ρ air fnction dif dif^ sm(dif^) index 0.001% 0.999% % % R m s R (ρ air ) RSU 3.5 % 1.64 % Table 11 Corrected concentration and it s associated ncertainty ρ air ρ air m R m ρ air corr ρair s(ρ air ) s R (ρ air ) ρ air corr RSUρ air corr 4.64 %,33 % where: c = s ( ρair ) + sr ( ρair ) So, the corresponding treness for the concentration ρ air = (0.184 ± 0.006) mg/l and ρ air = (0.38 ± 0.006) mg/l measred for 30 times nder the same condition of measrement is: E n = 0.05 and E n = 0.13 The reslts of the recovery stdy, sing two different breath alcohol concentrations: mg/l and 0.38 mg/l are presented in Table 1: Tabel 1 Treness - Score E n No. ρ air, c ρair, E n mg/l mg/l

16 100 Mirela Adelaida Anghel The accracy is confirmed [9] de to vales for E n less then 1, for both reference materials. 7. Smmary and Conclsions Breath alcohol analyzers are widely accepted as legal measrement instrments sed for determination of the mass concentration of alcohol in exhaled breath. Nowadays, Traffic Road Department from Romanian Ministry of Interior is sing abot one thosand and few hndred of electronic devices for testing breath alcohol concentration. Gas Concentration Laboratory from National Institte of Metrology has started a project to prepare ethanol in air standards in order to provide the following control procedres: initial verification of new evidential breath analyzers, periodic verification, performance test and calibrations. Dring two years of sstained research activity different alcohol concentrations were prepared and the associated ncertainties according to the latest standards were evalated [1,]. The reslts obtained show that National Institte of Metrology standards, prepared according to the Eropean and international standards and with the knowledge and eqipment existent in Romanian laboratory, have the necessary accracy and can be sed to transmit the measring nit, mg/l, to breath alcohol analyzers. R E F E R E N C E S [1] Gide to the expression of ncertainty in measrement, first edition, 1995, ISO/BIPM/IEC/IFCC/IUPAC/IUPAP/IOML (pblished by ISO) [] Erachem/CITAC Gide, Qantifying Uncertainty in Analytical Measrement, nd edition 000, Pblished by LGC, UK [3] BIPM, IEL, IFCC, ISO, IUPAC, OIML, International Vocablary of Basic and General Terms in Metrology, ISO, Geneva, Switzerland,nd end., 1993 [4] International Committee of Legal Metrology OIML R 16: Evidential breath analyzers, 1998 [5] DIN VDE 0405 Determination of Breath Alcohol Concentration, 1995 [6] Dbowski, K.M., Breath-alcohol simlators. Scientific basis and actal performance, J. Anal. Toxicol., (1979), v3, p 177 [7] OIML Blletin, volme XLVIII, nmber 3, Jly 007 Traceability of breath alcohol concentrations in Romania, Mirela Anghel [8] A.Slemeyer A Depletion Compensated Wet Bath Simlator For Calibrating Evidential Breath Alcohol Analyzers, University of Applied Science of Giessen [9] A. Slemeyer Calibrating Gas Generator, University of Applied Science of Giessen, 1998 [10] ISO 858:1991, Shewhart control chart