Uncertainty Evaluation of Toluene Determination in Room Air by Thermal Desorption Gas Chromatography

Transcription

1 International Conference on Civil, Transportation and Environment (ICCTE 06) ncertainty Evalation of Tolene Determination in Room Air by Thermal Desorption Gas Chromatography Xiaoyan Wen, a,yanhi Gao, b, Caiyn Sn, c Department of Basic Crriclm, North China Institte of Science and Technology, Langfang, China Department of Basic Crriclm, North China Institte of Science and Technology, Langfang, China Department of Basic Crriclm, North China Institte of Science and Technology, Langfang, China a xiaoyanwen_ncist@6.com, bgaogao_050@6.com, cyncai@ncist.ed.cn Keywords: ncertainty, Tolene, Thermal desorption, Gas chromatography Abstract. The method of ncertainty evalation of determination of tolene in room air has been established. Gas chromatography with thermal desorption was sed for determining of tolene. The main factors which can affect on the analytical reslts were identified. The measrement repeatability, standard soltion preparation process, calibration crve fitting, sampling process and the instrment are the main sorces of tolene ncertainty. The ncertainty of calibration crve fitting has been identified as the greatest effect on the analytical reslts. Introdction Tolene is a common organic solvent with high toxicity. Determination of the tolene in the air by thermal desorption gas chromatography has been proved as sensitive and reliable []. Many factors, sch as sampling process, preparation of standard soltion, calibration crve fitting, instrment measrement, repeatability of analysis reslts, will affect the confidence and accracy of the analytical reslts. ncertainty evalation shold be sed for controlling of the determination [, ]. Measrement process Sampling tbe with Filled Poly-,6 diphenyl phenyl ether (Tenax), was sed for concentration of tolene in ambient air and indoor air nder the normal temperatre. Sampling tbe will be heated after connected into the thermal desorption instrment, and desorption tolene can be determined by gas chromatography with flame ionization detector (FID). Mathematical model The concentration of tolene can be identified as: The sampling volme, Vt, converted into standard condition sampling volme V0 V0 Vt T0 P 7 + t P0 () The concentration of tolene in air can be calclated: Ci m m0 V0 () Then 06. The athors - Pblished by Atlantis Press

2 C i m m0 T0 P Vt ( 7 + t) P 0 () Which: C i : concentration of tolene in air, mg/ m m: the mass of tolene in sampling tbe, mg m 0 : the mass of tolene in blank sampling tbe, mg P 0 : atmospheric pressre nder standard conditions, 0.kPa. P: sampling point pressre, kpa T: sampling point temperatre, C T 0 : the absolte temperatre nder standard conditions, 7K V t : volme of sampling, m Sorces of ncertainty The main sorces of measrement ncertainties were from sb-ncertainties of sample preparation, calibration soltion preparation, calibration crve fitting and instrment measrements, etc. ncertainty of repeatability of analysis reslts tolene Table ncertainty of repeatability of analysis reslts Reslt (mg/m ) mean vale (mg/m ) standard deviation From Table, the ncertainty of repeatability of analysis reslts can be calclated: s repeatability. mg/ m (4) n Relative standard ncertainty: rel( repeatability ) 4 repeatability (5) c ncertainty of calibration crve fitting The concentration of tolene was calclated by sing calibration crve with the area of the peak of tolene. Chromatographic pre tolene was dilted to give the standard soltions which concentrations of µg/ ml, respectively. The standard soltion were detected by gas chromatography, regression eqation of tolene, A 6.58 c , was fitted with the peak area and concentration, slope is 6.58, intercept is 48.9, correlation coefficient r The standard deviation of calibration crve can be calclated: ( y) n m ( yij yi) i j Sy / x (6) m n Which: y ij : response of instrment measrement on sample

3 y i : response which calclated by calibration crve n: nmber of diferent concentration of standard soltion for detection m: the nmber of repeated measrements of each sample y. A ( ) 5048 ncertainty of calibration crve: ( xsample x) ( i ) S( y/ x) + + b n N x x ( crve) ' (7) ( crve ) ( ) µg ml Which: n : the nmber of repeated measrements of each sample b: slope of calibration crve N: nmber of diferent concentration of standard soltion for detection, involve the repeat times of analysis x i : mean vale of concentrations calclated by calibration crve x sample : mean vale of concentrations of standard soltion. Relative standard ncertainty: 44. rel( crve) ncertainty of preparation of standard soltion The concentration of certified reference material, tolene, is.00mg / ml, and the relative standard ncertainty is %. The ncertainty of prity of certified reference material shold be: 005. rel( prity) µL syringe: The given allowed deviation of capacity is.8%, calclated with the niform distribtion, the relative ncertainty shold be: 008. rel( syringe) ml measring pipet: According to instrment calibration certificate, the given allowed deviation of capacity is 0.05 ml, calclated with the niform distribtion, the relative ncertainty shold be: 005. rel( pipet ) ml volmetric flask: According to instrment calibration certificate, the given allowed deviation of capacity is 0.0 ml, calclated with the niform distribtion, the relative ncertainty shold be: 4

4 0. rel( flask ) ncertainty of preparation of standard soltion: rel( soltion ) ncertainty of sampling Sampling process. Pmping air 0.L/ min for 0min, the temperatre is 5 C, atmospheric pressre P 0.kPa. The standard condition sampling volme shold be: T0 P 7 0. V0 Vt 0. 95L 7 + t P ncertainty of flowmeter. According to instrment calibration certificate, the given error of indication of flowmeter is 5%, error of flow repeatability is %, error of flow stability is 5%. Assmed niform distribtion: ( Rv) / R ncertainty of thermometer. According to instrment calibration certificate, the given ncertainty of thermometer is 0. C(coverage factor k), calclated with the niform distribtion, the relative ncertainty shold be: 0. ( t) 0.5 ( t) 0.5 C, T 5 ncertainty of air pressre gage. According to instrment calibration certificate, the given error of indication of flowmeter is ±0.6 hpa, calclated with the niform distribtion, the relative ncertainty shold be: 0.6 p) 0.46 ( p) 0.46 hpa,.4 0 P 0 ncertainty of sampling: ( 4 [ ] [ ] / rel( sampling) ( R ( v) / R + t ()/ T + ( p)/ P ) ( ) 004. ncertainty of instrments ncertainty of gas chromatography. According to instrment calibration certificate, the given error of stability of oven temperatre is 0.%, error of repeatability is 0.9%, ncertainty of FID is.8% (coverage factor k), the relative ncertainty of FID shold be The ncertainty of GC shold be: 5

5 rel ( GC) ( ) ncertainty of thermo desorption. According to instrment calibration certificate, the given ncertainty of thermo desorption is %, the relative ncertainty shold be: 00. rel( TD ) 00. ncertainty of instrments: rel( instrment ) ( ) 00. The combined relative standard ncertainty Relative standard ncertainty can be combined: ( C) crel rel( repeatability) rel( crve) rel( soltion) rel( sampling) rel( instrment) The expanded ncertainty Becase of the measrement error is obeyed to normal distribtion, confidence level p95%, coverage factor k, expanded ncertainty shold be: k ( C) rel rel The concentration of tolene in air is 0.05mg/m, the expanded ncertainty shold be: mg/m Expression of reslt The determination of concentration of tolene in air by sing GC, the reslt can be representing: C 005. ± mg/m, confidence probability is 95% tolene Conclsion The ncertainty of determination of tolene in room air mainly originates from the change of the calibration crve fitting. A reasonable and accrate calibration crve can be sed to control the degree of ncertainty. The ncertainty evalation of determination of tolene may be helpfl for improving the confidence and accracy of the test reslts, and redce the ncertainty of the test reslts. Acknowledgements This work was financially spported by the Fndamental Research Fnds for the Central niversities (405045). 6

6 References [] S. Sándor, T. László, L. József, S. Attila,A. Róza. Simltaneos Determination of Styrene, Tolene, and Xylene Metabolites in rine by Gas Chromatography/Mass Spectrometry. Archives of Toxicology, 00, 76(0): [] Q.S. He, X.J. Lai, Y.Y. Yang, Z.W. Shao. Analysis and Evalation of ncertainty of Tolene in Water. Jornal of China Jiliang niversity, 005, 6(4): [] Y. P. Wang. ncertainty Evalation of Determination Reslt of Tolene in Carbon Bislphide by Gas Phase Chromatography. Technology & Development of Chemical Indstry, 009, 8(7):