Evaluation of the uncertainty in the calibration device for gas chromatographs

Thread Content

According to the calibration specification JJG700-1999 for gas chromatographs, the main parameters that are calibrated for such instruments include column oven temperature stability, reproducibility of programmed temperature changes, baseline noise, baseline drift, sensitivity or detection limit, quantitative reproducibility, and carrier gas flow rate stability. Using a microsyringe and standard substances to determine the sensitivity or detection limit of a gas chromatograph: Using a chromatographic detector and a platinum resistance thermometer to assess the stability of the column box temperature and the repeatability of programmed temperature ramps ； Use a soap film flow meter to verify the stability of the carrier gas flow rate. Therefore, the uncertainty in the calibration of a gas chromatograph is related to these factors. With the continuous development of chromatography technology, there are an increasing number of detectors for gas chromatographs, with dozens of types available today. However, the commonly used detectors include the Thermal Conductivity Detector (TCD), Flame Ionization Detector (FID), Flame Photometric Detector (FPD), Electron Capture Detector (ECD), and Nitrogen Phosphorus Detector (NPD). The JJG700-1999 calibration specification for \"Gas Chromatographs\" mainly covers the calibration of these 5 types of detectors. Due to the different principles of each detector, their formulas for calculating results also vary. 　　I. Mathematical Model Gas chromatograph detectors are divided into two categories: one is the concentration-type detectors, which include the thermal conductivity detector (TCD) and the electron capture detector (ECD) ； The second category is mass-type detectors, including the Flame Ionization Detector (FID), Flame Photometric Detector (FPD), and Nitrogen Phosphorus Detector (NPD). 1. Concentration-type detector, whose response value is related to the flow rate of the carrier gas; the formula for calculating sensitivity is: S = (AFC)/W (1), where S represents sensitivity, in mV·mL/mg ； A —— Peak area of the solute in the standard substance, μV·s ； FC — carrier gas flow rate, mL/min ； W —— Sampling amount of the reference material, in g. 2. Mass-type detector, whose response value is independent of the carrier gas flow rate; generally, the detection limit is calculated using equation (2): D = (2NW)/A. In equation (2), D represents the detection limit, in g/s ； N — baseline noise, A ； A —— Peak area of the solute in the standard substance, A·s ； W —— Sampling amount of the reference material, g ； Due to the different response mechanisms of FPD for sulfur determination, its response value is proportional to the square of the standard substance concentration, and the detection limit of FPD for sulfur determination is calculated using equation (3). http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311062AA.JPG Where: D —— detection limit, g/s ； N — baseline noise, mm ； h — peak height of sulfur in the standard material, mm ； W1/4 —— the peak width at 1/4 of the height of the sulfur chromatogram peak, s ； Wns —— the injection amount of sulfur in the standard material, in g. 　　II. Sources and analysis of uncertainty: According to the propagation of uncertainty, it is obtained from equation (1): http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311062AB.JPG It is obtained from equation (2): http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311063AA.JPG It is obtained from equation (3): http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311063AB.JPG The various uncertainty components are ** with respect to each other, and the sensitivity coefficient is +1. 1. Sources of uncertainty (1) Type B uncertainty a. Relative uncertainty of the reference material, Urel1. The relative uncertainty of the reference material is the main source of uncertainty in the calibration of gas chromatographs, and it has a direct impact on the calibration results. This relative uncertainty is usually provided in the certificate for the reference material. For the reference materials GBW(E)130101–GBW(E)130104 listed in the reference material catalog, the uncertainty associated with their determined values is 3%, with a coverage factor of k=3; therefore, Urel1.1 = 0.03 ÷ 3 = 0.01. If a methane standard in nitrogen is used for calibration, the uncertainty in the determined value of methane in nitrogen is 1%, with a coverage factor of k=2; therefore, Urel1.2 = 0.01 ÷ 2 = 0.005. b. The relative uncertainty Urel2 of the calibration value of the soap film flow meter is given in the calibration certificate. The uncertainty in the calibration value of such a flow meter is generally 1%, with a coverage factor of k=2; thus, Urel2 = 0.01 ÷ 2 = 0.005. c. The relative uncertainty Urel3 associated with the calibration of micropipettes: The volume markings on micropipettes represent one of the major sources of uncertainty, so they must be calibrated before use. The regulations specify that calibration generally involves 6 weighings with a relative standard deviation of 1%. (When calculating the test results, the calibrated value should be used) Urel3=0.01÷http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311063AC.JPG=0.004. (2) Class A uncertainty: a. The relative uncertainty Urel4 of the peak area or peak height measurements is primarily determined by the repeatability of sampling. The procedure specifies that 6 samples should be taken, with a quantitative repeatability of 3%; therefore, Urel4 = 0.03 ÷ http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311063AC.JPG = 0.0122. b. The uncertainty Urel5 of the carrier gas flow rate measurements is determined according to the procedure, which requires 6 measurements of the carrier gas flow rate, with a relative standard deviation of 1%. Therefore, Urel5 = 0.01 ÷ http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311063AC.JPG = 0.004. c. The uncertainty of baseline noise measurement, Urel6, arises mainly from the uncertainty associated with the calibration of the caliper and the uncertainty in measurements taken using that caliper; it is generally around 0.02. If baseline noise is recorded using a chromatography workstation, its uncertainty is better than 0.01. 2. The uncertainty analysis (SA)/A represents the relative uncertainty Urel4 of peak area measurement; (SFc)/FC is the relative uncertainty of flow rate measurement, which includes the relative uncertainty Urel2 of the calibration value of the soap film flow meter and the uncertainty Urel5 of the carrier gas flow rate. (SW)/W is the relative uncertainty of the standard material, encompassing the uncertainty Urel1 of the standard material itself and the uncertainty Urel3 associated with the calibration of the micropipette. Additionally, uncertainties arising from differences in the visual acuity of the person performing sampling, as well as temperature differences during the calibration and use of the micropipette, are also considered. Results from trainer-led evaluations show that these factors introduce negligible uncertainties, and the uncertainty Urel6 for baseline noise measurement is zero. 　　III. Combination of uncertainties
TCD: http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311063AD.JPG; U = 0.0176. With the inclusion factor k set to 2, U = kuc = 2 × 0.0176 = 3.6%.
ECD: http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311063AE.JPG; U = 0.0266. With k set to 2, U = kuc = 2 × 0.0266 = 5.4%.
FID, FPD (phosphorus), NPD: http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311063AF.JPG; U = 0.0258. With k set to 2, U = kuc = 2 × 0.0258 = 5.2%.
FPD (sulfur): http://zgjl.chinajl.com.cn/zgjl/zgjl/watu/0311063AG.JPG; U = 0.0232. With k set to 2, U = kuc = 2 × 0.0232 = 4.7%.

IV. Conclusion
In the calibration of gas chromatographs, if good sampling techniques are used or gas reference materials are employed, along with sampling via a six-way valve and baseline noise measurement by a chromatography workstation, the expanded uncertainty of the calibration results for gas chromatographs is always below 5%. In accordance with the **measurement technical specification JJF1059-1999 \"Evaluation and Expression of Measurement Uncertainty\", an analysis and evaluation of the uncertainty for the calibration devices of gas chromatographs were carried out. The results obtained were generally consistent with those from actual calibrations, thereby providing a feasible and effective method for assessing the reliability of the calibration results of gas chromatographs. http://www.jlbjb.com/edu/show.asp?id=1712