UOP 555-2010 Trace Impurities in Benzene by GC.pdf

1、 IT IS THE USERS RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TO DETERMINE THE APPLICABILITY OF REGULATORY LIMITATIONS PRIOR TO USE. EFFECTIVE HEALTH AND SAFETY PRACTICES ARE TO BE FOLLOWED WHEN UTILIZING THIS PROCEDURE. FAILURE TO UTILIZE THIS PROCEDURE IN THE MANNER PRESCRIB

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3、All rights reserved. Nonconfidential UOP Methods are available from ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959, USA. The UOP Methods may be obtained through the ASTM website, www.astm.org, or by contacting Customer Service at serviceastm.org, 610.832.9

4、555 FAX, or 610.832.9585 PHONE. Trace Impurities in Benzene by GC UOP Method 555-10 Scope This gas chromatographic method is for determining individual and total trace hydrocarbon impurities in olefin-free, high-purity benzene. C8 and lower boiling paraffins, naphthenes, toluene, and C8 aromatics ar

5、e determined. The lower limit of quantitation for any single component is 1 mg/kg (mass-ppm). Benzene purity can also be determined by ASTM Method D4492, where the lower detection limit is 50 mg/kg per component. Although benzene is not listed in their scopes, ASTM Methods D2360, D5917, and D7504 ma

6、y also be used to measure impurities in benzene. All of these ASTM Methods determine the non-aromatics as a total whereas this method also provides a distribution. References ASTM Method D2360, “Trace Impurities in Monocyclic Aromatic Hydrocarbons by Gas Chromatography,” www.astm.org ASTM D4307, “Pr

7、eparation of Liquid Blends for use as Analytical Standards,” www.astm.org ASTM Method D4492, “Standard Test Method for Analysis of Benzene by Gas Chromatography,” www.astm.org ASTM Method D5917, “Trace Impurities in Monocyclic Aromatic Hydrocarbons by Gas Chromatography and External Calibration,” ww

8、w.astm.org ASTM Method D7504, “Trace Impurities in Monocyclic Aromatic Hydrocarbons by Gas Chromatography and Effective Carbon Number,” www.astm.org Scanlon, J. T. and Willis, D. E., Journal of Chromatographic Science, 23, 333-340 (1985) UOP Method 999, “Precision Statements in UOP Methods,” www.ast

9、m.org Outline of Method The sample is injected into a gas chromatograph that is equipped with an autoinjector, a fused silica capillary column internally coated with cross-linked methyl silicone, and a flame ionization detector. The concentrations of individual or group impurities are determined by

10、the external standard method of quantitation, wherein peak areas of the sample components are compared to the peak areas of a 2 of 11 555-10 calibration blend analyzed under identical conditions and injection volumes. See Note for an alternative means of calibration and calculation. Apparatus Refere

11、nces to catalog numbers and suppliers are included as a convenience to the method user. Other suppliers may be used. Balance, analytical, readable to 0.0001 g Chromatographic column, 50 m of 0.21-mm ID fused silica capillary, internally coated to a film thickness of 0.5-m with cross-linked methyl si

12、licone, Agilent Technologies, Cat. No. 19091S-001 Gas chromatograph, temperature programmable, built for capillary column chromatography, utilizing a split injection system having a glass injection port insert, and equipped with a flame ionization detector that will give a minimum peak height respon

13、se of 10 times the background noise for 1 mg/kg n-octane when operated at the recommended conditions, Agilent Technologies, Model 7890 Data system, electronic, for obtaining peak areas. This device must integrate areas at a sufficiently fast rate so that narrow peaks typically resulting from use of

14、a capillary column can be accurately measured. Agilent Technologies, ChemStation. Hood, fume Leak detector, gas, Alltech Associates, Cat. No. 21-250 Refrigerator, flammable storage or explosion proof Regulator, air, two-stage, high purity, delivery pressure range 30-700 kPa (4-100 psi), Matheson Tri

15、-Gas, Model 3122-590 Regulator, hydrogen, two-stage, high purity, delivery pressure range 30-700 kPa (4-100 psi), Matheson Tri-Gas, Model 3122-350 Regulator, nitrogen, two-stage, high purity, delivery pressure range 30-700 kPa (4-100 psi), Matheson Tri-Gas, Model 3122-580 Sample injector, any syring

16、e or injector capable of injecting a repeatable 0.5-L volume of sample. The use of an automatic injection device is required to achieve necessary repeatable injection volumes. See Note and Appendix. Agilent Technologies, Model 7683. Reagent and Materials References to catalog numbers and suppliers a

17、re included as a convenience to the method user. Other suppliers may be used. Air, zero gas, total hydrocarbons less than 2.0 ppm as methane, local supply Benzene, 99.9% minimum purity, Sigma-Aldrich, Cat. No. 270709. CAUTION: Benzene is especially hazardous. Perform all work with benzene in a fume

18、hood, using appropriate personal protection equipment, as directed by local regulations and the MSDS. Gas purifier, for hydrogen, to remove oxygen and moisture from carrier gas, VICI Mat/Sen, Cat. No. P200-1 3 of 11 555-10 Hydrogen, zero gas, 99.99% minimum purity, total hydrocarbons less than 0.5 p

19、pm as methane, local supply Nitrogen, zero gas, 99.99% minimum purity, total hydrocarbons less than 0.5 ppm as methane, local supply n-Octane, 99.0% minimum purity, Sigma-Aldrich, Cat. No. 74821 Pipets, disposable, Pasteur, VWR, Cat. No. 14673-043 Pipet bulbs, VWR, Cat. No. 15001-362 Syringe, replac

20、ement, for recommended sample injector, 5-L, Agilent Technologies, Cat. No. 5181-1273 Vials, 22-mL, with polyseal-lined caps, VWR, Cat. No. 16087-068 Vials, autosampler, for recommended sample injector, with caps, Agilent Technologies, Cat. No. 5182-0864 Calibration Preparation of Calibration Blend

21、Quantitative results are based on the injection of repeatable volumes of both the calibration blend and the sample. Absolute response factors, derived from the calibration blend, are used to relate the peak areas of each known component to mg/kg. 1. Prepare a stock solution as described in ASTM Meth

22、od D4307 to contain approximately 1.5 mass-% n-octane in benzene. Thoroughly mix the solution by shaking. Record all weights to the nearest 0.1 mg. Obtain the purest benzene possible to prepare the blend. Analyze it, looking for impurities that elute at the n-octane site. If impurities in the benzen

23、e are present at this site, the concentration must be accounted for in the calculation of the respective concentrations of the octane in the blend. This blend will be used as the stock solution in the preparation of the actual calibration blend. Label this mixture as the stock solution. 2. Prepare t

24、he calibration blend to contain approximately 1.5 mass-% of the stock solution in benzene. Thoroughly mix the calibration blend by shaking. Record all weights to the nearest 0.1 mg. If refrigerated, the stock solution and calibration blend should remain stable for five months. The benzene solutions

25、may freeze in the refrigerator. If frozen, thaw at room temperature; do not heat. 3. Calculate the concentration of n-octane in the calibration blend to the nearest mg/kg using Equation 1. Using the above dilutions, the resulting calibration blend should contain approximately 225 mg/kg of n-octane.

26、EDC BA10M6+= (1) where: A = mass of n-octane in the stock solution, g B = mass of stock solution in the calibration blend, g C = total mass of the stock solution prepared, g D = total mass of the calibration blend prepared, g E = concentration of n-octane, if any, in the benzene as analyzed as descr

27、ibed in Appendix B; see first bullet under Step 1 above, mg/kg M = concentration of n-octane in the calibration blend, mg/kg 4 of 11 555-10 106 = factor to convert to mg/kg 4. Analyze the calibration blend in triplicate as described under Chromatographic Technique. The peak areas from each of the tr

28、iplicate runs should not deviate from the average by more than 3% (relative) of the value. If greater deviations occur, make certain that there are no problems with the equipment and then make additional runs until the required repeatability is obtained on three consecutive runs. Confirm the stabili

29、ty of the chromatographic system by analyzing the calibration blend again at the end of a series of analyses. If the results differ by more than 5% from the average of the triplicate runs, a problem has developed with the chromatographic system, and the series of samples must be rerun after the prob

30、lem is resolved. Typical problems to look for include a leaky septum and a dirty or partially plugged syringe. 5. Use the average peak areas to calculate the absolute response factor for n-octane, to three significant figures, using Equation 2. PML = (2) where: L = absolute response factor for n-oct

31、ane M = concentration of n-octane in the calibration blend, from Equation 1, mg/kg P = average peak area for n-octane in the calibration blend 6. Determine the response factor daily or each time analyses are performed. Procedure The analyst is expected to be familiar with general laboratory practice

32、s, the technique of gas chromatography, and the equipment being used. Chromatographic Technique 1. Install the gas purifier in the supply line between the carrier gas source and the carrier gas inlets on the gas chromatograph. Column life is significantly reduced if the gas purifier is not used. 2.

33、Install the fused silica capillary column in the gas chromatograph according to the column and gas chromatograph manufacturers instructions. CAUTION: Hydrogen leakage into the confined volume of the column oven can cause a violent explosion. Therefore, it is mandatory to check for leaks each time a

34、connection is made and periodically thereafter. 3. Establish the recommended operating conditions as given in Table 1. Different conditions may be used provided they produce the required sensitivity and chromatographic separations equivalent to those shown in the Typical Chromatogram (Figures 1-3).

35、4. Program the column oven to 250C (see Table 1) and maintain this temperature until a stable baseline has been obtained at the required sensitivity. 5. Cool the column oven to a stabilized 32C. 6. Mix the sample by shaking. Fill an autosampler vial with an aliquot of the sample and place in the aut

36、osampler (or autoinjector) tray. Multiple samples may be prepared in advance for unattended operation. 7. Inject nominally 0.5 L (repeatable) of sample into the gas chromatograph and start the data system and the column oven programming sequence. 5 of 11 555-10 The use of an autoinjector or autosamp

37、ler automates the injection of the sample into the GC, starts the data system, and the GC oven program simultaneously. To minimize cross contamination in trace level analyses, an injection of carbon disulfide is to be made between each sample or blend. Also use carbon disulfide in the syringe wash v

38、ial, and replace it after every series of injections. 8. Identify the components in the resultant chromatogram and determine the areas of the impurity peaks. A typical chromatogram is shown in Figure 1. Expanded regions are given in Figures 2 and 3. Unidentified impurities are summed and reported as

39、 a composite. Table 1 Recommended Operating Conditions Carrier gas hydrogen Mode constant flow Column flow rate 1.2 mL/min Head pressure 32C 138 kPa gauge (20 psig) Linear velocity 32C 36 cm/sec Split flow 50 mL/min Injection port temperature 250C Column temperature program Initial temperature 32C I

40、nitial hold time 6 min Programming rate 5C/min Final hold temperature 52C Final Time 14 min Rate 2 20C/min Final Temperature 2 250C Final hold time 2 0 min Detector flame ionization Detector temperature 250C Hydrogen flow rate* 39 mL/min Air flow rate* 450 mL/min Makeup gas nitrogen Makeup gas flow

41、rate* 45 mL/min Sample size 0.5 L, repeatable *Consult the manufacturers instrument manual for suggested flow rates. Calculations Obtain peak areas for each individual component or group of components except benzene and calculate the composition of the sample to the nearest mg/kg using Equation 3. S

42、ince the flame ionization detector does not respond equally on a mass basis to all the components determined, relative response factors are required to correct the responses of the components. It has been found that using effective carbon number factors, sometimes called theoretical factors, provide

43、s accurate quantitation. The theoretical response factors, relative to the n-octane external standard, were calculated using the effective carbon number (ECN) concept as described by Scanlon and Willis. These factors are listed in Table 2 and are used in Equation 3. Component, mg/kg = S L F (3) wher

44、e: 6 of 11 555-10 F = response factor for component, relative to n-octane L = absolute response factor for n-octane, previously defined, Equation 2 S = peak area of individual component or group of components Table 2 Theoretical Response Factors, Relative to n-Octane, Mass Basis All components excep

45、t as listed below (all non-aromatics) 1.003 Toluene 0.922 Ethylbenzene, m-xylene, p-xylene, and o-xylene 0.929 Report each measured component or group of components to the nearest mg/kg. Note The external standard method of quantitation is preferred for best efficiency when analyzing multiple sample

46、s. It does, however, require the use of an autoinjector (or autosampler) for best precision. If an autoinjector is not available or if only one or two samples are to be analyzed, the internal standard technique may be a suitable alternative. In this technique, the peak areas for the impurity compone

47、nts are compared to the peak area for a known amount of internal standard weighed into each sample. The procedure for using the internal standard technique is described in the Appendix of this method. The external standard technique will be the referee in case of dispute. Precision Precision stateme

48、nts were determined using UOP Method 999, “Precision Statements in UOP Methods,” from precision data obtained using an autosampler. Repeatability and Site Precision A nested design was carried out for determining impurities in benzene by two analysts, with each analyst performing analyses on two sep

49、arate days, performing three analyses each day for a total of 12 analyses. Using a stepwise analysis of variance procedure, the within-day estimated standard deviations (esd) were calculated at the concentration means listed in Table 3. Two analyses performed in one laboratory by the same analyst on the same day should not differ by more than the repeatability allowable differences shown in Table 3 with 95% confidence. Two analyses performed in one laboratory by different analysts on different days should not differ by more than the site pr