UOP 931-2010 Trace Impurities in Mixed Xylenes by GC.pdf

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4、2.9585 PHONE. Trace Impurities in Mixed Xylenes by GC UOP Method 931-10 Scope This method is for determining trace impurities in high-purity mixed xylenes by gas chromatography (GC). Specific trace impurities determined include non-aromatic hydrocarbons, benzene, toluene and individual C9 and C10 ar

5、omatic compounds. C10 or higher non-aromatics, if present, may interfere with the determination of benzene, but it can be determined by UOP Method 543. The lower limit of quantitation for any single component is 1 mg/kg (mass-ppm). Impurities at concentrations above 500 mg/kg should be determined by

6、 UOP Method 744. References ASTM D4307, “Preparation of Liquid Blends for use as Analytical Standards,” www.astm.org UOP Method 543, “Non-aromatic Hydrocarbons in High-Purity Aromatics by Gas Chromatography,” www.astm.org UOP Method 744, “Aromatics in Hydrocarbons by Gas Chromatography,” www.astm.or

7、g UOP Method 999, “Precision Statements in UOP Methods,” www.astm.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 poly(ethylene glycol), and a flame ionization detector. The concentrat

8、ions of individual or group impurities are determined by the external standard (ESTD) method of quantitation, wherein peak areas of the sample components are compared to the peak areas of a calibration blend analyzed under identical conditions and injection volumes. See Note for alternative internal

9、 standard (ISTD) calibration and calculation techniques. Definition Mixed xylenes. For purposes of this method. mixed xylenes are defined to include ethylbenzene in addition to p-, m-, and o-xylene. 2 of 12 931-10 Apparatus References to catalog numbers and suppliers are included as a convenience to

10、 the method user. Other suppliers may be used. Balance, analytical, readable to 0.0001 g Chromatographic column, 60 m of 0.32-mm ID fused silica capillary, internally coated to a film thickness of 0.50 m with cross-linked poly(ethylene glycol), Restek, Cat. No. 10642 Gas chromatograph, temperature p

11、rogrammable, 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 response of 10 times the background noise for 1 mg/kg benzene when operated at the recom

12、mended 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 a capillary column can be accurately measured. Agilent Technologies, ChemStation. L

13、eak 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-Gas, Model 3122-590 Regulator, hydrogen, two-stage, high purity, delivery pressure range 30-7

14、00 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 syringe or injector capable of injecting a repeatable 0.5-L volume of sample. The use of an automati

15、c injection device is required to achieve necessary repeatable injection volumes. See Note and Appendix. Agilent Technologies, Model 7683. Reagents and Materials References to catalog numbers and suppliers are included as a convenience to the method user. Other suppliers may be used. Air, zero gas,

16、total hydrocarbons less than 2.0 ppm as methane, local supply Benzene, 99.9% minimum purity, Sigma-Aldrich, Cat. No. 270709 Carbon disulfide, low organic impurity, VWR, Cat. No. AA40910-AP 2-Ethyltoluene(1-methyl-2-ethylbenzene), 99% minimum purity, Sigma-Aldrich, Cat. No. E49401 Gas purifier, for h

17、ydrogen, to remove oxygen and moisture from carrier gas, VICI Mat/Sen, Cat. No. P200-1 n-Heptane, 99% minimum purity, Sigma-Aldrich Chemical, Cat. No. H219-8 Hydrogen, zero gas, 99.99% minimum purity, total hydrocarbons less than 0.5 ppm as methane, local supply Nitrogen, zero gas, 99.99% minimum pu

18、rity, total hydrocarbons less than 0.5 ppm as methane, local supply 3 of 12 931-10 Pipets, disposable, Pasteur, VWR, Cat. No. 14673-043 Pipet bulbs, VWR, Cat. No. 15001-362 Syringe, replacement, for recommended sample injector, 5-L, Agilent Technologies, Cat. No. 5181-1273 Toluene, 99.9% minimum pur

19、ity, Sigma-Aldrich, Cat. No. E49401 n-Undecane, 99% minimum purity, Sigma-Aldrich, Cat. No. U407 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 Calibra

20、tion Blend 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 i

21、n ASTM Method D4307 to contain approximately 1.5 mass-% each of benzene, toluene, n-undecane and 1-methyl-2-ethylbenzene in n-heptane. Thoroughly mix the solution by shaking. Record all weights to the nearest 0.1 mg. Obtain the purest n-heptane possible to prepare the blend. Analyze it, looking for

22、impurities that elute at the benzene, toluene, n-undecane and 1-methyl-2-ethylbenzene sites. If impurities in the heptane are present at any of the sites, their concentrations must be accounted for in the calculation of the respective concentrations of the named components in the blend. Analyze the

23、n-heptane, if needed, using the technique in the Appendix of this method and determine the concentrations of benzene, toluene, n-undecane and 1-methyl-2-ethylbenzene in the n-heptane. This blend will be used as the stock solution in the preparation of the actual calibration blend. Label this mixture

24、 as the stock solution. 2. Prepare the calibration blend to contain approximately 1.5 mass-% of the stock solution in heptane. 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

25、 several months. 3. Calculate the concentration of benzene, toluene, n-undecane and 1-methyl-2-ethylbenzene in the calibration blend to the nearest mg/kg using Equation 1. Using the above dilutions, the resulting calibration blend should contain approximately 230 mg/kg of each added component. ECDAB

26、10M6+= (1) where: A = mass of benzene, toluene, n-undecane, or 1-methyl-2-ethylbenzene 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 benzene, to

27、luene, n-undecane or 1-methyl-2-ethylbenzene, if any, in the n-heptane as analyzed as described in Appendix A; see first bullet under Step 1 above, mg/kg 4 of 12 931-10 M = concentration of benzene, toluene, n-undecane, or 1-methyl-2-ethylbenzene in the calibration blend, mg/kg 106 = factor to conve

28、rt to mg/kg 4. Analyze the calibration blend in triplicate as described under Chromatographic Technique. The peak areas from each of the triplicate 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 w

29、ith the equipment and then make additional runs until the required repeatability is obtained on three consecutive runs. Confirm the stability 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 aver

30、age of the triplicate runs, a problem has developed with the chromatographic system, and the series of samples must be rerun after the problem 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 abs

31、olute response factor for benzene, toluene n-undecane and 1-methyl-2-ethylbenzene, to three significant figures, using Equation 2. PML = (2) where: L = absolute response factor for benzene, toluene, n-undecane, or 1-methyl-2-ethylbenzene M = concentration of benzene, toluene, n-undecane, or 1-methyl

32、2-ethylbenzene in the calibration blend, from Equation 1, mg/kg P = average peak area for benzene, toluene, n-undecane, or 1-methyl-2-ethylbenzene in the calibration blend 6. Determine the response factor daily or each time analyses are performed. 7. Use the absolute response factor for n-undecane

33、to calculate the concentration of the non-aromatics. Use the absolute response factor for 1-methyl-2-ethylbenzene to calculate the concentration of all C9, C10, and heavy aromatics. Procedure The analyst is expected to be familiar with general laboratory practices, the technique of gas chromatograph

34、y, 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. Install the fused silica capillary co

35、lumn 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 connection is made and periodically t

36、hereafter. 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 and 2). 5 of 12 931-10 4. Program the col

37、umn oven to 240C (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 50C. 6. Mix the sample by shaking. Fill an autosampler vial with an aliquot of the sample and place in the autosampler (or autoi

38、njector) tray. Multiple samples may be prepared in advance for unattended operation. Samples are stable in the vials for several hours. Table 1 Recommended Operating Conditions Carrier gas hydrogen Mode constant flow Flow rate 2.3 mL/min Head pressure 50C 68 kPa gauge (10 psig) Linear velocity 50C 3

39、8 cm/sec Split flow 50 mL/min Injection port temperature 230C Column temperature program Initial temperature 50C Initial hold time 5 min Programming rate 8C/min Final hold temperature 100C Final Time 0 min Rate 2 2C/min Final Temperature 2 120C Final hold time 2 0 min Rate 3 20C/min Final Temperatur

40、e 3 230C Final hold time 3 5 min Detector flame ionization Detector temperature 250C Hydrogen flow ratea 38 mL/min Air flow ratea 450 mL/min Makeup gas nitrogen Makeup gas flow ratea 45 mL/min Sample size 0.5 L, repeatable aConsult the manufacturers instrument manual for suggested flow rates. 7. Inj

41、ect nominally 0.5 L (repeatable) of sample into the gas chromatograph and start the data system and the column oven programming sequence. The use of an autoinjector or autosampler automates the injection of the sample into the GC, starts the data system, and the GC oven program simultaneously. To mi

42、nimize 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 vial, and replace it after every series of injections. CAUTION: Carbon disulfide is a flammable and toxic compound. Use appro

43、priate care and personal protection equipment (PPE) when handling. 8. Identify the components in the resultant chromatogram and determine the areas of the impurity peaks. 6 of 12 931-10 A typical chromatogram is shown in Figure 1. The C9 and C10 aromatics region is expanded in Figure 2. Non-aromatic

44、s are determined as a composite group that includes all unidentified components eluting before ethylbenzene. The composite also includes benzene and toluene, if present. For some samples, benzene and/or toluene may be present and clearly separated from the non-aromatic components. In this case, benz

45、ene and/or toluene may be reported separately from the non-aromatics composite. The area of the n-propylbenzene peak must be measured by performing a tangent skim on the tail of the o-xylene peak. Heavy aromatics are defined as a composite group that includes all unidentified components eluting afte

46、r o-xylene. Calculations Obtain peak areas for each individual component or group of components except ethylbenzene and xylenes and calculate the composition of the sample to the nearest mg/kg using Equation 3: Component, mg/kg = 0.795 LS (3) where: L = absolute response factor, previously defined,

47、Equation 2 S = peak area of individual component or group of components 0.795 = correction for the difference in density between the n-heptane external standard and the xylenes sample, 0.684 0.860 Report each measured component or group of components to the nearest mg/kg. 7 of 12 931-10 Note The ext

48、ernal standard method of quantitation is preferred for best efficiency when analyzing multiple samples. 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 standar

49、d technique may be a suitable alternative. In this technique, the peak areas for the impurity components 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 statements were determined using UOP Method 999, “Precision Statements in UOP Methods,” from precision data obtained using an autosampler. Rep