UOP 720-2008 Impurities in and Purity of High Purity p-Xylene by GC.pdf

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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

2、ED HEREIN CAN BE HAZARDOUS. MATERIAL SAFETY DATA SHEETS (MSDS) OR EXPERIMENTAL MATERIAL SAFETY DATA SHEETS (EMSDS) FOR ALL OF THE MATERIALS USED IN THIS PROCEDURE SHOULD BE REVIEWED FOR SELECTION OF THE APPROPRIATE PERSONAL PROTECTION EQUIPMENT (PPE). COPYRIGHT 1972, 1974, 1987, 1993, 2008 UOP LLC.

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. Impurities in, and Purity of, High Purity p-Xylene by GC UOP Method 720-08 Scope This method is for determining trace hydrocarbon impurities in high purity p-xylene. The lower limit of detection for any single component is 0.002 mass-%. The method can also be used for

5、 determining the total purity of the p-xylene. Mixed xylene isomers and/or diethylbenzenes in other process streams should be determined using UOP Method 744, “Aromatics in Hydrocarbons by Gas Chromatography.” References ASTM Practice D4307, “Preparation of Liquid Blends for Use as Analytical Standa

6、rds,” www.astm.org UOP Method 744, “Aromatics in Hydrocarbons by Gas Chromatography,” www.astm.org UOP Method 999, “Precision Statements in UOP Methods,” www.astm.org Outline of Method The sample to be analyzed is injected into a gas chromatograph (GC) that is equipped with a fused silica capillary

7、column, a capillary injection port, and a flame ionization detector (FID). The concentrations of individual or group impurities are determined by the external standard method of quantitation, wherein peak areas of the sample components are compared to the peak areas of a calibration blend analyzed u

8、nder identical conditions and injection volumes. The concentration of the major component is then determined by subtracting the total impurities from 100%. See Note 1 for alternative means of calibration and calculation. Apparatus References to catalog numbers and suppliers are included as a conveni

9、ence to the method user. Other suppliers may be used. Balance, readability 0.1-mg Bottle, 120-mL, clear, round, glass, with TFE lined closure, VWR, Cat. No. 16151-286 2 of 15 720-08 Chromatographic column, 60 m of 0.32-mm ID fused silica capillary, internally coated to a film thickness of 0.50 m wit

10、h cross-linked Carbowax 20M, Supelco, Cat. No. 2-4085 (Supelcowax 10). See Note 2. Gas chromatograph, temperature programmable, built for capillary column chromatography, utilizing a split injection system having a glass injection port insert and equipped with an FID that will give a minimum peak he

11、ight response of five times the background noise for 0.002 mass-% o-xylene when operated under 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 r

12、esulting from use of a capillary column can be accurately measured. Agilent Technologies, ChemStation Refrigerator/Freezer, explosion-proof or flammable storage, VWR, Cat. No. 55700-340 Regulator, air, two-stage, high purity, delivery pressure range 30-700 kPa (4-100 psi), Matheson Tri-Gas, Model 31

13、22-590 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 syringe or injector

14、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 Appendix. Agilent Technologies, Model 7683. Reagents and Materials References to catalog numbers and suppliers are included as a conve

15、nience to the method user. Other suppliers may be used. Air, total hydrocarbons less than 2.0 ppm as methane (zero gas) Carbon disulfide, low organic impurity, VWR, Cat. No. AA40910-AP p-Diethylbenzene, 99.9% minimum purity, Chemsampco, Cat. No. 1135.72-1. See Note 3. Ethylbenzene, 99.9% minimum pur

16、ity, Chemsampco, Cat. No. 2379.03-1 Gas purifier, for hydrogen, to remove oxygen and moisture from carrier gas, VICI Mat/Sen, Cat. No. P200-1 Hydrogen, 99.95% minimum purity, total hydrocarbons less than 0.5 ppm as methane (zero gas) Nitrogen, 99.99% minimum purity, total hydrocarbons less than 0.5

17、ppm as methane (zero gas) n-Nonane, 99.9% minimum purity, Chemsampco, Cat. No. 6780.00-1 Pipet bulbs, VWR, Cat. No. 15001-362 Pipets, disposable, Pasteur, VWR, Cat. No. 14673-043 Syringe, replacement, for recommended sample injector, 5-L, Agilent Technologies, Cat. No. 5181-1273 Vials, autosampler,

18、for recommended sample injector, Agilent Technologies, Cat. No. 5182-0864 m-Xylene, 99.9% minimum purity, Chemsampco, Cat. No. 1240.12-1 3 of 15 720-08 o-Xylene, 99.9% minimum purity, Chemsampco, Cat. No. 1240.02-1 p-Xylene, 99% minimum purity, Chemsampco, Cat. No. 1240.21. See Purification of p-Xyl

19、ene. Calibration Purification of p-Xylene p-Xylene containing no other measurable C8aromatic impurities must be prepared to be used in a calibration blend. Prepare the purified p-xylene as follows. 1. Cool approximately 500 mL of commercial p-xylene in an explosion-proof freezer at 10 5C until appro

20、ximately 1/2 to 3/4 of the p-xylene crystallizes. This requires about 5 hours. 2. Remove the p-xylene from the freezer and decant and discard the liquid portion. 3. Allow the frozen p-xylene to thaw, and then repeat the above crystallization step on the remaining p-xylene several times until no ethy

21、lbenzene, m- and o-xylene are detected in the p-xylene, as indicated by gas chromatographic analysis (refer to Chromatographic Technique). See Note 3. Typically 4 or 5 recrystallizations are required. Preparation of Calibration Blend Quantitative results are based on the injection of repeatable volu

22、mes of both sample and calibration blend. Absolute response factors, derived from the calibration blend, are used to relate the peak areas of each known component to mass-ppm. 1. Prepare a stock solution as described in ASTM Method D4307 to contain approximately 1.5 mass-% each of n-nonane, ethylben

23、zene, m-xylene, o-xylene, and p-diethylbenzene in the purified p-xylene (see Purification of p-Xylene). Thoroughly mix the solution by shaking. Record all weights to the nearest 0.1 mg. This blend will be used as the stock solution in the preparation of the actual calibration blend. Label this mixtu

24、re as the stock solution. 2. Prepare the calibration blend to contain approximately 1.5 mass-% of the stock solution in purified p-xylene. 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 remai

25、n stable for several months. 3. Calculate the concentration of n-nonane, ethylbenzene, m-xylene, o-xylene, and p-diethylbenzene in the calibration blend to the nearest 0.0001 mass-% using Equation 1. Using the above percentages, the resulting calibration blend should contain approximately 0.023 mass

26、-% of each added component. CDAB100M = (1) where: A = mass of n-nonane, ethylbenzene, m-xylene, o-xylene, or p-diethylbenzene in the stock solution, g B = mass of base blend in the calibration blend, g C = total mass of the stock solution, g D = total mass of the calibration blend, g 4 of 15 720-08

27、M = concentration of n-nonane, ethylbenzene, m-xylene, o-xylene, or p-diethylbenzene in the calibration blend, mass-% 100 = factor to convert to percent 4. Analyze the calibration blend in triplicate as described under Chromatographic Technique. The peak areas from each of the triplicate runs should

28、 not deviate from the average by more than 3% 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 stability of the chromatographic syste

29、m by analyzing the external standard 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 problem is resolved. Typical proble

30、ms 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-nonane, ethylbenzene, m-xylene, o-xylene, and p-diethylbenzene, to three significant figures, using Equation 2. PML = (2) where: L = absolute

31、response factor for n-nonane, ethylbenzene, m-xylene, o-xylene, or p-diethylbenzene M = concentration of n-nonane, ethylbenzene, m-xylene, o-xylene, or p-diethylbenzene in the calibration blend, mass-% P = average peak area for n-nonane, ethylbenzene, m-xylene, o-xylene, or p-diethylbenzene in the c

32、alibration blend Use the absolute response factor for n-nonane to calculate the concentration of the non-aromatics, benzene, and toluene composite. Use the absolute response factor for ethylbenzene, m-xylene, and o-xylene to calculate the concentration of each of these components. Use the absolute r

33、esponse factor for p-diethylbenzene to calculate the concentration of the composite of all peaks eluting after m-xylene, excluding o-xylene. Procedure The analyst is expected to be familiar with general laboratory practices, the technique of gas chromatography, and the equipment being used. Chromato

34、graphic 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 column in the gas chromatograph according t

35、o 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 thereafter. 3. Establish the recommended o

36、perating 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 (Figure 1). 5 of 15 720-08 Table 1 Recommended Operating Conditions Carrier gas hydrogen Mode

37、 constant pressure Head pressure 35 kPa gauge (5 psig) Linear velocity 70C 17 cm/sec Equivalent flow 70C 0.86 mL/min Split flow 150 mL/min Injection port temperature 230C Column temperature program Initial temperature 70C Initial hold timea29 min Programming rate 10C/min Final hold temperature 240C

38、Final hold time 0 min Detector flame ionization Detector temperature 240C Hydrogen flow rateb30 mL/min Air flow rateb350 mL/min Makeup gas nitrogen Makeup gas flow rateb30 mL/min Sample size 0.5 L, repeatable aAdjust the initial time so that the programming rate does not begin until after the m-xyle

39、ne elutes. bConsult the manufacturers instrument manual for suggested flow rates. 4. Program the column 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 70C. 6. Mix the sample by sh

40、aking. Fill an autosampler vial with an aliquot of the sample and place in the autosampler (or autoinjector) tray. Multiple samples may be prepared in advance for unattended operation. Samples are stable in the vials for several hours. 7. Inject nominally 0.5 L of sample into the gas chromatograph a

41、nd start the integrator and the column oven programming sequence. When using an autosampler or autoinjector, the injection sequence of a GC is typically automated, performing the injection, and starting the data system and column temperature program simultaneously. To minimize cross contamination in

42、 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. 8. Identify the components in the resultant chromatogram and determine the areas of the impurity pe

43、aks. A typical chromatogram is shown in Figure 1. The area of the m-xylene peak must be measured by performing a tangent skim on the tail of the p-xylene peak. See Note 2. 6 of 15 720-08 Non-aromatics are determined as a composite group that includes all components eluting before ethylbenzene. The c

44、omposite also includes benzene and toluene, if present. See Note 4. C9+ aromatics are determined as a composite group that includes all components eluting after m-xylene excluding o-xylene. Calculations Obtain peak areas for each individual component or group of components and calculate the composit

45、ion of the sample to the nearest mass-% using Equation 3: Component, mass-% = LS (3) where: L = absolute response factor, previously defined, Equation 2 S = peak area of individual component or group of components Calculate the concentration of p-xylene in the sample by subtracting the sum of all th

46、e measured components from 100%. Report each measured component or group of components to the nearest 0.001 mass-%. Report the concentration of p-xylene to the nearest 0.01 mass-%. Notes 1. The external standard method of quantitation is preferred for best efficiency when analyzing multiple samples.

47、 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 component

48、s 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 Appendix A of this method. Newer GCs may have a wide dynamic range such that sufficient sensitivity for the small impurities can b

49、e obtained without exceeding the instruments capabilities to measure the major component. If that can be validated, see Appendix B, then the total normalization technique of quantitation may be used. The external standard technique will be the reference in case of dispute. 2. The column must be capable of separating the p- and m-xylene in a sample so that the distance from the baseline to the valley between the peaks is not more than 20% of the m-xylene peak height when the m-xylene is at a concentration of 0.005 mass-%. 3. p-Xylen