UOP 971-2000 TRACE NITROGEN IN LIGHT AROMATIC HYDROCARBONS BY CHEMILUMINESCENCE.pdf

1、TRACE NITROGEN IN LIGHT AROMATIC HYDROCARBONSBY CHEMILUMINESCENCEUOP Method 971-00SCOPEThis method is for determining total nitrogen in light aromatic hydrocarbons, such as benzene. It isapplicable to quantitative determination of nitrogen at a level of 30 to 400 ng/g. Samples containing morethan 40

2、0 ng/g nitrogen can be analyzed according to ASTM D 4629 or D 6069. Halogen and sulfur-containing compounds can interfere if any one or all of these elements are present in a sample inconcentration greater than 10%.OUTLINE OF METHODThe sample is injected by an autosampler into a quartz-wool filled s

3、ample boat. The volatile componentsare allowed to evaporate into an argon stream that carries them into the combustion furnace. The boat isthen moved into the furnace and the residual material is combusted in oxygen. The organic material isconverted to carbon dioxide and water. The nitrogen in the s

4、ample is converted to nitric oxide that is furtherreacted in the detector with ozone to produce nitrogen dioxide that is generated in an excited state. Theexcited state nitrogen dioxide molecules relax to the ground state, producing light (chemiluminescence),that is detected by a photomultiplier tub

5、e. The signal is proportional to the total nitrogen in the sample. Thedetector is operated at reduced pressure to decrease the probability of excited state nitrogen dioxidemolecules colliding with other molecules before emitting light, thus increasing sensitivity and signal-to-noise ratio.This proce

6、dure is not sensitive to diatomic nitrogen (N2) and has reduced sensitivity to compounds withnitrogen-nitrogen bonds that decompose to produce nitrogen gas. The relative responses of some nitrogencompounds have been determined and tabulated by Jones and Daughton.APPARATUSReferences to catalog number

7、s and suppliers are included as a convenience to the method user. Othersuppliers may be used.IT IS THE USERS RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TODETERMINE THE APPLICABILITY OF REGULATORY LIMITATIONS PRIOR TO USE. EFFECTIVE HEALTH ANDSAFETY PRACTICES ARE TO BE FOLLOW

8、ED WHEN UTILIZING THIS PROCEDURE. FAILURE TO UTILIZE THISPROCEDURE IN THE MANNER PRESCRIBED HEREIN CAN BE HAZARDOUS. MATERIAL SAFETY DATA SHEETS(MSDS) OR EXPERIMENTAL MATERIAL SAFETY DATA SHEETS (EMSDS) FOR ALL OF THE MATERIALS USED INTHIS PROCEDURE SHOULD BE REVIEWED FOR SELECTION OF THE APPROPRIAT

9、E PERSONAL PROTECTIONEQUIPMENT (PPE). COPYRIGHT 2000 UOP LLCALL RIGHTS RESERVEDUOP Methods are available through ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken PA 19428-2959,United States. The Methods may be obtained through the ASTM website, www.astm.org, or by contacting

10、 Customer Service atserviceastm.org, 610.832.9555 FAX, or 610.832.9585 PHONE2 of 8971-00Chemiluminescence analyzer, Mitsubishi TN-110 analyzer with an automatic boat controller module,Cosa Instruments. This method was developed using the Mitsubishi analyzer. The procedure foranalysis may be differen

11、t for other instruments. The Mitsubishi analyzer must be equipped with thefollowing accessories:Autosampler, Mitsubishi Cat. No. ASC-150L, Cosa InstrumentsAutosampler syringes, gas tight, 100-L, Mitsubishi Cat. No. MSSG10, Cosa InstrumentsAutosampler vials, rinse, Mitsubishi Cat. No. TX3LSW, Cosa In

12、strumentsAutosampler vials, sample, Mitsubishi Cat. No. TX3LSV, Cosa InstrumentsVacuum pump, 50 L/min minimum flow capacity, Sinku Kiko GLS-050 (Mitsubishi), CosaInstruments. See NOTE 1.Filter capsule, particulate, HEPA-Cap 36, glass fiber, Fisher Scientific, Cat. No. 09-744-12. Connectthis filter b

13、efore the membrane drier.Flasks, volumetric, glass, Class A, 25-, 50 (two required), and 100-mL, Fisher Scientific, Cat. Nos.10-210A, B and C, respectivelyMembrane drier, Perma Pure, MD-110-12F. See NOTE 2.Regulator, air, stainless steel, two-stage, high purity, Matheson Gas Products, Model 3810-590

14、Regulator, argon, stainless steel, two-stage, high purity, Matheson Gas Products, Model 3810-580Regulator, oxygen, stainless steel, two-stage, high purity, Matheson Gas Products, Model 3810-540Syringes, gas tight, 50 and 100 L, Fisher Scientific, Cat. Nos. 14-824-30 and 13-684-100. Thesyringe used f

15、or pure pyridine should not be used for low nitrogen samples without first beingthoroughly cleaned.REAGENTS AND MATERIALSReferences to catalog numbers and suppliers are included as a convenience to the method user. Othersuppliers may be used.Air, compressed, dry, oil-free air for Perma Pure membrane

16、 drierAluminum foil, optional, see NOTE 3, local supplyArgon, high purity, 99.98+%, Matheson Gas ProductsBenzene, low nitrogen, as an alternative to isooctane, local supplyCatalyst, Mitsubishi Cat. No. TN5CAT, Cosa InstrumentsIsooctane, should be as low in residual nitrogen as possible to minimize t

17、he blank value, Pesticide grade,Fisher Scientific, O297-4. If available, benzene containing very low concentrations of nitrogencompounds could be substituted. The blank value must be determined for each new bottle of solventused for making standards.3 of 8971-00Oxygen, high purity, 99.7+%, Matheson

18、Gas ProductsPyridine, 99+%, Aldrich, Cat. No. 36,057-0Quartz Wool, Mitsubishi Cat. No. TNQWL, Cosa InstrumentsToluene, Fisher Scientific, Cat. No. T324-4PROCEDUREPreparation Of Standard SolutionsPyridine stock solution, 100 g N/mL1. Dispense 58 L of pyridine using a microliter syringe into a clean,

19、dry 100 mL volumetric flask.2. Dilute to the mark with toluene. Cap and mix well.The identity below shows the calculation of the concentration of nitrogen in the stock solution:()( )( )( )()()=58 0.978 14.01 1000100gN/mL79.10 100where:0.978 = density of pyridine at 20C, mg/L14.01 = molecular mass of

20、 nitrogen, g/mol58 = volume of pyridine, L100 = dilution volume, mL79.10 = molecular mass of pyridine, g/mol1000 = factor to convert mg to gCalibration standard solution, 50 ng N/mL1. Dispense 25 L of the pyridine stock solution with a microliter syringe into a clean, dry 50-mLvolumetric flask.2. Di

21、lute to the mark with a low nitrogen solvent, either isooctane or benzene. Cap the flask and mixwell. The actual concentration of this standard will be 1/2000thof the pyridine stock solution.Calibration standard solution, 100 ng N/mL1. Dispense 50 L of the pyridine stock solution with a microliter s

22、yringe into a clean, dry 50-mLvolumetric flask.2. Dilute to the mark with the low nitrogen solvent used above. Cap the flask and mix well. The actual concentration of this standard will be 1/1000thof the pyridine stock solution.4 of 8971-00Calibration standard solution, 200 ng N/mL1. Dispense 50 L o

23、f the pyridine stock solution with a microliter syringe into a clean, dry 25-mLvolumetric flask.2. Dilute to the mark with a low nitrogen solvent used above. Cap the flask and mix well. The actual concentration of this standard will be 1/500th of the pyridine stock solution.The stock solution can be

24、 retained, if refrigerated, for up to three months. The standard solutions can beretained, if refrigerated, for one week. Do not refrigerate stock and standard solutions if prepared in benzeneas the benzene will freeze, causing erratic results.SamplingSome organic nitrogen compounds will adsorb on t

25、he walls of metal sample containers. It is, therefore,recommended that analysis by this procedure be available in the refinery where sampling takes place. Ifsamples must be shipped to another site for analysis, use pressure-tight glass containers and the fastest formof transport available. Analyze s

26、amples as soon as possible after sampling.Instrument Set Up1. Set up the instrument according to the manufacturers instructions. Connect the filter capsule andmembrane drier in series in the line from the combustion tube to the detector. Allow the instrument towarm-up and the baseline to stabilize b

27、efore injecting samples. Suggested Operating Conditions forthe Mitsubishi TN-110 analyzer are listed in Table 1.Table 1Suggested Operating Conditions for Mitsubishi TN-110:Furnace inlet temperature 600CCatalyst bed temperature 900CArgon carrier 170 mL/minOxygen 110 mL/minOzone 190 mL/minIntegrate de

28、lay 0 secSlope start 200Slope end 20End thrsh 10 %Normal end OffTimer end 325 secBase correct OffMinimum area 5000Base line 25 %Interval end 350 sec2. Disconnect the tubing from the combustion tube to the detector to prevent carbon build-up inmembrane drier, tubing, and detector during the burn prof

29、iling steps.5 of 8971-003. Determine proper settings for automatic boat controller programs for 40 L injection of calibrationstandard solvent and for 80 L injections of each type of hydrocarbon sample (benzene, toluene,ethylbenzene, etc.). See NOTES 4 and 5. Different hydrocarbons require different

30、temperatures in order to vaporize at a controlled rate and burncompletely. The programs need to be set the first time the instrument is run. Future runs can use the same settings aslong as the instrument parameters (flow rates, temperatures, etc.) are not changed.4. Reconnect the tubing from combust

31、ion tube to the detector.CalibrationCalibrate daily when in use.1. Fill autosampler vials, one each, with the solvent used to make the standard solutions (solvent blank)and the 50, 100, and 200 ng/mL nitrogen calibration standards.2. Cycle the empty boat in and out of the combustion furnace to remov

32、e any residual contamination.3. Run a calibration program with multiple 40 L injections of the solvent blank and each standard. It isrecommended to inject each standard or blank at least 5 times. The injections contain 0, 2, 4 and 8 ngof added nitrogen. Relative standard deviation (RSD), as calculat

33、ed by the instrument for the blank, should be within 25%.The RSD for the standards should be within 15%. Smaller, 40 L injections are used for standards to minimize residual nitrogen from the solvent duringcalibration.4. Create a regression line using the instrument software using the blank and the

34、three standards. Set theregression line to “y=bx” instead of “y=bx+c”. The calibration corrects for the residual nitrogen content of the solvent used to make the standards(solvent blank, often greater than 50 ng/g) by generating the regression line as a standard addition to thesolvent blank to calib

35、rate the sensitivity to added nitrogen. The slope generated is used to create acalibration line starting at the origin, i.e., zero signal for zero nitrogen.Sample Analysis1. Enter the sample parameters into the software to create a sample method. Set injection volume to 80L. Each sample should be in

36、jected at least four times. More than five injections are recommended for samples expected to be below 100 ng/g. The 80 L injection volume is necessary for the required sensitivity. Do not inject less than 80 L forsamples containing less than 200 ng/g.2. Cycle the empty boat in and out of the combus

37、tion furnace to remove any residual contamination.3. Fill the autosampler vials with the samples and place the vials onto the autosampler rack.4. Inject and analyze the samples using the automatic boat controller parameters optimized for 80 Linjections. If the detector becomes contaminated (trace of

38、f scale), continue to inject blanks until the responsestabilizes.6 of 8971-00CALCULATIONSAll calculations are performed by the software, and results are displayed and printed in ppb (ng/g) or ppm(g/g) as appropriate. The density is input during sample data entry and is used by the instrument to conv

39、ertresults from wt/vol to wt/wt.REPORTReport mass-ppb (ng/g) results to the nearest unit.NOTES AND PRECAUTIONS1. The vacuum pump is required to reduce the pressure in the detector to improve sensitivity, and also todraw the combustion product gas into the detector. This requires a pump capable of ha

40、ndling highflow rates. The pump specified provides for a minimum of 50 L/min capacity (60 L/min for operationwith 60 Hz power). Pump stability is very important as pressure fluctuations will strongly affectsensitivity.2. The Perma Pure membrane drier is used to remove the water produced during combu

41、stion. If notremoved, the water vapor would enter the detector and result in a reduction of the chemiluminescenceintensity. The membrane drier consists of a thin walled NafionTMtube within a larger plastic tube. Thecombustion product gas flows through the Nafion tube. Dry air or nitrogen is flowed c

42、ounter-currentthrough the outer plastic tube. The Nafion membrane allows water to pass through, and be carriedaway by the air stream on the other side. The membrane drier was determined to be superior to thephosphoric acid scrubber to remove water without reducing sensitivity.3. Since this method is

43、 for use with very low levels of nitrogen, care must be taken to preventcontamination of the sample. The containers used for the samples and standards must not contaminatethe solutions. The autosampler vial septum could also be a source of nitrogen contamination. Fill anautosampler vial with isoocta

44、ne and cap. Analyze immediately from this container, and again, severalhours thereafter. Any increase in nitrogen level is an indication of contamination from the container.If all available septa cause a high level of nitrogen contamination, use aluminum foil in place of theseptum. The samples shoul

45、d not be left uncovered any longer than necessary. This prevents loss ofvolatile components and/or entry of contaminants.4. The sample must not burn too quickly. Incomplete combustion will result in carbon monoxide, andeven particulate carbon, entering the detector and causing a positive interferenc

46、e. The sample shouldevaporate and burn completely at least 10 seconds before the boat is inserted into the furnace.5. This method can be extended to non-aromatic hydrocarbons by creating an appropriate automatic boatcontroller program for the hydrocarbon matrix. The program should be checked to conf

47、irm properevaporation and combustion before it is used for analysis.6. An instrument using a constant rate injector can be substituted for the autosampler and boat drivesystem. Follow the manufacturers directions for recommended conditions. With direct injectioninstruments, it is recommended that th

48、e same injection volume be used for both the standards and thesamples.7 of 8971-00PRECISIONThe precision statements were determined using UOP Method 999.ASTM RepeatabilityA nested design was carried out for determining nitrogen in benzene with two analysts in one laboratory.Each analyst carried out

49、tests at two concentrations on two separate days, performing four tests each day.The total number of tests at each concentration was 16. Using a stepwise analysis of variance procedure, thewithin-day estimated standard deviation (esd) was calculated for each concentration and is listed in Table 2.Two tests performed by the same analyst on the same day should not differ by more than the ASTMallowable difference shown in Table 2 with 95% confidence.UOP RepeatabilityA nested design was carried out for determining nitrogen in benzene with two analysts in one l