1、BASIC NITROGEN IN LPG BY ION CHROMATOGRAPHYUOP Method 939-96SCOPEThis method is for determining basic nitrogen in liquefied petroleum gas (LPG). The method determinesbasic nitrogen species that are extractable by aqueous acid, including ammonia and alkyl amines, whichtypically adsorb on the stainles
2、s steel surface of a LPG cylinder. This method is limited to ammonia,primary amines containing four or less carbon atoms, secondary amines containing eight or less carbonatoms and tertiary amines with nine or less carbon atoms. All linear aliphatic substituents of the amineanalytes must contain less
3、 than five carbon atoms. Combined nitrogen can be determined using UOPMethod 936. Nitriles are not detected by this method. Amides and/or ionic sodium, if present at aconcentration of 10 mass-ppm or greater, will interfere with the quantitation of ammonia. Potassium ions inthe acid extract can also
4、interfere with the detection of methylamine and ethylamine. The procedure yieldsquantitative basic nitrogen results in the range of 0.1 mass-ppm to 3 mass-%.OUTLINE OF METHODNitrogen compounds are extracted from an LPG sample using a solution of acetic acid in water accordingto the procedure specifi
5、ed in UOP Method 936. The sample extract is diluted and injected into an ionchromatograph equipped with a cation exchange column, conductivity suppressor device and conductivitydetector. Analytes are separated by cation exchange. Analyte column retention is based primarily on ionicsize and charge. T
6、he column effluent enters a conductivity suppressor device, where the backgroundconductivity of the column effluent is reduced and the analytes are converted into more conductive forms.The analyte effluent from the conductivity suppressor device flows to a conductivity detector wheresolution conduct
7、ance is continuously measured. The solution conductance is proportional to ionicconcentration and the inherent mobility of each ion present in the detector flow cell.An external standard method of quantitation is used. The best quantitation is achieved by matchingabsolute calibration blends to known
8、 sample analyte composition. If the analyte composition of a sample isunknown, representative response factors are determined by analyzing calibration blends that provide arepresentative mean detector response.IT IS THE USERS RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TODETE
9、RMINE THE APPLICABILITY OF REGULATORY LIMITATIONS PRIOR TO USE. EFFECTIVE HEALTH ANDSAFETY PRACTICES ARE TO BE FOLLOWED 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
10、DATA SHEETS (EMSDS) FOR ALL OF THE MATERIALS USED INTHIS PROCEDURE SHOULD BE REVIEWED FOR SELECTION OF THE APPROPRIATE PERSONAL PROTECTIONEQUIPMENT (PPE). COPYRIGHT 1996 UOP LLCALL RIGHTS RESERVEDUOP Methods are available through ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohoc
11、ken PA 19428-2959,United States. The Methods may be obtained through the ASTM website, www.astm.org, or by contacting Customer Service atserviceastm.org, 610.832.9555 FAX, or 610.832.9585 PHONE.2 of 12939-96APPARATUSReferences to catalog numbers and suppliers are included as a convenience to the met
12、hod user. Othersuppliers may be used.In addition to the equipment listed below, all of the acid extraction related apparatus specified in UOPMethod 936 is also required for this method.Balance, readability 0.1-mgCation MicroMembrane Suppressor, conductivity suppressor device, Dionex CMMS-II, Dionex,
13、 Cat. No.43021Cation MicroMembrane Suppressor installation kit, includes one 4-L pressurizable regenerant reservoir,one 0 to 30 psi pressure regulator, and all tubing and fittings required to install the CMMS-II, Dionex,Cat. No. 38018Chromatographic column, IonPac CS3 Analytical, dimensions 250-mm l
14、ong by 4-mm ID, Dionex, Cat.No. 37024Cylinders, graduated, borosilicate glass, 10-, 50-, 250-, 500- and 1000-mL, Fisher Scientific, Cat. Nos.08-556-A, C, E, F and G, respectivelyFilter apparatus, all glass filter, 47-mm diameter, 300-mL capacity, one-liter flask, Millipore, Cat. No.XX1504700Filters,
15、 Nylon-66, 47-mm diameter, 0.45-m pore size, Rainin Instrument, Cat. No. 38-114Flask, Erlenmeyer, 4-L Pyrex glass, Fisher Scientific, Cat. No. 10-040PFlask, filtering, 4-L Pyrex glass with tubulation, Fisher Scientific, Cat. No. 10-180HFlasks, volumetric, 2000-mL, with caps, Fisher Scientific, Cat.
16、No. 10-210-5HIntegrator, electronic, for obtaining peak areasIon chromatograph, equipped with a Dionex APM-2 analytical pump, a Dionex Universal Sampler withauxiliary sampling pump, a Dionex CDM-2 conductivity detector and a Dionex AdvancedChromatography Module CHA-2 system controllerMagnetic stir b
17、ar, 0.5-inch diameter x 3-inch length, octagonal shape, covered with Teflon fluorocarbonresin, Fisher Scientific, Cat. No. 14-511-85Mobile Phase Reservoir, 4-L pressurizable mobile phase reservoir and plastic tubing, Dionex, Cat. No.380183 of 12939-96Pasteur pipet, 146-mm length disposable borosilic
18、ate glass, Fisher Scientific, Cat. No. 13-678-20ARecorder, 10-mV full scaleRegulator, nitrogen, two-stage, Matheson Gas Products, Model 3122-580Reservoir, 4-L, propylene bottle, Dionex, Cat. No. 39164Sample vials, for dilution of acid extract, Kimble Scientific, Cat. No. 60957-1, with Quorpack black
19、phenolic caps and polyseal liners, Cat. No. 02-883-5G, all available from Fisher ScientificStir plate, magnetic stirrer with 7- x 7-inch ceramic stirring surface, Fisher Scientific, Cat. No. 11-496-79Test tubes, 16- x 100-mm borosilicate glass culture tubes with plain end, Fisher Scientific, Cat. No
20、. 14-961-29Tubing, vacuum, 1/4-inch ID (6.4-mm), Fisher Scientific, Cat. No. 14-173CVacuum pump, house vacuum can be substitutedValve, needle, Supelco, Cat. No. 2-2116Vials, 6 dramWater purification system, Barnstead NANOpure II, 3 module, equipped with Pretreatment (D0835),Ultrapure (D0809) and Org
21、anic Free (D0820) replacement cartridges, Fisher ScientificREAGENTS AND MATERIALSAll reagents shall conform to the specifications established by the Committee on Analytical Reagents ofthe American Chemical Society, when such specifications are available, unless otherwise specified.References to wate
22、r mean distilled and deionized water that is further treated with a Barnstead NANOpureII or other equivalent water purification system.References to catalog numbers and suppliers are included as a convenience to the method user. Othersuppliers may be used.In addition to the reagents listed below, th
23、e reagents specified in UOP Method 936 are also required forthis method.Acetic acid, approximately 99.7% purity, Fisher Certified, Fisher Scientific, Cat. No. A38-212Acetic acid, approximately 0.4-N. Prepare two liters by slowly adding 46 mL of concentrated acetic acidfrom a graduated cylinder to ap
24、proximately 1500 mL of water in a 2000-mL volumetric flask. After theheat of dilution dissipates, dilute to volume with water and mix thoroughly by capping the flask andinverting at least 10 times.4 of 12939-96Ammonium chloride, 99.998% pure, Aldrich Chemical, Cat. No. 25,413-4Dibutylamine, 99+% pur
25、e, Aldrich Chemical, Cat. No. 24,000-1Diethylamine, redistilled, 99.5% pure, Aldrich Chemical, Cat. No. 38,645-6Dimethylamine hydrochloride, 99% pure, Aldrich Chemical, Cat. No. 12,636-5Dipropylamine, 99+% pure, Aldrich Chemical, Cat. No. 24,008-7Hydrochloric acid, Fisher Reagent ACS HCl, Fisher Sci
26、entific, Cat. No. A144-500Hydrochloric acid, approximately 18-mM. Prepare four liters by slowly adding 6 mL of concentratedhydrochloric acid from a graduated cylinder to 4 liters of water contained in a 4-L filtering flask. Mixsolution thoroughly with a magnetic stir bar on a stir plate. Filter one
27、liter at a time through the filterapparatus, under vacuum. Combine the filtrate in the 4-L mobile phase reservoir.Nitrogen, zero gas, total hydrocarbons less than 0.5 ppm as methaneTetrabutylammonium hydroxide, 40 mass-% solution in water, Aldrich Chemical, Cat. No. 17,878-0Tetrabutylammonium hydrox
28、ide, approximately 100-mM. Prepare four liters by slowly adding 275 mL of40 mass-% solution of tetrabutylammonium hydroxide (in water) to 3725 mL of water in a 4-literErlenmeyer flask. Filter one liter at a time through the Millipore vacuum filter apparatus. Combinefiltrate into 4-liter regenerant r
29、eservoir.Water, ionically pure, 18 megohm-cm organic-free, prepared with NANOpure II water purificationsystemPROCEDURESample Extraction1. Obtain an acetic acid extract of the sample according to the procedure specified in UOP Method 936.2. Decant a portion of the acid extract from the volumetric fla
30、sk into a 6-dram sample vial and analyzethe acid extract by ion chromatography (see Sample Analysis). A single extraction will provide enough extract for determining both combined nitrogen (UOP Method 936)and basic nitrogen (this method).Chromatographic Technique1. Assemble and test all chromatograp
31、hic system components (see Fig. 1) per the manufacturersspecifications.2. Install the analytical column and suppressor device.3. Fill the mobile phase reservoir with four liters of 18-mM HCl, and pump mobile phase through thechromatographic system at a flow rate of 1.0 mL/minute.5 of 12939-964. Fill
32、 the regenerant reservoir with 100-mM tetrabutylammonium hydroxide, and establish a flow rateof approximately 3 to 4 mL/min of regenerant solution through the cation micromembrane suppressordevice. Flow is accomplished by applying 5 to 10 psig pressure of nitrogen gas to the regenerantreservoir.5. S
33、et the conductivity detector sensitivity at one micro-Siemen (S), full scale. The detector baselineshould level off at a steady flat baseline within one hour. When the system is fully equilibrated, the background conductivity should be between 3 and 10 S.Sample Analysis1. Weigh approximately 2 grams
34、 of LPG-acetic acid extract into a 6-dram vial and record the mass tothe nearest 0.1 mg.2. Add 8 grams of water to sample and record the total diluted sample mass (sample plus water) to thenearest 0.1 mg. Cap the solution and mix thoroughly by inverting 10 times. This dilution is generally sufficien
35、t to bring the sample into the calibration range of the procedure. However,if further dilution is required, use 0.04-N acetic acid as the diluent to maintain the matrix match, and modifyEq. 4 to reflect the additional dilution.3. Inject 50 L of the diluted sample solution into the ion chromatograph
36、and immediately start therecorder and integrator. Collect data on the integrator until the last analyte has fully eluted. If the sample content is unknown, collect data for a maximum of 50 minutes.4. Identify the components of interest by comparing the chromatogram obtained with chromatograms ofstan
37、dard pures that have been prepared and analyzed according to Calibration. When calibrating withRepresentative Calibration Standard Blends (Calibration Procedure 2), determine the followingSpecies Group peak areas: 1) ammonium site, 2) sum of peak areas beyond ammonium site throughdimethylamine site,
38、 3) sum of peak areas beyond dimethylamine site through diethylamine site, 4)sum of peak areas beyond diethylamine site through dipropylamine site and 5) sum of any remainingpeak areas.CalibrationCalibration factors are required, as detector response varies for each analyte. Detector response ismeas
39、ured in micro-Siemens (S) units of conductivity. It is proportional to the ionic concentration and theinherent mobility of each analyte ion type present when measured using the conductivity detector, flow cellelectrode. Calibration of the method is performed in one of two ways, dependent upon whethe
40、r or not theapproximate analyte composition of the sample is known.Calibration Procedure 1Use this procedure when the approximate analyte composition is known and the analytes of interest arechromatographically resolved.1. Prepare a Stock Calibration Blend by weighing in one sample vial, 0.1 g of ea
41、ch individual analytestandard pure, and record the mass of each analyte (A) to the nearest 0.1 mg.6 of 12939-962. Dissolve the standard pures in approximately 10 g of 0.4-N acetic acid, recording the total mass (B) ofthis Stock Calibration Blend to the nearest 0.1 mg. Cap and mix the Stock Calibrati
42、on Blendthoroughly, to ensure that all components are brought into solution.3. Determine the mass-ppm nitrogen for each analyte in the Stock Calibration Blend, to three significantfigures, using Eq. 1.610 ADE=100B(1)where:A = mass of each analyte, gB = total mass of Stock Calibration Blend, gD = % n
43、itrogen in each analyte (see NOTE)E = mass-ppm nitrogen for each analyte in the Stock Calibration Blend100 = factor to convert % to mass fraction106= factor to convert g to mass-ppm (g)4. Perform serial dilutions by mass (sample diluent: 0.4-N acetic acid) of the Stock Calibration Blend,prepared in
44、Step 2, to prepare an Absolute Calibration Blend Standard with known analyte basicnitrogen concentrations ranging between 0.5 to 1.5 mass-ppm. Mix each serial dilution thoroughly. For best precision, serial dilutions must be performed at a dilution ratio of 1:100 or less.5. Determine the basic nitro
45、gen concentration for each analyte in each serial dilution, to three significantfigures, using Eq. 2. The final serial dilution standard is named the Absolute Calibration BlendStandard.EGF=H(2)where:E = mass-ppm nitrogen for each analyte in the Stock Calibration BlendF = mass-ppm nitrogen for each a
46、nalyte in the Absolute Calibration Blend Standard(Calibration Procedure 1) or in the Representative Calibration Blend Standard(Calibration Procedure 2)G = mass of Stock Calibration Blend used, gH = total mass of Absolute Calibration Blend Standard (Calibration Procedure 1) orRepresentative Calibrati
47、on Blend Standard (Calibration Procedure 2), g6. Inject 50 L of Absolute Calibration Blend Standard into the ion chromatograph, that is operatingaccording to the procedure in Chromatographic Technique, and immediately start the integrator andchart recorder. Collect data on the integrator until the l
48、ast analyte has eluted.7 of 12939-967. Identify the analytes on the resulting chromatogram and determine their individual peak areas. Analytes typically elute in the order of their ionic radius. For example, tertiary amines will elute beforesecondary amines of the same carbon number. Watch for inter
49、fering sodium and potassium peaks thatmay be detected. Correct identification of peaks must be confirmed by preparing individual analytestandards and analyzing each separately.8. Determine the analyte response factor, to three significant figures, for each compound in the AbsoluteCalibration Blend Standard using Eq. 3.FJ=K(3)where:F = previously defined, Eq. 2J = response factor for each analyteK = peak area of analyte, from chromatogram9. Analyze the Absolute Calibration Blend Standard in duplicate each day a sample analysis isperformed.10. Average the Response Factors obtain
