1、Designation: E1511 93 (Reapproved 2010)Standard Practice forTesting Conductivity Detectors Used in Liquid and IonChromatography1This standard is issued under the fixed designation E1511; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revisi
2、on, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the testing of the performance ofconductivity detectors used as the detection com
3、ponent of aliquid or ion chromatography system.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.2. Referenced Documents2.1 ASTM Standards:2E1151 Practice for Ion Chromatography Terms and Rela-tionships3. Terminology3.1 See Pr
4、actice E1151.3.2 Definitions:3.2.1 cell constantthe cell constant (K) of a conductivitycell is equal to 1/A,sok = GK.3.2.1.1 DiscussionIf the cell constant of the flow-throughcell used is equal to one, then the conductivity equals theconductance. Although the cell constant is often specified forcond
5、uctivity detectors, there is little practical value in knowingthe constant as long as the detector is properly calibrated forconductivity.3.2.2 conductancethe conductance (G) of a solution is theinverse of the resistance measured between two electrodes in acell, expressed in units of siemens (S), eq
6、ual to inverse ohms.3.2.2.1 DiscussionThe term resistance refers specificallyto the dc resistance to ionic current, independent of thecapacitive reactance at the interfaces between the electrodesand the solution.3.2.3 conductivitysince the conductance is dependent onboth the conductive properties of
7、 the solution and on thedimensions of the electrodes and the cell, the conductivity (k)of the solution is defined to be independent of electrode andcell dimensions. Specifically,k5G1A(1)where:1 = the distance between two planer disk electrodes, andA = the electrodes surface area.3.2.3.1 DiscussionIn
8、 liquid and ion chromatography, celldimensions are commonly measured in centimetres, so theunits of k are S/cm. (Alternatively, the SI units of S/m may beused. S/m = 100 S/cm.)3.2.4 driftthe average slope of the noise envelope ex-pressed in nano siemens per centimetre per hour as measuredover a peri
9、od of 1 h.3.2.5 equivalent conductivityof an ionic solute, the con-tribution of the solute to the total conductivity of the solution,measured in microsiemens per centimetre, divided by itsconcentration in milliequivalents/litre.3.2.6 flow dependence ratethe change in measured con-ductivity as a func
10、tion of flow rate.3.2.7 limiting equivalent conductivityof an ionic solute,its equivalent conductivity extrapolated to infinite dilution.3.2.8 linear rangeof a conductivity detector for a givensolute in a specific solvent, the concentration range of solutefor which the detector response factor is wi
11、thin 5 % of theresponse factor in the middle of the range as determined fromthe linearity plot specified in Section 11.3.2.8.1 DiscussionThe lower limit may be limited bynoise, and the upper limit by deviation from linearity. (Theupper limit may instead be limited by the maximum full-scaledeflection
12、 on the detectors least sensitive output range.)3.2.9 long-term noisethe maximum amplitude in nanosiemens per centimetre for all random variations of the detectoroutput of frequencies between 2 and 60 cycles per hour.3.2.9.1 DiscussionLong-term noise represents noise thatcan be mistaken for eluting
13、peaks.1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and Separation Science and is the direct responsibility of Subcom-mittee E13.19 on Separation Science.Current edition approved Nov. 1, 2010. Published November 2010. Originallyapproved in 1993. Last previou
14、s edition approved in 2005 as E1511 - 93(2005).DOI: 10.1520/E1511-93R10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe AS
15、TM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.10 minimum detectabilityof a conductivity detector,that concentration of solute in a specific solvent that corre-sponds to twice the short-term noise.3.2.10.1 DiscussionB
16、ecause of the difficulty of pumpingsolvents through the chromatographic system without anycontamination of the solvents from the system, this quantitycan only be measured with solutes retained by a column. Sinceminimum detectability is dependent on the chromatographicsystem used, it is not measured
17、in this practice. However, if theminimum detectability of a solute is measured on one systemwith one detector, the minimum detectability can be predictedwhen other detectors are tested on the same system bycomparing the measured values of short-term noise.3.2.11 response factorof a conductivity dete
18、ctor, the mea-sured conductivity response of a solute divided by the soluteconcentration.3.2.12 response time of the detectorthe time required forthe output of the detector to change from 10 to 90 % of the newequilibrium value when the composition of the eluent ischanged in a stepwise manner, within
19、 the linear range of thedetector.3.2.12.1 DiscussionAslow response time has the effect oflimiting resolution for efficient peaks such as early elutingpeaks and those from highly efficient columns or microborecolumns. Response time is generally dependent on threefactors: (a) cell volume, (b) volume o
20、f heat transfer tubingleading to the cell, and (c) electronic filtering of the output.3.2.13 sensitivitythe detector response divided by concen-tration, which is also the response factor (11.1.1).3.2.13.1 DiscussionSensitivity is therefore by definitionthe same for all properly calibrated conductivi
21、ty detectors.(Sensitivity is often confused with minimum detectability,which is dependent on both sensitivity and noise.) Therefore,the calibration of the detector should be measured, and ifnecessary, adjusted. Follow the manufacturers procedure forcalibrating the detector. The procedure in Section
22、9 is used bymany manufacturers and is useful for the tests in this practice.3.2.14 short-term noisethe maximum amplitude in nanosiemens per centimetre for all random variations of the detectoroutput of a frequency greater than one cycle per minute.3.2.14.1 DiscussionShort-term noise determines thesm
23、allest signal detectable by a conductivity detector, limits theprecision available for the determination of trace samples, andmay set the lower limit of linearity.4. Summary of Practice4.1 Four different tests are performed to characterize adetector.4.1.1 Noise and drift are measured while a solutio
24、n isflowing through the detector cell. The test is performed usingtwo different solutions: deionized water (DI) and 1 mMpotassium chloride (KCl).4.1.2 Linear range is determined by preparing a plot ofresponse factor versus the log of solute concentration usingstandard solutions of KCl and hydrochlor
25、ic acid (HCl) assolutes.4.1.3 Dependence of response on flow rate is measured bypumping 1 mM KCl through the conductivity cell at severalflow rates and measuring the detector output.4.1.4 Response time is measured by measuring the timerequired for the detector output to change from that measuredwith
26、 DI water to that measured with 1 mM KCl.5. Significance and Use5.1 This practice is intended to describe the performance ofa conductivity detector independent of the chromatographicsystem in terms that the analyst can use to predict overallsystem performance when the detector is coupled to thecolum
27、n and other chromatography system components.5.2 Although it is possible to observe each of the severalcharacteristics of a detector under different and unique condi-tions, it is the intent of this practice that a complete set ofdetector specifications should be obtained at the same operat-ing condi
28、tions, including the setup used for testing, flow rates,and temperatures. It should be noted that to specify a detectorscapability completely, its performance should be measured atseveral sets of conditions within the useful range of thedetector. The terms and tests described in this practice aresuf
29、ficiently general so that they may be used at whateverconditions may be chosen for other reasons.6. Reagents6.1 Reagent chemicals are reagent grade or better.6.1.1 Deionized Water, (DI water), 18 M-ohm.6.1.2 Potassium Chloride, (KCl) dry powder.6.1.3 Hydrochloric Acid, (HCl) standard 0.1000 N soluti
30、on.7. Preparation of Standards7.1 Potassium Chloride Standards:7.1.1 Prepare a 10-mM KCl standard stock solution. Weighout 0.7455 g KCl (desiccated) and dissolve it in 18 M-ohm DIwater in a 1-L plastic volumetric flask. Fill the flask to 1 L withDI water.7.1.2 Prepare KCl standards from the 10-mM KC
31、l standardstock solution. Using accurate Class A pipettes, pipette thevolumes of the 10-mM standard stock solution listed belowinto 100-mL plastic volumetric flasks. For the 1-mM KClstandard, fill a 100-mLplastic volumetric flask with the 10-mMKCl solution and transfer to a 1-L plastic volumetric fl
32、ask. Fillto the line with DI water.KCl Concentration,mmVolume in 100 mL DI Water,mL0.05 0.50.1 10.2 20.5 51 100 mL in 1 L2205510 mM No dilution7.2 Hydrochloric Acid Standards:7.2.1 Prepare a 2.00-mM HCl standard stock solution bydiluting 20.0 mL of standard 0.1000 N HCl into a 1-L plasticvolumetric
33、flask and filling to the line with DI water. Ifstandard 0.1000 N HCl is not available, a 0.10-mM HClsolution can be prepared by diluting 8.3 mL of 12 N (37 %)concentration HCl into 1 L of DI water. (The concentration ofthis solution will be less accurate than that prepared from0.1000 N HCl standard.
34、)E1511 93 (2010)27.2.2 Prepare the following HCl calibration standards fromthe 2.00-mM HCl standard stock solution. Use accurate ClassA pipettes and 100-mL plastic volumetric flasks.HCl Concentration,mMVolume in 100 mL DI Water,mL0.02 10.04 20.1 50.2 100.4 20152 No dilution8. Instrumentation Set-Up8
35、.1 Set up the chromatographic system according to themanufacturers recommendation. Also, passivate the conduc-tivity cell using the manufacturers recommended procedure.Set the flow rate on the pump to 1.0 mL/min or to the flow ratenormally used in your application. Fill the eluent bottle with 18M-oh
36、m DI water. Connect the outlet of the pump to theinjection valve, and the outlet of the injection valve directly tothe conductivity cell using as short a length of tubing as ispractical. (Standard 0.25-mm (0.01-in.) internal diameterHPLC tubing may be used.) Do not install any columns orsuppressors.
37、 To ensure smooth operation of the pump, it isnecessary to supply more pressure than that normally providedby the detector cell and the standard tubing alone. This isaccomplished by installing a 1-m coil of 0.25-mm internaldiameter narrow bore tubing between the pump and theinjection valve. Increase
38、 the length or decrease the diameter ofthe tubing if the pressure is not high enough to produce smoothpump operation. Generally, 500 to 1 000 psi will be sufficient.The waste line connected to the cell outlet should be ofsufficient length to provide enough backpressure on the cell toprevent the form
39、ation of bubbles inside the cell. Inserting 20cm of 0.25-mm internal diameter tubing between the cell andwaste line should provide sufficient backpressure.8.2 Install a sample injection loop of approximately 200 Lon the injection valve. This can be constructed from1mof0.5-mm (0.02-in.) internal diam
40、eter tubing. During the testsdescribed in Sections 6 and 7, observe the recorder trace andverify that a plateau is reached after injection of the standardsolutions. If no plateau is reached, then a larger sampleinjection loop is needed.8.3 If the conductivity detector has a setting for temperatureco
41、mpensation, set it to 2.0. If not, the DI water eluent and allof the test solutions should be thermostated as close as possibleto 25C. Or, the detector cell may be thermostated at a highertemperature but be calibrated as if the cell were at 25C. If thecell is thermostated, ensure that the cell tempe
42、rature hasstabilized. Refer to the manufacturers procedure for celltemperature stabilization. Turn off any output filtering on thedetector. The output from the detector should be monitored ona strip-chart recorder, integrator, or computer. The calibrationand linearity tests can be performed with a v
43、oltmeter monitor-ing the detector output or, on some detectors, the output ismonitored on the front panel readout.9. Calibration9.1 MethodThe detector is calibrated by adjusting thedetector output to 147.0 S/cm for a 1-mM solution ofpotassium chloride flowing through the conductivity cell at 1mL/min
44、.9.1.1 Set up the chromatographic system according to theinstructions in Section 8. Turn on the pump and ensure that thepump is pumping smoothly.9.1.2 Monitor the detector output. The conductivity shouldbe below 1S/cm. If it is higher, continue flushing out thesystem to remove leftover salts until t
45、he conductivity stabilizesbelow 1 S/cm. (A higher reading is an indication of either anincompletely cleaned flow system or of poor deionized waterquality and may compromise noise and linearity tests.) Fill thesample injection loop with DI water and inject. Note theminimum conductivity reported durin
46、g the elution of the DIwater through the detector cell. Either calibrate the detector tozero using the injected DI water or subtract the conductivity ofinjected DI water from all subsequent measurements.9.1.3 Fill the sample injection loop with the 1-mM KClstandard and inject the standard. Note the
47、maximum conduc-tivity reported during the elution through the detector cell ofthe 1-mM KCl standard. It should be 147.0 S/cm. If it is not,follow the manufacturers procedure for calibrating the con-ductivity detector so that the reading will be 147.0 S/cm.9.1.4 Some conductivity detectors do not rep
48、ort conductiv-ity directly in siemens, but instead provide a voltage outputproportional to conductivity. Instead of adjusting the detectoroutput, calibrate these detectors by recording the knownconductivity of the calibration solution (147.0 S/cm for 1 mMKCl), the detector sensitivity range, and the
49、 measured voltageoutput. (Be sure to subtract the voltage output for a blank of DIwater.) Divide the known conductivity by the net voltageoutput reading and multiply all subsequent voltage outputreadings by this value.10. Noise and Drift10.1 Method of MeasurementNoise and drift are mea-sured under two conditions. Pure DI water is pumped throughthe conductivity cell at 1 mL/min and the noise and driftmeasured. The procedure is then repeated using 1 mM KCl.The detector output may be sensitive to temperature changes. Itis worthwhile to perform th
