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Difference between ion chromatography and liquid chromatography

Classification from the chromatographic principle, ion chromatography is a kind of liquid chromatography, but due to some differences in structure and analysis objects between them, it is generally regarded as an independent chromatographic category. The difference between them is mainly seen from two aspects, that is, the structure of the instrument and the scope of application.

1) In terms of instrument structure, both IC and LC have solvent delivery systems, sample injection systems, detection systems, and signal recording and processing systems. However, due to the different mobile phases , the detection methods and signals are different. The processing requirements are different, and there are some differences in each component.

A. Ion chromatography generally uses an aqueous solution of acid, base and salt as the mobile phase, so usually non-metallic materials are used as the material of the entire system, and Peek plastic is generally used as the pump body, flow path and valve body and other parts that require high pressure resistance. The polytetrafluoroethylene material is used as a detector, an external pipeline, etc., and the system is required to be resistant to acid and base.However, liquid chromatography generally uses organic solvents as eluents, so most of them still use metal pumps, which can withstand any type of organic solvents, but acid or base solvents are prone to corrosion when used.

With the wide application in the biological field, metals have a certain adsorption effect on some biologically active compounds, and some liquid chromatography in biological applications also widely uses Peek materials as the pump body, flow path and valve, so that the they have a certain versatility.

B.  At present, the suppressed type ion chromatography is widely used ,which has a suppressor , while ordinary liquid chromatography does not have a similar device.

C. Another difference between them is the detector. In general, liquid chromatography uses a UV-visible photometric detector; while ion chromatography is more common with a conductivity detector.

2) Another difference between them lies in the analysis object. Generally, when liquid chromatography is used, the measured object is required to have a certain photometric absorption, so it can generally be used for the analysis of organic compounds.Ion chromatography mainly uses the characteristics of ionization of ions in aqueous solution to generate conductivity, so it is mainly used for the analysis of inorganic ions.

D.Column Differences. Due to the relationship of the eluent, ion chromatography columns are generally polymer columns, mainly styrene and divinylbenzene columns, with a higher column efficiency of about 30,000, while conventional liquid chromatography columns are more commonly used silica gel The matrix column is mainly reversed-phase, and the column efficiency is generally above 60,000.

E .gas path
Due to the different designs of different manufacturers, some instruments rely on pneumatics, and are stabilized by gas protection eluent (NaOH system), and the gas consumption is not large; while in liquid chromatography, only ELSD and MS detectors use gas sources. And it uses a lot of gas.

F mobile phase (eluent)
Ion chromatography generally uses a strong acid and strong base water system, the pH can be 0-14, and some reversed-phase organic solvents are added, and some inorganic salts are also used. The liquid chromatography is mostly organic phase, the pH is mostly between 2-8.5, there are many kinds of organic reagents, and the use of mobile phase additives is more flexible.


Precautions for starting these laboratory instruments-Liquid Chromatography

Back to the laboratory, the first thing is of course to turn on the instrument, but the instrument that has been resting for such a long time may have a “post-holiday syndrome” like you, and it might be awkward, so how to turn on your instrument correctly?

#01.Liquid Chromatography
  • Prepare the mobile phase according to the experiment, prepare the pump head cleaning solution 10% methanol aqueous solution, and prepare the autosampler needle washing solution 10% methanol aqueous solution;
  • Turn on the power switches of the pump, autosampler, column thermostat, and detector in turn;
  • Check if there are any bubbles inside the infusion tube, if there are, they should be discharged through the drain valve in time
  • Turn on the computer, double-click the SmartLab 2.0 icon on the desktop to enter the software;
  • The pump discharges bubbles (open the Purge valve and discharge the bubbles separately for each channel), remember to close the Purge valve after the bubbles are discharged;
  • After starting up, remove the cinematographic column, connect the entire pipeline, and clean the system with mobile phase for 30 minutes.
  • Connect the chromatographic column, set the ratio and flow rate of the mobile phase, remember that the flow rate setting principle is from low to high, first set the flow rate of 0.2ml/min, and run at this flow rate for more than 30 minutes, and slowly increase the flow rate until it reaches the final use Flow rate, balance system; (If buffer salts are used in the mobile phase, please rinse the flow path with 10% methanol or acetonitrile water for 15 minutes, and then change to the ratio of mobile phase analysis);
  • Set the column oven temperature
  • Cleaning the autosampler: click the injector flush button to repeatedly flush the injector
  • Turn on the deuterium lamp or tungsten lamp, set the detection wavelength, acquisition frequency and time constant; (it is recommended to turn on the pump for 5 minutes before lighting)
#02.Common precautions for liquid chromatography
  • The liquid storage bottle needs to be thoroughly cleaned before use to ensure cleanliness, otherwise it may cause system blockage
  • The pressure gradually changes (increases or decreases) for a period of time after starting up. This is often because the mobile phase has not been well balanced in the chromatographic column and the oven temperature has not been constant. These are not instrumental issues, as long as you balance it for a while, it will be stable. If you are using a gradient program, since the ratio of the mobile phase is changing, the pressure will follow the change.
  • No pressure or instantaneous pressure change after starting up (>3MPa): The reason is nothing more than the following four conditions: (1) There is air bubbles; (2) Leaking; (3) Check valve is not good; (4) Pump working phase is incorrect ; (5) The sinker is blocked, which can be eliminated one by one.

How does a conductivity detector work?

Working principle and structure of conductivity detector of ion chromatograph

The conductivity detector of the ion chromatograph is based on the conductivity of the ionic compound solution, and its conductivity is related to the nature and concentration of the ions. The conductivity detector is a general-purpose detector for ion chromatography. Generally speaking, ionic substances can be measured by the conductivity method, but the conductivity of the solution is the sum of its various ions, and the solvent itself is used for ion separation.

High conductivity will cover the conductivity of the ions in the medium to be measured, so it can only be detected when one ion conductivity is absolutely dominant

#01.The working principle of the conductivity detector
  • When a voltage is applied to the two electrodes of the conductivity cell, the anions in the solution move to the anode, and the cations move to the cathode. The number of ions in the electrolyte solution and the movement rate of ions determine the resistance of the solution. The movement rate of ions depends on the charge and size of the ions, the type of medium, the temperature of the solution, and the ion concentration. The applied voltage can be a DC voltage, a sine wave or a square wave voltage. When the applied voltage is determined, the current value in the circuit can be measured, that is, the conductance value can be measured.
#02.Conductivity detector structure
  • The conductivity detector is composed of a conductivity cell, an electronic circuit, a conversion sensitivity device and a digital display device. The conductivity cell is the core part. The basic structure of the conductivity cell is to place two electrodes in the effluent of the chromatographic column, and then measure the conductivity of the solution through an electronic circuit. The detection volume can reach microliters or even nanoscale.
#03.The characteristics of the conductivity detector
  • High sensitivity, the lower limit of detection is generally nanogram level, sometimes up to picogram level.
  • It has good selectivity and can measure extremely trace amounts of electroactive substances in a large number of non-electroactive substances.
  • Wide linear range.
  • simple structure, easy to operate automatically

*The product in the picture is IC6600

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