Ion chromatography instrument instructions

Ion chromatographic instruments are generally composed of mobile phase transport systems, sample introduction systems, separation systems, suppression or derivatization systems, detection systems, and data processing systems.

I. Ion chromatography mobile phase transport system Ion chromatography instrument transport system includes liquid storage tanks, high pressure infusion pumps, gradient elution devices, etc., and high-performance liquid chromatography transport system is basically similar.

1. The storage of the solvent in the reservoir is mainly used to supply a sufficient quantity and meet the requirements of the mobile phase. The requirements for the solvent reservoir are:

(1) Must have sufficient volume to ensure adequate fluid supply for repeated analyses;

(2) convenient degassing;

(3) Can withstand a certain amount of pressure;

(4) The selected material is inert to the solvents used.

Since the mobile phase of ions is usually an aqueous solution of acid, alkali, salt or complex, the liquid storage system is generally made of glass or PTFE. The volume is generally 0.5 to 4 L. The solvent must be degassed before use. . Because the column is operated with pressure, bubbles are easily released in the flow path, causing the detector noise to increase, making the baseline unstable and the instrument unable to work properly. This is even more prominent when the mobile phase contains organic solvents.

There are many methods of degassing. The following methods are used in ion chromatography:

(1) Low-pressure degassing method: The vacuum pump and vacuum pump can be used to heat or blow nitrogen at the same time. This method is especially suitable for the eluent formulated with pure water solvent.

(2) Blow helium or nitrogen degassing method: helium or nitrogen is introduced into the rinsing liquid under reduced pressure, and the air in the rinsing liquid can be discharged under a certain pressure.

(3) Ultrasonic degassing method: The rinsing agent is placed in an ultrasonic cleaning tank and ultrasonically degassed with water as a medium. General ultrasound about 30min, can achieve the purpose of degassing. The new ion chromatograph has an online degassing device on the high pressure pump to automatically degas the eluent automatically online.

2. High-pressure infusion pump The high-pressure infusion pump is an important part of the ion chromatograph. It will input the mobile phase to the separation system and allow the sample to complete the separation process in the column system. High pressure pumps for ion chromatography should have the following properties:

(1) Flow stability: The flow accuracy is usually required to be about ± 1% in order to ensure the repetition of retention time and the accuracy of qualitative and quantitative analysis.

(2) There is a certain output pressure, ion chromatography generally works at 20 MPa, slightly lower than high performance liquid chromatography.

(3) Corrosion resistance to acids, bases, and buffers. Unlike high performance liquid chromatography, all eluents in ion chromatography contain acids or bases, and pumps should be made of fully plastic Peek materials.

(4) Small pressure fluctuations, convenient replacement of solvents, small dead volume, ease of cleaning and replacement of solvents.

(5) The flow rate is optionally within a certain range and can meet certain accuracy requirements.

(6) Some infusion pumps have a gradient elution function.

At present, the reciprocating plunger pump is widely used in ion chromatography, and only the low pressure ion chromatography uses a peristaltic pump. However, the peristaltic pump can endure too little pressure and problems may arise in the actual operation.

Due to the high reciprocating frequency of the plunger of the reciprocating plunger pump, the wear resistance of the seal ring and the rigidity and accuracy of the one-way valve are all very high. The seal ring is generally made of PTFE additive material, and the ball, seat and plunger of the one-way valve are made of artificial gem material. Reciprocating pumps have single and double plungers.

In general, the double plunger flow is more stable and the pulsation is small, but the structure is complicated and the price is relatively high.

High-performance liquid chromatography using stainless steel or titanium Alloy material, compared with high-performance liquid chromatography, ion chromatography pump is a fully plastic system, thus the acid, alkali, salt anti-pollution properties, and to ensure the determination of metal ions Accuracy.

a. The single plunger pump takes some necessary measures such as adding dampers, special design of the cam shape and advanced electronic technology to obtain satisfactory results. Early ion chromatography and some simple ion chromatographs often used single plunger pump systems.

b. Double-plunger pump The double-plunger pump, which uses two pump heads in parallel, uses a 180° phase difference to reduce pressure and flow fluctuations. Therefore, the double plunger pump can directly enter the separation system without a damper, and can perform low pressure gradient elution. Most high-end ion chromatographs now use double-plunger pumps.

At present, the latest type of microporous ion chromatograph has a column diameter of 2mm, which is 1/4 of the cross section of a conventional 4mm id column. Therefore, under the condition that the line speed is constant, the flow rate is conventional. One-fourth of the chromatogram, so it uses the pump's flow rate is 0.01 ~ 2.50ml/min. Its main feature is that the plunger is relatively small, but its structure is similar to that of a conventional ion chromatograph. Due to the reduced flow rate, the amount of solvent can be greatly reduced. If the sample is injected in the same amount, the sensitivity can be increased by 4 times.

3, gradient elution device gradient elution and similar temperature increase in the gas chromatograph, chromatographic separation brings great convenience, but the ion chromatography conductivity detector is an overall nature of the detector, so gradient elution is generally only Hydroxide ion-containing eluents can only be detected using suppressed conductivity detection. Gradient elution technology can improve the degree of separation, shorten the analysis time, and reduce the detection limit. It is an extremely important means for the separation of complex mixtures, especially those with large differences in retention strength. In addition, the new suppressor removes CO2 in the eluent through degassing. The background conductivity of the carbonate eluent is very low, so that the sensitivity is greatly increased, and carbonate gradient elution can also be achieved. Ion chromatography gradient elution can be divided into two kinds: low pressure gradient and high pressure gradient, which are respectively introduced as follows:

a. Low-pressure gradient b. High-pressure gradient It is composed of two high-pressure infusion pumps, gradient program controllers, mixers and other components. The two pumps respectively input the two eluents into the mixer, and after thorough mixing, they enter the chromatographic separation system. It is also referred to as a pumped high pressure mixture.

The gradient elution solvent mixer must have the features of small volume, no dead zone, easy cleaning, high mixing efficiency, and repeated gradient elution with short lag time.

At present, Dionex's latest eluent generator EG40 can automatically generate eluent as long as pure water is added. It can control the current to achieve the eluent gradient.

Second, ion chromatography sample system Ion chromatography injection is divided into three types: namely, pneumatic, manual and automatic injection methods.

1. The manual injection valve adopts six-way valve for manual injection. Its working principle is the same as that of HPLC, but its injection volume is larger than that of HPLC, which is generally 50μl. The sample is first filled with the dosing tube in a low pressure state. When the valve is rotated clockwise to another location, the fixed volume of sample stored in the dosing tube is sent to the separation system.

2. Pneumatic sampling valve Pneumatic valve adopts certain helium or nitrogen gas pressure as power, through the two-way four-pass loading quantitative tube, sampling and injection, which effectively reduces the error caused by manual injection due to different actions. .

3. Autosampler Autosampler is a series of operations such as sampling, sample injection, and cleaning under the control of chromatographic workstation. The operator only needs to load the sample into the sampler in sequence. The working procedure for automatic disc injection is as follows:

(1) The motor drives the storage tray to rotate. The sample to be analyzed is placed directly under the sampling needle.

(2) The motor rotates forward, the screw drives the slider downward, inserts the sampling needle into the plastic cover of the sample, the slider continues to move downwards, pushes the bottle cap into the bottle, and the sample flows through the pipe under the extrusion of the bottle cap. Valve quantification tube completes sampling action.

(3) Injection valve switch to complete injection.

(4) The motor is reversed, the screw drives the slider upwards, and the sampling needle returns to its original position.

Automatic injection can achieve a wide range of sample injection volume.

III. Ion chromatography separation system Ion chromatography is a separation analysis method, so the separation system is the core and foundation of ion chromatography. The ion chromatography column is the "heart" of the ion chromatograph and requires high column efficiency, good selectivity, and fast analysis speed. Ion chromatography is a kind of liquid-solid chromatography, which is a type of high-performance liquid chromatography. However, column packing and separation mechanism have their own characteristics. The study of ion chromatography column is also a hot topic in the field of ion chromatography. The particle size of the ion chromatographic column packing is generally between 5 and 25 μm, which is slightly larger than the column packing of the high-performance liquid chromatography. Therefore, the pressure of the ion chromatography column packing is smaller than that of the high performance liquid chromatography, and is usually monodisperse and spherical.

1. High-molecular polymer filler The most widely used filler in ion chromatography is polystyrene divinylbenzene copolymer. The cation exchange column generally uses sulfonic acid or carboxylic acid functional groups, and the anion exchange column packing uses quaternary amine functional groups or tertiary amine functional groups. Ion-rejection column packings are mainly fully sulfonated polystyrene divinylbenzene copolymers. Such ion exchange resins can be used in the pH range of 0-14. If the use of high cross-linking materials to improve, but also compatible with organic solvents, to resist organic pollution. In general, the exchange capacity of ion-exchange chromatography columns is very low.

2. Silica gel type ion chromatography packing This packing is made of porous silica column packing and is a typical thin shell packing used for anion exchange chromatography. It was prepared by coating SiO2 microspheres with quaternary amine-functionalized methyl methacrylate. Cation exchange resins are prepared by coating low molecular weight sulfonated fluorocarbon polymers onto silica particles. The pH of these fillers is in the range of 4 to 8, and is generally used in single-column ion chromatography columns.

3. Column structure The general analysis column has an inner diameter of 4mm and a length of 100-250mm. The columns have two fastening screws. High-grade instruments, especially cationic columns, are generally made of PTFE to prevent metal interference. With the development of ion chromatography, the small-diameter column has attracted people's attention. The 2mm column can not only reduce the solvent consumption, but also increase the sensitivity by 4 times for the same injection volume.

IV. Suppression System for Ion Chromatography For the suppression (double column) ion chromatography system, the suppression system is an extremely important part, and it is also the most important feature of ion chromatography that is different from high performance liquid chromatography. The development of suppressors has undergone a number of developmental stages, and currently commercial ion chromatographs also employ different suppression methods and related research results.

1. Resin Packing Suppression Column This suppression system uses a high exchange capacity cation resin packed column (anion suppression), and the resin is converted to hydrogen form by sulfuric acid. It inhibits the capacity is not high, requires regular regeneration, and the dead volume is relatively large, and the weak acid ions are often unable to accurately quantify due to the effect of ion repulsion.

This type of suppressor is basically not used at present, but the United States Alttech company to improve this type of suppressor, so that the need to regenerate the packed column will change color, and the use of electrochemical regeneration, greatly improving the traditional method, improve the performance of the suppressor .

The opposite is true for cation suppression, which uses a high exchange capacity anion resin as the packed column.

2, fiber suppressor This inhibition system uses cation-exchanged hollow fiber as a suppressor, the external sulfuric acid as a regenerant, can continuously eluent regeneration, this suppressor's dead volume is relatively large, inhibit capacity is not high .

3, micro-membrane suppressor This inhibition system uses a cation exchange flat film, the middle through the eluent, while the outer side of the sulfuric acid regeneration solution. This suppressor has a relatively high exchange capacity, a small dead volume, and gradient elution.

4. Electrolysis suppressor This inhibition system uses a cation-exchanged flat membrane to regenerate the eluent by means of electrolytically generated H. Early inhibitors of this type were invented by Tian Zhaowu of Xiamen University in China and were put into production, but they need to be added with sulfuric acid regularly to supplement H. The United States Dionex company has improved this type of suppressor to make it self-regenerating, as long as it can be regenerated by eluent self-circulation or deionized water electrolysis. The suppression capacity can be controlled by changing the current size, and the dead volume is very small. small. US Alttech uses a packed column suppressor, which is regenerated by suitable electrolysis. It also has convenient and efficient functions. Its latest DSPlus type suppressor is based on chemical suppression and electrochemical regeneration, and then carries out carbon dioxide elimination. The background conductance can be effectively reduced to achieve gradient elution of different carbonates.

V. Detection System of Ion Chromatography Conductivity detection is the most commonly used type of ion chromatography detection method. Due to the effect of the equivalent capacitance in the conductivity cell, the relationship between the voltage and current applied to the cell is non-linear, which presents great difficulties in measuring the conductance value. In addition, the background conductance value in the mobile phase is very high, and accurately measuring the signal of the component to be measured from a large background value is also an important issue in conductance detection. At present, more methods are used: bipolar pulse chemical suppression conductance detection, five-electrode detection, and analog signal AC phase-locked amplification.

1. Bipolar pulse conductance detection adopts variable frequency bipolar pulsed chemical suppression conductance detection method, represented by Dionex Corporation of USA. The detection method effectively suppresses the influence of the equivalent capacitance of the conductance cell and the background phase conductance of the mobile phase, and has high measurement sensitivity, wide linear range, and good stability.

It uses the 8085 chip as the central processor (CPU), and controls other units through the processor input and output unit (PIO). After the CPU clock is divided and triggered, it generates a bipolar pulse, which is sent to the conductivity cell after shaping; the conductivity cell returns. The signal is sampled after the second pulse and converted into a DC signal. This signal and the temperature measurement signal are alternately fed into the voltage frequency converter, and the digital signal is sent to the CPU. When performing compensation, the CPU processes this signal and sends it back to the amplifier circuit through the D/A (compensation) converter to compensate the original signal until the comparison output is in the "OK" state. The output of the signal is also sent to the CPU through the V/F inverter circuit. After the CPU processes it, it passes through a D/A (output) inverter circuit and outputs it to the data processor via the driver.

2. The five-electrode conductance test adopts a five-electrode conductance measurement technology, which can effectively eliminate the influence of double-layer capacitance and electrolytic effect. The structural feature is that four electrodes are arranged in the flow path, and the constant voltage between the two measuring electrodes is maintained in the circuit design without being affected by changes in load resistance, inter-electrode resistance, and electric double layer capacitance. Therefore, the current change between the two measuring electrodes can be taken out from both ends of the load resistor for amplification and display. The fifth electrode is a shield electrode, which helps to improve the stability of the measurement. The five-electrode conductivity detector effectively eliminates the effects of polarization and electrolysis, and can still achieve extremely low noise levels with high background conductance, making it suitable for non-chemical suppression conductivity detectors.

In addition, ion chromatography can also use ultraviolet, visible light, fluorescence, ampere and other high performance liquid chromatography commonly used detectors, and its principle is similar to conventional high performance liquid chromatography detection.

VI. Data Processing System of Ion Chromatography Ion chromatography is generally not very efficient. Compared with gas chromatography and high performance liquid chromatography, the degree of ion chromatographic separation is generally not high. It does not require high data acquisition speed, so it can be used Other types of data processing systems can also be used in ion chromatography. Moreover, in the conventional ion analysis, the peak shape of the chromatogram peak is ideal, and the peak height quantitative analysis method can be used for analysis. The main data processing system is:

1. Recorder recorder requires full-scale travel time ≤ 1s, high input impedance, good shielding, and stable paper speed. With a double pen recorder, high and trace concentrations of components in the sample can be measured simultaneously, as well as dual detector analysis.

2. Automatic integrator It is an instrument that analyzes chromatographic information and prints chromatograms through A/D conversion, using a fixed program. The use of an automatic integrator greatly reduces the cumbersome procedures for chromatographic manual processing in recorders.

3, data workstation through A / D conversion, the data collected in the computer, and then through the analysis of the collected data to obtain the relevant chromatographic information. With the popularity of personal computers, data workstations will be widely used.

PeakNet, an ion chromatography data workstation from Dionex Corporation of the United States, can process multiple chromatographic information at the same time, and can control the system of ion chromatography such as pumps, detectors, and autosamplers.

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