Abstract: Analysts should have some general information about the ion to be measured. First, they should understand the molecular structure and properties of the compound to be tested and the matrix of the sample, such as inorganic or organic ions, the charge number of the ion, whether it is acid or alkali, hydrophilic or hydrophobic , Whether it is a surface-active compound, etc. The hydrophobicity and hydration energy of the ions to be tested are the main factors that determine which separation method to choose.
Ions with high hydration energy and weak hydrophobicity, such as Cl- or K, are best separated by HPIC. Ions with low hydration energy and high hydrophobicity, such as perchloric acid (ClO4-) or tetrabutylammonium, are best separated by a hydrophilic ion exchange separation column or MPIC. Ions with a certain hydrophobicity and a significant hydration energy with pKa values ​​between 1 and 7, such as acetate or propionate, are best separated by HPICE. Some ions can be separated by either anion exchange or cation exchange, such as amino acids, alkaloids and transition metals.
Many ions can be detected in multiple ways. For example, when measuring transition metals, a single column method can be used directly with conductivity or pulsed amperometric detectors, or post-column derivatization can be used to react metal ions with PAR or other color reagents, and then UV / VIS detection. The general rule is: for acids and bases without ultraviolet or visible absorption and strong dissociation, it is best to use a conductivity detector; for ions with electrochemical activity and weak dissociation, it is best to use an amperometric detector; for the ions themselves or after passing through the column The complex formed after the reaction has ions and compounds that absorb or produce fluorescence in the ultraviolet, and it is best to use a UV / VIS or fluorescence detector. If there are several options for the problem to be solved, the determination of the analysis plan is mainly determined by the type of substrate, the selectivity, the complexity of the process, and whether it is economical. Table 5-3 and Table 5-4 summarize the separation and detection methods available for various types of ions.
The development of ion chromatography column packing has promoted the rapid development of ion chromatography applications, and has provided many possibilities for the development of various ion analysis methods. In particular, the commercialization of hydrophilic, high-efficiency, high-capacity column packings in aqueous solutions of pH 0-14 and 100% organic solvents (organic solvents for reversed-phase high-performance liquid chromatography) makes the application of ion exchange separation even more expand. Common ions in the form of ions in aqueous solution, including inorganic and organic ions, with weak acid salts (Na2CO3 / NaHCO3, KOH, NaOH) or strong acids (H2SO4, methanesulfonic acid, HNO3, HCl) as the mobile phase, anions Exchange or cation exchange separation, conductivity detection, is already a mature method, there are mature chromatographic conditions can refer to. For near-neutral water-soluble organic "large" molecules (relatively common to small molecules), if the compound to be tested is a weak acid, the weak acid will exist as an anion in a strong alkaline solution, so it is stronger The base is the mobile phase and is separated by anion exchange; if the compound to be tested is a weak base, since it will exist as a cation in a strong acid solution, a stronger acid is used as the mobile phase for cation exchange separation; if the ion to be tested is hydrophobic Stronger, due to the adsorption between the stationary phase and longer retention time or peak tailing, you can add an appropriate amount of organic solvent to the mobile phase to weaken the adsorption, shorten the retention time, improve peak shape and selectivity. Ion-pair chromatography can also be used for the separation of these compounds, but the mobile phase generally contains more complex ion-pair reagents. In addition, high-capacity columns and weak eluents can be selected for weakly retained ions to enhance retention, and vice versa for strongly retained ions. Table 5-3 and Table 5-4 list the two main detectors commonly used in ion chromatography: electrochemical detectors (including conductivity and amperage) and optical detectors. Ions in the form of ions in aqueous solutions, ie strong acids or bases, should use conductivity detectors. For compounds that have absorbing groups for ultraviolet or visible light or after post-column derivatization (less pre-column derivatization is used in ICs) to generate light-absorbing groups, use optical detectors. Compounds with oxidation or reduction reaction groups under applied voltage can be detected by DC ampere or pulse ampere. For some complex samples, in order to get more information in one injection, two or three detectors can be used in series.
2 Optimization of chromatographic conditions
2.1 Improve resolution
(1) Dilute the sample
For samples with complex composition, if the affinity of the tested ions to the resin is quite different, several injections must be made and rinsed with different concentrations or strengths of eluent or gradient. If the concentration difference between the ions to be measured is large and the affinity for the stationary phase is large, the simplest way to increase the resolution is to dilute the sample or do sample preparation. For example, the separation of SO42- and Cl- in brine. If the sample is injected directly, under the commonly used chromatographic conditions for analyzing anions, the chromatographic peaks are wide and trailing. After 30 minutes, the elution of Cl- continues, indicating that the injection volume has exceeded the capacity of the separation column. In this case, no more injections can be made until the stable baseline is restored. If the sample is diluted 10 times before injection, a good separation between Cl- and trace SO42- can be obtained. The maximum injection volume recommended for anion analysis is generally 30% of the static column capacity. Above this range, large flat-headed or shoulder peaks will appear.
Table 5-3 Selection of separation method and detector (anion)
Analytical ion separation method detector
Inorganic
Anionic hydrophilic strong acid F-, Cl-, NO2-, Br-, SO32-, NO3-, PO43-, SO42-, ClO-, ClO2-, ClO3-, BrO4-, low molecular weight organic acid anion exchange conductivity, UV
SO32-ion repulsion ampere
Arsenate, selenate, selenite anion exchange conductance
Arsenite Ion Repel Ampere
Weak acid BO32-, CO32- ion repulsion conductance
SiO32-ion exchange, ion repulsion column derivation, VIS
Hydrophobic CN-, HS- (high ionic strength matrix)
BF4-, S2O32-, SCN-, ClO4-, I- ion repulsion
Anion exchange, ion pair ampere
Conductance
Condensed phosphoric acid
The sequestering agent is not complexed
Complexed anion exchange
Derivation after anion exchange column, VIS
Conductance
Metal complex Au (CN) 2-, Au (CN) 4-, Fe (CN) 64-, Fe (CN) 63-EDTA-Cu ion pair
Anion exchange conductance
Conductance
organic
Anionic carboxylic acid monovalent fatty acid, C <5 (acid digestion sample, brine, high ionic strength matrix) ion repulsion conductivity
Fatty acid, C> 5 aromatic acid ion pair / anion exchange conductivity, UV
Mono to trivalent monobasic, dibasic and tribasic carboxylic acid inorganic anions
Hydroxy carboxylic acid, binary and tricarboxylic acid alcohol anion exchange
Ion repulsion conductance
Conductance
Sulfonic acid alkyl sulfonate, aromatic sulfonate ion pair, anion exchange conductivity, UV
Alcohol C <6 ion repulsion ampere
Table 5-4 Selection of separation method and detector (cation)
Analytical ion separation method detector
Inorganic cations Li, Na, K, Rb, Cs, Mg2, Ca2, Sr2, Ba2, NH4 cation exchange conductance
Transition metals Cu2, Ni2, Zn2, Co2, Cd2, Pb2, Mn2, Fe2, Fe3, Sn2, Sn4, Cr3, V4, V5, UO2, Hg2
Al3
Cr6 (CrO42-) anion exchange / cation exchange
Cation exchange
Derivation of VIS after anion exchange column
Conductance
Derivative VIS
Derivative VIS
Lanthanide metals La3, Ce3, Pr3, Nd3, Sm3, Eu3, Gd3, Tb3, Dy3, Ho3, Er3, Tm3, Yb3, Lu3 anion exchange, cation exchange column-derived VIS
Organic cation low molecular weight alkyl amine, alcohol amine, alkali metal and alkaline earth metal cation exchange conductance, ampere
High molecular weight alkylamine, aromatic amine, cyclohexylamine, quaternary amine, polyamine cation exchange, ion pair conductivity, ultraviolet, ampere
(2) Change the separation and detection method
If the ions to be tested have similar or similar affinity to the stationary phase, the effect of sample dilution is often unsatisfactory. In this case, in addition to selecting the appropriate mobile phase, consideration should also be given to selecting the appropriate separation method and detection method. For example, NO3- and ClO3-, due to their similar charge number and ionic radius, were co-eluted on the anion exchange separation column. However, ClO3- is more hydrophobic than NO3- and can be easily separated on the ion-pair chromatography column. Another example is that the retention time of NO2- and Cl- on the anion exchange separation column is similar, the concentration of Cl- in common samples is much greater than that of NO2-, making the separation more difficult, but NO2- has strong UV absorption, while Cl- is very Weak, so you should use ultraviolet as a detector to measure NO2-, use conductivity to detect Cl-, or connect two detectors in series to detect Cl- and NO2- at the same time. For the analysis of organic acids in high-concentration strong acids, if ion repulsion is used, since strong acids are not retained, the elimination in the dead volume will not interfere with the separation of organic acids.
(3) Sample preparation
For the determination of trace ions in high-concentration substrates, such as the determination of anions in seawater, the best method is to pre-treat the sample appropriately. The pretreatment methods for removing excess Cl- are: passing the sample through the Ag-type pretreatment column to remove Cl-, or adding AgNO3 to the sample to precipitate Cl- before injection; valve switching technology can also be used, the method is to make the sample weakly retained The components and more than 90% of Cl- enter the waste liquid, and only about 10% of Cl- and components with a retention time greater than Cl- enter the separation column for separation.
(4) Select the appropriate eluent
Ion chromatography separation is based on the competition between the elution ions and the sample ions for the effective exchange capacity of the resin. In order to obtain effective competition, the sample ions and the elution ions should have similar affinity. The following is an example to illustrate the general principles of selecting eluent. When using CO32- / HCO3- as the eluent, the ions eluted before Cl- are weakly retained ions, including monovalent inorganic anions, short carbon chain monocarboxylic acids and some weakly dissociated components, such as F-, HCOO -, CH3COO-, AsO2-, CN- and S2-, etc. For the separation of HCOO-, CH3COO-, and F-, Cl-, etc., the weaker elution ions should be selected. Commonly used weak elution ions are HCO3-, OH-, and B4O72-. Because HCO3- and OH- easily absorb CO2 in the air, CO2 will be converted into CO32- in an alkaline solution, and the leaching strength of CO32- is greater than that of HCO3- and OH-, which is not conducive to the separation of the above-mentioned weakly retained ions. B4O72- is also a weak eluent ion, but the solution is stable and is the recommended eluent for the separation of weakly retained ions. The medium-intensity carbonate eluent has a low elution efficiency for high-affinity components. There are two cases of ions with strong affinity for ion exchange resins. One is that the charge of ions is large, such as PO43-, AsO43- and polyphosphate. The other is that the ionic radius is large and the hydrophobicity is strong, such as I -, SCN-, S2O32-, benzoic acid and citric acid. For the former, increase the concentration of eluent or choose strong eluent ion. For the latter case, the recommended method is to add organic modifiers (such as methanol, acetonitrile, p-cyanophenol, etc.) to the eluent or use hydrophilic columns. The role of organic modifiers is to reduce sample ions and ions. The non-ion exchange effect between the exchange resins occupies the hydrophobic position of the resin, reduces the adsorption of hydrophobic ions on the resin, thereby shortening the retention time, reducing the peak tailing, and increasing the measurement sensitivity.
In ion chromatography, different eluent additives can be added to improve selectivity. This eluent additive only affects the interaction between the resin and the measured ions, but not the ion exchange. For ions with strong affinity to resin, such as some polarizable ions, I- and ClO4-, and hydrophobic ions, benzoic acid and triethylamine, etc., add an appropriate amount of polar organic solvent to the eluent Such as methanol or acetonitrile, can shorten the retention time of these components and improve the asymmetry of the peak shape. In order to reduce the non-ion exchange effect between sample ions and resin, and reduce the adsorption of hydrophobic ions by resin, in anion analysis, p-cyanophenol can be added to the eluent. For example, when measuring trace amounts of I- and SCN- in 1% NaCl, adding cyanophenol to occupy the resin's adsorption position for I- and SCN- will reduce peak tailing and increase the sensitivity of the measurement. In IC, the monovalent eluent ion elutes the monovalent test ion, and the bivalent eluent ion elutes the bivalent test ion. The effect of the change of the eluent concentration on the retention time of the divalent and multivalent test ion is greater than one Price to be measured ion. If the retention time of polyvalent ions is too long, increasing the concentration of eluent is a better method.
2.2 Reduce retention time
The requirements for shortening the analysis time and increasing the resolution are sometimes contradictory. Under the premise of getting better separation results, the analysis time is naturally as short as possible. In order to shorten the analysis time, the capacity of the separation column, the flow rate of the eluent, and the strength of the eluent can be changed. Organic modifiers and gradient eluent technology can be added to the eluent.
The simplest of the above methods is to reduce the capacity of the separation column, or use a short column. For example, it takes 18 min to separate NO3- and SO42- with a 3 × 500 mm separation column, while it takes only 9 min to use the same concentration of eluent with a 3 × 250 mm separation column. However, the separation of NO3- and SO42- is not good, and a better separation can be obtained if a slightly weaker eluent is used.
A large injection volume is beneficial to improve the detection sensitivity, but it leads to a large system dead volume, that is, a large negative water peak, thus delaying the peak time of sample ions. For example, when NaOH is used as the eluent on the AS11 column of Dionex, and the injection volume is 25 μL, 250 μL, and 750 μL, the retention time of F- is 2.0 min, 2.5 min, and 3.6 min, respectively. In order to reduce the retention time, it is best to use a small injection volume.
Increasing the flow rate of the eluent can shorten the analysis time, but the increase in the flow rate is limited by the highest pressure that the system can withstand. The change in flow rate has a greater impact on the resolution of the components that are not completely ion-exchanged, such as For the separation between Br- and NO2-, the resolution decreases a lot when the flow rate increases, and the separation mechanism is mainly ion exchanged NO3- and SO42-, even at very high flow rates, the separation between them is still very good .
Increasing the eluent strength has the same effect on resolution as shortening the separation column or increasing the eluent flow rate. The use of stronger leaching ions can accelerate the elution of ions, but for weakly and moderately retained ions, the resolution will be reduced. When using weak eluent (such as B4O72-) to separate weakly retained sample ions, weakly retained ions such as quinate, F-, lactate, acetate, propionate, formate, butyrate , Methanesulfonate, pyruvate, valerate, monochloroacetate, BrO3- and Cl-, etc. were well separated. But the general sample contains some ions with strong affinity for anion exchange resin, such as SO42-, PO43-, oxalate, etc. If they are rinsed with equal concentration, they will be eluted after one hour or even longer. . In this case, after 3-5 injections, use a high concentration of strong eluent as a sample injection to push out the strongly retained components from the column, or wash the column with a stronger eluent half an hour. Adding organic improvers to the eluent can shorten retention time and reduce peak tailing.
2.3 Improve detection sensitivity
The first is to operate according to the instructions. Only when the instrument is in the best working state and a stable baseline is obtained can the detector sensitivity be set at a higher sensitivity. This is the simplest way to improve the detection sensitivity, but at this time the baseline noise also varies The increase.
The second method is to increase the injection volume. Direct injection, the upper limit of injection volume depends on the time between the shortest retention time chromatographic peak and the dead volume (commonly referred to as water negative peak in IC), for example, using IonPacCS12A column, 12mmol / L sulfuric acid as eluent. The injection volume is 1300 μL, and alkali metals and alkaline earth metals with low μg / L can be detected directly by conductivity, see Figure 5-11. In the figure, the retention time of Li (the peak with the smallest retention time) is 4.1 min, the water negative peak is 1.6 min, and the interval between the Li peak and the water negative peak is 2.5 min, so a large volume of injection can be used directly. In the anion analysis, if CO32- / HCO3- is used as the mobile phase, since the F-peak (the peak with the shortest retention time) is close to the water negative peak, if the injection volume is increased, the water negative peak increases, and the F- peak even It cannot be separated from the negative water peak; on the other hand, because the retention time of F- is generally less than 2min, if the injection volume is greater than 1ml, the flow rate is 1mL / min-2mL / min, F- does not have enough time to participate in the chromatographic process, so the peak Quantification of tailing is difficult. If a hydrophilic stationary phase is used and NaOH is used as the eluent, especially during gradient elution, due to the low NaOH concentration at the beginning of the gradient elution, and because the background solution after passing through the suppressor is low conductivity water, almost There is no negative water peak. In this case, the injection volume can be increased appropriately. Figure 5-12 shows that if the injection volume is 1000 μL, common anions of 0.1 μg / L can be directly determined.
The third method is to use a concentration column, but it is generally only used for the determination of trace components in cleaner samples. When using a concentration column, be careful not to overload the separation column. The dynamic ion exchange capacity of the column is less than 30% of the theoretical value. When enriching F- with a concentration column, if the sample also contains strongly retained ions, such as SO42- or PO43-, recovery of F- is not good. The reason for this is that SO42- or PO43- in the sample also acts as an elution ion, which can elute weakly retained F-parts. For weakly retained ions, if the column capacity of the concentration column is not large enough, the results obtained by increasing the injection volume method are better than those using the concentration column.
The fourth method is to use microporous columns. The diameter of standard columns commonly used in ion chromatography is 4 mm, and the diameter of microporous columns is 2 mm. Because the volume of the microporous column is 4 times smaller than that of the standard column, entering a sample of the same (and standard) quality in the microporous column will produce a signal 4 times that of the standard column in the detector. From a kinetic point of view, at the same flow linear velocity, a column with a larger internal diameter has a larger elution volume than a column with a smaller internal diameter, so the sample is diluted more in the column with a larger internal diameter than the internal diameter. The small column is serious. In addition, the column with a smaller inner diameter is easier to elute when performing ion exchange, and the dead volume is smaller. Therefore, even if the injection volume is large, there will be no serious tailing of the chromatographic peak. When concentrating the column, some ions to be measured cannot be quantitatively retained due to the high matrix. And the amount of eluent is only a quarter of the standard column, thus reducing the consumption of eluent.
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