WO1990005913A1

WO1990005913A1 - Method for the chromatographic determination of cations

Info

Publication number: WO1990005913A1
Application number: PCT/EP1989/001395
Authority: WO
Inventors: Hans-Heinrich Schaper
Original assignee: Schaper Hans Heinrich
Priority date: 1988-11-18
Filing date: 1989-11-18
Publication date: 1990-05-31
Also published as: DE3839035A1

Abstract

The invention concerns a method for the chromatographic determination of monovalent and polyvalent cations, in particular for the simultaneous determination of alkali and alkaline earth ions in wine or mineral water. At least one polybasic carboxylic acid with a hydrophobic residue is applied to a reversed-phase column so that at least a part of the stationary phase of the column is coated with the carboxylic acid. The substance containing the cations to be determined is then applied to the column and the cations are eluted from the column with an eluent. The column is advantageously coated with at least one alkylsuccinic acid. The polybasic carboxylic acids used to coat the column can be substituted in the α-position of the carboxyl groups with a substituent having a - I or a - M effect. The invention also concerns a reversed-phase material coated with the polybasic carboxylic acid as well as a particularly advantageous solution for applying and eluting the reversed-phase material.

Description

Chromatographic determination method

of cations

The invention relates to a method for the chromatographic determination of mono- and polyvalent cations, in particular for the simultaneous determination of alkali and alkaline earth ions in wine or mineral water.

Chromatographic methods in which the monovalent cations are separated with nitric or hydrochloric acid components of the eluent are known to date. Polyvalent cations remain on the column, since the elution power of saltpetre or

Hydrochloric acid is not sufficient for elution of these ions.

Polyvalent cations were separated using the previously known methods with complexing organic compounds which compete with the column material as chelating ligands and with ethylenediamine, which in its protonated form competes with the cations adsorbed on the column. The high elution power of these eluent components causes the monovalent cations, in particular the alkali cations, to be eluted as a common fraction in or shortly after the dead volume of the respective chromatography columns.

In so-called ion pair chromatography, a non-polar material based on silica gel or on a polymer is used as the stationary phase. In the eluent are organic at least partially contain hydrophobic and ionic molecules that adsorb to the reverse phase column material. Aliphatic sulfonic acids have hitherto been used as the elution reagent, that is to say as the counter ion for the cations to be separated.

With these systems, which in the short name for example RP ₄ , RP ₈ , RP ₁₈ resp. PRP-1 are called, a separation of mono- and bivalent cations is possible, but with an unsatisfactory resolution. Particularly with regard to the analysis of mineral waters or wines, particular problems arise because, for example, very high sodium concentrations occur in addition to smaller potassium concentrations and lithium concentrations in the range of individual ppm, for example in the mineral waters. When analyzing wine, there is usually an inverse concentration ratio between sodium and potassium ions; the potassium content predominates here by far, so that these systems in particular present particular problems in the analysis.

The object of the invention is to propose a method and a column material with which a simultaneous separation of mono- and multivalent cations, in particular also alkali and alkaline earth ions, is possible.

This object is achieved according to the invention in that a so-called reversed-phase column is at least partially covered with at least one polybasic carboxylic acid which has a hydrophobic residue, in that the substance which contains cations is applied to the column and then eluted.

Alkyl or cycloalkyldicarboxylic acids are particularly suitable here. In addition, aromatic dicarboxylic acids can also be used, especially if a non-polar polymer-based material, such as the PRP-1 column, is used as the stationary phase. The invention relates generally to the use of polybasic carboxylic acids and not only to the use of dicarboxylic acids, although the discussion below relies essentially on examples in which dicarboxylic acids are used as polybasic carboxylic acids.

Examples of these carboxylic acids are alkyl and aryl derivatives of malonic acid and succinic acid as well as 1,2-dicarboxyl-5-alkylcyclohexanes, 1,2-dicarboxyl-alkylcyclopentanes, alkyl derivatives of phthalic acid and alkylnaphthalenedicarboxylic acids.

A particularly good interaction of the hydrophobic part of the carboxylic acid with the column material, as occurs, for example, in the PRP-1 column (here styrene-divinyl-benzene is used), results with aromatic, dibasic and polybasic carboxylic acids, such as naphthalene -di -, - tri- and -tetracarboxylic acids, since charge-transfer complexes can form here with the aromatic components of the column material.

The decisive factor in the selection of the corresponding polybasic carboxylic acid is on the one hand the interaction of the hydrophobic residue with the non-polar column material and on the other hand the complex formation constant with the mono- and polyvalent cations to be separated. With regard to the complex formation constant, it has been found that particularly favorable values are achieved when the chelate complexes of the polybasic carboxylic acids form a six- or seven-membered ring together with the cation. For carboxylic acid derivatives in which chelate complexes are possible which have only four ring members including the metal atom, the complex formation constant is generally too small.

For sufficient anchoring of the hydrophobic residue on the non-polar reversed-phase column material, for example, Al kylreste with four to sixteen carbon atoms, but especially with six to 10 carbon atoms. Hexylmalonic acid and hexylsuccinic acid can be named as examples. Aromatic, polybasic carboxylic acids can also be adsorbed onto the non-polar column material via appropriate alkyl chains.

If alkyl chains are used as the hydrophobic residues of the polybasic carboxylic acids which have more than ten carbon atoms, then these carboxylic acids are very well anchored to the column material, so that the separation mechanism of the column shifts from ion pair chromatography more towards ion exchange chromatography.

However, expectant with longer carbon chain of the alkyl group enhances also the + I effect on the carboxyl groups, so that their _pK increases. Since the deprotonated carboxylate groups are responsible for the chelation, an improvement in the separation performance is achieved if a substituent with - I and / or - M effect is present on at least one of the carbon atoms α to the carboxyl groups.

Examples of such substituents are in particular the nitro group or halogen radicals, here in particular bromine radicals.

The polybasic carboxylic acid is expediently applied to the column in a polar solvent, in particular in an alcoholic solution. Again, the use of a methanolic solution is particularly recommended.

In the column prepared in this way, the elution is then preferably carried out with the aid of a mono- or dicarboxylic acid in the eluent, the pK _S value or first pK _S value of which is less than 3, preferably less than 2. Stand by the specialist The above-mentioned measures are available for influencing the pK _S value.

In the event of dynamic loading of the column with the polybasic carboxylic acid, for example in the case of a medium to weak interaction of the polybasic carboxylic acid with the column material, the polybasic carboxylic acid will have to be added to the eluent.

It has proven particularly expedient to carry out the elution using trifluoroacetic acid components in the eluent or else using oxalic acid components. These acids or their corresponding bases compete with the polybasic carboxylic acids fixed on the column and thus lead to elution of the cations adsorbed on the column.

The elution effect can be improved by adding so-called elution enhancers, which can be formed in particular by a diamine, to the elution solution. The diamines are in the pH range in which the column according to the invention is usually used in protonated form and thus compete with the cations adsorbed on the column.

One of the preferred diamines is the 1,7-diaminoheptane.

In particular, if the column is used according to the ion-pair chromatography method, in which the shorter-chain hydrophobic residues of the polybasic carboxylic acid are used in particular, it is advisable to ensure the eluent the polybasic carboxylic acid in order to ensure constant loading of the column with the polybasic carboxylic acid to add and to cover the acid with the polybasic carboxylic acid already with the eluent. It is then expedient to coat the column with the same solution and carry out the elution.

For the purposes of mineral water and wine analysis, it has proven particularly expedient if the column is coated with a solution of hexylsuccinic acid and oxalic acid in water, whereupon the substance which is to be analyzed for the cation content is applied to the column and then the elution is carried out with the solution used to load the column.

Concentrations between 1 and 4 mmol and with respect to succinic acid 0, 5 and 4 mmol oxalic acid have proven to be particularly advantageous with respect to hexylsuccinic acid. Especially in the difficult analysis of wine and mineral waters, a content of approx. 2 mmol hexylsuccinic acid and approx. 1 mmol oxalic acid is recommended.

A reversed-phase column is preferably used as the column material in the process, the stationary phase of which is a non-polar material based on silica gel, to which alkyl groups, such as RP ₄ , RP ₈ and RP ₁₈ , are bound. Alternatively, a reversed-phase column can be used, the stationary phase of which is formed by a non-polar polymer-based material, such as, for example, PRP-1.

The second aspect of the invention relates to the creation of a modified reversed-phase material, in particular a column, which is suitable for the chromatographic determination of mono- and polyvalent cations, in particular for the simultaneous determination of alkali and alkaline earth metal ions, in wine or mineral waters. According to the present invention, this column material is characterized in that it is at least partially at least one polybasic carboxylic acid, which has a hydrophobic residue, is occupied. The hydrophobic residue is held on the reversed phase material by non-covalent forces.

Non-polar materials based on silica gel, which has alkyl groups, such as, for example, the RP ₄ , RP ₈ and RP ₁₈ materials, are particularly suitable as reversed phase material. A column material whose stationary phase is based on a non-polar polymer-based material, such as, for example, PRP-1, can also be used with very good results.

The reversed phase material will preferably be coated with an alkyl or cycloalkyl dicarboxylic acid.

Column material, which is especially prepared for the analysis of mineral waters and wines, preferably has an occupancy with alkyl succinic acid, in particular hexyl succinic acid.

Preferred solutions for covering and eluting reversed-phase material in the chromatographic determination of mono- and polyvalent cations, in particular in the simultaneous determination of alkali and alkaline earth ions in wine or mineral water, comprise at least one polybasic carboxylic acid which has a hydrophobic residue, and a mono- or dicarboxylic acid, the pK _S value or first pK _S value of which is less than 3. The solvent of the solution will preferably be a polar solvent. The polybasic carboxylic acid here is preferably an alkyl or cycloalkyl dicarboxylic acid, it also being possible to use aromatic derivatives of dicarboxylic acids or aromatic dicarboxylic acids themselves. One of the preferred solutions contains hexylsuccinic acid and oxalic acid in water, the concentrations of which are particularly useful for problematic analysis of wine and mineral water

Hexylsuccinic acid is approximately 2 mmol and the concentration of oxalic acid is approximately 1 mmol in water.

In the following, some explanations are given on the probable separation mechanisms, which, however, are not to be understood as a restriction of the scope of protection.

The size of the chelate rings formed by the polybasic carboxylic acid with the cations is of importance in relation to the separation properties of the system. When maleic acid derivatives are used, a grouping (1,4-dicarboxylic acid) is formed which can obviously complex metal ions better than, for example, the corresponding bases of monoalkylcarboxylic acids.

It can be assumed that complexing the cation (chelation) forms a seven-membered ring, the structure of which is given below in a general form:

R ₁ , R ₂ Δ AlkYlrest, H or residues of a copolymer Measurements have indicated that the seven-membered ring that is likely to be formed is more stable than a four-membered ring that can result from the described use of chemically bound weak cation exchangers and of alkyl monocarboxylic acids as ion-pairing reagents; Such a four-ring complex is given below using the example of 1-octane carboxylic acid:

These four-ring complexes presumably have much smaller complex formation constants.

A possible coordination of two carboxylic acid groups of a monocarboxylic acid to the respective metal ion does not appear to change that much, although it must also be taken into account that the dissociation of the carboxylic acid is suppressed with increasing concentration.

When using the so-called Schomburg column, its carboxylic acid groups can also form four-ring chelates with the metal ions, so that in addition to the possible seven rings, these also contribute to the complex stability.

The chelate complexes of 1,3-dicarboxylic acids, for example of alkylmalonic acids, also have satisfactory complex formation constants.

Due to the different ionic radii and coordination geometries of the cations, complexes can form for calcium and magnesium by complexing with the carboxylic acid exchange groups, which can differ in the coordination numbers and complex stabilities. This is one of the reasons for the selectivity of this exchange material. For the sake of simplicity, the existing aquo complexes and the complexes of the cations with organic acids are not taken into account in this consideration.

It is clear from the retention times that the complex stabilities of magnesium with the exchange groups are smaller than that of calcium.

As calculations and tests show, polycarboxylic acids only have a negligible complexing effect if

at most one carboxyl group is mainly dissociated. This is the case with the applied pH, which is in the acidic range. In this case, only the lone pairs of oxygen contribute to the complexation. However, the negative charge present in the dissociated state of the acid group is better able to stabilize the complexes significantly. It is therefore assumed that no 1: 1 complexes form between the metal ion and the acid group. The respective metal ion, with probably two or more groups, in each case forms chelate complexes, for example in the following form

R ₁ , R ₂

Residues of the copolymer, alkyl residues or hydrogen It follows from these explanations that the ion pair reagents should have at least two carboxy exchange groups in order to be able to form the relatively stable chelate rings, preferably 6- and 7-rings, together with the cations. At the same time, a hydrophobic residue is necessary so that interactions with the non-polar column material are possible.

The practical application of the invention in the analysis of mineral water and wine is given below by way of example:

The demands placed on the ion pair reagents are met, for example, by alkyl succinic acids.

For this compound and for the polybasic carboxylic acids, dodecyl and hexadecyl succinic acid also used in the examples, the instructions for the preparation should first be given:

A synthetic route starting from n-alcohol is used, which is outlined in the diagram of FIG. 1.

The respective methyl esters and not, as described in the literature, the ethyl esters are used, since they turn out to be much easier to saponify.

The following table 1 shows the characteristic data of the alkyl succinic acids obtained:

The melting points are determined in open glass capillaries using a melting point apparatus with a paraffin oil bath. The ¹ H-NMR spectra are recorded with a Bruker WP 80 (80 MHz) with tetramethylsilane (TMS) as the internal standard.

A Metrohm 670 tritroprocessor is used to determine the pK _S values.

Due to the different hydrophobicity of the substances, the measurement is carried out for hexylsuccinic acid in water, for dodecyl- or hexadecylsuccinic acid in 80 or 90% methanol. The influence of the methanol concentration is negligible.

Separation systems that optimally use the synthesized alkyl succinic acids will now be described in the following:

Experimental part:

Devices: Bischoff model 2200 HPLC pump

Rheodyne injector 7125, 5-20 μl ESA Ion Chem conductivity detector model 5400

Waters conductivity detector 430

Membrane filtration system (see 3.1.1) HP Integrator 3390 A

Lens ice pen L 6510 Columns: Superspher RP ₁₈ 3 to 4 μ (Merck)

LiChrospher RP ₁₈ 5 μ, 10 μ (Merck) LiChrosorb RP ₁₈ 5 μ, 10 μ (Merck) Nucleosil RP ₁₈ 5 μ (Bischoff)

every 250 x 4 mm with the respective guard column 50 x 4 mm

Reagents: oxalic acid x 2H ₂ O pa (Merck)

1,3-diaminopropane

1,4-diaminobutane

1,5-diaminopentane

1,6-diaminohexane

1,7-diaminoheptane

(all p.a. Fluka)

Hexyl, dodecyl, hexadecylsuccinic acid

Conditions: HPLC pump: Flow: 1.0 ml / min.

Integrator: Chart speed: 0.2

Recorder: 20 cm / h

Conductivity detector: Gain: 0.005

Range: 500

A reversed-phase material based on silica gel proves to be sufficient here, which has significantly lower costs than a polymer material.

Of the columns used, the LiChrospher RP 18 with spherical 5 μ material proves to be the most suitable. A further reduction in particle size in the Superspher RP 18 does not result in better separation performance. The irregular material also tested in the LiChrosorb RP 18 and the spherical Nucleosil RP 18 follow a poorer separation yourself. As described above, the hexylsuccinic acid can form seven rings with the respective cations, two or more groups also being possible.

The conductivity detection is the suitable detection system for the developed system because it can detect mono- and bivalent cations equally.

It should be noted that there are no hydrated ions here, but mainly ion pairs, which reduce the conductivity due to their electrical neutrality. There is therefore an indirect conductivity measurement.

The conductivity detectors from Bischoff and Waters used here electronically suppress the basic conductivity. This more recent development is to be seen as an alternative to the suppressor method, where an anion exchange results in a "chemical" suppression of the basic conductivity.

The tartaric acid previously used in the eluent did not result in a separation of the bivalent cations in the ion pair system to be developed, hexylsuccinic acid initially being used as the ion pair reagent. At the present pH value of 2.6, the tartaric acid has only a negligible complexing effect, so that the selectivity for the separation of the bivalent ions from the cation exchange material comes, which is underlined by comparative tests with hydrochloric, formic and acetic acid.

There is therefore only a separation of the monovalent cations from one another and from the bivalent ones, which in turn are not separated. Thus, it appears that whatever monocarboxylic acid residues used in the Schomburg column, another separation mechanism is present, which is to be assumed even by the different _pK values.

For this reason, the following two conditions are to be used for the ion pair system organic acids linked: low _pK values, so that a complexing action with respect to the alkaline earth metal ions may be present; Different complex stabilities with calcium and magnesium so that the retention of the bivalent cations is different.

The tartaric acid of the pK _a values ago seen similar organic acids such as citric, malic and succinic acid are eligible for the separation of mono- and divalent cations less in question since they do not meet the above criteria entirely satisfactory.

Polycarboxylic acids such as maleic acid or polybasic inorganic acids such as phosphoric acid, which only have the first pK _s stage in the pH range around 2.0, also prove to be less useful. Organic acids are preferably used, which Substituents with predominantly or -I -M effects have to the pK _s - values decrease.

These include Nitrophthalic acids, various nitrosalicylic acids, sulfosalicylic acid as well as mono- and dihalogenated succinic acids.

Although these compounds have now significantly lower pK _a values than the unsubstituted acids, the complex stabilities of the complexes with calcium and magnesium are small, so that they are less suitable for the particular intended here separation.

Also various Pyridinmono- and dicarboxylic acids and Benzoldi-, tri- and tetracarboxylic _s values are due to their low pK tested. But they also do not result in a good separation of calcium and magnesium.

In the present application, mainly oxalic acid and trifluoroacetic acid have good complexing properties, especially also in the pH range 2.6-3.5. With oxalic acid, a better separation of the alkaline earth ions is achieved, so that it is particularly suitable for the ion pair system.

The optimal concentration range of the oxalic acid in the eluent is approximately 1 mmol in the present example. Smaller concentrations lead to too long retention times of the alkaline earth ions, because on the one hand the elution power of the eluent is reduced and on the other hand the capacity of the exchange groups is increased by a higher degree of dissociation. Larger concentrations of oxalic acid reduce the degree of dissociation and at the same time increase the elution power; A sufficient separation may then no longer exist. In processes as are carried out according to the prior art, 5-10 mmol of an alkyl sulfonic acid are usually used as the ion pair reagent.

In the system according to the invention, hexylsuccinic acid concentrations around 2 mmol have been found to be optimal.

Due to the already pronounced hydrophobic content, only approx. 4 mmol are still soluble in hot water, in contrast to e.g. to the more polar octane sulfonic acid.

Lower concentrations result in less good separation, larger amounts of retention times of the bivalent cations are too long. There are also disturbing system peaks in the conductivity measurement at higher concentrations of hexylsuccinic acid. The system with 2 mmol of hexylsuccinic acid and 1 mmol of oxalic acid has been found to be particularly favorable in experiments.

2 shows the result of a simultaneous separation of alkali and alkaline earth ions as well as zinc, strontium and manganese.

The developed separation system is designed in such a way that it is also possible to determine sodium and potassium in their unfavorable conditions in wine and mineral water. At the same time, the bivalent cations should be determinable with acceptable retention times. If the conditions are not so unfavorable or if the excess component is determined, the elution power of the eluent, as already mentioned, should not be increased by the oxalic acid concentration.

As a further possibility, the elution power of the eluent can be regulated by adjusting the pH. The alkali and alkaline earth hydroxides are not well suited for this, since these cations can then no longer be detected with sufficient sensitivity. Divalent cations such as ethylenediammonium should now be used, because diethanolamine or other monovalent amines, for example, first have a greater influence on the alkali ions and secondly they increase the capacity of the exchanger, because increasing the pH value means that more exchange groups are dissociated, so that retention times increase.

As mentioned several times, ethylenediamine (EN) is practically a dication in the pH range below about 6. As there are no comparisons with higher homologues or other polyamines in the literature besides the frequently used EN, they should also be tested. Therefore compounds such as 1,3-diaminopropane to 1,7-diaminoheptane are used. These have pK _s values shifted even further into the "alkaline", so that they are also present in the "acidic" form as a dication. If the non-polar portion of the diamines becomes larger, the interactions with the RP ₁₈ column material increase, which is documented by shorter retention times.

From 1,5-diaminopentane to 1,7-diaminoheptane, the influence of diamine on retention is approximately the same. However, a system peak occurs with 1,5-diaminopentane and 1,6-diaminohexane, which can interfere with the determination. This problem does not exist with 1,7-diaminoheptane. Compared to ethylenediamine, only a smaller concentration is necessary with a more sensitive measurement.

Basically, it can be said that a larger addition of diamine reduces the basic conductivity of the respective eluent by forming ion pairs. These electrically neutral ion pairs do not contribute to the basic conductivity, as is confirmed by this decrease. This makes the presence of ion pairs clear, otherwise the diamines with their high equivalent conductivity would have to increase the basic conductivity of the eluent. Relaxation and electrophoretic effects etc. can also support this reduction in basic conductivity due to the resulting smaller ion mobility. At the same time, however, the sensitivity is deteriorated, which is why a small amount of the diamine is preferably added if measurements are still to be made sensitively.

The best solution has been found to be raising the pH of the eluent by 0.05 - 0.1 pH units from 2.92 to 2.97 - 3.02 with 1,7-diamine heptane.

As previously described, ion exchange chromatography can be viewed as a limiting case of ion pair chromatography. In order to be able to carry out this borderline case with ion pair reagents, dodecyl and hexadecyl succinic acid have been produced.

The greater hydrophobic fraction compared to hexylsuccinic acid means that these compounds are almost no longer water-soluble and have a comparatively stronger interaction with the non-polar column material.

An approx. 1 mmol 50% methanolic solution of dodecyl and an approx. 1 mmol methanolic solution of hexadecyl succinic acid are now produced.

One of these two solutions is used to "load" the

HPLC column used by the column with a flow of 1.0 ml / min. is rinsed for about 1 hour.

Then the eluent with 1 mmol oxalic acid, which has been tried and tested in hexylsuccinic acid, is used. Even after two days of rinsing with the eluent, there is no reduction in the retention times; this entails a reduction in the consumption of ion pair reagent. From this it can be seen that the long-chain succinic acids have practically no solubility in the eluent due to the strong interactions with the non-polar column material. However, organic solvents such as acetonitrile or prolonged rinsing with methanol make them desorbable again, so that the RP ₁₈ column can then be used again for other problems.

Table 2 summarizes the pK _a values of the three acids listed.

Table 2: pK _a values comparison of alkyl succinic

Hexylbern- Dodecylbern- Hexadecylbern-succinic acid rock acidic acid

It is believed that the alkyl chains are moved with the Hexylbernsteinsäure in higher pH ranges, so that compared with the latter, a much smaller part of the carboxyl groups present dissociated by their + I-effects, the pK _a values compared.

The capacity of the separation column is therefore too low to separate the bivalent ions as well as the monovalent ones.

Another reason for the poorer selectivity towards hexylsuccinic acid is due to the fact that the longer alkyl chains have greater freedom in the molecular structure, so that the probability of the possible two seven-membered rings mentioned is lower. The longer-chain alkyl succinic acids are therefore less suitable, because adjusting the eluent pH to pH ranges favorable for these acids is not possible due to the loss of sensitivity. If only the bivalent cations are to be separated, the post-column derivatization can be used instead of the conductivity detection. With this detection method, the pH value can be adjusted so that this separation is possible again.

The insertion of substituents with -I and / or -M effec- th in the alkyl dicarboxylic acids, the pK _a values can, however, move into favorable for the separation areas.

The applicability in mineral water and wine analysis is described using the previously described separation system with hexylsuccinic acid and oxalic acid.

Experimental part:

Devices: Bischoff model 2200 HPLC pump

Rheodyne injector 7125, 5-20 μl Waters conductivity detector 430

Membrane filtration system see 3.1.1

HP Integrator 3390 A.

Column: LiChrospher RP ₁₈ 5 μ (Merck)

250 x 4 mm with guard column 50 x 4 mm

Reagents: oxalic acid x 2H ₂ O pa (Merck)

1,7-diaminoheptane p.a. (Fluka)

Hexylsuccinic acid

Conditions; HPLC pump: Flow: 1.0 ml / min.

Integrator: Chart speed: 0.2

Conductivity detector: Gain: 0.005

Range: 500

Eluent: 2 mmol hexylsuccinic acid

+ 1 mmol oxalic acid pH 2.92 An LF detector is now used here, which electronically keeps the temperature constant at 35º C and also suppresses the basic conductivity of the eluent. Basically, the examined samples should be diluted as much as possible to avoid peak overlaps.

This risk is particularly present in mineral water or wine samples, since either sodium or potassium is present in large excess.

Interferences in the magnesium determination, such as have occurred in methods according to the prior art, are not present here in the conductivity detection, as illustrated in FIG. 3. Fig. 4 shows the sodium determination, which is possible with this system in contrast to the Schomburg column and other single column systems even without a so-called suppressor system.

The chromatogram of the calcium and magnesium determination in mineral water is shown in FIG. 5 and that of the potassium and lithium determination in FIG. 6.

With this system, too, detection of conductivity is not possible for manganese determination, since the necessary sensitivity is not sufficient. If manganese is to be determined, it has proven to be advantageous to connect the equipment for post-column derivatization (reagent pump, UV detector, etc.) after the conductivity detection. The peristaltic pump is not sufficient for this, since it is the limiting factor for the sensitivity due to strong pulsation. A reaction pump from Merck,

Darmstadt, can be used, which is characterized by low-pulse pumping of the reagent (PAR-Zn-EDTA). The following sample preparation is necessary:

25 ml of mineral water are mixed with stirring fish and approx. 100 mg Na ₂ S ₂ O ₃ in a small beaker and kept at the boil for about 5 minutes on the magnetic stirrer. After cooling, the membrane is filtered and acidified with 3-4 drops of 25% HCl (pH 2.0 - 2.5) and degassed in an ultrasonic bath.

The specified Na ₂ S ₂ O ₃ addition and the heating time must be observed, since smaller additions and shorter times result in poor results. The carbon dioxide and hydrogen carbonate contained in mineral water must be removed before injecting, otherwise the column packing can be damaged.

If only the main cations potassium, calcium and magnesium or sodium, calcium and magnesium are to be determined in wine and mineral water samples, 1,7-diaminoheptane is added to the eluent to increase the elution power, which shortens the retention times. This enables the determination of the respective cations more quickly.

The deterioration in sensitivity resulting from the addition of diamine is irrelevant for the determination of the main cations. Raising the pH of the eluent by 0.1 pH units by adding the diamine has been found to be optimal. If only the alkaline earth ions are to be analyzed, a further addition is possible to reduce the analysis time. From this it becomes clear that the developed system can be adapted to the existing separation problems. The determination ranges of the cations are listed in Table 3.

The range around ≤ 0.1 ppm sodium should be avoided for the determination, since contaminated results can result from contamination of the glass devices used. Contamination is negligible in the area used here. The samples are diluted 1: 5 to 1: 1000 for mineral water samples and 1:20 to 1: 1500 for wine samples.

Due to the different linearity ranges and strongly differing contents, a simultaneous evaluation is not possible, so that each sample has to be injected several times. The trace elements occasionally found in wine and mineral water could not be detected.

Atomic absorption spectrometry (AAS) is used as the reference method for the analyzed ions.

Tables 4 and 5 show the contents of the wine and mineral water samples examined in comparison with the AAS values.

From the ratios in Table 6, there is a very good agreement between ion chromatography and AAS for the determination of potassium, calcium and magnesium in wine.

When determining sodium, the IC provides up to 11% higher values, which presumably result from organic components in the wine.

Table 7 shows the ratios of IC to AAS for the sodium, potassium, lithium, calcium and magnesium determination in mineral water. There is also a good agreement here.

In addition to the Schomburg column, the system according to the invention has crystallized out as a further possibility for simultaneously determining alkali and alkaline earth ions. The separation system is also much cheaper and more universal, as shown above.

The ion pair reagent is removed by rinsing with 20% methanol, so that the column is also available for other separations.

Commercially available cation exchangers with sulfone and carboxylic acid exchange groups are not suitable for separation, since they only allow either the determination of the alkali or alkaline earth ions. For the same reason, ion pair chromatographic systems with alkyl sulfonic and alkyl carboxylic acids are not suitable. It is justified that a sterically favorable seven-membered ring is able to achieve the selectivity between mono- and bivalent as well as within these two types of ions.

Therefore, alkyl succinic acids are synthesized that have this important group (1,4-dicarboxylic acid). They also have a non-polar alkyl residue, which allows their use as ion pair reagents in ion pair chromatography.

Based on this, a separation system with hexylsuccinic acid and oxalic acid is developed, which is suitable for the determination of alkali and alkaline earth. Oxalic acid has been found to be optimal acid, because it has the one hand, low _pK values and on the other hand, still sufficient in the "acid" Differences in the complex stabilities to the divalent cations. Many organic acids tested only meet the first condition and are therefore less suitable. The pH value of the eluent plays a decisive role: firstly, it determines the capacity of the column via the degree of dissociation of the carboxylic acid groups of hexylsuccinic acid, and secondly, it controls the elution power of the eluent via the increasing or decreasing complexing properties of oxalic acid.

A solution of approx. 2 mmol hexylsuccinic acid and approx. 1 mmol oxalic acid has been found to be a favorable concentration range for the eluent.

With this system, the main cations and the occasional trace elements (e.g. lithium, manganese) in wine and mineral water samples can be determined simultaneously and easily.

If only the alkaline earth ions and, depending on the food, sodium (mineral water) or potassium (wine) are determined in the samples, the elution power of the above eluent is increased by adding 1,7-diaminoheptane. This can significantly shorten the analysis time. The comparison of the system with the atomic absorption spectrometry used as a reference method shows a good agreement, only slightly higher contents are found for sodium, which is probably due to the organic components.

The longer-chain alkyl succinic acids, which were also tested, can be well and permanently adsorbed after rinsing the column with the methanolic solutions. However, they do not possess the required for bivalent cations selectivity, which is due to shifted to the "Alkaline" _pK values. This shift also means that the capacity is much smaller. It is not possible to adjust the pH to pH ranges favorable for these acids, since the sensitivity of the conductivity detection used here drastically decreases. However, this problem can be avoided with post-column drainage.

The insertion of substituents with -I and / or -M effec- th in the alkyl succinic acids, the pK _a values can be moved into favorable for the separation areas.

In summary it can be said that with the developed

Ion pair system from hexylsuccinic acid and oxalic acid a very good separation of alkali, alkaline earth ions and some other cations is achieved.

In addition, the good applicability in mineral water and wine analysis documents that it is a powerful separation system.

The system described can be used in mineral water and wine analysis despite the unfavorable ratios of sodium and potassium and is therefore more universal and also cheaper than the Schomburg column mentioned.

From the separation systems developed and used in this work, it becomes clear that ion chromatography is suitable as a fast method to be adopted in routine analysis more than before by simultaneous determination of ion groups. This gives you advantages, speed, cost and time savings even more.

Of course, the method according to the invention is not limited to mineral water or wine analysis, but rather lets are generally used for a cation analysis, especially for the simultaneous determination of mono- and polyvalent cations next to each other. In addition to liquid samples, ashes can of course also be analyzed, which have been prepared in the usual way for ion chromatography.

Fig. 1: Synthetic route of the n-alkyl succinic acids

Fig. 2: The simultaneous determination of mono- and

bivalent cations

Eluent: 2 mmol hexylsuccinic acid +

1 mmol oxalic acid

Column: LiChrospher RP ₁₈ 5 μ (Merck)

Detection: conductivity

Sensitivity: 2.5 μSxFS

Standard solution: 1 ppm Na, K, Zn, Mn, Mg, Ca, Sr as well

0.25 ppm Li

Fig. 3: Calcium and magnesium determination in wine

Eluent: 2 mmol hexylsuccinic acid +

1 mmol oxalic acid

Column: LiChrospher RP ₁₈ 5 μ (Merck)

Detection: conductivity

Sensitivity: 2.5 μS x FS

Calcium peak: 1.01 ppm Approx

Magnesium peak: 0.80 ppm Mg

Fig. 4: Sodium determination in wine

Eluent: 2 mmol hexylsuccinic acid +

1 mmol oxalic acid

Column: LiChrospher RPis 5 μ (Merck)

Detection: conductivity

Sensitivity: 2.5 μS x FS

Calcium peak: 1.01 ppm Approx

Sodium peak: 0.64 ppm Na Fig. 6: Potassium and lithium determination in mineral water

Eluent: 2 mmol hexylsuccinic acid +

1 mmol oxalic acid

Column: LiChrospher RP ₁₈ 5 μ (Merck)

Detection: conductivity

Sensitivity: 2.5 μS x FS

Potassium peak: 1.66 ppm K

Lithium peak: 62 ppb Li

Claims

EXPECTATIONS

1. A method for the chromatographic determination of mono- and polyvalent cations, in particular for the simultaneous determination of alkali and alkaline earth ions in wine or mineral water, characterized in that

a reversed phase column is at least partially covered with at least one polybasic carboxylic acid which has a hydrophobic residue,

- the substance containing the cations is applied to the column and

- is then eluted.

2. The method according to claim 1, characterized in that the column is coated with at least one alkyl or cycloalkyldicarboxylic acid.

3. The method according to claim 1 or 2, characterized in that the column is coated with at least one alkyl succinic acid, the alkyl radical contains 4 to 16 carbon atoms, in particular 6 to 10 carbon atoms.

4. The method according to any one of the preceding claims, characterized in that the column is coated with hexylsuccinic acid.

Process according to one of the preceding claims, characterized in that the column is coated with at least one polybasic carboxylic acid which has a substituent with - I and / or - M effect on at least one of the carbon atoms 0 (to the carboxyl groups ().

6. The method according to claim 5, characterized in that the substituent is a nitro group or a halogen radical, in particular a bromine radical.

7. The method according to any one of the preceding claims, characterized in that the polybasic carboxylic acid is applied to the column in a polar solvent, in particular in methanolic solution.

8. The method according to any one of the preceding claims, characterized in that the elution is carried out with the aid of a mono- or dicarboxylic acid whose _pK a value or first _pKa less than 3, preferably less than 2, is.

9. The method according to any one of the preceding claims, characterized in that the elution is carried out with the aid of trifluoroacetic acid.

10. The method according to any one of the preceding claims, characterized in that the elution is carried out with the aid of oxalic acid.

11. The method according to any one of the preceding claims, characterized in that the elution solution additionally contains an elution enhancer, in particular a diamine, preferably 1,7-diaminoheptane.

12. The method according to any one of the preceding claims, characterized in that the column, in particular in the case of polybasic carboxylic acids with a short-chain hydrophobic residue, is simultaneously coated with the polybasic carboxylic acid and the eluent and that the elution is carried out simultaneously with the polybasic carboxylic acid and the eluent .

13. The method according to any one of the preceding claims, characterized in that the covering of the column and the elution is carried out with the same solution of carboxylic acid and eluent.

14. The method according to any one of the preceding claims, characterized in that

the column is coated with a solution of hexylsuccinic acid and oxalic acid in water, - The substance containing the cations is applied to the column and - then eluted with the solution used to coat the column.

15. The method according to claim 14, characterized in that the solution contains between 1 and 4 mmol of hexylsuccinic acid and between 0, 5 and 4 mmol of oxalic acid.

16. The method according to claim 14 or 15, characterized in that the solution contains about 2 mmol of hexylsuccinic acid and about 1 mmol of oxalic acid.

17. The method according to any one of the preceding claims, characterized in that the chromatographic determination is carried out with a reversed-phase column, the stationary phase of which is a nonpolar material based on silica gel which has alkyl groups, such as RP ₄ , RP ₈ , RP _{18 '} is.

18. The method according to any one of the preceding claims, characterized in that the chromatographic determination is carried out with a reversed-phase column, the stationary phase of which is a non-polar polymer-based material such as PRP-1.

19. Reversed-phase material, in particular column, for the chromatographic determination of mono- and polyvalent cations, in particular for the simultaneous determination of alkali and alkaline earth ions in wine and mineral water, characterized in that their column material is at least partially occupied by at least one polybasic carboxylic acid which has a hydrophobic residue by non-covalent binding.

20. Reversed phase material according to claim 19, characterized in that its column material is at least partially coated with at least one alkyl or cycloalkyldicarboxylic acid.

21. Reversed phase material according to one of claims 20 or 21, characterized in that its column material is coated with at least one alkyl succinic acid, in particular hexyl succinic acid.

22. Solution for covering and eluting reversed-phase material in the chromatographic determination of mono- and polyvalent cations, in particular in the simultaneous determination of alkali and alkaline earth metal ions in wine or mineral water, characterized in that the solution contains at least one polybasic carboxylic acid a hydrophobic group and a mono- or dicarboxylic acid whose _pK a value or first pK _s value is less than 3, comprises in a polar solvent.

23. Solution according to claim 22, characterized in that the polybasic carboxylic acid is an alkyl or cycloalkyldicarboxylic acid.

24. Solution according to one of claims 22 or 23, characterized in that it comprises a solution of hexylsuccinic acid and oxalic acid in water.

25. Solution according to claim 24, characterized in that it contains about 2 mmol of hexylsuccinic acid and about 1 mmol of oxalic acid in water.

26. Use of at least one polybasic carboxylic acid which has a hydrophobic residue for the chromatographic determination of mono- and polyvalent cations, in particular for the method according to one of claims 1 to 18.