Ion-chromatography of metals with post-column ion displacement

Talanta, Vol.

8, pp. 835-840, 1990

0039-9140/90 $3.00 + 0.00

Printedin Great Britain. All ri&ts reserved

Copyright 0

1990 Pergamon Press plc

ION-CHROMATOGRAPHY OF METALS WITH POST-COLUMN ION DISPLACEMENT C%PIS~NE J. BOWL= and LAURENCEW. BADER Department of Chemistry, University of Saskatchewan, Saskatoon, Sask. Canada S7N OWO KENNETH

W. JACKSON*

Wadsworth Center for Laboratories and Research, New York State Department of Health, and School of Public Health, State University of New York, Albany, NY 12201-0509, U.S.A. (Received

25 September

1989. Revised

I December

1989. Accepted

27 December

1989)

Sunnnary-A post-column reagent mixture of Eriochrome Black T and magnesium-EDTA complex is added to the eluent from an ion-chromatograph. Eluted metal cations displace the magnesium, which then forms a complex with the Eriochrome Black T. The absorbance of this complex is measured at 520 nm. Detection limits for several cations are in the pg/ml range.

Ion-chromatographs commonly use conductivity detection and a suppressor column to remove the background conductivity of eluent ions. For cation determinations, the simplest suppression reaction involves a strong-base anion-exchange resin in its hydroxide form to convert the protons in an acidic eluent into water. Typical mobile phases are dilute mineral acids, but for alkaline-earth metal ion separations the mobile phase should contain a doubly charged ion of high equivalent conductance, such as the ethylenediammonium ion. This acts as a “driving ion” and promotes exchange with the metal ions on the column.’ Transition metal ions can be eluted in a reasonable time by inclusion of a weak complexing agent, such as tartrate, oxalate or citrate in the mobile phase.2 For the determination of alkaline-earth metal ions, a zinc nitrate/nitric acid mixture has been used as eluent, zinc hydroxide being precipitated by use of a hydroxide-form suppressor column.3 A problem with basic suppressor resins is the likelihood of metal hydroxide precipitation, and this has led to other types of eluent/ suppressor systems being used. Nordmeyer et ~1.~ determined alkaline-earth and bivalent transition-metal cations with barium nitrate, barium chloride or lead nitrate solutions as eluents. The suppressor column was loaded with sulfate ions, to precipitate the eluent cations as their sulfates. A column in hydrogen-ion form was placed between the suppressor and detector *Author for correspondence.

to minimize the-effect of pH on the baseline and to increase the sensitivity. A lead nitrate eluent with an iodate suppressor (to precipitate lead iodate) was later used,’ and this also allowed barium to be determined. The hydrogen-ion form post-suppressor was replaced by a hydroxide-form column. Overall, the success of suppressed ion-chromatography for cation determinations has been limited. Although the technique is satisfactory for alkali and alkalineearth metals, many other cations (including those of transition metals) are determined with poor selectivity because of band broadening within the suppressor column. If a low-conductivity mobile phase is used, then its suppression may not be necessary. Sevenich and Fritz6 overcame some of the limitations above in use of an unsuppressed system for the determination of 25 metal ions. The eluent contained a mixture of ethylenediammonium and tartrate ions which provided sharper peaks and hence improved selectivity for many cations. However, some metals gave badly tailing peaks due to slow complexing and decomplexing equilibria. The difficulties of conductivity detection have led to other systems being used, of which ultraviolet-visible molecular absorption spectrometry has been the most common. It is a very suitable detector for flow-through systems, already well established for high-performance liquid chromatography (HPLC). When it is used for cation chromatography, however, postcolumn derivatization is necessary. A complexing agent, such as 4-(2-pyridylazo)resorcinol

(PAR), is introduced into the flowing stream after the ion-exchange column,’ and the resulting metal complexes absorb light in the visible region of the spectrum. The advantage of PAR over many other chelating agents is the high molar absorptivity of its metal complexes and the wide range of metals that react with it7 This post-column derivatization system has certain disadvantages, however. Although PAR reacts with many cations, each complex has maxims absorption at a different wavelength. The broad absorption bands overlap, allowing all the complexes to be detected at the same wavelength, but this is a compromise, and the sensitivity may not be optimum for all analytes. A novel derivatization system was introduced by Argue110 and Fritz’ for the determination of calcium and magnesium ions. The reagent consisted of an equimolar mixture of PAR and zinc-EDTA complex. Analyte ions then displaced zinc from its EDTA complex, which allowed formation of the Zn-PAR complex, which was measured spectrophotometrically. A similar system was described by Yan and Schwedt9 Compromise detection conditions are then not required because the same absorbing species is detected fur all ions. In this paper we describe the introduction of a post-column reagent stream containing Eriochrome Black T in its doubly dissociated form (HEBT’-) and Mg-EDTA. The following post-column iondisplacement (PCID) reaction (e.g., for a bivalent cation M2+) then takes place: M2+ + HEBT2- + Mg-EDTA2FL Mg-EBT-

+ M-EDTA2-

+ H+

(1)

An advantage of this system is that Mg-EDTA is one of the weakest metal-EDTA complexes.‘a

Hence, the equilibrium is always strongly to the right, permitting the analyte ion to displace magnesium, which then forms the EBT complex. Detection is by measu~ment of the increase in absorbance of Mg-EBT- at 520 nm, or the decrease in absorbance of HEBT*- at 610 run. A possible disadvantage of the earlier Zn-EDTA/PAR system is that some metal-PAR complexes have higher formation constants than that of Zn-PAR.” This could prevent the formation of Zn-PAR in the presence of some analytes. EXPERIMENTAL

The liquid chromatograph, incorporating PCID, is shown schematically in Fig. 1. A Bio-Rad Model 1330 HPLC pump (Bio-Rad, Mississauga, Ontario, Canada) was used to deliver the mobile phase. Samples were introduced into the mubile phase stream by a Rheodyne Model 7125 injection valve (Bio-Rad), fitted with a 20-4 sample loop. A Wescan high-speed cation-chromatography column was used (Wescan Instruments, Santa Clara, CA, U.S.A.) and the detector was a Bio-Rad Model 1305A variable wavelength absorbance monitor, fitted with a tungsten lamp as light-source. Chromatograms were recorded on a Fisher Recordall Series 5000 chart recorder. Postcolumn reagent was pumped by nitrogen pressure from a l~-~ Nalgene bottle inside a constant-pressure vessel (10 psig; this system was based on that designed by Fritz and Willis).” The mobile phase and reagent streams were both pumped through l/16-in. bore stainless-steel tubing. The mixing cell for the two streams was either a “T-piece” or a “Y-piece” INJECTION VALVE

MIXING CELL

REAGENT--

-3

Fig. 1. Schematic diagram of tie chromatograph and PCfD system.

Ion-chromatography of metals with post-column ion displacement

with OS-mm bore channels. These cells were each machined out of a single block of Perspex, drilled to accommodate standard low-pressure stainless-steel or polyethylene fittings. The “Tpiece” was used for most of the work described in this paper. Back-pressure was provided in both the mobile phase and reagent streams by inserting dummy columns (50 cm x 3 mm bore) packed with silica gel. Tygon tubing (0.5 mm bore) was used between the “T-” or “Y-piece” and the detector. The mixing coil was a 2-m length of the same tubing. Reagents Throughout the work, reagents of the highest available purity were used. All dilutions were made with glass-distilled water. Ethylenediamine ltartar~~ acid (IDA / TA) mo bile phase. This contained 1.5 x 10V3&f ethylenediamine and 2 x 10T3M tartaric acid, adjusted to pH 4 with nitric acid. Ammonium a-hydroxyisobutyrate (0UPM) (HZZ3A) mobile phase. This was prepared by dissolving 9.4 g of a-hydroxyisobutyric acid (Aldrich Chemical Co., Milwaukee, WI 53233) in almost 1000 ml of water and adding 7 ml of concentrated ammonia solution. After adjustment to pH 4 with nitric acid, the solution was made up to 1000 ml with water. Post-cozen ion-d~pia~ement reagent. This contained 2 x 10p4M Eriochrome Bfack T and 2 x 10m4M Mg-EDTA, adjusted to pH 10 (for use with EDA/TA mobile phase) or pH 12 (for HIBA mobile phase), with an ammonia/ ammonium chloride buffer. This was sufficient to give the mixed stream a pH of 10. The Mg-EDTA solution was prepared by titrating 0.02M Mg2+ with 0.02M EDTA at pH 10 to an Eriochrome Black T end-point, in order to ensure equimolar concentrations of magnesium and EDTA. An indictor-fry solution was then made by mixing the appropriate volumes of the components. The titration was repeated whenever a new batch of either component was used. Metal standard solutions. Appropriate amounts of the pure metal or metal salt were dissolved in nitric acid to produce lOOO-mg/l. stock metal solutions. More dilute solutions for injection into the chromatograph were prepared in 1% nitric acid. Procedure The flow-rates used were: mobile phase 1.0 ml/min (EDA/TA) or 0.5 ml/min (HIBA);

837

post-column reagent 0.3 ml/min. The peak areas for the absorbance at 520 nm, and the retention times, were measured. RESULTS AND DISCWSSION

AppIic~i~ity of the post -bourn ment method

ion -displace-

For application to a particular cation, the equilibrium in equation (1) should lie strongly to the right, i.e., the conditional formation constant of the analyte metal-EDTA chelate must be greater than that of Mg-EDTA. Tables of conditional formation constants show this to be the case for most metals.‘O The iondisplacement reaction is unsuitable for some cations, even though the conditional formation constants are high. For example, Cd’+ forms a carbonate precipitate, Pb2+ a hydroxide precipitate, and Mn2+ a green colour, because of oxidation to Mn3+ by dissolved oxygen (and formation of a green complex). This complex results in negative absorbance after correction for the blank. This was reflected in a Job plot (at 520 nm) which showed a minimum (the Job plots for all the other metals showed an appropriate maximum). Optimization of ~tr~ental

par~eters

Choice of mobile phase. When a cationexchange column is used, the mobile phase generally includes an acidic anion, often as its ammonium salt. The acids most commonly employed are citric, tartaric and oxalic. As many as 13 metal ions have been separated in 34 min on a strong cation-exchanger with a tartrate mobile phase at pH 2.75.’ These acids are often used in conjunction with weak chelating agents, such as ethylenediamine, to enhance the retention properties of the stationary phase. Recently,12 u-hydroxyisobuty~c acid has been shown to give good resolution for several metal species on a cation-exchange column. For the present work, two mobile phases were investigated; ethylenediamine/ tartaric acid (EDA/TA) and a-hydroxyisobutyric acid (HIBA). pH. For the mixed stream, the optimum pH would be that favoring formation of the analyte metal-EDTA complex and the Mg-EBT complex. At pH below 7, EBT forms a red colloid that does not react with metal ions. Therefore, for the mixed stream, a pH of 10 was chosen as a compromise for all the metal ions determined.

CHRISTINE J. E~OWLES et al.

838

The pH of the mobile phase affected the retention of metal ions. The aim was to select the pH which would give the longest retention times in order to obtain the best resolution. For each mobile phase system, the detector response for Mg2+ was monitored while the pH was varied, with the reagent concentration and flowrates kept constant. The mixed stream was maintained at pH 10 by adjusting the reagent pH. The reagent flow-rate was 0.3 ml/min (for the EDA/TA system) or 0.5 ml/min (for HIBA), and the mobile phase flow-rates were 1.Oml/min (EDA/TA) and 0.5 ml/min (HIBA). The results (Table 1) show that the retention time increased as the pH decreased, until pH 4-5, below which there was little further change in retention time. Hence, pH 4 was chosen as the optimum. The reagent pH that was needed to maintain the mixed stream at its optimum pH depended on the buffering capacity of the mobile phase. For EDA/TA, the reagent pH was 10, but for HIBA (higher buffering capacity), a reagent pH of 12 was needed. Reagent concentration. With EBT used alone as reagent, at varied concentration, excess of 4 x 10m2M magnesium solution was injected into the chromatograph. The flow-rates of the EDA/TA mobile phase and reagent were 1.0 and 0.5 ml/min respectively, and the pH conditions were optimum. The absorbance of the resulting Mg-EBT complex (520 nm) increased with increasing reagent concentration and then levelled off at 2 x 10w4M EBT. This EBT concentration was chosen as the optimum. Flow-rates. Under optimized conditions of pH and reagent concentration, several alkaline earth and transition metal cations were eluted separately to study the effect of flow-rate on retention time. The ion-displacement reaction is fast, so the reagent flow-rate was not critical (provided the 2-m coil was present when HIBA was used). A fast mobile-phase flow-rate is desirable, because it leads to shorter retention times and faster analysis, but this could degrade

Table 2. Retention times of individually chromatographed cations Retention time, min Cation

EDA/TA mobile phase

HIBA mobile phase

Ca2+

2.8 1.6 l.l-3.1* 1.7 1.7 1.5 3.4 1.5

21.0 9.6 4.4/1.4? 13.2 12.0 9.0 38.0 6.6

$ Fez+ Mg2+ Ni*+ srr+ Zn2+

*Concentration-dependent. tDouble peak.

resolution. With EDA/TA, it was possible to use a high mobile-phase flow-rate (1 ml/min) and resolve the alkaline-earth cations MgZ+, Ca2+ and S?+, but transition metal cations were not resolved (Table 2). The use of lower flowrates did not allow transition metal cations to be resolved. The 2-m mixing coil was not required when EDA/TA was used as mobile phase. The fastest HIBA flow-rate which gave satisfactory resolution was 0.5 ml/min. When various metal cations were eluted individually, they gave the retention times shown in Table 2. With these optimized flow-rates (1 ml/min for EDA/TA, or 0.5 ml/min for HIBA) the highest detector response was still achieved with the previously optimized EBT reagent concentration (2 x 10b4M), i.e., the optimum reagent concentration is unaffected by change in flow-rate. Detection limits

Except for Co2+ and Fe2+, both mobile phases gave similar detection limits when cations were eluted separately (Table 3). However, the HIBA mobile phase made the detection limit difficult to measure for Cu2+ (which gave a double peak). Although the same species Table 3. Detection limits of individually chromatographed cations Detection limit*, mgll.

Table 1. Effect of mobile-phase pH on the retention time of Mg2+ Retention time, min PH 6.1 6.0 5.2 5.0 4.0 3.5

EDA/TA mobile phase

HIBA mobile phase

0.9 11.0 1.6 1.7

11.5 12.0 12.1

Cation

EDA/TA mobile phase

HIBA mobile phase

Ca2+ co2+

1.0 11 19 1.0 0.3 1.5 1.6 2.5

1.0 2.1

;$ Mg2’ Ni2+ Sr2+ Zn2+

*Concentration corresponding deviation of a blank.

6.6 0.5 1.4 8.0 1.6 to 3 times the standard

Ion-chromatography

of metals with post-column ion displacement

(Mg-EBT) is always measured in the ion-displacement method, metal ions do not all give the same detection limit, because this depends on the extent of the ion-displacement reaction. Also, some metal ions (notably A13+,Cu2+ ) “block” EBT by forming particularly stable complexes with it. I3 The major contribution to noise was the post-column mixing of reagents. Hence, it is highly likely that the lower noise levels associated with the introduction of reagents through a membrane reactorI would lead to improved detection limits. ikfultielement analysis The use of EDA/TA was not studied further, because of its limited applicability. All multielement studies were done with HIBA as mobile phase. When mixtures of metal ions were separated, the use of HIBA resulted in alteration of some retention times, presumably because of an interaction on the column or in the eluent. For example, Ni2+ and Zn2+ were co-eluted at the retention time previously established for Zn2+ (Table 2). When Zn2+, Mg2+ and Ca2+ were present as a mixture, the chromatogram shown in Fig. 2a was obtained. All the peaks were resolved and the calculated separation factor for Zn2+/Mg+ was acceptable (a = 1.33).

(a)

However, when SrZ+ was added, the retention times for Mgr+ and Ca2+ decreased (Fig. 2b), and the separation of Zn2+ and MgZ+ was poorer (~1= 0.5). Multielement calibration showed linear response up to at least !&lo mg/l. for all cations examined (20 ~1 injection). A commercial multivitamin tablet was analyzed by dissolving it in 50 ml of (1 + 1) hydrochloric acid, diluting accurately to 250 ml with Millipore water, allowing the insoluble binders to settle, and taking two fractions of supematant liquid for analysis. (a) A 2-ml portion was diluted to 100 ml with HIBA solution, and (b) a 3-ml portion was diluted to 50 ml. Aliquots (20 ~1) were injected into the chromatograph and the chromatograms shown in Fig. 3 were obtained. Table 4 shows the results obtained for multielement calibration, and these show reasonable agreement with the manufacturer’s quoted amounts. The results for Ca2+ show the poorest agreement, but the narrow precision limits suggest this is due to variability between the tablets. CONCLUSION

The purpose of this research was to demonstrate the feasibility of PCID for multielement

@I

i

839

Mg

‘+

4 2+

Sr2+ n

Ca ‘+

Ca ‘+

II

11

1

0

4

6

RETENTION

”

1”

12 TlMUmin

16

L

’

I

20

0

I

I

4

I

11

8

i

11

I”’

12

16

RETENTION

1

20

24

’

“1

26

TIME/min

Fig. 2. Typical chromatograms for multielement analysis, with HIBA mobile phase; (a) a mixture of Zn2+, Mg2+ and Ca2+; (b) a mixture of Zn2+, Mg+, Ca2+ and Ss+.

32

840 Table 4. Analysis of multivitamin tablets

Cation CaZS coz+ CU2f Mg2+ Zn2+

(a)

*Precision limits are f two standard deviations. VNot detected. {Detected, but not quantified.

high running costs of plasma spectrometers. Al~ou~ less sensitive, ion~hromato~aphy with PCID would be a lower-priced alternative, with easy automation and providing sequential multielement analysis. An additional advantage for samples such as natural waters could be the ability to detect different oxidation states of a given element.

4 am

M~ufact~r’s quoted amount per Amount found per tablet,* tablet, mg mg 158&S 143&2 162 2.6 x IO-’ ND? 2 NQI 106f2 100*2 100 18f5 16+3 15

0~0 RETENTION TIME/min

Fig 3. Chromatograms obtained from two dilutions of a commercial m~tivitamin tablet. Peaks (from left to right) are Cuz+ (negative peak} ?Zn *+, Mg2+ and Ca*+.

determinations. It was possible to determine only about four cations simultaneously, because the chromatographic efficiency was quite low. With this PCID detection system it should be possible to determine more elements simultaneously by use of other stationary phases, such as chemically modified reversed phases. The requirements of the post-column reagent mixture are a metal chelate with a lower fo~ation constant than the ~rresponding chelates of the analyte metals, together with an indicator that will form a radiation-absorbing complex with the displaced cation. This introduces the possibility of choosing other reagent mixtures for the analysis of various cation mixtures by PCID detection. Atomic spectrometric techniques are most commonly used for trace metal determinations, but multielement analysis requires the high capital expense and

AcknowZedge~nts-T~s research was supported by the Natural Sciences and Engineering Research Council of Canada. We are esatefirl to F. Cantwell for loan of the pressure vessels used in the instrument.

REFERENCES 1. K. Robards, E. Patsalides and S. Dilli, J. Chromntog., 1987, 411, 1. 2. D. Yan and G. Schwedt, Z. Anal. Chem., 1985,320,325. 3. J. W. Wimbcrley, Anal. Chem., 1981, S3, 2137. 4. F. R. Nordmeyer, L. D. Hansen, D. J. Eatough, D. K. Rollins and J. D. Lamb, ibid., 1980, 52, 852. 5. J. D. Lamb, L. D. Hansen, G. G. Patch and F. R. Nordmeyer, ibid., 1981, 53, 749. 6. G. J. Sevenich and J. S. Fritz, ibid, 1983, 55, 12. 7. J. S. Fritz and J. N. Story, ibid., 1974, 46, 825. 8. M. D. Arguello and J, S. Fritz, &id., 1977, 49, 1595. 9. D. Yan and G. Schwedt, Z. And. Chem., 1985,3u1,252. IO. K. L. Cheng, K. Ueno and T. Imamura, CRC Handbook of Organic Analytical Reagents, CRC Press, Boca Raton, Florida, 1982. 11. J. S. Fritz and R. B. Willis, .I. Chromotog., 1973,79, 107. 12. R. M. Cassidy, S. Elchuk and P. K. Dasgupta, Anal. Chem., 1987, 59, 85. 13. R. Pfibil, Applied CompIexometry, p. 22. Pergamon Press, Oxford, 1982. 14. P. K. Dasgupta, Approaches to Ionic Chr~atography, in ion Chromatography, J. G. Tarter ted.), Dekker, New York, 1987,