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Contamination of aqueous eluents due to corrosion of stainless-steel chromatographic components
Journal of Chromatography, 407 (1987) 133-140 ElsevierSciencePublishersB.V.. Amsterdam - Printed in The Netherlands
CHROM. 19 805 OF AQUEOUS ELUENTS DUE TO CORROSION CHROMATOGRAPHIC COMPONENTS
CONTAMINATION STAINLESS-STEEL
OF
P. R. HADDAD* and R. C. L. FOLEY Department ofAnalytical Chemistry, University of New South Wales, P.O. Box I, Kensington, N.S. W. M33 (Australia) (Received June 27th, 1987)
SUMMARY
Typical eluents used for ion-exchange separations of proteins and inorganic anions and cations are examined for the presence of metal ions resulting from corrosion of stainless-steel chromatographic components. Eluents studied include nitric acid, ethylenediamine-tartrate, ethylenediamine-citrate, phthalate, gluconate-borate and tris(hydroxymethyl)aminomethane-sodium chloride. These were repeatedly cycled through the chromatographic system in order to permit possible corrosion products (iron, chromium, manganese, molybdenum and nickel) to accumulate to detectable levels (i.e. 0.3-1.5 ppb). It was found that chromatographic hardware components such as the pump and injector did not contribute to detectable levels of the above corrosion products to the eluents, especially after passivation with 6 A4 nitric acid. However, when a stainless-steel column with metallic frits was included in the eluent flow-path, detectable quantities of metal ions, particularly Fe3+, were observed with low pH eluents exhibiting oxidation or complexation properties.
INTRODUCTION
Those components of a liquid chromatograph which are in contact with the eluent stream are generally constructed of materials which are designed to be chemically inert. The most commonly used material is stainless steel because of its favourable mechanical properties and excellent corrosion resistance. Several studies have been directed towards determination of the stability of stainless steel in chromatographic eluents. Moweryl examined corrosion of 316 stainless steel in the presence of acetonitrile or methanol mobile phases and observed that some components with small diameter openings (such as regulators and capillary tubing) were extremely susceptible to corrosion. This was attributed to erosion of the protective oxide layer on the steel surface as a result of the high fluid velocity and it was found that corrosion of the exposed metal surface followed unless the mobile phase contained a suitable corrosion inhibitor such as lithium nitrate, Sadek et aL2 and Trumbore et al3 have shown that proteins may become irreversibly adsorbed onto stainless-steel chromatographic components, particularly column frits: this process is presumably 0021-9673/87/%03.50
0
1987 Elsevier Science Publishers B.V.
134
P. R. HADDAD,
R. C. L. FOLEY
due to the presence of free metal ions at the metallic surface. Shih and Carr4 reported the occurrence of ligand-exchange reactions between injected metal-diethyldithiocarbamate complexes and nickel ions from stainless-steel fiits and Hutchins et ~1.~ observed the same effect with the low surface area column dispersion plates in radial compression columns. In the former case, treatment of the frit with ethyltrimethoxysilane minimised ligand-exchange reactions. The above studies have concentrated on the mechanical effects of corrosion (in terms of the failure of chromatographic devices such as regulators) or on the loss of solute resulting from reactions with corrosion products. In this paper we report quantitative data on the levels of metal ions introduced into aqueous eluents as a result of corrosion reactions of stainless steel, with particular emphasis on eluents used for ion chromatography. EXPERIMENTAL
Equippnen t
The chromatographic equipment was newly purchased for these studies and consisted of a Waters Assoc. (Milford, MA, U.S.A.) Model M501 pump and Model U6K injector. When required, these items were passivated by pumping 100 ml of 6 M nitric acid through both components, followed by 1 1 of water. Two empty chromatographic columns were also employed, these being Waters Assoc. IC Pak A (stainless steel, 50 x 4.6 mm I.D.) and IC Pak C (unidentified polyene material, 50 x 4.6 mm I.D.) which were obtained by removing the packing material from used columns. The end frits were retained in both columns. The eluent flow-path used was determined by the particular experiment in progress and details are provided in Results and discussion. Analyses for iron, chromium, manganese, nickel and molybdenum were performed on a Labtam PlasmaLab inductively coupled plasma atomic emission spectrometer using wavelengths of 259.94, 267.72, 257.61, 231.60 and 202.03 nm, respectively. The instrumental conditions were carefully adjusted to optimise the response for individual determinations of the above metal ions. Results were corrected for instrumental drift and matrix background correction was also applied. Reagents and procedures
The following reagents were of analytical grade and were used to prepare the eluents listed in Table I: nitric acid, boric acid, potassium hydrogen phthalate and sodium chloride from Ajax Chemicals (Sydney, Australia), ethylenediamine, tartaric acid and sodium tetraborate from May and Baker (Dagenham, U.K.), citric acid from B.D.H. Chemicals (Poole, England), sodium gluconate from Fluka (Switzerland), and tris(hydroxymethyl)aminomethane from Merck (Darmstadt, F.R.G.). These reagents were used without further purification with the exception of nitric acid which was doubly distilled from PTFE apparatus, and ethylenediamine which was doubly distilled. Water for the preparation of eluents was treated by passage through a Millipore (Bedford, MA, U.S.A.) Mill&Q water purification apparatus. Eluents were prepared in polypropylene volumetric ware (obtained from Kartell, Italy) and the eluent reservoir in the chromatographic system was constructed of the same material. The general procedure adopted for each eluent was to take 200
136
P. R. HADbAD,
R. C. L. FOLEY
TABLE II TYPICAL COMPOSITION
OF TYPE.9 304 AND 316 STAINLESS STEEL
Data taken from ref. 12. Element
Cunteni5 (%I
Carbon Manganese Phosphorus Sulphur Silicon Chromium Nickel Molybdenum
Type 304
Type 316
10.08 2.00 0.045 0.030 1.00 18.00-20.00 8.0&12.00 -
10.08 2,oo 0.045 0.030 1.00 16.00-20.00 10.00-14.00 2.00-3.00
graphy is generally of the 304 or 3 16 types and typical compositions of these materials are given in Table II. These materials are corrosion resistant by virtue of a protective coating of chromium-rich oxides which forms on the surface’. This coating can develop gradually during usage or can be formed rapidly by exposing the surface to relatively strong nitric acid solutions. If the latter method is used, the surface is said to be “passivated”. Consideration of the composition of the steel suggests that the ions most likely to be produced by corrosion reactions are iron, chromium, manganese, molybdenum and nickel. These metal ions could be leached from the metallic surface through either direct oxidation or by complexation reactions with eluent components. The
TABLE III SIDE-REACTION COEFFICIENTS (log b:MeJ FOR COMPLEXATION IONS WITH ELUENT COMPONENTS
OF THE STUDIED METAL
Data for calculation of these values was taken from refs. 13 and 14. Metal ion
Fe2+ Fe3+ Mn’+ Cr3 + Ni2 + Mo3+
log
%ULJ
Tartrate (1.3 mM, pH 3.2)
Citrate (10 mM, pH 3.1)
Phthalate (1.0 mM, pH 4.2)
0.01 2.49 0.01 *
0.01 5.44 0.01 *
l
0.005
0.09 *
l
t 0.04 2.48 0.01 l
Ethylenediamine (0.5 mM, pH 3.2)
(3.5 mM, pH 3.1)
** * f*
* * **
2.52 **
3.16 * *
l
_I I. f,r “,
l No stability constant found. ** Insignificant complexation between metal ion and ligand under the experimental conditions used.
CONTAMINATlON
OF AQUEOUS ELUENTS
137
latter mechanism could be expected to be most prevalent with eluents containing strong complexing agents such as citrate, tartrate, phthalate and ethylenediamine.
TableIII listsvaluesof side-reaction coefficients for complexation of the above metal ions with the eluents listed in Table I and shows that significant complexation iron, chromium and, to a lesser extent, nickel can be expected.
of
Analytical methodology
A requirement for this study was the availability of analytical methodology for the above five metal ions, with sensitivity in the low ppb* range. Moreover, it was necessary that this sensitivity be attainable in samples containing a significant proportion of organic matter. Carbon furnace atomic absorption spectrometry (CFAAS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were both evaluated for this task and it was found that ICP-AES produced superior results in terms of sensitivity and precision in those eluents containing organic components, since in the former method this resulted in the appearance of high non-atomic background absorption signals. Under carefully optimised conditions, ICP-AES gave detection limits of 8,9,2, 11 and 11 ppb for iron, chromium, manganese, molybdenum and nickel ions, respectively. Since it was expected that the contamination levels would be low, it was considered necessary to pass each eluent through the chromatographic system a number of times in order to permit ‘any _eorrosion products to accumulate in sufficient concentration to permit reliable analysis. This approach could be applicable only if leaching of metal ions from the surface occurred at a relatively constant rate, at least within the number of cycles used in the experiment. To investigate this effect, the nitric acid eluent was recirculated through the pump, injector and IC Pak A column housing and samples withdrawn at regular intervals. The results for iron and nickel are shown in Fig. 1, from which it can be seen that a progressive increase in the concentrations of these species was observed. In the case of nickel, the rate of increase of concentration was almost constant, whereas the increase in the iron concentration accelerated slightly over the number of eluent cycles considered. These results suggest that measurement of corrosion products by the proposed method of multiple eluent cycles would be reliable and that any error incurred for iron would be an exaggeration rather than an understatement of the concentration likely to be produced by a single passage of the eluent. Corrosion studies
Fig. 2 shows a schematic illustration of the experimental design used in this study. Three different flow paths were possible, depending on the nature of the column used, and each of these could be tested with the pump and injector passivated or unpassivated. The results are given in Table IV which shows the average amount of metal ion resulting from a single pass of the eluent through the chromatographic system. No results are provided for the gluconate-borate eluent because the relatively high level of organic material present in this eluent caused excessive instability of the analytical signals obtained by ICP-AES. However, consideration of the high pH of
* The American billion ( 10y) is meant throughout the article.
138
P. R. HADDAD,
R. C. L. FOLEY
zwo-2ooo"p ,o g1500s e z1000ttl (5 u 500-
0
Fig. 1. Accumulation of iron and nickel during repeated cylcles of the nitric acid eluent. Eluent: 10 mM nitric acid. Chromatographic system: pump, injector, IC Pak A column housing (flow-path A in Fig. 2). Flow-rate: 1 ml/min.
Fig. 2. Schematic illustration of the experimental design showing the alternative flow-paths used. Flowpaths: (A) empty Waters stainless-steel IC Pak A column housing, 5.0 x 4.6 mm I.D., containing stainless-steel frits; (B) PTFE tubing, 0.5 mm I.D.; (C) empty Waters polyene IC Pak C column housing, 5.0 x 4.6 mm I.D., containing non-metallic frits.
TABLE IV AVERAGE ELUENT
CONCENTRATIONS
OF METAL IONS OBSERVED
FOR A SINGLE CYCLE OF
Eluent identities are given in Table I. The flow-paths refer to Fig. 2. The blank levels of metal ions present in the eluent have been subtracted. Eluen t
HNOs
EDA-TA EDA-CA
KHP TRIS-NaCl
Pump passivated
No No Yes No No No No Yes No Yes
Flow path
A B C A B A B C A B
Number of eluent cycles
1.5 7.3 1.2 5.8 7.5 6.4 3.4 1.5 6.6 5.9
Metal concentration (ppb) per eluent cycle Fe
Cr
Mn
MO
Ni
311.8 cl.0 2.1 9.5 1.9 20.0 3.8 2.9
11.0 4.4 11.0 Cl.5 Cl.3 Cl.4 < 2.6 < 1.2 3.2 Cl.5
33.6 co.3 co.3 co.3 0.4 co.3 CO.6 co.3 co.3 co.3
10.8 -=z1.5 < 1.5 < 1.6 < 1.5 < 1.7 ~3.2 i 1.5 < 1.7 i 1.9
11.7 < 1.5 < 1.5 cl.6 < 1.5 Cl.7 < 3.2 Cl.5 < 1.7 Cl.9
CONTAMINATION OF AQUEOUS ELUENTS
139
this eluent (pH 8.5), together with the very low solubilities at this pH of the hydroxides of the metal ions under study and the weakly compiexing nature of the gluconate-borate complex, suggests that it would be .most unlikely that any of the metal ions studied would be solubilised by this eluent. Several inferences may be drawn from Table IV. The first is that passage of an oxidising or complexing eluent through an unpassivated pump and injector produced only minute levels of metal ions. When a metallic column housing was included in the flow-path, significant concentrations of metal ions resulted, particularly with an oxidising eluent. The complexing eluents produced lower concentrations of metal ions and the levels observed were in general agreement with those predicted from consideration of the side-reaction coefficients listed in Table III. The polyene column did not exert any influence on the levels of metal ions observed, however the stainless-steel column was clearly a source of corrosion. It is interesting to speculate on the source of the corrosion products observed with the stainless-steel column. The most probable source is the column frits which have a very high surface area in comparison to the rest of the column and the external chromatographic system and inevitably also contain stress points at which the rate of corrosion would be accelerated. Our calculations show that if the eluent wets the entire surface of the column frits, then these frits account for approximately 96% of the total metal surface in contact with the eluent and should therefore be the prime source of eluent contamination. The levels of metal ions given in Table IV should be viewed from a standpoint which considers the total levels of these species from all sources. The reagents used for the preparation of eluents can be expected to contain residual levels of metal ions: examination of the manufacturers’ specifications shows that the levels of iron and nickel would be in the range of l-5 ppb in the eluents tested. The exception to this
is the tris(hydroxymethyl)aminomethane-sodium chloride eluent which can be expected to contain about 50-60 ppb of iron as a contaminant of sodium chloride: actual measurementsof the background iron level in this eluent revealed a concentration of 59.5 ppb. CONCLUSIONS
The results of this study show that oxidising or complexing eluents used for cation-exchange separations can be contaminated with detectable levels of iron, chromium and nickel from stainless-steel column frits. Accordingly, cation-exchange columns designed for use with these eluents should be fitted with non-metallic frits, or alternatively, metallic frits should be deactivated by passivation or silanisation reactions. Eluents typically used for ion chromatography of anions did not show any contamination from the metallic components of the chromatographic system and this result indicates that metallic frits are suitable for use in anion-exchange columns. Chromatographic hardware components such as pumps and injectors did not contribute any significant levels of the metal ions, particularly when metallic surfaces had been passivated by treatment with nitric acid. Corrosion of these components therefore does not represent a problem in ion chromatographic methods.
140
P. R. HADDAD,
R. C. L. FOLEY
ACKNOWLEDGEMENT
We gratefully acknowledge ments from Mr. R. J. Finlayson.
technical assistance with the ICP-AES measure-
REFERENCES I R. A. Mowery, Jr., J. Chromatogr. Ski., 23 (1985) 22. 2 P.,D. Sadek, P. W. Carr, L. D. Bowers and L. C. Haddad, Anal. Biochem., 144 (1985) 128. 3 C. N. Trumbore, R. D. Tremblay, J. T. Penrose, M. Mercer and F. M. Kelleher, J. Chromatogr., 280 (1983) 43. 4 Y-T. Shih and P. W. Carr, Tulanta, 28 (1981) 411. 5 S. R. Hutchins, P. R. Haddad and S. Dilli, .I. Chromatogr., 252 (1982) 185. 6 P. R. Haddad and A. L. Heckenberg, J. Chromatogr., 252 (1982) 177. 7 P. R. Haddad, P. E. Jackson and A. L. Heckenberg, J. Chromatogr., 346 (1985) 139. 8 J. S. Fritz, D. T. Gjerde and R. M. Becker, Anal. Chem., 52 (1980) 1519. 9 G. J. Sevenich and J. S. Fritz, Anal. Chem., 55 (1983) 12. 10 G. Schwedt and D. R. Yan, Fresenius’ 2. Anal. Chem., 320 (1985) 325. 11 S. H. Chang, K. M. Gooding and F. E. Regnier, JI. Chromatogr., 125 (1976) 103. 12 R. C. Weast (Editor) CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 62nd ed., 1981, p. F-122. 13 R. M. Smith and A. E. Martell, Criricul StubiZity Constants, Plenum Press, New York, 1976. 14 J. Inczedy, Analytical Applications of Complex Equilibria, Ellis Horwood, Chichester, 1976.
CHROM. 19 805 OF AQUEOUS ELUENTS DUE TO CORROSION CHROMATOGRAPHIC COMPONENTS
CONTAMINATION STAINLESS-STEEL
OF
P. R. HADDAD* and R. C. L. FOLEY Department ofAnalytical Chemistry, University of New South Wales, P.O. Box I, Kensington, N.S. W. M33 (Australia) (Received June 27th, 1987)
SUMMARY
Typical eluents used for ion-exchange separations of proteins and inorganic anions and cations are examined for the presence of metal ions resulting from corrosion of stainless-steel chromatographic components. Eluents studied include nitric acid, ethylenediamine-tartrate, ethylenediamine-citrate, phthalate, gluconate-borate and tris(hydroxymethyl)aminomethane-sodium chloride. These were repeatedly cycled through the chromatographic system in order to permit possible corrosion products (iron, chromium, manganese, molybdenum and nickel) to accumulate to detectable levels (i.e. 0.3-1.5 ppb). It was found that chromatographic hardware components such as the pump and injector did not contribute to detectable levels of the above corrosion products to the eluents, especially after passivation with 6 A4 nitric acid. However, when a stainless-steel column with metallic frits was included in the eluent flow-path, detectable quantities of metal ions, particularly Fe3+, were observed with low pH eluents exhibiting oxidation or complexation properties.
INTRODUCTION
Those components of a liquid chromatograph which are in contact with the eluent stream are generally constructed of materials which are designed to be chemically inert. The most commonly used material is stainless steel because of its favourable mechanical properties and excellent corrosion resistance. Several studies have been directed towards determination of the stability of stainless steel in chromatographic eluents. Moweryl examined corrosion of 316 stainless steel in the presence of acetonitrile or methanol mobile phases and observed that some components with small diameter openings (such as regulators and capillary tubing) were extremely susceptible to corrosion. This was attributed to erosion of the protective oxide layer on the steel surface as a result of the high fluid velocity and it was found that corrosion of the exposed metal surface followed unless the mobile phase contained a suitable corrosion inhibitor such as lithium nitrate, Sadek et aL2 and Trumbore et al3 have shown that proteins may become irreversibly adsorbed onto stainless-steel chromatographic components, particularly column frits: this process is presumably 0021-9673/87/%03.50
0
1987 Elsevier Science Publishers B.V.
134
P. R. HADDAD,
R. C. L. FOLEY
due to the presence of free metal ions at the metallic surface. Shih and Carr4 reported the occurrence of ligand-exchange reactions between injected metal-diethyldithiocarbamate complexes and nickel ions from stainless-steel fiits and Hutchins et ~1.~ observed the same effect with the low surface area column dispersion plates in radial compression columns. In the former case, treatment of the frit with ethyltrimethoxysilane minimised ligand-exchange reactions. The above studies have concentrated on the mechanical effects of corrosion (in terms of the failure of chromatographic devices such as regulators) or on the loss of solute resulting from reactions with corrosion products. In this paper we report quantitative data on the levels of metal ions introduced into aqueous eluents as a result of corrosion reactions of stainless steel, with particular emphasis on eluents used for ion chromatography. EXPERIMENTAL
Equippnen t
The chromatographic equipment was newly purchased for these studies and consisted of a Waters Assoc. (Milford, MA, U.S.A.) Model M501 pump and Model U6K injector. When required, these items were passivated by pumping 100 ml of 6 M nitric acid through both components, followed by 1 1 of water. Two empty chromatographic columns were also employed, these being Waters Assoc. IC Pak A (stainless steel, 50 x 4.6 mm I.D.) and IC Pak C (unidentified polyene material, 50 x 4.6 mm I.D.) which were obtained by removing the packing material from used columns. The end frits were retained in both columns. The eluent flow-path used was determined by the particular experiment in progress and details are provided in Results and discussion. Analyses for iron, chromium, manganese, nickel and molybdenum were performed on a Labtam PlasmaLab inductively coupled plasma atomic emission spectrometer using wavelengths of 259.94, 267.72, 257.61, 231.60 and 202.03 nm, respectively. The instrumental conditions were carefully adjusted to optimise the response for individual determinations of the above metal ions. Results were corrected for instrumental drift and matrix background correction was also applied. Reagents and procedures
The following reagents were of analytical grade and were used to prepare the eluents listed in Table I: nitric acid, boric acid, potassium hydrogen phthalate and sodium chloride from Ajax Chemicals (Sydney, Australia), ethylenediamine, tartaric acid and sodium tetraborate from May and Baker (Dagenham, U.K.), citric acid from B.D.H. Chemicals (Poole, England), sodium gluconate from Fluka (Switzerland), and tris(hydroxymethyl)aminomethane from Merck (Darmstadt, F.R.G.). These reagents were used without further purification with the exception of nitric acid which was doubly distilled from PTFE apparatus, and ethylenediamine which was doubly distilled. Water for the preparation of eluents was treated by passage through a Millipore (Bedford, MA, U.S.A.) Mill&Q water purification apparatus. Eluents were prepared in polypropylene volumetric ware (obtained from Kartell, Italy) and the eluent reservoir in the chromatographic system was constructed of the same material. The general procedure adopted for each eluent was to take 200
136
P. R. HADbAD,
R. C. L. FOLEY
TABLE II TYPICAL COMPOSITION
OF TYPE.9 304 AND 316 STAINLESS STEEL
Data taken from ref. 12. Element
Cunteni5 (%I
Carbon Manganese Phosphorus Sulphur Silicon Chromium Nickel Molybdenum
Type 304
Type 316
10.08 2.00 0.045 0.030 1.00 18.00-20.00 8.0&12.00 -
10.08 2,oo 0.045 0.030 1.00 16.00-20.00 10.00-14.00 2.00-3.00
graphy is generally of the 304 or 3 16 types and typical compositions of these materials are given in Table II. These materials are corrosion resistant by virtue of a protective coating of chromium-rich oxides which forms on the surface’. This coating can develop gradually during usage or can be formed rapidly by exposing the surface to relatively strong nitric acid solutions. If the latter method is used, the surface is said to be “passivated”. Consideration of the composition of the steel suggests that the ions most likely to be produced by corrosion reactions are iron, chromium, manganese, molybdenum and nickel. These metal ions could be leached from the metallic surface through either direct oxidation or by complexation reactions with eluent components. The
TABLE III SIDE-REACTION COEFFICIENTS (log b:MeJ FOR COMPLEXATION IONS WITH ELUENT COMPONENTS
OF THE STUDIED METAL
Data for calculation of these values was taken from refs. 13 and 14. Metal ion
Fe2+ Fe3+ Mn’+ Cr3 + Ni2 + Mo3+
log
%ULJ
Tartrate (1.3 mM, pH 3.2)
Citrate (10 mM, pH 3.1)
Phthalate (1.0 mM, pH 4.2)
0.01 2.49 0.01 *
0.01 5.44 0.01 *
l
0.005
0.09 *
l
t 0.04 2.48 0.01 l
Ethylenediamine (0.5 mM, pH 3.2)
(3.5 mM, pH 3.1)
** * f*
* * **
2.52 **
3.16 * *
l
_I I. f,r “,
l No stability constant found. ** Insignificant complexation between metal ion and ligand under the experimental conditions used.
CONTAMINATlON
OF AQUEOUS ELUENTS
137
latter mechanism could be expected to be most prevalent with eluents containing strong complexing agents such as citrate, tartrate, phthalate and ethylenediamine.
TableIII listsvaluesof side-reaction coefficients for complexation of the above metal ions with the eluents listed in Table I and shows that significant complexation iron, chromium and, to a lesser extent, nickel can be expected.
of
Analytical methodology
A requirement for this study was the availability of analytical methodology for the above five metal ions, with sensitivity in the low ppb* range. Moreover, it was necessary that this sensitivity be attainable in samples containing a significant proportion of organic matter. Carbon furnace atomic absorption spectrometry (CFAAS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were both evaluated for this task and it was found that ICP-AES produced superior results in terms of sensitivity and precision in those eluents containing organic components, since in the former method this resulted in the appearance of high non-atomic background absorption signals. Under carefully optimised conditions, ICP-AES gave detection limits of 8,9,2, 11 and 11 ppb for iron, chromium, manganese, molybdenum and nickel ions, respectively. Since it was expected that the contamination levels would be low, it was considered necessary to pass each eluent through the chromatographic system a number of times in order to permit ‘any _eorrosion products to accumulate in sufficient concentration to permit reliable analysis. This approach could be applicable only if leaching of metal ions from the surface occurred at a relatively constant rate, at least within the number of cycles used in the experiment. To investigate this effect, the nitric acid eluent was recirculated through the pump, injector and IC Pak A column housing and samples withdrawn at regular intervals. The results for iron and nickel are shown in Fig. 1, from which it can be seen that a progressive increase in the concentrations of these species was observed. In the case of nickel, the rate of increase of concentration was almost constant, whereas the increase in the iron concentration accelerated slightly over the number of eluent cycles considered. These results suggest that measurement of corrosion products by the proposed method of multiple eluent cycles would be reliable and that any error incurred for iron would be an exaggeration rather than an understatement of the concentration likely to be produced by a single passage of the eluent. Corrosion studies
Fig. 2 shows a schematic illustration of the experimental design used in this study. Three different flow paths were possible, depending on the nature of the column used, and each of these could be tested with the pump and injector passivated or unpassivated. The results are given in Table IV which shows the average amount of metal ion resulting from a single pass of the eluent through the chromatographic system. No results are provided for the gluconate-borate eluent because the relatively high level of organic material present in this eluent caused excessive instability of the analytical signals obtained by ICP-AES. However, consideration of the high pH of
* The American billion ( 10y) is meant throughout the article.
138
P. R. HADDAD,
R. C. L. FOLEY
zwo-2ooo"p ,o g1500s e z1000ttl (5 u 500-
0
Fig. 1. Accumulation of iron and nickel during repeated cylcles of the nitric acid eluent. Eluent: 10 mM nitric acid. Chromatographic system: pump, injector, IC Pak A column housing (flow-path A in Fig. 2). Flow-rate: 1 ml/min.
Fig. 2. Schematic illustration of the experimental design showing the alternative flow-paths used. Flowpaths: (A) empty Waters stainless-steel IC Pak A column housing, 5.0 x 4.6 mm I.D., containing stainless-steel frits; (B) PTFE tubing, 0.5 mm I.D.; (C) empty Waters polyene IC Pak C column housing, 5.0 x 4.6 mm I.D., containing non-metallic frits.
TABLE IV AVERAGE ELUENT
CONCENTRATIONS
OF METAL IONS OBSERVED
FOR A SINGLE CYCLE OF
Eluent identities are given in Table I. The flow-paths refer to Fig. 2. The blank levels of metal ions present in the eluent have been subtracted. Eluen t
HNOs
EDA-TA EDA-CA
KHP TRIS-NaCl
Pump passivated
No No Yes No No No No Yes No Yes
Flow path
A B C A B A B C A B
Number of eluent cycles
1.5 7.3 1.2 5.8 7.5 6.4 3.4 1.5 6.6 5.9
Metal concentration (ppb) per eluent cycle Fe
Cr
Mn
MO
Ni
311.8 cl.0 2.1 9.5 1.9 20.0 3.8 2.9
11.0 4.4 11.0 Cl.5 Cl.3 Cl.4 < 2.6 < 1.2 3.2 Cl.5
33.6 co.3 co.3 co.3 0.4 co.3 CO.6 co.3 co.3 co.3
10.8 -=z1.5 < 1.5 < 1.6 < 1.5 < 1.7 ~3.2 i 1.5 < 1.7 i 1.9
11.7 < 1.5 < 1.5 cl.6 < 1.5 Cl.7 < 3.2 Cl.5 < 1.7 Cl.9
CONTAMINATION OF AQUEOUS ELUENTS
139
this eluent (pH 8.5), together with the very low solubilities at this pH of the hydroxides of the metal ions under study and the weakly compiexing nature of the gluconate-borate complex, suggests that it would be .most unlikely that any of the metal ions studied would be solubilised by this eluent. Several inferences may be drawn from Table IV. The first is that passage of an oxidising or complexing eluent through an unpassivated pump and injector produced only minute levels of metal ions. When a metallic column housing was included in the flow-path, significant concentrations of metal ions resulted, particularly with an oxidising eluent. The complexing eluents produced lower concentrations of metal ions and the levels observed were in general agreement with those predicted from consideration of the side-reaction coefficients listed in Table III. The polyene column did not exert any influence on the levels of metal ions observed, however the stainless-steel column was clearly a source of corrosion. It is interesting to speculate on the source of the corrosion products observed with the stainless-steel column. The most probable source is the column frits which have a very high surface area in comparison to the rest of the column and the external chromatographic system and inevitably also contain stress points at which the rate of corrosion would be accelerated. Our calculations show that if the eluent wets the entire surface of the column frits, then these frits account for approximately 96% of the total metal surface in contact with the eluent and should therefore be the prime source of eluent contamination. The levels of metal ions given in Table IV should be viewed from a standpoint which considers the total levels of these species from all sources. The reagents used for the preparation of eluents can be expected to contain residual levels of metal ions: examination of the manufacturers’ specifications shows that the levels of iron and nickel would be in the range of l-5 ppb in the eluents tested. The exception to this
is the tris(hydroxymethyl)aminomethane-sodium chloride eluent which can be expected to contain about 50-60 ppb of iron as a contaminant of sodium chloride: actual measurementsof the background iron level in this eluent revealed a concentration of 59.5 ppb. CONCLUSIONS
The results of this study show that oxidising or complexing eluents used for cation-exchange separations can be contaminated with detectable levels of iron, chromium and nickel from stainless-steel column frits. Accordingly, cation-exchange columns designed for use with these eluents should be fitted with non-metallic frits, or alternatively, metallic frits should be deactivated by passivation or silanisation reactions. Eluents typically used for ion chromatography of anions did not show any contamination from the metallic components of the chromatographic system and this result indicates that metallic frits are suitable for use in anion-exchange columns. Chromatographic hardware components such as pumps and injectors did not contribute any significant levels of the metal ions, particularly when metallic surfaces had been passivated by treatment with nitric acid. Corrosion of these components therefore does not represent a problem in ion chromatographic methods.
140
P. R. HADDAD,
R. C. L. FOLEY
ACKNOWLEDGEMENT
We gratefully acknowledge ments from Mr. R. J. Finlayson.
technical assistance with the ICP-AES measure-
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