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Speciation of chromium in aquatic samples by coupled column ion chromatography-inductively coupled plasma-mass spectrometry
ANALmcA
CHIMICA
ACM
ELSEVIER
Analytica Chimica Acta 318 (1996) 335-343
Speciation of chromium in aquatic samples by coupled column ion chromatography-inductively coupled plasma-mass spectrometry Mari Pantsar-Kallio,
Pentti KG. Manninen
*
Lahti Control and Research Laboratory, Niemenkatu 73 C, FIN-15210, Lahti, Finland Received
10 May 1995; revised 29 August 1995; accepted 8 September
1995
Abstract A method is described that utilizes inductively coupled plasma mass spectrometry as an element-selective detector for coupled column ion chromatographic determination of chromium species in aquatic samples. The coupled column system consists of a cation guard column and an anion column. Species conversion is eliminated by minimizing the sample pretreatment and by using dilute nitric acid eluents. The smallest concentration for quantitation is 0.3 pg/l for Cr(III) and 0.5 pg/l for CrNI). Keywords:
Chromium
speciation;
Inductively
coupled plasma-mass
spectrometry;
1. Introduction Chromium is widely distributed in the environment because of many industrial applications - for example, in the galvanization, steel, leather and paint industries. Chromium compounds are also added to cooling water for corrosion control. As a result chromium is found in the waste waters from many industrial processes. Chromium exists primarily in two oxidation states, trivalent Cr(II1) and hexavalent Cr(VI). The uncomplexed trivalent species is the chromic ion Cr3+, which is soluble in acidic solutions as Cr(H,O)i+, but precipitates as hydroxide in alkaline solution. In seawater, C&II) can also exist as Cr(OH); [l]. The
’ Corresponding
author.
0003-2670/96/$15.00 0 1996 Elsevier Science B.V. AI1 rights reserved SSDIOOO3-2670(95)00448-3
Ion chromatography
hexavalent species exists primarily as the chromate (CrO:- ) or dichromate (Cr,O;- ) ion, depending upon the pH of the solution. In either state, Cr(V1) is a strong oxidizer and therefore harmful in environmental and biological systems. Due to the toxicity of chromium(V1) the speciation of chromium has provided a major challenge for many years. Many speciation methods concentrate on determining the amount of either inorganic C&II) or Cr(VI), and then calculating the amount of the other species by difference after total chromium measurement [2,3]. For determination of hexavalent chromium, sensitive and highly selective colorimetric methods are well established [4,5]. Other methods separate the inorganic species and measure the concentrations directly. In addition to selective volatilization of the species in atomic absorption measurement [6-81, methods for hyphenated liquid
336
M. Pantsar-Kallio,
P.K.G. Manninen /Analytica
chromatographic separation have been developed using flame atomic absorption [9,10], direct current plasma emission [ 111, chemiluminescence [121 or visible spectrometry with postcolumn derivatization [ 131 for detection. Although inductively coupled plasma mass spectrometry (ICP-MS) offers extremely low detection limits as well as multielement capability, and gives isotope information, there are only a few speciation studies for chromium utilizing ICP-MS [14-161. One of the earliest hyphenated methods combining ion pairing LC with plasma emission spectroscopy, was introduced by Krull et al. in 1982 [17]. After that, liquid chromatographic methods, mainly ion pairing [15] and ion chromatography [11,18], have been used for chromium speciation in many studies. An interesting study by Beere and Jones [18] applied an anion exchange column containing a small proportion of cation exchange groups with potassium chloride eluent and a chemiluminescence detector. Urasa and Nam [ll] applied either an anion or a cation column to separate the species. However, the eluents used were very strong for speciation analysis - 1 M NaOH and 1 M HCl. In their method one of the chromium species was retained in the column, while the other eluted in the void volume, as in some other speciation studied [19]. However, there was no guarantee that there might not be other chromium species in the samples, that also eluted in the void volume. The assumption that the chromium exists only as the dissolved inorganic ions in aquatic samples, might in many cases be too strict, because dissolved organic and other dissolved chromium compounds are very likely to occur in the environment. For example, Beaublen et al. [20] found in their speciation study on the Great Lakes that about 10% of the dissolved chromium appeared in the colloidal/organic form. Some other speciation studies, applying HPLC, like EDTA [17] or use complexing ions tetrabutylammonium acetate (TBAA) [21] in the eluent. However, this may change the balance between the species. This paper reports a coupled column ion chromatography-inductively coupled plasma mass spectrometry (IC-ICP-MS) method for determination of the chromium species. The separated fractions can be divided into three parts: those that are retained in the
Chimica Acta 318 (1996) 335-343
cation or in the anion column and those that elute in the void volume through these columns. No reagents were added to the samples prior to analysis and the eluent composition was kept as simple as possible, to prevent species conversion.
2. Experimental 2.1. Ion chromatography A Pharmacia LKB gradient pump (Model 2249) having a reagent delivery module with a beaded mixing coil and a flow-through cell was used. The anion column was an IC-Pak Anion having trimethyl ammonium functionalized groups on polymethacrylate. The two different-sized cation columns used were a Guard-Pak CM/D (3.9 X 5 mm, 5 pm) and IC-Pak CM/D (3.9 X 150 mm) containing sulfonic acid groups on polybutadiene maleic anhydride silica. All the columns were from Waters. The flow-rate of the eluent was 1.0-2.5 mi/min. The gradient programs used with different columns are presented in Table 1. The injection loop volume was 0.5 ml.
Table 1 IC-gradient programs for different columns and column pairs. Cation column 1 is the Cation Guard Pak CM/D and cation column 2 is the IC-Pak CM/D Anion column
Cation column 1
Cation column 2
Anion-cation 1 (column pair)
Anion-cation 2 (column pair)
2.0 ml/min 0.0-2.0 2.5-8.0 8.2-15.0 2.5 ml/mm 0.0-0.5 1.0-4.0 2.0 ml/min 0.0-1.0 1.5-7.5 1.5 ml/min
min min min
5 mM HNO, 10 mM HNO, 40 mM HNO,
min min
0.4 mM HNO, 5 mM HNO,
min min
0.4 mM HNO, 10 mM HNO,
0.0-2.0 2.2-3.0 3.2-6.0 6.2-12.0 1.5 ml/min
min min min min
0.4 3 10 40
0.0-2.0 2.5-10.0 12.2-22.0
min min min
2 mM HNO, 10 mM HNO, 40 mM HNO,
mM mM mM mM
HNO, HNO, HNO, HNO,
M. Pantsar-Kallio, Table 2 ICP-MS instrumental
All the parts in the pump material.
1350 13.5 0.90 0.80 4 500 650 52 3.45 2.90
were
metal-free
Chimica Acta 318 (1996) 335-343
337
The mass calibration for ICP-MS was done daily, and the response calibration twice a week. The instrument was tuned with 10 pg/l indium at mass 115 prior to analysis, to the count rate of 2.3 X lo6 counts/second.
parameters
RF Power, W Outer gas flow, 1 mir-’ Intermediate gas flow, 1 min- ’ Nebulizer gas flow, 1 min-’ Spray chamber temperature, “C Dwell time per channel, ms Total acquisition time, s Single ion monitoring mode, m/z Detector voltage, JIT 1, mV Detector voltage, JIT 2, mV
2.2. Inductively
P.K.G. Manninen/Analytica
3. Results and discussion
PEEK
coupled plasma mass spectrometry
A Fisons Plasma Quad PQ II + with a concentric nebulizer, a Scott-type quartz spray chamber and a Fassel-type quartz torch were used for detection. The data were collected by monitoring m/z 52 using the single ion monitoring mode (SIM). The operating conditions are listed in Table 2.
It was found necessary to concentrate both the cationic and the anionic species in the column, in order to get lower detection limits and to ensure, that the different species did not coelute in the void volume into the ICP-MS. This was done by coupling an anion and a cation column. The study was divided into the three following parts: (1) optimization of the cation column conditions, (2) optimization of the anion column conditions, (3) optimization of anioncation column pair conditions. Concentrating both the cationic and the anionic species in the columns gives much lower detection limits than are obtained if the other species elutes in the void volume. 3.1. Optimization of the cation Guard-Pak column conditions
CM/D
2.3. Reagents The eluent for ion chromatographic separations was nitric acid (Merck, Darmstadt) diluted with water, passed through Milli-Q mixed-bed resin (Millipore, Milford) to 0.4-40 mM. The chromium standards were made up from pro analysi grade K,Cr,O, and Cr(NO,), .9H,O from Merck. The C&II) concentration was verified by GFAAS and comparing the results with standards made up from Cr(NO,), (Titrisol Merck Art. 9948). 2.4. Procedures The columns were purged with water for 5-10 min between injections. When the columns were not in use, 0.1 ml/min of 0.4 mM HNO, was pumped through them. Before starting the analysis, the cation columns had to be cleaned with 10 mM HNO,, and anion column with 40 mM HNO,. The air was removed from the eluents by bubbling nitrogen through for 15 min and then keeping the eluent in an ultra sonic bath for 10 min.
The Guard-Pak CM/D column is usually used as a cation precolumn concentrator, and its dimensions are only 3.9 X 5.0 mm. The optimum start of the gradient would have been water, because there was an attempt to use as dilute eluents as possible to prevent species conversions, by allowing the separation conditions to be as close to the natural conditions of the sample as possible. However, the column did not retain C&II) at neutral pH, so 0.4 mM HNO, was chosen as the first eluent. The Cr(V1) was not retained in the Guard-Pak CM/D column and eluted in the void volume. The C&II) was eluted from the column with 5 mM HNO,. The flow-rate was high, 2.5 ml/min, giving very sharp peaks for both species and a total analysis time of only 4 min. The separation is presented in Fig. 1. The linearity was good for C&II) in lake water using the Guard-Pak CM/D column to only 10 pg/l due to the competing ions and the small capacity of this short column. With higher concentrations, the C&II) peak broke up into a double peak, and some C&II) was not retained in the column at all,
M. Pantsar-Kallio,
338
38
P.K.G. Manninen /Analytica
75
i13 Time
Chimica Acta 318 (1996) 335-343
150
i8S
225
263
-J
300
(5)
Fig. 1. Separation of 10 pg/l C&II) and C&I) spiked in lake water, using the cation Guard-Pak are described in Table 1, and the ICP-MS conditions in Table 2.
CM/D
column. The gradient conditions
in the column. For the standard solutions prepared in Milli-Q water, both species were linear up to 500
eluting at the void volume, thus preventing also the determination of Cr(V1). For Cr(V1) the method was linear up to 200 pg/l, when all the (XII) retained
lJg/l.
WCC0
j!jC30
R
a 4LTo50 \Y
2
1co50
” ?
I
3?000
5
2.:030 i
co;0
3003
_~-_-__-_---_______-___-_-__i_____-_---_____________ 33
is
1%
il3. Tint:
l:i3
225
263
1s)
Fig. 2. Separation of 10 pg/l C&II) and Cr(VI) spiked in lake water, using the cation IC Pak CM/D described in Table 1, and the ICP-MS conditions in Table 2.
column. The gradient conditions
are
M. Pantsar-Kallio,
3.2. Optimization umn conditions
P.K.G. Manninen /Analytica
of the cation IC-Pak CM/D
col-
This column differed only in its size compared with the Guard-Pak CM/D column. The optimum gradient started again with 0.4 mM HNO,, but the last eluent strength had to be raised to 10 mM, in order to get reasonable analysis times. Also, the optimum flow-rate was decreased to 2.0 ml/mm and the total analysis time was 6 min. The separation is presented in Fig. 2. The linearity for the peak areas was good for both species spiked in lake water to at least 500 pg/l, Cr(V1) eluting again in the void volume. The method can easily be applied to analysis of Cr(II1) and to the analysis of both species, C&II) and Cr(VI), if there are no other chromium species in the sample that elute in the void volume. 3.3. Optimization tions
of the IC-Pak Anion column condi-
In the IC-PAK Anion column, there was found some unrepeatable retention also for (XII); some C&II) eluted in the void volume when starting the gradient with water or 0.4-4 mM HNO,, and the
200
450 Time..(s)
339
rest eluted after increasing the eluent strength to 10 mM. However, all the C&II) could be eluted from the column in the void volume with 5 mM HNO,. After that, the Ct(VI) was eluted with 40 mM HNO,. This method is also suitable for simultaneous speciation of chromium, if one can be sure that there are no other chromium species eluting in the void volume except the C&II) ion. The method was linear for both species up to at least 500 pg/l. With the flow-rate of 2.0 ml/min, the analysis was completed in 15 min, as presented in Fig. 3. 3.4. Optimization conditions
of the anion-cation
column pair
The third part of the study was the coupling of the anion and the cation columns. The idea was: (1) to pump the chromium species to the anion column, (2) to elute the cationic C&II) species from the column and (3) to concentrate them in the cation column, (4) to elute the cationic C&II) species to the ICP-MS, and finally (5) to elute the anionic Cr(VI) species from the anion column through the cation column to the ICP-MS. The procedure would separate the chromium species into at least three fractions:
0 00 153
Chimica Acta 318 (1996) 335-343
'600
Fig. 3. Separation of 10 pg/l Cr(III) and Cr6$ spiked in lake water, described in Table 1, and the ICP-MS conditions in Table 2.
-P-_I----_i50.
using the IC-Pak
500
lC50
Anion column.
The gradient
conditions
are
340
M. Pantsar-Kallio,
P.K.G. Manninen/Analytica
Chimica Acta 318 (1996) 335-343
CdW 50030 i 45030
~0000
I
T
i i
1
35000 I R
sooop-
;,
C 0 25000 ::
T
;
-
20000
5003i 0.00r-
188 375
563 Time
i50
538
1125
1313
Is)
Fig. 4. Separation of 10 pg/l C&II) and Cr(VI) spiked in lake water, using the coupled IC-Pak Anion-Cation Guard Pak column pair. The gradient conditions are described in Table 1, and the ICP-MS conditions in Table 2. The back pressure was 1500 psi and both the Cr(IIIl and Cr(VI) peaks in the chromatogram break up.
sure of 2300 psi with a 2 ml/min flow-rate, and 1500 psi with a 1.5 ml/min flow-rate and the peaks broke up in the chromatogram as shown in Fig. 4;
cationic, anionic, and those that are not retained in either the cation or the anion column. Coupling of two long columns gave a back pres-
55000
50000
45000
,” 3COOO :: , PSC(iO s 2SOOO
i 5000
10000
5000
0.00 84
167
281 Time
374
468
561
655
(I)
Fig. 5. Separation of 10 pg/l C&II) and Cr&I) spiked in lake water, using the coupled Anion-Cation gradient conditions are described in Table 1 and the ICP-MS conditions in Table 2.
Guard-Pak
CM/D
column pair. The
M. Pantsar-Kallio, Elucnt
P.K.G. Manninen/Analytica
pump
column
anion
cation
guard
column
ICPMS
Fig. 6. Schematic system employed.
diagram
of the coupled
column
IC-ICP-MS
although with both columns separately, the separation succeeded. When using the IC-Pak Anion-Cation Guard-Pak CM/D column pair, the back pressure decreases to 1000 psi when the flow-rate was 1.5 ml/mm, and both the cationic and the anionic species could be concentrated and separated in the columns. The sam-
Chimica Acta 318 (1996) 335-343
341
ples were pumped into the anion column with 0.4 mm I-INO,. After 2 min, the HNO, strength was increased to 3.2 mM, which eluted most of the C&II) cations from the anion column to the cation column. An increase of the HNO, concentration to 10 mM eluted all the C&II) ions from the anion and the cation columns in a very sharp zone into the ICP-MS. After that, the Cr(VI) ions were eluted from the anion column with 40 mM HNO,. Good linearity was obtained for Cr(II1) in a range of 0.3-200 pg/l and for Cr(VI) in a range of 0.5-500 pg/l. The extended dynamic linear range of C&II) compared with that obtained by the cation Guard-Pak column alone can be explained by the fact that the weaker cations elute from the cation column with 0.4 mM HNO,, while most of the C&II) is still retained in the anion column, thus leaving more sites free for C&II). The chromatogram for lake water with the coupled column system is presented in Fig. 5, and the schematic diagram of the coupled column ICICP-MS system employed is presented in Fig. 6. The stability of the species under elution conditions used, was tested by injecting only one of the species in time - and no species conversion was found to occur.
i2ci5
R .;, ICCCC
Fig. 7. Speciation of chromium in lake water using the coupled IC-Pak Anion-Cation Guard-Pak concentration was 2.4 pg/l and Cr(VI) 0.45 pg/l. The conditions were described in Tables 1 and 2.
CM/D
column
pair. The Cr(III)
M. Pantsar-Kallio,
342
P.K.G. Manninen /Anaiytica
Chimica Acta 318 (1996) 335-343
Table 3 The comparison of the results (n = 5) obtained for CrGII) and Cr(VI) spiked in lake water by coupled column ICP-MS, GFAAS and the spectrophotometric method. The spike concentrations in lake water are 1, 5 and 10 pg/l for both species. With GFAAS the total chromium concentration was measured Cr(III) IC-ICP-MS Lake water + 1 pg/l
RSD
Lake water + 5 pg/l
RSD
Lake water + 10 pg/l
RSD
CttVI) IC-ICP-MS
1.39 pg/l 8.34% 5.32 wg/l 4.62% 10.85 /.rg/l 5.94%
1.29 fig/l 6.41% 5.27 pg/l l::;Y;g/ 1.92%
3.5. The detection limits and quantitation The detection limits using the coupled IC-Pak Anion-Cation Guard-Pak CM/D column pair were for 0.3 /..4.g/l for Cr3+ and 0.5 pg Cr(VI)/l CrOi-/Cr,Oqin lake water sample, determined as three times the standard deviation of the blank. In this analysis, a 0.5 ml injection loop was used; and when using larger loops, much lower detection limits could be obtained, because both species concentrate in the columns. Fox example with 1 ml injection loop the detection limits were 0.1 pg/l for Cr(II1) and 0.2 pug/l for Cr(V1). The chromatogram for speciation of chromium in lake water near the detection limit, containing 2.4 as CrOt-/ pg/l Cr3+ and 0.45 pg Cr(VI)/l Cr,Of-, is presented in Fig. 7. The chromium species were quantitated by comparing the peak areas with the external standards after background substraction. The integration of the peaks was done manually and the background subtracted was integrated near the peaks of interest. For Cr(II1) also the analyte blank, accumulated to the columns from the eluent used for purification of the instrument between analysis, was subtracted. For Cr(V1) no blank problem existed. 3.6. Comparison of the results by coupled column IC-ICP-MS with GFAAS and the calorimetric method In Table 3, the results obtained for Cr(II1) and Cr(V1) spiked in lake water, by coupled column IC-ICP-MS, are compared with those obtained for total chromium concentration by GFAAS, and for
Cr(VI) coloring
Cr GFAAS
< det. limit
2.52 /&g/l 2.01% 10.81 /&l 4.24% 20.96 /q/l 3.07%
4.64 /.&I 22% 9.79 jAg/l 10.70%
Cr(VI) by the spectrophotometric method measuring the absorbance of @VI) complex with 1,5diphenylcarbazide at 540 nm in pH 1. The stability of the species was not very good with the spectrophotometric method, due to high acid concentration, and the results for the spiked lake water samples are too low (Table 3). The results obtained by coupled column IC-ICP-MS are fully comparable with those obtained by GFAAS, although the RSDs for coupled column IC-ICP-MS are a little higher than for GFAAS.
4. Conclusions The work described in this paper applies a coupled column ion chromatographic separation and concentration of chromium species prior to on-line ICP-MS detection without species conversion. With the development of this highly sensitive method, the chromium species in the samples can be fractionated into at least three parts: those that are retained in the anion or in the cation column and those that elute in the void volume through these columns. The research into quantitation of organic chromium species is continuing in our laboratory together with the study of chromium species stability.
References [I] T.M. Florence and G.E. Batley, CRC Crit. Rev. Anal. Chem., 9 (1980) 219. [2] P. Battistoni, S. Bomadre, G. Fava and G. Gobbi, Talanta, 1983, 30, 15.
M. Pantsar-Kallio,
P.K.G. Manninen/Analytica
[3] M. Sperling, X. Yin and B. WeIz, Analyst, 117 (1992) 629. [4] Standard Methods for the Examination of Water and Wastewater, 17th edn., APHA, AWWA, WPCF, Washington, DC, 1989. [5] Annual Book of ASTM Standards, Chromium-Total D 168786, Vol. 11.02, Water, American Society for Testing and Materials, Philadelphia, PA, 1990. [6] S. Arpadjan and V. Krivan, Anal. Chem., 58 (1986) 2611. [7] W.R. Wolf, J. Chromatogr., 134 (1977) 159. [8] R.J. Lloyd, R.M. Barnes, P.C. Uden and W.G. Elliot, Anal, Chem., 50 (1978) 2025. [9] J. Posta, H. Bemdt, S. Luo and G. Schaldach, Anal. Chem., 55 (1993) 2590. [lo] M. Sperling, X. Yin and B. WeIz, .I. Chromatogr., 640 (1993) 73. [ll] I.T. Urasa and S.H. Nam, J. Chromatogr. Sci., 27 (1989) 30. [12] B. Gammelgaard, 0. Jons and B. Nielsen, Analyst, 117 (1992) 637.
Chimica Acta 318 (1996) 335-343
343
[13] E. Pobozy, M. Trojanowicz and P.R. Worsfold, Anal. Len., 25 (1992) 1373. [14] D.T. Gjerde, D.R. Wiederin, F.G. Smith and B.M. Mattson, J. Chromatogr., 640 (1993) 73. [15] N. Jakubowski, B. Jepkens, D. Stuewer and H. Bemdt, J. Anal. At. Spectrom., 9 (1994) 193. [16] R. Roehl and M. AIforquem., At. Spectrosc., 11 (1990) 210. [17] IS. Krull, D. Bushee, R.N. Savage, R.G. Schleicher and S.B. Smith Jr., Anal. Lett., 15 (1982) 267. [18] H.G. Beere and P. Jones, Anal. Chim. Acta, 293 (1994) 237. [19] E.J. Arar, S.E. Long, T.D. Martin and S. Gold, Environ. Sci. Technol., 26 (1992) 1944. [20] S. Beablen, J. Nriagu, D. Blowes and G. Lawson, Environ. Sci. Technol., 28 (1994) 730. [21] A. Syty, R.G. Christensen and T.C. Rains, At. Spectrosc., 7 (1986) 89.
CHIMICA
ACM
ELSEVIER
Analytica Chimica Acta 318 (1996) 335-343
Speciation of chromium in aquatic samples by coupled column ion chromatography-inductively coupled plasma-mass spectrometry Mari Pantsar-Kallio,
Pentti KG. Manninen
*
Lahti Control and Research Laboratory, Niemenkatu 73 C, FIN-15210, Lahti, Finland Received
10 May 1995; revised 29 August 1995; accepted 8 September
1995
Abstract A method is described that utilizes inductively coupled plasma mass spectrometry as an element-selective detector for coupled column ion chromatographic determination of chromium species in aquatic samples. The coupled column system consists of a cation guard column and an anion column. Species conversion is eliminated by minimizing the sample pretreatment and by using dilute nitric acid eluents. The smallest concentration for quantitation is 0.3 pg/l for Cr(III) and 0.5 pg/l for CrNI). Keywords:
Chromium
speciation;
Inductively
coupled plasma-mass
spectrometry;
1. Introduction Chromium is widely distributed in the environment because of many industrial applications - for example, in the galvanization, steel, leather and paint industries. Chromium compounds are also added to cooling water for corrosion control. As a result chromium is found in the waste waters from many industrial processes. Chromium exists primarily in two oxidation states, trivalent Cr(II1) and hexavalent Cr(VI). The uncomplexed trivalent species is the chromic ion Cr3+, which is soluble in acidic solutions as Cr(H,O)i+, but precipitates as hydroxide in alkaline solution. In seawater, C&II) can also exist as Cr(OH); [l]. The
’ Corresponding
author.
0003-2670/96/$15.00 0 1996 Elsevier Science B.V. AI1 rights reserved SSDIOOO3-2670(95)00448-3
Ion chromatography
hexavalent species exists primarily as the chromate (CrO:- ) or dichromate (Cr,O;- ) ion, depending upon the pH of the solution. In either state, Cr(V1) is a strong oxidizer and therefore harmful in environmental and biological systems. Due to the toxicity of chromium(V1) the speciation of chromium has provided a major challenge for many years. Many speciation methods concentrate on determining the amount of either inorganic C&II) or Cr(VI), and then calculating the amount of the other species by difference after total chromium measurement [2,3]. For determination of hexavalent chromium, sensitive and highly selective colorimetric methods are well established [4,5]. Other methods separate the inorganic species and measure the concentrations directly. In addition to selective volatilization of the species in atomic absorption measurement [6-81, methods for hyphenated liquid
336
M. Pantsar-Kallio,
P.K.G. Manninen /Analytica
chromatographic separation have been developed using flame atomic absorption [9,10], direct current plasma emission [ 111, chemiluminescence [121 or visible spectrometry with postcolumn derivatization [ 131 for detection. Although inductively coupled plasma mass spectrometry (ICP-MS) offers extremely low detection limits as well as multielement capability, and gives isotope information, there are only a few speciation studies for chromium utilizing ICP-MS [14-161. One of the earliest hyphenated methods combining ion pairing LC with plasma emission spectroscopy, was introduced by Krull et al. in 1982 [17]. After that, liquid chromatographic methods, mainly ion pairing [15] and ion chromatography [11,18], have been used for chromium speciation in many studies. An interesting study by Beere and Jones [18] applied an anion exchange column containing a small proportion of cation exchange groups with potassium chloride eluent and a chemiluminescence detector. Urasa and Nam [ll] applied either an anion or a cation column to separate the species. However, the eluents used were very strong for speciation analysis - 1 M NaOH and 1 M HCl. In their method one of the chromium species was retained in the column, while the other eluted in the void volume, as in some other speciation studied [19]. However, there was no guarantee that there might not be other chromium species in the samples, that also eluted in the void volume. The assumption that the chromium exists only as the dissolved inorganic ions in aquatic samples, might in many cases be too strict, because dissolved organic and other dissolved chromium compounds are very likely to occur in the environment. For example, Beaublen et al. [20] found in their speciation study on the Great Lakes that about 10% of the dissolved chromium appeared in the colloidal/organic form. Some other speciation studies, applying HPLC, like EDTA [17] or use complexing ions tetrabutylammonium acetate (TBAA) [21] in the eluent. However, this may change the balance between the species. This paper reports a coupled column ion chromatography-inductively coupled plasma mass spectrometry (IC-ICP-MS) method for determination of the chromium species. The separated fractions can be divided into three parts: those that are retained in the
Chimica Acta 318 (1996) 335-343
cation or in the anion column and those that elute in the void volume through these columns. No reagents were added to the samples prior to analysis and the eluent composition was kept as simple as possible, to prevent species conversion.
2. Experimental 2.1. Ion chromatography A Pharmacia LKB gradient pump (Model 2249) having a reagent delivery module with a beaded mixing coil and a flow-through cell was used. The anion column was an IC-Pak Anion having trimethyl ammonium functionalized groups on polymethacrylate. The two different-sized cation columns used were a Guard-Pak CM/D (3.9 X 5 mm, 5 pm) and IC-Pak CM/D (3.9 X 150 mm) containing sulfonic acid groups on polybutadiene maleic anhydride silica. All the columns were from Waters. The flow-rate of the eluent was 1.0-2.5 mi/min. The gradient programs used with different columns are presented in Table 1. The injection loop volume was 0.5 ml.
Table 1 IC-gradient programs for different columns and column pairs. Cation column 1 is the Cation Guard Pak CM/D and cation column 2 is the IC-Pak CM/D Anion column
Cation column 1
Cation column 2
Anion-cation 1 (column pair)
Anion-cation 2 (column pair)
2.0 ml/min 0.0-2.0 2.5-8.0 8.2-15.0 2.5 ml/mm 0.0-0.5 1.0-4.0 2.0 ml/min 0.0-1.0 1.5-7.5 1.5 ml/min
min min min
5 mM HNO, 10 mM HNO, 40 mM HNO,
min min
0.4 mM HNO, 5 mM HNO,
min min
0.4 mM HNO, 10 mM HNO,
0.0-2.0 2.2-3.0 3.2-6.0 6.2-12.0 1.5 ml/min
min min min min
0.4 3 10 40
0.0-2.0 2.5-10.0 12.2-22.0
min min min
2 mM HNO, 10 mM HNO, 40 mM HNO,
mM mM mM mM
HNO, HNO, HNO, HNO,
M. Pantsar-Kallio, Table 2 ICP-MS instrumental
All the parts in the pump material.
1350 13.5 0.90 0.80 4 500 650 52 3.45 2.90
were
metal-free
Chimica Acta 318 (1996) 335-343
337
The mass calibration for ICP-MS was done daily, and the response calibration twice a week. The instrument was tuned with 10 pg/l indium at mass 115 prior to analysis, to the count rate of 2.3 X lo6 counts/second.
parameters
RF Power, W Outer gas flow, 1 mir-’ Intermediate gas flow, 1 min- ’ Nebulizer gas flow, 1 min-’ Spray chamber temperature, “C Dwell time per channel, ms Total acquisition time, s Single ion monitoring mode, m/z Detector voltage, JIT 1, mV Detector voltage, JIT 2, mV
2.2. Inductively
P.K.G. Manninen/Analytica
3. Results and discussion
PEEK
coupled plasma mass spectrometry
A Fisons Plasma Quad PQ II + with a concentric nebulizer, a Scott-type quartz spray chamber and a Fassel-type quartz torch were used for detection. The data were collected by monitoring m/z 52 using the single ion monitoring mode (SIM). The operating conditions are listed in Table 2.
It was found necessary to concentrate both the cationic and the anionic species in the column, in order to get lower detection limits and to ensure, that the different species did not coelute in the void volume into the ICP-MS. This was done by coupling an anion and a cation column. The study was divided into the three following parts: (1) optimization of the cation column conditions, (2) optimization of the anion column conditions, (3) optimization of anioncation column pair conditions. Concentrating both the cationic and the anionic species in the columns gives much lower detection limits than are obtained if the other species elutes in the void volume. 3.1. Optimization of the cation Guard-Pak column conditions
CM/D
2.3. Reagents The eluent for ion chromatographic separations was nitric acid (Merck, Darmstadt) diluted with water, passed through Milli-Q mixed-bed resin (Millipore, Milford) to 0.4-40 mM. The chromium standards were made up from pro analysi grade K,Cr,O, and Cr(NO,), .9H,O from Merck. The C&II) concentration was verified by GFAAS and comparing the results with standards made up from Cr(NO,), (Titrisol Merck Art. 9948). 2.4. Procedures The columns were purged with water for 5-10 min between injections. When the columns were not in use, 0.1 ml/min of 0.4 mM HNO, was pumped through them. Before starting the analysis, the cation columns had to be cleaned with 10 mM HNO,, and anion column with 40 mM HNO,. The air was removed from the eluents by bubbling nitrogen through for 15 min and then keeping the eluent in an ultra sonic bath for 10 min.
The Guard-Pak CM/D column is usually used as a cation precolumn concentrator, and its dimensions are only 3.9 X 5.0 mm. The optimum start of the gradient would have been water, because there was an attempt to use as dilute eluents as possible to prevent species conversions, by allowing the separation conditions to be as close to the natural conditions of the sample as possible. However, the column did not retain C&II) at neutral pH, so 0.4 mM HNO, was chosen as the first eluent. The Cr(V1) was not retained in the Guard-Pak CM/D column and eluted in the void volume. The C&II) was eluted from the column with 5 mM HNO,. The flow-rate was high, 2.5 ml/min, giving very sharp peaks for both species and a total analysis time of only 4 min. The separation is presented in Fig. 1. The linearity was good for C&II) in lake water using the Guard-Pak CM/D column to only 10 pg/l due to the competing ions and the small capacity of this short column. With higher concentrations, the C&II) peak broke up into a double peak, and some C&II) was not retained in the column at all,
M. Pantsar-Kallio,
338
38
P.K.G. Manninen /Analytica
75
i13 Time
Chimica Acta 318 (1996) 335-343
150
i8S
225
263
-J
300
(5)
Fig. 1. Separation of 10 pg/l C&II) and C&I) spiked in lake water, using the cation Guard-Pak are described in Table 1, and the ICP-MS conditions in Table 2.
CM/D
column. The gradient conditions
in the column. For the standard solutions prepared in Milli-Q water, both species were linear up to 500
eluting at the void volume, thus preventing also the determination of Cr(V1). For Cr(V1) the method was linear up to 200 pg/l, when all the (XII) retained
lJg/l.
WCC0
j!jC30
R
a 4LTo50 \Y
2
1co50
” ?
I
3?000
5
2.:030 i
co;0
3003
_~-_-__-_---_______-___-_-__i_____-_---_____________ 33
is
1%
il3. Tint:
l:i3
225
263
1s)
Fig. 2. Separation of 10 pg/l C&II) and Cr(VI) spiked in lake water, using the cation IC Pak CM/D described in Table 1, and the ICP-MS conditions in Table 2.
column. The gradient conditions
are
M. Pantsar-Kallio,
3.2. Optimization umn conditions
P.K.G. Manninen /Analytica
of the cation IC-Pak CM/D
col-
This column differed only in its size compared with the Guard-Pak CM/D column. The optimum gradient started again with 0.4 mM HNO,, but the last eluent strength had to be raised to 10 mM, in order to get reasonable analysis times. Also, the optimum flow-rate was decreased to 2.0 ml/mm and the total analysis time was 6 min. The separation is presented in Fig. 2. The linearity for the peak areas was good for both species spiked in lake water to at least 500 pg/l, Cr(V1) eluting again in the void volume. The method can easily be applied to analysis of Cr(II1) and to the analysis of both species, C&II) and Cr(VI), if there are no other chromium species in the sample that elute in the void volume. 3.3. Optimization tions
of the IC-Pak Anion column condi-
In the IC-PAK Anion column, there was found some unrepeatable retention also for (XII); some C&II) eluted in the void volume when starting the gradient with water or 0.4-4 mM HNO,, and the
200
450 Time..(s)
339
rest eluted after increasing the eluent strength to 10 mM. However, all the C&II) could be eluted from the column in the void volume with 5 mM HNO,. After that, the Ct(VI) was eluted with 40 mM HNO,. This method is also suitable for simultaneous speciation of chromium, if one can be sure that there are no other chromium species eluting in the void volume except the C&II) ion. The method was linear for both species up to at least 500 pg/l. With the flow-rate of 2.0 ml/min, the analysis was completed in 15 min, as presented in Fig. 3. 3.4. Optimization conditions
of the anion-cation
column pair
The third part of the study was the coupling of the anion and the cation columns. The idea was: (1) to pump the chromium species to the anion column, (2) to elute the cationic C&II) species from the column and (3) to concentrate them in the cation column, (4) to elute the cationic C&II) species to the ICP-MS, and finally (5) to elute the anionic Cr(VI) species from the anion column through the cation column to the ICP-MS. The procedure would separate the chromium species into at least three fractions:
0 00 153
Chimica Acta 318 (1996) 335-343
'600
Fig. 3. Separation of 10 pg/l Cr(III) and Cr6$ spiked in lake water, described in Table 1, and the ICP-MS conditions in Table 2.
-P-_I----_i50.
using the IC-Pak
500
lC50
Anion column.
The gradient
conditions
are
340
M. Pantsar-Kallio,
P.K.G. Manninen/Analytica
Chimica Acta 318 (1996) 335-343
CdW 50030 i 45030
~0000
I
T
i i
1
35000 I R
sooop-
;,
C 0 25000 ::
T
;
-
20000
5003i 0.00r-
188 375
563 Time
i50
538
1125
1313
Is)
Fig. 4. Separation of 10 pg/l C&II) and Cr(VI) spiked in lake water, using the coupled IC-Pak Anion-Cation Guard Pak column pair. The gradient conditions are described in Table 1, and the ICP-MS conditions in Table 2. The back pressure was 1500 psi and both the Cr(IIIl and Cr(VI) peaks in the chromatogram break up.
sure of 2300 psi with a 2 ml/min flow-rate, and 1500 psi with a 1.5 ml/min flow-rate and the peaks broke up in the chromatogram as shown in Fig. 4;
cationic, anionic, and those that are not retained in either the cation or the anion column. Coupling of two long columns gave a back pres-
55000
50000
45000
,” 3COOO :: , PSC(iO s 2SOOO
i 5000
10000
5000
0.00 84
167
281 Time
374
468
561
655
(I)
Fig. 5. Separation of 10 pg/l C&II) and Cr&I) spiked in lake water, using the coupled Anion-Cation gradient conditions are described in Table 1 and the ICP-MS conditions in Table 2.
Guard-Pak
CM/D
column pair. The
M. Pantsar-Kallio, Elucnt
P.K.G. Manninen/Analytica
pump
column
anion
cation
guard
column
ICPMS
Fig. 6. Schematic system employed.
diagram
of the coupled
column
IC-ICP-MS
although with both columns separately, the separation succeeded. When using the IC-Pak Anion-Cation Guard-Pak CM/D column pair, the back pressure decreases to 1000 psi when the flow-rate was 1.5 ml/mm, and both the cationic and the anionic species could be concentrated and separated in the columns. The sam-
Chimica Acta 318 (1996) 335-343
341
ples were pumped into the anion column with 0.4 mm I-INO,. After 2 min, the HNO, strength was increased to 3.2 mM, which eluted most of the C&II) cations from the anion column to the cation column. An increase of the HNO, concentration to 10 mM eluted all the C&II) ions from the anion and the cation columns in a very sharp zone into the ICP-MS. After that, the Cr(VI) ions were eluted from the anion column with 40 mM HNO,. Good linearity was obtained for Cr(II1) in a range of 0.3-200 pg/l and for Cr(VI) in a range of 0.5-500 pg/l. The extended dynamic linear range of C&II) compared with that obtained by the cation Guard-Pak column alone can be explained by the fact that the weaker cations elute from the cation column with 0.4 mM HNO,, while most of the C&II) is still retained in the anion column, thus leaving more sites free for C&II). The chromatogram for lake water with the coupled column system is presented in Fig. 5, and the schematic diagram of the coupled column ICICP-MS system employed is presented in Fig. 6. The stability of the species under elution conditions used, was tested by injecting only one of the species in time - and no species conversion was found to occur.
i2ci5
R .;, ICCCC
Fig. 7. Speciation of chromium in lake water using the coupled IC-Pak Anion-Cation Guard-Pak concentration was 2.4 pg/l and Cr(VI) 0.45 pg/l. The conditions were described in Tables 1 and 2.
CM/D
column
pair. The Cr(III)
M. Pantsar-Kallio,
342
P.K.G. Manninen /Anaiytica
Chimica Acta 318 (1996) 335-343
Table 3 The comparison of the results (n = 5) obtained for CrGII) and Cr(VI) spiked in lake water by coupled column ICP-MS, GFAAS and the spectrophotometric method. The spike concentrations in lake water are 1, 5 and 10 pg/l for both species. With GFAAS the total chromium concentration was measured Cr(III) IC-ICP-MS Lake water + 1 pg/l
RSD
Lake water + 5 pg/l
RSD
Lake water + 10 pg/l
RSD
CttVI) IC-ICP-MS
1.39 pg/l 8.34% 5.32 wg/l 4.62% 10.85 /.rg/l 5.94%
1.29 fig/l 6.41% 5.27 pg/l l::;Y;g/ 1.92%
3.5. The detection limits and quantitation The detection limits using the coupled IC-Pak Anion-Cation Guard-Pak CM/D column pair were for 0.3 /..4.g/l for Cr3+ and 0.5 pg Cr(VI)/l CrOi-/Cr,Oqin lake water sample, determined as three times the standard deviation of the blank. In this analysis, a 0.5 ml injection loop was used; and when using larger loops, much lower detection limits could be obtained, because both species concentrate in the columns. Fox example with 1 ml injection loop the detection limits were 0.1 pg/l for Cr(II1) and 0.2 pug/l for Cr(V1). The chromatogram for speciation of chromium in lake water near the detection limit, containing 2.4 as CrOt-/ pg/l Cr3+ and 0.45 pg Cr(VI)/l Cr,Of-, is presented in Fig. 7. The chromium species were quantitated by comparing the peak areas with the external standards after background substraction. The integration of the peaks was done manually and the background subtracted was integrated near the peaks of interest. For Cr(II1) also the analyte blank, accumulated to the columns from the eluent used for purification of the instrument between analysis, was subtracted. For Cr(V1) no blank problem existed. 3.6. Comparison of the results by coupled column IC-ICP-MS with GFAAS and the calorimetric method In Table 3, the results obtained for Cr(II1) and Cr(V1) spiked in lake water, by coupled column IC-ICP-MS, are compared with those obtained for total chromium concentration by GFAAS, and for
Cr(VI) coloring
Cr GFAAS
< det. limit
2.52 /&g/l 2.01% 10.81 /&l 4.24% 20.96 /q/l 3.07%
4.64 /.&I 22% 9.79 jAg/l 10.70%
Cr(VI) by the spectrophotometric method measuring the absorbance of @VI) complex with 1,5diphenylcarbazide at 540 nm in pH 1. The stability of the species was not very good with the spectrophotometric method, due to high acid concentration, and the results for the spiked lake water samples are too low (Table 3). The results obtained by coupled column IC-ICP-MS are fully comparable with those obtained by GFAAS, although the RSDs for coupled column IC-ICP-MS are a little higher than for GFAAS.
4. Conclusions The work described in this paper applies a coupled column ion chromatographic separation and concentration of chromium species prior to on-line ICP-MS detection without species conversion. With the development of this highly sensitive method, the chromium species in the samples can be fractionated into at least three parts: those that are retained in the anion or in the cation column and those that elute in the void volume through these columns. The research into quantitation of organic chromium species is continuing in our laboratory together with the study of chromium species stability.
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M. Pantsar-Kallio,
P.K.G. Manninen/Analytica
[3] M. Sperling, X. Yin and B. WeIz, Analyst, 117 (1992) 629. [4] Standard Methods for the Examination of Water and Wastewater, 17th edn., APHA, AWWA, WPCF, Washington, DC, 1989. [5] Annual Book of ASTM Standards, Chromium-Total D 168786, Vol. 11.02, Water, American Society for Testing and Materials, Philadelphia, PA, 1990. [6] S. Arpadjan and V. Krivan, Anal. Chem., 58 (1986) 2611. [7] W.R. Wolf, J. Chromatogr., 134 (1977) 159. [8] R.J. Lloyd, R.M. Barnes, P.C. Uden and W.G. Elliot, Anal, Chem., 50 (1978) 2025. [9] J. Posta, H. Bemdt, S. Luo and G. Schaldach, Anal. Chem., 55 (1993) 2590. [lo] M. Sperling, X. Yin and B. WeIz, .I. Chromatogr., 640 (1993) 73. [ll] I.T. Urasa and S.H. Nam, J. Chromatogr. Sci., 27 (1989) 30. [12] B. Gammelgaard, 0. Jons and B. Nielsen, Analyst, 117 (1992) 637.
Chimica Acta 318 (1996) 335-343
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[13] E. Pobozy, M. Trojanowicz and P.R. Worsfold, Anal. Len., 25 (1992) 1373. [14] D.T. Gjerde, D.R. Wiederin, F.G. Smith and B.M. Mattson, J. Chromatogr., 640 (1993) 73. [15] N. Jakubowski, B. Jepkens, D. Stuewer and H. Bemdt, J. Anal. At. Spectrom., 9 (1994) 193. [16] R. Roehl and M. AIforquem., At. Spectrosc., 11 (1990) 210. [17] IS. Krull, D. Bushee, R.N. Savage, R.G. Schleicher and S.B. Smith Jr., Anal. Lett., 15 (1982) 267. [18] H.G. Beere and P. Jones, Anal. Chim. Acta, 293 (1994) 237. [19] E.J. Arar, S.E. Long, T.D. Martin and S. Gold, Environ. Sci. Technol., 26 (1992) 1944. [20] S. Beablen, J. Nriagu, D. Blowes and G. Lawson, Environ. Sci. Technol., 28 (1994) 730. [21] A. Syty, R.G. Christensen and T.C. Rains, At. Spectrosc., 7 (1986) 89.