Anal. Chem. 1991, 63,520-522
520
TECHNICAL NOTES Determination of Vanadium in Seawater by Inductively Coupled Plasma Atomic Emission Spectrometry Using Chelating Resin Column Preconcentration Virginie D u p o n t , Yves Auger,* Catherine Jeandel,' and Michel W a r t e l Analytical and Marine Chemistry, C8, University of Lille, 59655 Villeneuve D'Ascq Cedex, France INTRODUCTION In this work, we have focused our attention on the analysis of vanadium in seawater. This element is the least studied among the transition elements in the natural environment (I). However, it plays an important role in biochemistry (2-5), and it is an environmental pollutant (6-8). Important industrial wastes are released in seashore waters in Northern France: it is of great interest to study the off-shore scattering of this element. In this work, we had to develop first a reliable method for the analysis of dissolved vanadium in seawater. High-salinity waters are known as environments in which the classical analysis methods for trace metal concentrations are difficult to apply. High concentrations of alkali metals, alkaline-earth metals, and halogens may be sources of chemical and physical interferences and therefore affect the accuracy of the measurements. This explains why several separation/preconcentration methods have been developed and adapted t o the determination of several trace elements (9). In the last decade, few works relative to vanadium have been published despite the improvement of techniques to analyze trace elements. The concentration range of dissolved vanadium is typically between 0.5 and 3 pg/kg (10-16). In marine conditions, vanadium is mostly dissolved (13-16): Particulate vanadium is negligible compared to the dissolved one (0.01% according to Collier (13)),except in coastal areas (1, 16). Furthermore, oceanic vertical profiles of vanadium concentrations display very small variations, except sometimes in surface or deep waters, where depletion or enrichment can be observed (13, 27, 28). T h e only direct determination of dissolved vanadium in seawater is performed by cathodic stripping voltammetry (CSV) (19, 20), but the performance of this CSV experiment is hard to evaluate, and the presence of natural organic surfactants in seawater lowers the sensitivity. All the methods currently used require a preconcentration step like (i) coprecipitation either with ferric hydroxide (16, 21) or with cobalt ammonium 1-pyrrolidinedithiocarbamate (ADPC) complex ( 1 3 , 2 2 )or (ii) adsorption on activated carbon (14) or on cation-exchange resin (23, 24). Among these preconcentration methods, we have chosen to retain the metal on Chelex resin. This is more selective than coprecipitation and easier and faster than organic solvent extractions. Vanadium is strongly chelated ( 2 5 , 2 6 )by the Chelex resin and is easily removed from seawater. In addition, the method used for sample packaging on resin and its storage is easy; vanadium is not altered, and its content in Chelex gel does not vary during the collecting campaign. After chemical separation, the preconcentrated vanadium can be analyzed by different techniques: neutron activation analysis (27),X-ray fluorescence spectrometry (28), atomic absorption emission (29), atomic absorption spectrometry (16, 30). Unfortunately, the vanadium fixation on Chelex remains strong in acidic media and the elution of this element becomes 'Present address: CNES/GRGS 18, Avenue E, Belin, 31055
Toulouse Cedex, France.
difficult. Greenberg e t al. (23,27) noticed a poor reproducibility of elution attempts. They made the analysis directly by neutron activation on solid resin samples. However, this technique is limited to specialized laboratories, having access to a nuclear reactor. Furthermore, it has a relatively low sample throughput. In this work, we present a n analytical procedure using inductively coupled plasma atomic emission spectrometry (ICP-AES). T h e simultaneous multielement analysis capability of ICP-AES enables the extrapolation of this analysis t o other trace metals. However, this does not solve the problem of the preliminary dissolution of vanadium fixed on Chelex resin. We propose here an original protocol, in which the resin is destroyed by acidic attack facilitated under the action of microwaves. EXPERIMENTAL APPROACH Vessel Preparation. Polyethylene bottles, which are used for sampling and for collecting seawater after filtration, are leached in a mixture of 10% nitric and hydrochloric acids for a week. They are rinsed with deionized Millipore Q grade water (resistivity > 15 MQ/cm). Polyethylene columns, which contain resin and their stoppers, Teflon connections, and borosilicate glass vials are all cleaned by using the same procedure. Reagents a n d Solutions. All the reagent and sample preparations are done in a class 100 clean air laboratory. Chelex 100 chelating resin, 100-200-mesh size, is obtained from Bio-Rad Laboratories. The reagents (HNO,, H2S04,Hz02)used are of ultrapure quality. Standard solutions are prepared with deionized Millipore Q grade water from Merck Titrisol solutions: lo00 mg/L for vanadium and 1010 mg/L for scandium (Aldrich Chemical Company). Working standards are prepared daily. Intermediate solutions are prepared by appropriate dilution of the stock solution. Sampling. Samples are collected by using "Go-Flo" bottles and filtered on a cellulose nitrate filter (0.45 pm) to separate the dissolved and particulate matter. They are stored in polyethylene flasks and acidified to pH 3-5 with ultrapure HNO,. Column Preparation. Chelex 100 resin (100-200 mesh, sodium form) is first washed with 2.5 M HNO,; then, it is loaded on the column. The column diameter is 6.5 mm, and its height is calculated knowing that about 25 mm per each liter of eluted seawater is needed. The excess acid is eluted by deionized Milli-Q water. Acidified seawater samples are then loaded on the column at a flow rate of 4 mL/min. Generally, 1 L is used for coastal waters and 2 L for low-concentration offshore waters (e.g., North Sea). Treatment of t h e Resin. When the sample has flowed through the column and vanadium is fixed on it, the resin is transferred to a container where it is attacked in a microdigester (microdigest 300 Prolabo) to recover the vanadium. The microwave method used is an open vessel decomposition technique, and the temperatures are limited by the boiling points of the reagents. This procedure is composed of two steps: the first digestive step consists of mineralizing the resin with sulfuric acid. Four milliliters of H2S04is used with a power of 120 W for 8 min ( T N 220 "C) and then 90 W for 4 min. Then, in the second step, an addition of HzOz oxidizes the carbon before removing the greatest quantity of acid. Two milliliters of 30% H202is used with a power of 210 W for 2 min ( T N 170 "C). It is worth noting that the introduction of Hz02as the oxidant just after the acid
0003-2700/9 110363-0520$02.50/0 0 199 1 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63,NO. 5, MARCH 1, 1991
521
Table I. Operating Conditions for ICP nebulizer
Instrumental Parameters Meinhard concentric glass nebulizer (type: Tr-30-A3)
rf' power
forward reflected gas flows
1200 w
selected analytical lines, nm 311.071 361.384
attack is important; otherwise, a kind of foam appears that overflows the container. The solution containing the vanadium, recovered after the resin digestion, is transferred to a 10-mL volumetric flask, and an internal standard of Sc is added to compensate the action of the residue of H2S04. ICP Operating Conditions. An ARL Model 3510 PC spectrometer is utilized in this study (Table I). Detection Limit. The detection limit is taken as the concentration for which the signal is twice the standard deviation of the background emission: we obtained a vanadium concentration of 4 pg/kg, with or without Sc. A vanadium concentration of 40 pg/ kg is the lowest measurable concentration for precision better than 10%. Thus, taking into account the preconcentration factor, we can measure down to 0.2 pg of vanadium/kg of seawater. Blanks. The blanks are treated identically as the samples. Two resin columns are connected in series. The one located below is the blank. The blank concentrations are below the detection limit. R E S U L T S AND D I S C U S S I O N Two experimental conditions have to be optimized to make our analytical method reliable: (i) adjustment of the analysis by ICP-AES from standard solutions and estimation of the standard deviation; (ii) vanadium fixation conditions on Chelex. Among the wavelengths able to be used by ICP-AES, we have selected 311.071 nm (31)because of its higher sensitivity and the absence of interference. The concentration values of dissolved vanadium in coastal waters of Northern France-which are considered as polluted areas-are expected to be as high as 5 parts per billion (ppb). As preconcentration multiplies the vanadium concentration in seawater by a factor of 100, we have calibrated the ICP in the range of 0-500 pg of vanadium/kg of seawater. In this concentration range, the curve corresponding to the intensity of the ICP signal as a function of the concentration of standards is a straight line (the root-mean-square (rms) error between calculated and found concentrations is about 1%). The results are reproducible within 2% in the case of various standards prepared with Milli-Q water, but the reproducibility decreases when the same samples are preconcentrated by the Chelex resin: the values found are always lower than those expected. We assume that this decrease of ICP sensitivity results from the presence of the sulfuric acid used for the resin attack: the higher acid concentration increases the dynamic viscosity, leading to a n increase of the mean diameter of the aerosol drops (14,32). When the volume of drops is large, the risk of losing these particles in the spray chamber before reaching the plasma is more important. This could result in the observed decrease of the ICP signal. In order to justify the errors obtained during the ICP analysis, Berman et al. (33)have also suggested that a more acidic aerosol would not be thoroughly dessolvated, due to
EXPECTED CONC\NTF!ATION
150
0 .l
D
0 .'s
0.2 Volumic
fraction
of
H2S04
Figure 1. (a) Vanadium concentration found by ICP-AES analysis for
more or less acidic samples having the same vandium concentration (150 ppb). (b) Vanadium concentration found by ICP-AES analysis by using different internal standards (scandium (0)and yttrium (V))in more or less acidic solutions having the same vanadium concentration (400 PPb).
Table 11. Reproducibility wt of seawater preconcentrated, kg
av
std dev
v, g l k g seawater" A
seawater" B
3.07 3.10 3.03 2.80
2.62 2.58 2.58 2.61
3.00 0.12
2.61 0.04
" Samples from Northern France coastal water: A, 51'
05' N, 2'
E; B, 51' 05' N, 2' 10' E.
increased "cooling" of the plasma. Choice of an Internal Standard. The decrease of the ICP signal is shown in Figure 1, when the concentration of H2S04 is increased in a solution of 150 pg/kg of vanadium. The experimental conditions followed to destroy the resin do not allow us to control the normality of the solution before the analysis, even when all the parameters are respected. For this reason, we had to use an internal standard element. This standard provides a measurement of the attenuation of the emitted signal versus acidity proportionally to that observed for vanadium. I t has to be poorly abundant in the initial sample, compared to the constant amount added during the experiment. Consequently, we have selected elements that are physically similar to vanadium in the periodic table and t h a t provide close ionization potentials (34)such as scandium and yttrium. Amounts of dissolved scandium and yttrium in seawater are very low (35),which makes them suitable as standards. Figure 1shows that the results are more accurate with scandium than with yttrium. Reproducibility. The reproducibility of this method has been tested with four replicates from two different seawaters preconcentrated on Chelex resin as described above. In each case, one 2-L sample and three 1-L samples have been analyzed. The results obtained after preconcentration are listed in Table 11. In each series, the results are consistent relative standard deviation (rsd) 3.9% and 1.5%) and reproducible.
522
Anal. Chem. 1991, 63, 522-525
limit is 0.25 gg/ kg of seawater before preconcentration. Results (Table IV) obtained hom the analysis of a seawater sample collected in the Mediterranean Sea by both analytical procedures are in good agreement, within 2%.
T a b l e 111. R e c o v e r y T e s t s Vaddedi
/-'glkg
Vfoundt
0.00 1.70 1.70 1.70 1.70 av Table
/-'glkg
