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Determination of Soluble Elements in Water by X-ray Fluorescence Spectrometry after Preconcentration with Polyvinylpyrrolidone-Thionalide R. Panayappan,“ D. L. Venerky, J. V. Gilfrich, and L. S. Birks Naval Research Laboratory, Washington, D.C. 20375 A method for the quantitative deterrninatlon of soluble elements such as Fe, Cu, Zn, Se, Cd, Sn, Te, Hg, and Pb in water Is described. Thls method Is based on preconcentratlng the dissolved elements with the combined organic preclpltating reagents polyvinylpyrroildone and thionalide, flltering the preclpltate to form a sultable x-ray sample, and analyzlng the sample by x-ray fluorescence. Large concentratlons of calcium and magnesium do not Interfere when the method is applied to natural and waste water samples. X-ray fluorescence analysis can achleve detectlon llmlts In the microgram-per-liter range for all elements tested.
Environmental concerns lead to a requirement for the analysis of large numbers of water samples. Generally it is impractical to transport the analytical equipment to the collection site; therefore the samples must be brought to the laboratory. Transporting water samples is both costly and awkward but, of more importance, trace concentrations may be lost to, or contamination leached from, the walls of the container. Particulate matter is readily collected by filtration a t the collection site, whereas the dissolved elements require a preconcentration scheme such as precipitation or ion exchange. Either of these produces a solid sample suitable for quantitative analysis and leads the analyst t o x-ray fluorescence as the method of choice ( I ) . One would prefer to collect all the elements of interest while avoiding interference from the alkali and alkaline-earth elements, some of which may be present at levels as high as 100 mg/L. Such interference may manifest itself in two ways: (a) the presence of large quantities of alkali and alkaline-earth elements may prevent efficient collection of other elements present in very low concentrations, or (b) the alkali and alkaline-earth elements, especially Ca, may be collected with the other elements to such an extent that the x-ray sensitivity for the other elements is lowered significantly. Ion-exchange and precipitation schemes for collecting the dissolved elements from water samples have been in use for some time. For example, Chelex 100, a general purpose ion-exchange resin sometimes used as an impregnant in filter paper to produce a convenient x-ray sample, is quite efficient a t collecting a variety of elements. Although its affinity for alkali and alkaline-earth elements is low, the high concentration of those elements present in natural water quickly saturates the exchange capacity of the resin and prevents the efficient collection of the elements of interest (2). On the other hand, the more common precipitation methods are not readily applicable to rapid, simple field use because they require (a) the use of reagents with short shelf-life [ammonium-1-pyrrolidine-dithiocarbamate (APDC) (3)], (b) long digestion periods [thionalide ( 4 ) ] , (c) higher concentrations of the elements of interest (mg/L) [disodium diethyldithiocarbamate (DDTC) (5)], or (d) they are limited to only specific types of water samples [1-(2-pyridylaz0)-2naphthol (PAN) ( 4 ) ] . This paper describes a simple new method which allows rapid and quantitative precipitation of trace metals and
metalloids from aqueous solutions with only negligible precipitation of alkali and alkaline-earth elements. It is applicable to field use and a variety of water types. The method employs a combination of two organic reagents, namely polyvinylpyrrolidone (PVP) and 2-mercapto-N-2-naphthylacetamide (thionalide), chosen t o provide (a) rapid precipitation of elements present a t low concentrations without heating, and (b) the formation of insoluble compounds rather than the soluble complexes formed with inorganic anions (chloride, cyanide, etc.) or organic compounds (humic acid, EDTA, etc.). This combination precipitates the elements in water rapidly a t room temperature and yields consistently higher recovery rates than other reagents reported in the literature. The precipitate is collected on a filter to form a suitable “thin sample” (6) for x-ray fluorescence analysis.
EXPERIMENTAL Reagents. Solutions of the two precipitants were prepared as follows. A 0.1 % thionalide solution was prepared by dissolving 0.5 g of the compound (J. T. Baker Chemical Co.) in 250 mL of glacial acetic acid and then diluting to 500 mL with water conformingto ASTM Type-I reagent water (7). (If the thionalide solution is to be stored below 22 “C, the acetic acid concentration should be increased to 2:l to prevent turbidity.) The PVP was prepared as a 1% aqueous solution using 360 OOO molecular-weight material from GAF Corporation (“PVP K-90”). Stock solutions containing 1 g/L of a number of individual elements were prepared from atomic-absorption solutions (“DILUT-IT” from J. T. Baker Chemical Co.) and diluted as necessary with reagent water. Apparatus. X-ray measurements were carried out with a Philips PW 1410 x-ray spectrometer under the conditions listed below: X-ray Tube. Cr target FAA 6013.5 operated at 45 kV (c.P.) and 45 mA. Crystal. LiF (200) or PET. Detector. Gas-flow proportional counter with 90% Ar, 10% CHI, operated at 1650 V, used alone or in tandem with a NaI(T1) scintillation counter operated at 950 V. Pulse-Height Analyzer. Differential mode with window set at one-tenth maximum (for the pulse amplitude distribution of the proportional counter). Counting Time: 100 seconds. Instrumental response (counts per secondlhg per cm2)for the elements of interest was determined from samples prepared by drying known volumes of the elemental stock solutions on the same type of filter paper used t o collect the precipitates. After drying the solutions, the elements are deposited throughout the volume of the filter paper and may require a correction for x-ray absorption of the emerging radiation (1). Procedure. The x-ray specimens were prepared from the precipitate which is formed by adding 15 mL of each precipitant solution to a suitable volume of the sample (usually 250 mL for the concentrations reported here, although volumes as large as 1 L were tested; the volume was chosen to be a reasonable compromisebetween a large starting volume and a short filtration time). After the reagents were added to the sample, the pH was adjusted to 4 with 4 N ammonium hydroxide. The resulting mixture was stirred for 15 min, allowed to settle for 5 min, and filtered using a 47-mm diameter, 0.45-pm pore-size Millipore filter. The x-ray sample consisting of the precipitate on the filter was allowed to dry and placed between 6 pm Mylar films which are
This article is not subject to U.S. Copyright. Published 1978 by the American Chemical Society
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Table I. Recovery Rates, Sensitivities, and Detection Limits Recovery rate: Element Felt cu2+ ZnZt Se( IV)" Cd2+ Sn(IV)" Te(1V)" Hg2 Pb2 +
+
%
90 90 80 95 70 95 95 95 95
100 s, Sensitivity,b 35 counts per 100 s/ detection limit,b Mg per L MIL 260 1.2 160 2.0 160 1.6 100 2.4 70 0.8 480 0.3 600 0.3 30 7.0 50 4.0
* Recovery rate was determined on four individual samples and the standard deviations were less than 5%. Each of these samples contained 50 mg of Ca and 1 5 mg of Mg per liter of solution. Assuming a 250-mL sample and the precipitate deposited over a 41-mm diameter on the 47-mm filter. " Assumed to be present in solution as the soluble hydrous oxide MO, .xH, 0 or the acid H,MO,.
Table 11. Reaulte on Water Samples (pg/L) River water Filtrate from Tap water (XRF only) effluent XRFO AASb Particulate Filtrate XRF AAS 50 Fe 26 52
