Chelex- 100 Ion-Exchange Filter Membranes for Preconcentration in X-ray Fluorescence Analysis of Water Ren6 E. Van Grieken," Catharlna M. Bresseleers, and Bruno M. Vanderborght Depadment of Chemistty, University of Antwerp (U.I.A.), 8-26 10 Wilrvk, Belglum
Fllterlng a 200mL water sample through a palr of Chelex-100 Ion-exchange membranes, under a 2-3 bar pressure, at pH 7 to 8 In not less than 20 mln, leads to an efflclent collection of many trace metals In a form that Is Ideal for subsequent x-ray fluorescence analysls. Slnce enrlchmenl factors around 1250 are obtalned, the detectlon llmlts are at the 1-ppb level. The preclslon Is estlmated to be In the 10-15% range. A perfectly linear relatlonshlp between concentratlon and measured x-ray yleld Is obtalned up to the total membrane palr capaclty of 0.07 mequlv. The slgnlflcant afflntly of Chelex-100 membranes for the-usually abundant-alkall and alkallne earth Ions, together wlth thelr llmlted capaclty, restrlcts the appllcablllty of the descrlbed technlque to samples wlth a very low alkall and alkaline earth content.
X-ray fluorescence is essentially a multielement, simple, rapid, and cheap analytical technique. Yet it is rarely applied to aqueous samples. Direct irradiation of aqueous samples in a measuring vial with a thin Mylar bottom leads to typical sensitivities for transition metals in the order of 0.3-5 ppm (1). When sensitivities in the ppb region are required, the trace metals must first be preconcentrated and the difficulty lies obviously in preparing targets containing all elements sufficiently enriched and distributed with sufficient uniformity to permit sensitive and reliable analysis. X-ray fluorescence has been employed in combination with precipitation and coprecipitation, evaporation, solvent extraction, chelation, and adsorption on activated carbon, use of ion-exchange resin or of silica gel with immobilized functional groups. One drawback of all these methods is that after the preconcentration they require a subsequent step to present the sample in a suitable form to the x-ray instrument. Ion-exchange resin loaded filter papers, however, which incorporate a finely divided ion-exchange resin in a polymer structure, are very thin (5 mg cm-') and provide a matrix of low atomic number elements. Hence they can be presented directly to the x-ray analysis unit after the trace metal collection is accomplished by a simple filtration, producing intimate contact between the solution to be analyzed and the resin fraction of the filter. Although several authors (2,3)had already mentioned the use of ion-exchange membranes, Campbell et al. (4) first devoted a comprehensive and excellent study to the characteristics of cation- and anion-exchange resin loaded papers. Several applications have since then been published 65-14) and reviewed (15). For cation collection all the previous authors, however, used acid ion-exchange filter papers which show a high affinity for the alkaline earth ions that are abundant in many water types. The limited capacity of the filter papers might therefore hamper their application considerably. Moreover, acid filters are optimally used in the lower pH range, requiring pH 1326
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUQUST 1977
adjustment of usually almost neutral natural water samples with a risk of contamination. Filtration through commercially available chelating ion-exchange resin loaded filter papers as a means of preconcentration for multielement x-ray analysis has not thoroughly been evaluated in the literature. Yet Chelex-100 papers such as the Acropor type CH membranes contain chelating iminodiacetate functional groups, and columns of this resin (Dowex A-1) show large distribution coefficients in the neutral pH range and have been used advantageously in preconcentrations from seawater (16,17). Since these membranes might thus have promising properties with respect to environmental water analysis, their important chemical and physical characteristics have been studied. One particularly attractive perspective was the eventual application of Chelex filters for continuous and in-line collections of ions from drinking water supplies and other rather neutral aqueous solutions. Our investigations have generally been directed towards this goal.
EXPERIMENTAL Apparatus. The x-ray apparatus was a Kevex-0810 system. It includes a Rontgen tube (usually operating at 40 kV-40 mA) and a Mo secondary target. The characteristic x-rays from the sample are counted by a Si(Li) detector with a 170-eV resolution at 5.9 keV. The electronic pulses are led through an amplifier, stored in a 4096 channel analyzer and recorded on magnetic tape for computer evaluation. The data reduction routine (18)was based on a subdivision of the spectra in energy regions of particular interest, on a background subtraction relying on the coherently and incoherently scattered peaks in the spectra and on experimentally determined coefficients to evaluate interelement interferences. Atomic absorption spectrometry was performed on a Perkin-Elmer 603 unit with a graphite furnace Perkin-Elmer HGA-74. Radioactivity counting was done using a 3 inch X 3 inch NaI(T1) detector or a liquid scintillation setup connected to a single channel analyzer. Reagents. All test solutions were prepared by dissolving weighed quantities of metals or metal salts in minimal amounts of HCl and diluting with doubly distilled water. In some cases, Titrisol standards were used. The solutions were prepared and stored in polyethylene containers only. Chelating ion-exchange filters of the type Acropor CH were obtained from Gelman Company, Ann Arbor, Mich. They contain a resin with iminodiacetate functional groups, as an integral part of a microporous plastic matrix. The 47-mm diameter filters weigh 91.6 k 0.3 mg, Le., 5.3 mg ern-'. Chemical analyses of the untreated Chelex-100 filter gave the following results (in weight percent): hydrogen, 8.9; carbon, 59.3; nitrogen, 9.3; oxygen, 11.2; chlorine, 7.1. About 4.2% of the material is not accounted for by these elements. Procedure. In order to achieve reasonable flow rates through the Chelex-100 membranes, all filtrations were performed in a pressure resistant Gelman No. 4280 stainless steel filter holder with a 200-mL capacity. Filtrations were usually carried out under a 2-3 bar pressure, provided by an air tank. Teflon rings assured the water tightness of the setup. Preconcentrations were carried out by passing a 200-mL water sample through a pair of Chelex-100 membranes, in not less than
Table I. Percent Collection Efficiency as a Function of pHa Collection efficiency Cation
pH= 4
5
6
7
8
9
10
K' Mn2* co2+ Ni2 cu2+ Zn'+ Rb'
58 60 55 57 52 49 46 67 67
40 57 59 64 59 61 63 68 68
74 78 73 76 66 69 66 72 72
71 90 83 88 76 74 77 84 84
68 92 87 92 74 80 80 85 80
67 66 60 64 57 54 68 73 52
28 25 27 29 26 25 43 41 19
+
Sr2+ Pb2+ a
Standard deviations amount o n the average to 6%
10 min, at a pH of 7-8. This procedure was selected in view of the results of the optimization study, described below. If necessary, precipitates were removed with a prefilter using 0.4-pm Nuclepore membranes. After loading, the Chelex-membranepairs were put in glassless slide frames and dried in a CaC12-NaOHdesiccator for more than 12 h. The filters were then suspended tightly on the frame, and offered a reproducible geometry for x-ray fluorescence analysis.
RESULTS AND DISCUSSION Numbers of Filters Required. One might obtain a more advantageous collection efficiency and capacity by using a stack of several ion-exchange membranes resulting, however, in a higher analysis cost. James (19) used three cation-exchange filters simultaneously to obtain complete recoveries. Experiments were conducted in which 100 mL of a 5 ppm Zn2+solution, at pH 8, were passed through either one, two, or five Chelex-100 filters. The average collection yields determined by both atomic absorption spectrometry of the solution and by x-ray fluorescence of the loaded membranes, amounted to 70 & 3 % ; 92 & 1% , and 99 f 170,respectively. All following experiments were carried out with two Chelex-100 filters, being considered as a suitable compromise between reasonable analysis sensitivity and analysis cost. Number of Filtrations Required. Campbell et al. ( 4 ) recommended passing the solution to be analyzed seven times successively through cation exchange papers to achieve high collection efficiencies. James (19),however, found satisfactory yields after one pass. When 100 mL of a 5-ppm Zn2+solution was filtered through a pair of Chelex-100 membranes up to five times consecutively, the collection yield amounted to 90 f 1% after the first pass and did not increase significantly afterwards. This was found from atomic absorption measurements on 2 mL of the eluate after each percolation. All following experiments therefore involved only one single filtration process, leading to satisfactory collection efficiencies for many elements, as will be seen below. Effect of pH. Two hundred-mL aliquots of solutions of various pH levels containing 0.5 or 0.25 ppm of K', Ca2+, Mn2+,Co2+,Ni2+,Cu2+,Zn2+,Rb', Sr2+,and Pb2+as chlorides or nitrates, were passed through a pair of Chelex-100 filters. The measured x-ray spectra were corrected for blank and absorption effects (see below). On the basis of the results given in Table I, one can conclude that the collection yields are usually optimal and satisfactorily high at pH 7 to 8, with no critical variations occurring in that region. In a series of previous experiments involving radioactive tracers (54Mn,6oCo,65Zn,59Fe) added separately to 5-ppm solutions and using other batches of Chelex-100 filters, significantly different collection efficiencies were noted in some cases. Also, the yields for Zn differed from those obtained in both previous paragraphs. Variations in pH dependence between different Chelex resin batches, however, have been suspected earlier (20). If confirmed, such an effect would
imply frequent and painstaking checking of collection yields in routing applications. The pH dependence of ion collections by larger amounts of Chelex-100 resin has been studied before. Riley and Taylor (16) found columns of Chelex-100 to adsorb quantitatively Co2+,Ni2+,CU", Zn2+,Pb2+,and Mn2+ at pH 7.6 from seawater. In batch experiments, Holynska (17) observed a constant and high recovery of Fe3+,Zn2+,and Pb2+between pH 3 and 8, while the collection efficiency for Cu2+appeared to decrease above pH 6. Leyden and Underwood (21)reported lower pH limits for Ca2+,Mn2+,Co2+,Ni2+,Cu2+,and Zn2+ around p H 3 and optimal and rather constant efficiencies between 5 and 7.5 for batch experiments on Dowex A-1. The low efficiency of Chelex-100 below pH 3 may be attributed to the protonation of the functional sites (the pK values for Chelex are around 2.8 and 8.3 (21, 22)). Hydrolysis might explain the decrease of efficiency at high pH levels. Effect of Solution Volume. A preconcentration step involving larger sample volumes obviously leads to higher enrichment factors, an advantage which can, in practice, be offset by lower collection yields, as Campbell et al. ( 4 ) demonstrated, and by the longer filtration time. In this work, a 200-mL sample volume was chosen, corresponding to the full capacity of the high-pressure filter holder. Experiments with 100- to 400-mL samples proved that the collection yields varied with the solution volume as can be predicted from simple theoretical considerations ( 4 ) . Effect of Filtration Rate. Campbell et al. (4) found the filtration rate did not affect the recovery of metal ions by cation-exchange papers between 120 and 4 mL min-l. Leyden et al. (23) and Holynska (17)found quantitative uptake of Cr3+,Zn2+,and Fe3+by Chelex-100 resin columns is realized within about 30 min. Since the coordination process is usually slow, an initial electrostatic interaction with the resin followed by chelation was proposed as a mechanism. For Chelex-100 filters, we found that variations in the flow rate up to 10 mL min-l did not affect the collection yields from a pH 7-8 solution. For higher filtration rates, however, a collection yield above 80% of the optimum was achieved only if the total final filter load amounted to less than 1/6 of the maximum capacity. Load Homogeneity and Effective Filtration Area. A 200-mL sample of a neutral Zn2+solution containing 65Znwas passed through a pair of Chelex-100 filters. The filters were then cut into 0.25-cm2 pieces, and the radioactivity of each was counted by NaI(T1) spectrometry. The filter load appeared to be homogeneous within 3%. The effective filtration area was 15.2 cm', Le., the area of the total filter (17.3 cm2) minus the Teflon sealing ring. This area is much larger than the 9.6-cm2flow-through region of the disk that supports the filters in the filter holder. Proportionality between X-ray Response a n d Ion Concentration. Of a standard solution of pH 7.8 containing K+, Ca2+,Mn2+,Co2+,Ni2+,Zn2+, Rb', Sr2+,and Pb2+ simultaneously, ten dilutions were prepared with concentrations ranging between 10 ppm and 10 ppb. Each of these solutions was run through a pair of Chelex-100 filters, the measured x-ray signal was corrected for blank and x-ray absorption effects, and the metal ion amounts collected on the fiiters were calculated. Some results are plotted in Figure 1. The data for K+ and Ca2+were erratic due t o low affinity for the ion exchanger and contamination effects, respectively. These dilution curves deviate from linearity a t the higher concentration end where the limited capacity of the Chelex filters is approached or exceeded, and at the lower end because of contamination effects. For all elements, a perfectly linear relation is found in the intermediate concentration range, implying the collection efficiency to be independent of the ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
1327
Table 11. Average Collection Yield, and Standard Deviation, at pH 7 . 8 Ion Mnzt CO'+ Ni" CIA2+ Collection yield, % Stand. dev., %
'"
I
Zn
85.8 2.5
I
1031
86.3 2.9
90.5 3.3
1
Figure 1. Relationship between determined and added concentrations for Ni2+, Mn2+, Zn2+, Rb', Sr2+,and Pb2+
ion concentration in the solution to be analyzed. Indeed, assuming a linear relationship with a zero intercept, the following correlation coefficients were calculated Mn, 0.9975; Co, 0.9988; Ni, 0.9985; Cu, 0.9983; Zn, 0.9985; Rb, 0.9998; Sr, 0.9997; and P b 0.9981. Ion Collection Yields. The slope of the straight curves in the intermediate regions of Figure 1 corresponds to the collection efficiency of the enrichment procedure. Assuming that the errors on the given concentrations of the solutions are negligible in comparison with the concentrations on the measured results, and assuming the standard deviations on the latter values to be identical, one can calculate the collection efficiencies given in Table 11. These values correspond within 5% with the data in Table I for pH 8. Qualitative conclusions about relative collection yields can be extracted by examining, in the right sides of the curves in Figure 1, for which ions a deviation from linearity occurs at the lowest concentration. From these and other graphs it was found that K+ and Rb+ seem to have the lowest affinity for Chelex-100 membranes. The affinity of the other ions can be arranged in the following order: Sr2+> Ni2+> Mn2+ 2 Pb2+ 2 Co2+ 2 Ca2+ 5 Zn2+ 1 Cu2+ Differences in the group Ni-Mn-Pb-Co-Ca-Zn-Cu are, however, of little or no significance. This order is not in contradiction with the data in Table I, if one considers the given uncertainties. Of course a t pH values differing from 7.8, other selectivity orders might exist. Some relative selectivity orders have been determined previously for the Chelex-100 resins in batch experiments. Van Willigen and Schonebaum (22)found stability constants with Chelex-100 in decreasing order for Cu2+ e Pb2+> Zn2+ 1328
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
75.2 2.9
Znl+
Rb'
SrZ+
PbZt
77.2 1.8
83.7 0.6
90.4 5.4
86.1
8.0
> Ca2+,after 120 h of equilibration. They mentioned a strong pH dependence for alkaline earth ions. Holynska (17 ) gives the order Cu2+> Zn2+> Pb2+at pH 5. Another source (24) lists Cu2+>> Pb2+> Ni2+> Zn2+> Co2+> Mn2+> Ca2+for nitrate or chloride media. In these experiments, the long equilibration time might enhance chelation relative to the ion-exchange mechanism for some ions, and distinct relative selectivity differences as a function of pH are to be expected (25-27). If one assumes the ion-exchanger weight to be half of the filter weight, and defines the distribution coefficients, D, as: amt. of m e t a l i o n / g resin D= (1) amt. of m e t a l i o n / m L solution collection efficiencies between 75% (for Cu2+)and 90% (for Ni2+ and Mn2+) correspond to log D values of 3.9 and 4.4, respectively. Leyden and Underwood (21) quoted log D data near 5 for Cu2+and Ni2+,4 for Mn2+,and 3.4 for Co2+ and Zn2+,for 48-h equilibration times. These results agree well on the average, but, again it appears that the affinity of the Chelex-100 membranes for Cu2+is unexpectedly low. Filter Capacity. The total ion concentrations for which the curves in Figure 1start to deviate from linearity illustrate that the total ion load of a pair of Chelex filters is limited to 0.064 mequiv. In another test, a 50-ppm Zn2+solution of pH 7.8 containing some carrier free 65Znwas filtered through a pair of Chelex-100 filters. The eluate was collected in 20-mL fractions. Comparing the radioactivity of each fraction with that of the original sample allowed to determine the Zn2+ quantity retained on the filters. A maximum load of 0.066 mequiv was found in this way. This corresponds to 0.8-g exchange capacity per g filter. Campbell et al. (4) mentioned a 0.20-mequiv capacity for a 3.5-cm diameter cation-exchange filter, Le., 1.7 g exchange capacity per g filter weight. Distribution over the Two Chelex-100 Filters. The distribution of the various ions between the two Chelex-100 filters was studied through the experiments from which the curves in Figure 1were derived, involving solutions at pH 7.8 of ten ions simultaneously, with a totalmetal ion concentration ranging from 100 to 2.5 ppm. Of each pair of Chelex-100 filters, the first one (uppermost during filtration) and second one were counted separately. Both filters were measured twice, namely with the front side (upper side during filtration) and with the back side facing the x-ray analysis system. The average of front- and back-side measurements were considered as representative for the filter load. Table I11 lists the ratios of the ion quantities on the first filter vs. those on the second. For high ion concentrations, exceeding the filter capacities to a large extent, both filters carry a similar load, except for the most strongly bound ions (Pb2+,Sr2+). At dilution, the first filter collects a larger fraction, and the ratios in Table I11 increase from ca. 2 for 10 ppm to ca. 4 when the total cation content is lower. Only the cations with the lowest affinity (K', Rb') are distributed about equally over both filters in these conditions. It was found that the distribution was more homogeneous when the solution to be analyzed was passed twice or more times through the Chelex filter pair. Measurements of the x-ray intensity from the front and back sides of the individual filters could yield information of the distribution of each ion in each filter, as in the procedure, proposed by Adams and Van Grieken (28), for deposition
Table 111. Distribution of Ions over the Pair of Chelex-100 Filters as a Function of the Concentration Level for Ten Ions Simultaneously present in Equal Concentrations ~~
Ratio of the quantities Ion
100 ppm
75 PPm
0.99 0.94 0.92 0.92 1.11 0.89 1.00 1.40 1.84
1.50 1.12 1.22 1.14 1.20 1.01 1.30 1.55 2.64
the first and second filter Total cation concentration 50 P P ~ 20 PPm 10 PPm
K' Ca2 Mn2 +
+
co2+
Ni2+
cuz+ Zn2+ Rbt
Sr2+ Pb2 +
-
011
0.36 0.79 1.03 0.93 0.99 0.97 0.89 0.41 1.39 1.84
-.
1.20 2.17 1.64 1.89 1.85 1.86 1.98 1.08 1.84 2.07
0.68 1.96 1.73 1.79 1.84 1.19 1.71 0.75 2.48 1.48
Table IV. X-ray Absorption Factors for Different Values of ora Absorption correction Element a = 1 a=2 a=4 a=K
Ca Mn
co Ni
cu Zn
Pb Sr
2.26 1.83 1.22 1.17 1.14 1.07 1.07 1.02 1.02
1.96 1.65 1.17 1.11 1.09 1.05 1.03 1.02 1.01
1.78 1.53 1.14 1.00 1.07 1.04 1.03 1.02 1.01
1.56 1.38 1.10 1.07 1.05 1.03 1.03 1.01 1.01 1001 Rb
I,S;$DexP(- X P X ) d P X (2) s;gDdPx
where Il and I2 = intensities from first and second filter respectively, in the absence of absorption effects; pD = filter thickness, in g cm-'; x = K~ cosec O1 + p z cosec O2 with K~ and w2 mass absorption coefficients, in cm2g-', of the exciting and characteristic x-rays, respectively; O1 and O2 = angle between the sample plane and the exciting and characteristic radiation, respectively. For a = 11/12,the absorption correction factor is given by:
I-,-+ I2 a + l I QI + exp(-XpD)
XPD
'
i - exp(-XpD)
0.95 1.79 3.95 3.87 3.93 3.59 3.64 1.02 4.58 3.72
1.17 5.55 1.98 4.95 3.99 3.35 3.54 1.17 3.40 3.68
cu
I
depth evaluation in aerosol loaded filters. However since Chelex-100 filters are quite thin, the relative difference in x-ray yield from both sides is too small even with a nonhomogeneous load, to allow significant conclusions. X-ray Absorption Effects. The x-ray absorption by the material collected on the filters is always negligible, since the maximum capacity of 0.06 mequiv per filter pair corresponds only to typically 50 Kg cm-' of metal ions. The absorption by the filters themselves depends on the distribution of the material being measured as a function of the depth p x within the filter pair. Assuming a homogeneous distribution within each filter of a pair, the total measured intensity for element A is given by: +
2.5 ppm
lC7t
a a = (ratio of ion loads on first and second filter of a pair; see text).
I = I1 SgDexP(- XPx)dPx SgDdPx
5 PPm
(3)
In the previous section, it was shown that cy depends on the ion type and concentration in the solution, and varies typically between 1and 4 (Table 111). Table IV represents some typical x-ray absorption corrections. The values were determined by transmission measurements through Chelex filters. The corrections appear to be significant for low 2 elements, particularly.
i
I
I
10-1
100
101
100
I
, 10-2
16'
,$
101
,A2
Na'-CONCENTRATION C t r meq / I 1
Flgure 2. Collection efficiency of Mn2+, CU*+,Rb', and Pb2+in the presence of increasing concentrations of NaCl (e) and NaN03 (+)
Takihg into account the conclusions from the previous section, a realistic value for a can be estimated and a suitable correction can be carried out. Uncertainties on the a values and on the distributions within each filter are expected to yield maximum uncertainties on the absorption correction factor of, e.g., approximately 30% for K, 10% for Mn, 4% for Cu, 1% for Pb. Influence of Alkali and Alkaline Earth Ions. The capacity of a Chelex-100 filter pair, limited to only 0.065 mequiv, together with the significant affinity for the-usually abundant-alkaline earth ions, might constitute a most serious drawback for the use of Chelex-100 filters. To investigate quantitatively the influence of alkali and alkaline earth ions, solutions with 0.5 ppm of K', Ca2+,Mn", Co2+,Ni", Cu2+, Zn", Pb2+,Rb', and Sr2+were analyzed after the addition of increasing concentrations of NaC1, KC1, CaC12and MgC12. Most experiments were run in duplicate. Figures 2 and 3 illustrate some typical results. The presence of 1000 ppm or 43 mequiv/L of Na' as NaCl lowers the collection efficiency for the transition metals and for Pb" and Sr" to 60-709'0, to 55% for Cu". The collection of Rb' and K+ is then down to about 5 % . In comparison with Na+-free solutions, the average collection efficiency for all the ions studied (without consideration of K', Rb', and Ca") decreases by 10% relANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
1329
3 >
100I
1
-
100
Rb
Figure 3. Collection efficiency of Mn2+,Cu2+, Rb', and Pb2+ in the presence of increasing concentrations of CaCI,
atively in the presence of 1 mequiv Na/L or 23 ppm, by 20% for 6 mequiv/L or 138 ppm and by 30% for 35 mequiv/L or 800 ppm Na. No significant differences were observed between the influence of NaCl and NaN03. Also both for KCl and for KN03 exactly the same results were found as for NaCl. The influence of Ca2+appears to be much more dramatic. Some results are illustrated in Figure 3. For 1000 ppm Ca2+ or 50 mequiv/L, the average collection efficiencies amount only to 3 % , A 1070, 20%, and 30% decrease is effectuated by the presence of 1.2, 2.6, and 4.2 ppm Ca2+,respectively. The effect on K', Rb', and maybe Cu2+is more pronounced than average while Pb2+and Sr2+appear least affected. The overall effect of Mg2+appeared to be less severe: its concentration can be almost a factor of two higher than for Ca2+to induce the same effect. The above results clearly point out that the applicability of Chelex-100 ion-exchange filters might be limited for solutions with high or variable alkali and alkaline earth content. Many applications of the resin have been reported involving the removal of di- and trivalent cations from solutions of alkalies and alkaline earths based upon a claimed selectivity of the resin towards ions of greater complexing ability. In these cases, however, the more selective chelation is probably more favored over the ion-exchange process because of long contact times, or the available resin capacity is not critically low. Blank Filter Impurities and Detection Limits. Typical impurity levels, in pg cm-', measured on a pair of Chelex filters and typical variations within one batch, were as follows: C1, 190 f 50; K, 1 f 0.6; Ca, 7 f 1;Ti, 7 f 0.1; Cr, 0.07 f 0.01; Mn, 0.07 f 0.01; Fe, 0.4 f 0.05; Ni, 0.05 f 0.01; Cu, 0.06 f 0.01; Zn, 0.16 f 0.02. It was observed, however, that different batches contained significantly different blank levels, particularly for Ti, and for W, that was abundant in some lots. In x-ray fluorescence, detection limits are usually defined as the amount corresponding to three times the standard deviation on the background. Assuming typical filter blank levels, the following interference-free detection limits are calculated, in ppb for a 200-mL solution and a 2000-9 counting time in the experimental conditions: K, 14.4;Ca, 17.5; Ti, 10.2; V, Cr, and Mn, 0.8; Fe, 1.5; Co, Ni, and Cu, 0.4; Zn, 0.6. It should be noted that, with the experimental procedure used, 1330
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
the detection limits for Cr, Fe, Co, Ni, Cu, and Zn were all between 2 and 3 ppb, assumingly because of varying contamination by the filter holder. Precision. From all experiments carried out in multiplicity on the same solutions, it appears that 10-15% is a reasonable estimate of the precision of the proposed procedure. Applicability. Chelex-100 ion-exchange filter papers can advantageously be used in combination with x-ray fluorescence for solutions with a low alkali and alkaline earth content, e.g., in cooling water systems, distilled water, rain water, etc. The enrichment factors, defined as the original water sample weight to the final specimen weight amount to 1250 in the proposed procedure. This allows analyses at the ppb level for many trace metals. Drinking water, however, contains typically some 20 ppm Na, 5 ppm K, 5 ppm Mg, and 60 ppm Ca in the dissolved form. Because of the effect of these ions, the heavy metal collection efficiencies from a 200-mL sample would, on the average, be reduced to only 16% of the normal value, mainly because of Ca2+. Small variations on the alkaline earth content will thus induce intolerable errors. If one would allow only a 20% collection efficiency decrease, relative to distilled water, a 8-mL water sample should be diluted to 200 mL and analyzed; the enrichment factor for typical drinking water would then only be 50, resulting in unfavorable detection limits. A similar reasoning holds for ground and surface waters, with similar Ca content. For seawater, with typically 13000 ppm Na, 400 ppm K, 1300 ppm Mg, 500 ppm Ca, the applicability of Chelex-100 filters seems even less promising at this stage. The problem of alkali and alkaline earth ion interferences might, however, be overcome by strongly increasing the equilibration time to favor the more selective chelation process over ion exchange, by adapting the pH or by adding a suitable complexant or precipitant prior to the collection step. This would, of course, exclude simple on-line collections, e.g. for drinking water, by means of Chelex-100 membranes. ACKNOWLEDGMENT Grateful acknowledgment is made to L. Van'tdack, H. Nullens, and H. Bosmans who contributed to some of the experimental work. We thank D. E. Leyden, University of Denver, Denver, Colo., and C. Gelman, Gelman Instrument Company, Ann Arbor, Mich., for stimulating discussions and correspondance and A. Sturesson, Lund Institute of Technology, Lund, Sweden, for the elemental analysis of a Chelex-100 filter. LITERATURE CITED (1) R. Van Grieken, K. Bresseleers, J. Smits, B. Vanderborght, and M. Vanderstappen, Adv. X-ray Anal., 19, 435 (1976). (2) W. T. Grubb and P. D. Zemany, Nature (London), 176, 221 (1955). (3) C. L. Luke, Anal. Chem., 36, 318 (1965). (4) W. J. Campbell. E. F. Spano and T. E. &een, Anal. Chem., 38, 987 (1966). (5) E. F. Spano and T. E. Green, Anal. Chem., 38, 1351 (1966). ( 6 ) E. F. Spano, T. E. Green, and W. J. Campbell, "Evaluation of a Combined Ion Exchange XRay Spectrographic Method for Determining Trace Metals in Tuwten , US. Department of the Interior, Bureau of Mines, Washington, D.C., 1964. (7) G. L. Hubbard and T. E. Green, Anal. Chem., 38, 428 (1966). (8) S. N. Ndam and V. C. Rose, Engineering, 44, (7), 2 (1966). (9) R. D. Walton, Dev. Appl. Spectrosc., 9, 287 (1971). (IO) S. L. Tackett, G. H. Bender, T. R. Brunner, D. J. Duncan, M. G. Fedak, R. F. Gentile, J. F. Hiller, K. A. Hooker, A. J. McAuley, K. L. Rollick, J. F. Sandolfini, J. C. Smith, J. D. Vojtko, P. H. Pekala and S. A. Williams, Anal. Lett., 6,355 (1973). (11) P. G. Burkhalter, "Trace metal water pollutants determined by X-ray fluorescence", Report NRL-7637, Naval Research Laboratory, Washington, D.C., 1973. (12) B. Holynska, M. Leszko and E. Nahlik, J . Radioanal. Chem., 13, 401 (1973). (13) A. Robert and R. Valles, Radiochem. Radioanal. Left., 15, 179 (1973). (14) C. H. Lochmuller, J. W. Galbraith, and R. L. Walter, Anal. Chem., 46, 440 (1974). (15) S. L. Law and W. J. Campbell, Adv. X-Ray Anal., 17, 279 (1973). (16) J P. Riley and H. J. Taylor, Anal. Chlm. Acta, 40, 479 (1968). (17) B. Holynska, Radiochem. Radioanal. Left., 17, 313 (1974).
(18) P. Van Espen and F. Adams, Anal. Chim. Acta, 7 5 , 6 1 (1974). (19) H. James, Analyst (London), 86, 274 (1973). (20) C. W. Biount, D.E. Leyden, T. L. Thomas, and S. M. Guill, Anal. Chem., 45, 1045 (1973). (21) D. E. Leyden and A. L. Underwood, J . Phys. Chem., 6 8 , 2095 (1964). (22) J. H. H. G. Van Willigen and R. C. Schonebaum, Recueil, 8 5 , 35 (1966). (23) D. E. Leyden, R. E. Channel, and C. W. Blunt, Anal. Chem.,44,607 (1972). (24) BioAad Laboratories Chemical Division, Technical Bulletin, 114, May 1972.
(25) (26) (27) (28)
C. Hertner-Wirguin and G. Markevils, J . Phys. Chem., 67, 2263 (1963). H. Luttrell, Jr., C. More, and C. T . Kenner, Anal. Chem., 43, 1370 (1971). S.L. Law, Am. Lab., July 1973, p 91. F. C. Adams and R. E. Van Grieken, Anal. Chem., 47, 1767 (1975).
for review February
'7
1977* Accepted
28,
1977.
Determination of Ammonia in Aqueous Solutions by Infrared Spectrometry Following Preconcentration on Zeolite 0. Yavur Ataman' and Harry B. Mark, Jr." Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1
A new method for the determlnatlon of ammonla In aqueous solutlons has been developed. I n thls method, NH4+species Is preconcentratedon reollte AAA matrlx In a batch process, and then a portion of the zeollte Is sampled by uslng a KBr pelletlng technique. The Infrared spectrum of the pellet Is taken and the lntenslty of the NH4+bendlng vlbratlon band at 1402 cm-' Is used to calculate the NH3-N concentration In the sample solutlon. The results of analyses had recoverles better than 95.0% and the preclslon of the method was varylng from 5 to 12% mean devlatlon for NH3-N concentratlons between 14.0 to 0.50 ppm. Thls Infrared method uslng preconcentratlon Is useful for fleld studles as It has the advantages of slmpltfylng the transportatlon and eliminating the necessity of preservatlon of the samples.
It is often desirable to determine the concentration of ammonia in aqueous systems because of the biological and environmental importance of this compound. Ammonia is a pollutant in the aquatic environment as well as in the air. For natural waters, ammonia exerts a stress in several ways. Among these ways we can mention its contribution to blue-green algae growth (I) causing eutrophication, toxicity to fish and aquatic life, causing corrosion of certain metals of construction, reducing the amount of dissolved oxygen in receiving waters due to nitrification process, and inhibiting the water and food intake for both mammals (2)and plants (3)affecting their appetites. Excess amounts of ammonia and ammonium salts decrease the pH in human blood, causing acidosis which can be monitored by the increase of ammonia in urine ( 4 ) . The well-known and most popular colorimetric method for ammonia determination was first proposed by Julius Nessler in 1856 (5). The Nessler method is not truly a colorimetric method because of the colloidal character of the colored species formed by Nessler reagent, K2Hg14. Temperature, alkalinity, time, purity of the chemicals used for the reagent, and the condition of the reagent are the factors affecting the color intensity (4). Usually, in order to eliminate several chemical interferences ( 4 ) ,a distillation step is necessary. The Nessler method is sensitive down to 20 ppb "3-N. Among the other well-established techniques for ammonia determination are the Berthelot method which is based on the blue color formed 'Present address, Department of Chemistry, M i d d l e East Technical
University, Ankara, Turkey.
by the reaction of phenol and sodium hypochlorite with ammonia (4), and the pyridine-pyrazolone method which was proposed by Kruse and Mellon (6). Several chemical and methodic interferences exist for the Berthelot (4, 7 ) and the pyridine-pyrazolone methods ( 4 , 8 ) , whose sensitivities are comparable to that of the Nessler method. The most significant one among the novel techniques for the determination of ammonia is the use of ammonia selective electrodes (9-12) which also has a claimed sensitivity limit of 20 ppb NHB-N. Some of the other interesting methods in this field are, the one which uses the rate of change of absorbance with time in the Berthelot method (131,Permutit method (141,the use of a nuclear backscatter analyzer (151, monitoring with differential pulse polarography of the reaction product of ammonia with formaldehyde in an acetate buffer (16), and a thermometric titration technique in which BrO- is used as titrant ( I 7 ) . Infrared spectrometry, on the other hand, despite the belief of some that it is becoming obsolete, is still one of the most important elements of industrial (18) and research laboratories. Although infrared spectrometry is commonly used for qualitative purposes, with the advent of on-line computers (I9,20),its quantitative power has gained new dimensions, along with new qualitative conveniences. Quantitative applications with infrared spectrometry are possible providing that the resolution associated with qualitative analysis is sacrificed for a better signal to noise ratio by using large slit values (21,22).This rule is very important, especially when samples less transparent than solutions such as alkali halide pellets and polymer films are to be analyzed. A critical review of the alkali halide pelletting technique for solid samples will be given in another paper (23). A very common way of improving the sensitivity of detection in a chemical analysis is preconcentration of the species to be determined on a matrix which will not interfere during analysis, so that the species could be determined on the matrix itself (24,25). In this study, a new technique for the quantitative determination of ammonia, in ammonium ion form, is based on a preconcentration step on a zeolite ion exchanger followed by quantitative infrared analysis of the sample and the zeolite matrix in a KBr pellet.
EXPERIMENTAL Apparatus. For the batch processes, wide mouthed and heavy walled Bel-Art polyethylene bottles with 120-mLcapacity were used. The bottles containing the sample were agitated in a ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
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