Traceable Values for Nitrate in Water Samples by Isotope Dilution

accurate and traceable determinations of nitrate in water samples, with an expanded uncertainty (coverage factor k ) 2) of 2-5%, and the detection lim...
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Anal. Chem. 1996, 68, 3231-3237

Traceable Values for Nitrate in Water Samples by Isotope Dilution Analysis Using a Small Thermionic Quadrupole Mass Spectrometer Jean-Claude Wolff, Philip D. P. Taylor,* and Paul De Bie`vre

Institute for Reference Materials and Measurements, European CommissionsJRC, B-2440 Geel, Belgium

An isotope dilution mass spectrometric procedure was developed for the determination of nitrate in water samples. The isotope dilution experiments were carried out using the Institute for Reference Materials and Measurements’s 15N-enriched nitrate spike reference material IRMM-627. Nitrate was isolated from the matrix by precipitating it as nitron nitrate, from which emission of negative thermal NO2- ions was found to be best. The ions were produced in the ion source of a small, low-cost, easy-to-handle thermionic quadrupole mass spectrometer equipped with a secondary electron multiplier coupled to an ion counter. The procedure developed was applied to the measurement of nitrate in a certified reference material (simulated rainwater, CRM 409 from Community Bureau of Reference), in sparkling mineral water, and in tap water. Results were compared with those obtained using ion chromatography. Good agreement (within 1%) was found between the concentration determined by isotope dilution mass spectrometry, the values from ion chromatography, and the certified value. The procedure developed allowed accurate and traceable determinations of nitrate in water samples, with an expanded uncertainty (coverage factor k ) 2) of 2-5%, and the detection limit was found to be 2 µmol kg-1. The quantification of nitrate is important for water quality evaluation in terms of agricultural, industrial, and biological pollution. Moreover, the potential health risk from nitrate, especially from its chemically and biologically active toxic metabolite nitrite, leads to increased stringency in nitrate monitoring of waters.1-3 Owing to the toxicity (risk of anemia, formation of carcinogenic nitrosamines), the nitrate content of drinking waters has been regulated by national authorities and by the European Community Directive 80/778/EEC, which states a maximum admissible concentration (MAC) of 50 mg of NO3- L-1. This directive gives also a guide level of 25 mg of NO3- L-1.4 The U.S. Environmental Protection Agency (USEPA), based on the World Health Organization (WHO) guideline (1984), adopted a MAC value of 45 mg of NO3- L-1. The WHO states also a guideline value of 10 mg of NO3- L-1 for water intended for bottlefed infants.3 (1) Selenka, F. Sicherh. Chem. Umwelt 1981, 1, 65-68. (2) Hodgson, E.; Levi, P. E. A textbook of modern toxicology; Elsevier: Amsterdam, 1987. (3) CEC-JRC, Scientific Assessment of EC Standards for Drinking Water Quality; Technical Report EUR 12427 EN, 1989; pp 92-97. (4) Council directive 80/778/EEC. Off. Pub. Eur. Commun. 1980, 229, 1129. S0003-2700(96)00368-X CCC: $12.00

© 1996 American Chemical Society

Achieving good analytical measurement capability for water analysis seems to become a necessity with “Europe 93” and the ratification of the Maastricht Treaty.5 It is of political importance that quality and comparability of measurement results be improved. Political and legal decisions, such as whether or not water is too polluted for consumption, are based on chemical measurements. The Institute for Reference Materials and Measurements’s (IRMM) International Measurement Evaluation Programme (IMEP)5,6 is intended to provide pictures of the “state-of-the-practice” in the assay of toxic or life-essential elements or species (in this case nitrate) in different matrices compared to a “certified” reference value obtained by an isotope-specific method, namely isotope dilution mass spectrometry (IDMS). As IDMS has been identified as a primary method of measurement by the Consultative Committee on Amount-of-Substance,7 it is of great interest to make this method available to a large number of laboratories. Now, small, easy-to-handle, low-cost thermionic quadrupole mass spectrometers are available commercially, and they have been proved to give good results.8,9 It is the aim of this paper to describe a procedure for determining nitrate in water samples by IDMS using such an instrument. Previous work has shown that nitrate undergoes decomposition and forms negative NO2- ions in a thermal ion source.10 Hence, the formation of NO2- was used to carry out isotope dilution analysis of nitrate in water,11 food,12 fruit juice,13 and Antarctic snow14,15 samples using negative thermal ionization in a singlefocusing magnetic field mass spectrometer with a double-filament ion source and a Faraday cup as detector. It turned out that the method described in this previous work cannot be transposed as such to this quadrupole instrument. The new procedure worked (5) De Bie`vre, P. Intern. J. Environ. Anal. Chem. 1993, 52, 1-15. (6) De Bie`vre, P.; Savory, J.; Lamberty, A.; Savory, G. Fresenius Z. Anal. Chem. 1988, 332, 718-721. (7) De Bie`vre, P.; Kaarls, R.; Peiser, H. S.; Rasberry, S. D.; Reed, W. P. Accred. Qual. Assur. 1996, 1, 3-13. (8) Heumann, K. G.; Schindlmeier, W.; Zeininger, H.; Schmidt, M. Fresenius Z. Anal. Chem. 1985, 320, 457-462. (9) Rener, M.; Lamberty, A.; De Bie`vre, P. Analusis 1992, 20, 229-234. (10) Unger, M.; Heumann, K. G. Int. J. Mass Spectrom. Ion Phys. 1983, 48, 373376. (11) Heumann, K. G.; Unger, M. Fresenius Z. Anal. Chem. 1983, 315, 454458. (12) Unger, M.; Heumann, K. G. Fresenius Z. Anal. Chem. 1985, 320, 525529. (13) Heumann, K. G.; Neubauer, J. Fresenius Z. Anal. Chem. 1988, 329, 795796. (14) Neubauer, J.; Heumann, K. G. Fresenius Z. Anal. Chem. 1988, 331, 170173. (15) Neubauer, J.; Heumann, K. G. Atmos. Environ. 1988, 22, 537-545.

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Table 1. Features of THQ-MS mass analyzer

detection system ion counter ion source sample introduction vacuum system computer system

quadrupole mass analyzer 200 mm × 8 mm rods 500 u mass range field radius, 3.45 mm (1) single Faraday cup in line with quadrupole axis (2) secondary electron multiplier (SEM) (17 discrete dynodes) mounted 90° off-axis (type 217, Balzers AG, Liechtenstein), coupled to an ion counter (Universal counter/timer 6006, Kontron Messtechnik GmbH, Eching, Germany) 25 ns dead-time individually adjustable source potentials for positive and negative ionization, manual/computer control sample turret for up to 13 samples, single, double, and triple filaments air-cooled 170 L s-1 turbomolecular pump with 6.4 m3 h-1 forevacuum pump HP 300 series (Hewlett Packard, Fort Collins, CO), 0.7-1 MB RAM IEEE and RS232 port

out will be presented and applied to a certified reference material (CRM 409, nitrate in simulated rainwater), to sparkling mineral water, and to tap water. Results will be compared with those obtained with the standard procedure, i.e., high-performance ion chromatography (HPIC). EXPERIMENTAL SECTION Instrumentation. (i) Mass Spectrometer. The mass spectrometric measurements were carried out using a thermionic quadrupole mass spectrometer (THQ-MS, Finnigan MAT, Bremen, Germany), the features of which are summarized in Table 1. (ii) Ion Chromatography. The ion chromatography system consisted of an isocratic HPLC pump, fitted with a titanium analytical pump head (Knauer, Berlin, Germany), an Ion-Pac AG12A guard column, an Ion-Pac AS12A separator column (conventional low-capacity anion-exchange resin in the HCO3form), an anion self-regenerating suppressor ASRS-1, and a CD20 conductivity detector with DS3-1 detection stabilizer cell (Dionex, Sunnyvale, CA). Chromatograms were recorded on a Shimadzu Chromatopac C-R6A integrator (Kyoto, Japan). The injection valve (Knauer), as well as all the tubings and connections, was made of inert poly(ether ether ketone) (PEEK). The injection volume was 20 µL, and the flow rate of the eluent (Na2CO3, 2.7 mM/ NaHCO3, 0.3 mM) was 1.5 mL min-1. The background conductivity was typically 14.1 µS. Reagents. The purest available chemicals were used. Solutions were prepared either with subboiled or deionized water (Milli-Q Plus System, Millipore S.A., Molsheim, France). All the weighings (preparation of calibration solutions, dilutions, and spikings) were performed in a room of controlled temperature and humidity and were corrected for buoyancy. For the isotope dilution experiments, the 15N-enriched nitrate spike isotopic reference material IRMM-627 has been used.16 IRMM-627 was prepared from KNO3 (Isotec Inc., Miamisburg, OH) in 10-4 mol L-1 NaOH solution (prepared from NaOH‚H2O suprapur (Merck, Darmstadt, Germany) and subboiled water) and stored in 5 mL holding quartz ampules.16 The isotopic composition of nitrate-nitrogen of IRMM-627 is f(15NO3-) ) 0.6460 ( 0.0050 and f(14NO3-) ) 0.3540 ( 0.0050, leading to an isotope ratio of n(14NO3-)/n(15NO3-) ) 0.548 ( 0.012. The concentration of nitrate, as determined by reverse IDMS, is c(NO3-) ) 3.124 ( 0.080 mmol kg-1. The stated uncertainties are expanded uncertainties, with k ) 2 (i.e., 95% confidence level). Procedure. The principles of IDMS as well as those of negative thermal ionization mass spectrometry (NTI-MS) have (16) Wolff, J.-C.; Dyckmans, B.; Taylor, P. D. P.; De Bie`vre, P. Submitted to Int. J. Mass Spectrom. Ion Processes.

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Analytical Chemistry, Vol. 68, No. 18, September 15, 1996

already been described extensively elsewhere.17,18,19 In this section, the procedural aspects related to the determination of nitrate will be explained. Sample preparation was carried out in an ultraclean chemical laboratory (UCCL, class 5 clean bench),20 and the final loading of the sample onto the filament was done in a class 100 clean bench to avoid any nitrate contamination. Sample Preparation. (i) Theoretical Aspects. The first step of analysis consisted of performing the spiking, i.e., the addition of a carefully weighed amount of IRMM-627 to the (water) sample. With the THQ mass spectrometer, the most accurate and precise measurements for isotope ratios were obtained in the range 0.1-10. Nitrate was isolated from the matrix by precipitation with the monoacid base 1,5-diphenylanilodihydrotriazol, C20H16N4, which Busch21 called nitron. The latter has different mesomeric structures,22 so that it is protonated and thus forms a fairly crystalline precipitate with NO3- that has interesting properties, i.e., high melting point, high stability, and nice crystallization. The solubility of nitron nitrate is 0.099 g L-1 at about 20 °C.23 However, interferences are caused by anions such as (given with the approximate solubilities of their nitron salts at about 20 °C) ClO4- (0.08 g L-1), I- (0.17 g L-1), SCN- (0.4 g L-1), CrO42- (0.6 g L-1), ClO3- (1.2 g L-1), NO2- (1.9 g L-1), Br- (6.1 g L-1), hexacyanoferrate(II), hexacyanoferrate(III), oxalates and considerable quantities of Cl-.23 If present in “large” amounts, e.g., for the analysis of sea water, these ions should be removed, for instance, by using anion-exchange resins. The most important interference was caused by nitrite, whose nitron also forms NO2thermal ions in the mass spectrometer. Hence, nitrite, which can be present in water samples at levels around 0.01-1 µg g-1 (and even higher in strongly polluted waters) was removed by the addition of amidosulfuric acid:

NH2SO3H + HNO2 f H2SO4 + N2 + H2O

(1)

Moreover, the spike isotopic reference material IRMM-627 itself contained about 2 µg g-1 nitrite. At least 1-2 mg of precipitate was needed for reasons of sample handling and nearly quantitative precipitation. In fact, (17) Heumann, K. G. In Inorganic Mass Spectrometry; Adams, F., Gijbels, R., Van Grieken, R., Eds.; John Wiley and Sons: New York, 1988; pp 301-376. (18) Fassett, J. D.; Paulsen, P. J. Anal. Chem. 1989, 61, 643A-649A. (19) De Bie`vre, P. In Trace element analysis in biological specimens; Herber, R. F. M., Stoeppler, M., Eds.; Elsevier: Amsterdam, 1994; pp 169-183. (20) Lamberty, A.; Moody, J. R.; Van Duffel, E.; De Bie`vre, P.; Broothaerts, J.; Taylor, P. D. P.; Lathen, C. Submitted to Fresenius J. Anal. Chem. (21) Busch, M. Ber. Dtsch. Chem. Ges. 1905, 38, 861-866. (22) Kuhn, R.; Kainer, H. Angew. Chem. 1953, 65, 442-446. (23) Jeffrey, G. M.; Basset, J.; Mendham, J.; Denney, R. C. Vogel’s Textbook of Quantitative Chemical Analysis, 5th ed.; Longman Scientific and Technical: 1989; p 484.

Awad et al.24 found limited recovery in the gravimetric microdetermination of nitrate using nitron (in that case, about 80%). (ii) Procedural Aspects. To meet the requirements of amount of precipitate and range of ratio, a preliminary analysis was done using Merckoquant nitrate test strips (Merck, Darmstadt, Germany) to estimate the nitrate content of the water sample. Generally, 0.5-4 g of IRMM-627 were added to 10-100 g of water sample in a quartz beaker. After homogenization to attain isotopic mixing, 0.2-1 mL of amidosulfuric acid solution (10% w/w) and a few milliliters of 1 M H2SO4, which favored the formation of fairly crystalline white needles,21,25 thus allowing better filtration, were added. The volume of the sample was reduced by evaporation in a fume hood of the UCCL to about 5 mL. In cases of strongly mineralized waters, e.g., the sparkling mineral water analyzed, a precipitate appeared while reducing the volume. This precipitate had to be eliminated by filtration using carefully washed membrane filters. To the warm solution was added 0.2-0.5 mL of a freshly prepared 10% (w/w) nitron acetate solution. The solution took a yellow-brownish color. It was stirred, and then needles of nitron nitrate precipitate started appearing while cooling down. The sample was placed into ice water for at least 2-3 h to allow nearly quantitative precipitation. The precipitate was filtered with slight suction using 1 µm PTFE membrane filters (Nuclepore Corp., Pleasanton, CA) prewetted with methanol. It was then washed using 2-5 mL of subboiled water at ∼0-5 °C in small portions. In fact, Gutbier25 showed that the amount of nitron nitrate resolubilized during the washing operation depends on the temperature and the amount of water used. The precipitate was dried first in a desiccator and then in a vacuum oven for 2 h at 60 °C. It was finally redissolved in a few microliters of ethanol (to give a suspension) and kept in a vial in a clean bench until loading onto the filament for mass spectrometric measurement. Mass Spectrometric Measurement Procedure. Nitratenitrogen isotope ratios were measured using negative thermal ions (NO2-) produced in a double-filament ion source. Approximately 50 µg of NO3-, deposited in 2 µL fractions of the nitron nitrate suspension, was loaded onto the evaporation filament while a current of 0.5 A was passed through it to fix the sample. Next, 10 µg of Ba2+ (as Ba(OH)2 in acetic acid solution), which served as an ionization enhancer, was deposited onto the ionization filament. After the barium solution had dried at room temperature, the filament was shortly heated to red-hot to transform the barium into its oxide. Both filaments were made of Re (H. Cross Co., thickness 0.025 mm, width 0.7 mm) and had previously been outgassed at 4 A during 25 min, 2-8 days prior to their use for measurement. Ionization of NO2 (eq 2), which was formed during the evaporation and subsequent decomposition of nitron nitrate (C20H16N4HNO3) into various products of undefined nature containing N, O, C and H, occurred at ∼1200 °C. The most efficient

NO2 + e- f NO2-

(electron affinity ) 3.91 eV) (2)

evaporation-ionization was reached by indirect heat transfer from the ionization to the evaporation filament, placed at a distance of 3 mm from each other. (24) Awad, W. I.; Hassan, S. S. M.; Zaki, M. T. M. Talanta 1971, 18, 219-224. (25) Gutbier, A. Z. Angew. Chem. 1905, 18, 494-499.

Total ion currents of 10-13 A, corresponding to 3 × 105 ions s-1, were obtained using the secondary electron multiplier (SEM) with ion counting as detection device. Small magnets were placed in the source region to minimize the background ion beam contribution. The isotope ratios were measured at mass positions 46 and 47 (14NO2- and 15NO2-). The ionization filament was heated stepwise (in 12 min from 800 mA (∼700 °C) to 1300-1350 mA (∼1200 °C)) until a sufficient ion beam intensity was reached. Collection was started immediately after the electromagnetic lenses of the ion source were focused. Each measurement consisted of 60 isotope ratios measured in six blocks of 10 scans (measuring time, 35 min) in the peak jumping mode. Integration time of each scan was 10 s, and baseline measurement was carried out at mass 48.5. Typical background ion beam contribution was about 1000 ions s-1. Ion Chromatography. The water samples were injected at least five times using disposable 2 mL plastic syringes. The injection loop was flushed with at least 1 mL of the sample to be analyzed, thus avoiding cross-contamination. Concentrations were calculated against a metrologically prepared sodium nitrate calibration solution (NaNO3 Suprapur, Merck, Darmstadt, Germany) of nearly the same concentration as the water sample to be analyzed (external calibration). The calibration solution was injected before and after the injection of the water sample in order to correct for any instrumental drift during measurement. RESULTS AND DISCUSSION Mass Spectrometric Measurement Procedure. The procedure described here differs in several points from earlier work.10,11 The THQ mass spectrometer did not allow measurement of isotope ratios with sufficiently high precision when using a Faraday cup as detector. In fact, the ion beam gradually decreased with time and did not last more than 10 min. Therefore, ion counting was chosen as detection device. As a consequence of using an ion counter, which has a much higher sensitivity than the Faraday cup, the background ion beam contribution, resulting from electrons which are not easily filtered out by the quadrupole mass analyzer, was relatively high, i.e., up to 3000 ions s-1. The use of small magnets placed in the source region decreased the background ion beam contribution by a factor of 3. The distance between ionization and evaporation filaments greatly affected the ion beam stability. For a too-short distance, i.e., 2 mm, the evaporation-decomposition of nitrate took place before the optimal ionization temperature was reached; hence, the ion beam, even with the ion counter, did not last more than 20 min. The distance was varied between 2 and 4 mm in 0.5 mm intervals, and it was found that the best ion beam stability was obtained for 3 mm. The ion beam stability was also affected by the outgassing of the filaments. In fact, it turned out that filaments had to be outgassed 2-8 days prior to their use for measurement to guarantee the best ion beam stability obtainable with the procedure described. The reason for this is not well understood. The ionization temperature was lowered from 1550 (refs 11 and 12) to 1400 °C, and to ∼1200 °C when barium was used as ionization aid. This lower temperature diminishes the mass fractionation effects in the ion source. A sufficient ion beam was Analytical Chemistry, Vol. 68, No. 18, September 15, 1996

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Table 2. Different Nitrate Compounds Investigated compound

ion beam intensity

NaNO3 Ca(NO3)2 La(NO3)3 Al(NO3)3 Cr(NO3)3 Fe(NO3)3 Ag(NO3)3 Pb(NO3)2 BiONO3 nitron HNO3

no ion beam observed no ion beam observed no ion beam observed 3.2 × 105 ions s-1 2.1 × 103 ions s-1 3.4 × 104 ions s-1 no ion beam observed no ion beam observed no ion beam observed >1 × 106 ions s-1

ionization temp (°C)

1 + 0.000761813 RO-corr ∼1250 ∼1500 ∼1450

It followed that the 17O-corrected ratio was given by

RO-corr )

∼1200

Table 3. Probabilities of Occurrence of the Isotopically Different Species at Masses 46 and 47 According to IUPAC Tabulated Values mass

isotopic species

probability of occurrence (p)

46 47

14N16O16O

0.991 603 065 0.000 377 708 0.000 377 708 0.003 642 599

14N16O17O 14N17O16O 15N16O16O

obtained during 60-75 min, with no significant fractionation trend (i.e., within the uncertainty of the determined mass fractionation factor, ∼1%). Apart from the nitron nitrate, several other nitrate salts were investigated to find the best ionisation technique (Table 2). In fact, it would be nice to measure nitrate-nitrogen isotope ratios directly from NaNO3 or KNO3 instead of via the nitron salt when characterizing a spike reference material, thus minimizing contamination and possible fractionation effects during sample preparation. As far as the alkali and the earth alkali nitrates are concerned, the results shown in Table 2 were expected, because these nitrates decompose into nitrites (eq 3) when heated. ∆

NaNO3 98 NaNO2 + 1/2O2

(3)

Unger et al.10 observed only a very weak ion current when measuring on a magnetic field mass spectrometer. The “heavy” metal nitrates, like Pb(NO3)2, normally decompose into their respective metal oxide, NO2, and oxygen. However, under the conditions investigated, no ion beam was observed. Besides nitron nitrate only Al(NO3)3 gave a reasonable ion beam. The reason for this is not well understood. Determination of the Mass Fractionation Factor. Prior to any determination of mass fractionation, the observed ratios, originating from the measurement of NO2- ions, had to be corrected for 17O contribution. To do so, the probabilities of occurrence (p) of the isotopically different NO2- ions (mass 46 to mass 51) were calculated from the IUPAC tabulated values of nitrogen and oxygen (Table 3). The observed ion beam intensity at m/z ) 47 (I47) consisted of

I47 ) I(15N16O16O) + I(14N17O16O) + I(14N16O17O) ) I(15N16O16O) + 2I46p(14N17O16O)/p(14N16O16O) (4) This equation was divided by the observed ion beam intensity at m/z ) 46 (I46, which is equal to I(14N16O16O)), yielding 3234

I47 p(14N17O16O) I(15N16O16O) 1 ) ) 14 16 16 + 2 14 16 16 ) I46 Robsd I( N O O) p( N O O)

Analytical Chemistry, Vol. 68, No. 18, September 15, 1996

Robsd (1 - 0.00761813Robsd)

(5)

It is well known that measured isotope ratios are biased because of the procedure and instrumentation used. Three main components can be distinguished: (1) the bias introduced during sample preparation (as precipitation or ion exchange processes are mass dependent); (2) the fractionation due to the measurement process, i.e., ion production (the lighter isotope of an element tends to evaporate more likely at lower temperature (Rayleigh distillation)), ion transfer, separation, and detection; and (3) the fractionation induced by experimental (instrumental) parameters such as type of mass spectrometer, type of detector, type of source or lenses, and impurities. The mass fractionation factor is defined as the ratio between the “true” isotope ratio and the observed one:

K ) Rtrue/R′obsd

(6)

Fractionation is a very complex process that is not yet fully understood. The main problem is that no reliable and appropriate thermodynamic data for the ionisation and evaporation processes of isotopic species under the “nonequilibrium” conditions in the thermal ion source are available. Next to the different theoretical fractionation models,26 an empirical method is used to correct for the bias introduced by isotopic fractionation in practice. The mass fractionation factor is derived by measuring a certified isotopic reference material under “exactly” the same conditions as the sample itself, i.e., measuring the spike reference material IRMM-627 with certified 14NO -/15NO - ratio. The mean observed ratio from eight 3 3 replicates was 0.550, leading thus to a K-factor of 0.995. The repeatability between the eight filaments measured was about 0.6% (Figure 1). Another experimental approach to determine K consisted of preparing metrologically a synthetic isotope mixture16 from a wellcharacterized natural nitrate solution and the 15N-enriched material IRMM-627. The isotope ratio of the prepared blend was measured, and K was calculated using the IDMS equation:

cx )

my Ry - KR′B 1 + Rx c mx y KR′B - Rx 1 + Ry

(7)

The variables are defined in Table 4. No K-factor was applied to the certified value of IRMM-627 (Ry), nor to the isotope ratio of natural nitrate (Rx), as the IUPAC value for nitrogen, i.e., 272.2 ( 6.7, was taken.27 This was done because all the isotope amount (26) Habfast, K. Int. J. Mass Spectrom. Ion Phys. 1983, 51, 165-189. (27) De Bie`vre, P.; Taylor, P. D. P. Int. J. Mass Spectrom. Ion Processes 1993, 123, 149-166.

Figure 1. Repeatability of the isotope ratio measurements of IRMM-627 using the THQ mass spectrometer with ion counting as detection. The error bars represent the repeatability of the 60 isotope ratios measured on one filament, whereas the area between the dotted lines represents the repeatability among eight filaments (i.e., external relative standard deviation, which is about 0.6%). Table 4. Input Data for the Calculation of the Mass Fractionation Factor Using a Synthetic Isotope Mixture

mass of natural nitrate solution mass of IRMM-627 concn of natural nitrate solution concn of IRMM-627 isotope ratio of natural nitrate solution isotope ratio of IRMM-627 measd isotope amount ratio of the blend

symbol (unit)

value

mx (g) my (g) cx (mmol kg-1) cy (mmol kg-1) Rx Ry R′B

0.9683 2.5448 17.9814 3.124 272.2 0.548 3.899

ratios were expressed as n(14NO3-)/n(15NO3-) after 17O correction (eq 5). Equation 7 was solved for K:

( [ (

)

cxmx 1 + Ry + Ry Rx cymy 1 + Rx K) cxmx 1 + Ry R′B 1 + cymy 1 + Rx

)]

Table 5. Weighings for the Blends Investigated, Corresponding Isotope Ratios Measured after 17O and K Correction, and Concentrations Calculated Using IDMS Equation with Their Respective Combined Uncertainty, uc (in Parentheses)

blend

mass sample (g)

mass IRMM-627 (g)

corrected c(NO3-) isotope ratio in sample of the blend (mmol kg-1)

951108A 951108B 951108C 951108C1P 951108C2P

10.159 (5) 20.457 (10) 20.133 (10) 20.133 (10) 20.133 (10)

CRM 409 2.3636 (5) 2.3077 (5) 4.0769 (10) 4.0769 (10) 4.0769 (10)

0.7152 (33) 0.8937 (64) 0.7378 (39) 0.7246 (45) 0.7611 (36)

0.0790 (37) 0.0793 (25) 0.0781 (34) 0.0727 (35) 0.0876 (34)

951204A 951204B

20.645 (5) 30.156 (5)

Tap Water 1.7240 (5) 1.0412 (5)

2.606 (10) 5.460 (29)

0.3515 (54) 0.3500 (53)

Beckerich Mineral Water 49.966 (10) 1.3837 (10) 2.530 (14) 102.3 (1) 1.7328 (10) 3.752 (20)

0.1122 (18) 0.1115 (17)

960208B 960208C

(8)

The input data are given in Table 4. A K factor of 0.994 was found by measuring this blend, which compared well, within the uncertainties of the determinations, with the one determined by directly measuring IRMM-627 (K ) 0.995). The mean K, i.e., 0.9946 ( 0.0013, from these determinations was taken for the subsequent calculations. Application to Water Samples. One of the samples consisted of the certified reference material CRM 409 (from Community Bureau of Reference), i.e., simulated rainwater having high mineral content, with a certified value for nitrate of 78.1 ( 1 µmol kg-1.28 The other samples analyzed were drinking waters, i.e.,

tap water from the south of Luxembourg and sparkling mineral water (Beckerich, Luxembourg). For all the samples, at least two spikings with IRMM-627 were carried out in order to verify the repeatability of the method (Table 5). The measured ratios of the blends, 17O- and K-corrected, with their corresponding combined uncertainty (uc), i.e., square root of the sum of the squares of the relative uncertainty on R (i.e., repeatability of the measurement) and K, are given in Table 5. All the uncertainties were calculated according to Eurachem29 and NIST30 guidelines. The concentration of the blends were calculated according to the IDMS equation (eq 7). The combined uncertainty was obtained by applying the uncertainty propagation law to this equation (Table 5 ). A complete orthodox uncertainty budget is given in Table 6 for one of the spiking experiments related to CRM 409.

(28) Reijnders, H. F. R.; Quevauviller, P.; Van Renterghem, D.; Griepink, B.; Van der Jagt, H. Fresenius J. Anal. Chem. 1994, 348, 439-444.

(29) Eurachem, Committee Draft, 1995. (30) Taylor, B. N.; Kuyatt, C. E. NIST Technical Note 1297, 1994.

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Figure 2. Statistical uncertainty magnification factor (M) and ratio of the blend as a function of the sample to spike ratio q for enrichments of 66% and 99%. Table 6. Uncertainty Budget: Type A Uncertainties Based on Statistical Methods for Data Treatment and Type B Uncertainties Based on Scientific Judgement Using the Relevant Information Available30 source of uncertainty mass of CRM 409, mx (g) certified nitrate concn of IRMM-627, cy (mmol kg-1) mass of IRMM-627, my (g) isotope ratio of CRM 409 (IUPAC value), Rx certified isotope ratio of IRMM-627, Ry measd isotope ratio of the mixing, RB (repeatability of 8 replicates) mass fractionation factor, K

typical combined type value uncertainty (of uc)

RSD

20.457 3.124

0.001 0.040

A B

0.00005 0.0128

2.307 7 272.2

0.0005 6.7

A B

0.00022 0.0246

0.548 0

0.0061

B

0.0111

0.8986

0.0063

A

0.0070

0.9946

0.0013

A + B 0.0013

total combined uncertainty (square root of the sum of the squares of relative uncertainties) total combined uncertainty (calculated from uncertainty propagation law)

0.0307 0.0312

It was shown that a nearly quantitative precipitation of nitron nitrate is crucial to obtain accurate results. In fact, for the blend 951108C, the precipitation was first carried out without reducing the volume of the solution from 25 to 5 mL (blend 951108C1P). A small amount of nitron nitrate precipitated, which yielded a lower isotope ratio and hence a lower nitrate concentration for CRM 409. This means that the heavier isotopes 15NO3- precipitated first. The sample preparation step, when not taking sufficient care, thus also yields unacceptable mass fractionation. The blend 951108C2P corresponds to the filtrate of 951108C1P, the volume of which was reduced to 5 mL. Some nitron nitrate, enriched in 14NO -, precipitated (Table 5). When redissolving the precipitates 3 from 951108C1P and 951108C2P in ethanol and homogenizing the solution, the value of 951108C given in Table 5 was found, which agreed with that of the blends 951108A and 951108B. For CRM 409, the combined uncertainty calculated using uncertainty propagation law, including type A (repeatability of measurements) and type B (nonstatistical) uncertainties, is rather high (about 4.5%). This is due mainly to the fact that the ratio of 3236

Analytical Chemistry, Vol. 68, No. 18, September 15, 1996

15N

spike

the blend measured is too close to the ratio of IRMM-627, thus giving rise to a large uncertainty magnification factor for all the spike components (Figure 2),31 i.e., the isotope ratio of IRMM627 as well as its concentration. For the measurements of tap water and Beckerich sparkling mineral water, the ratios of the blends were more favorable, involving a smaller uncertainty magnification factor, so that a combined uncertainty of 1.5% could be reached. As can be seen from Figure 2, ratios larger than 2 have to be chosen in order to achieve an uncertainty smaller than 2%. This fact, as well as the amount of nitron nitrate precipitate needed (at least 1-2 mg), limits the detection limit of the method. The latter was determined to be about 2 µmol kg-1, considering a reasonable amount of water sample to be investigated and allowing an expanded uncertainty of about 5%. The mean nitrate concentration from the different spiking experiments for CRM 409, Beckerich sparkling mineral water, and the tap water is given in Table 7. The combined uncertainty of the mean nitrate concentration was calculated by averaging the combined uncertainty on each concentration following

u j c(cj) )

1 n

x

n

∑u i)1

2 c

(c)(Bi)

(9)

where (Bi) refers to the different spikings (blends) and n to their number. The expanded uncertainty is given with a coverage factor of k ) 2, leading hence to relative expanded uncertainties of 4.74% for CRM 409, 2.23% for Beckerich sparkling mineral water, and 2.16% for tap water (Table 7). The results compare well with those obtained by HPIC and with the certified value for CRM 409 (Table 7). The combined uncertainty with a coverage factor k ) 2 for the value obtained by HPIC was calculated by taking the square root of the sum of the repeatability of the measurement of the (31) Wolff, J.-C.; Taylor, P. D. P.; De Bie`vre, P. Internal Report GE/R/SIM/8/ 95, 1995.

Table 7. Final Results: Comparison of the Nitrate Concentration and Expanded Uncertainty (Coverage Factor, k ) 2) Obtained by the Different Measurement Methods water sample

c(NO3-) from IDMS (mmol kg-1)

c(NO3-) from HPIC (mmol kg-1)

c(NO3-) (mmol kg-1)28

CRM 409, simulated rain water (three replicates) Beckerich sparkling mineral water (Luxembourg) (two replicates) tap water (south of Luxembourg) (two replicates)

0.0788 (37) 0.1119 (25) 0.3508 (75)

0.0805 (24) 0.1184 (52) 0.3458 (74)

0.0781 (10)a 0.0903b

a

Certified value.

b

Stated value.

calibration solution and the repeatability of the measurement of the water sample and adding the relative uncertainties from the weighings and dilutions as well as those from the nonlinearity of the detector. As the nitrate concentrations of calibration solution and water sample were comparable c(NO3-)calib ≈ c(NO3-)sample, the response of the detector was quite similar, thus minimizing the uncertainty. CONCLUSION The method described here allowed an accurate and traceable determination of nitrate in water samples using a low-cost, easyto-handle quadrupole thermal ionization mass spectrometer. The spike reference material IRMM-627, which has been calibrated against synthetically prepared isotope mixtures, is traceable to the international SI unit for amount of substancesthe mole.16 The use of this material to carry out the isotope dilution analysis of CRM 409, Beckerich sparkling mineral water, and the tap water rendered the measurements performed also traceable to the SI system. By using IRMM-627, expanded uncertainties with a coverage factor k ) 2 of 2-5% were obtained, and the detection limit was found to be about 2 µmol kg-1 when spiking a sufficient amount of water sample. This detection limit is mainly due to the “limited” enrichment of IRMM-627 and the fact that a certain amount of nitron nitrate precipitate is needed. A more enriched spike reference material is under preparation.32 This will allow a (32) Wolff, J.-C.; Dyckmans, B.; Taylor, P. D. P; De Bie`vre, P. Unpublished work.

larger measuring range from the point of view of the isotope ratios of the blends investigated (Figure 2)31 and thus a better detection limit. IRMM-627 can then be used to determine the mass fractionation factor of the procedure, because its ratio is well suited for this purpose, whereas the more enriched material will then be used to carry out isotope dilution experiments at lower concentration levels (