Measuring SI-traceable nitrate concentrations in water by a primary method of measurement, isotope dilution mass spectrometry

ANALWICA CHIMICA ACTA

ELSEVIER

Analytica

Chimica

Acta 346 (1997) 3-15

Measuring S&traceable nitrate concentrations in water by a primary method of measurement, isotope dilution mass spectrometry Jean-Claude

Wolff, Philip D.P. Taylor*, Paul De Bibre

Institute for Reference Materials and ~e~urements, Received

European Commi.wion - JRC, B-2440 &eel, Belgium

31 July 1996; received in revised form 28 October

1996; accepted

30 October

1996

Abstract Simulated

tap-, river- and sparkling mineral-water were analysed for their nitrate content, using isotope dilution mass (IDMS). After the removal of nitrite with amidosulfuric acid, nitrate was isolated selectively from the matrix by it with nitron. The nitron-nitrate formed was used to produce NOzi thermal ions from a double-filament

rain-,

spectrometry precipitating

arrangement in the ion source of a quadrupole thermionic mass spectrometer and from a triple-filament arrangement in the source of an NBS-type magnetic-field thermionic mass spectrometer. Both measurement procedures are discussed and compared. The IDMS experiments were performed using the “N-enriched nitrate species-specific spike isotopic reference material, IRMM-629, which is traceable to the international SI unit system in the shortest possible way. It was shown how isotope dilution combined with negative thermal-ionisation mass spectrometry yielded accurate and traceable values for nitrate along an unbroken chain of a transparent procedure with full orthodox uncertainty evaluation for each step. Expanded uncertainties (coverage factor k=2) of 2%-5% were obtained. Keywords:

Nitrate; Isotope-dilution

mass spectrometry;

Negative thermal ionisation; -..

1. Introduction The use of nitrate as an impo~ant nutrient in agriculture has been increasing markedly since the last few decades, i.e. from two million tons in 1955 to 11 million tons per year in 1988 [l]. It has been estimated that up to 40% of applied nitrates enter water sources as run-off or leachate, and thus, drinking water supplies. Hence, several studies in Denmark, Germany 1’21and in the United States [33 revealed that nitrate concentrations in drinking water supplies

*Corresponding

author. Fax: +32 (0) 14 584 273.

~3-267~97/$17,~ 0 1997 Elsevier Science B.V. All rights reserved. PII SOOO3-2670(96)00544-Z

Traceability;

Isotopic reference

materials;

Water

exceeded periodically or continuously the maximum admissible concentration (MAC) of 50 mg l-l, fixed by the European Community directive EEC/80/778 [4]. The World Health Org~isation (WHO) and the US Environmental Protection Agency [5] have adopted a MAC value of 45 mg l-‘, which is based on a review by Walton [6]. This review does not document any case of methaemoglobinaemia, the acute toxic effect (especially for infants) of nitrate ingestion [1,7], below that level. Concern also exists about the role nitrate might play in carcinogenesis [ 1,7]. This potential health risk from nitrate and, especially, from its chemically and biologically active toxic metabolite nitrite, which is formed in the human

4

J.-C. Wolff et al./Analytica

body, leads to increased stringency for the determination of nitrate in drinking waters. Numerous research papers are describing new or improved methods yielding better sensitivity and detection limits in the determination of nitrate [8]. But, does sensitivity and detection limit rhyme with accuracy? As nowadays, quality assurance, reliability and comparability of measurement results are very much in the “focus of analytical chemistry”, the following question can be asked: “how can analytical chemists provide political and legal authorities with comparable, reliable and accurate results?” In physical measurements, comparability was set up by BIPM/ CGPM (Bureau International des Poids et Mesures / Confkrence G&&ale des Poids et Mesures) by ensuring traceability of measurements in SI units (kg, s, m, A,...). Since CGPM has introduced the mole in 197 1 as the SI unit for the amount of substance, chemists have the opportunity to assure traceability of chemical measurements in the SI system. Since 1990, organisations, such as EURACHEM and CITAC (Co-operation on International Traceability in Analytical Chemistry), have been found in order to improve comparability and traceability of chemical measurements. The Community Bureau of Reference (BCR) recently issued certified reference materials (CRM) for the quality control of nitrate analysis in fresh- [9] and rainwater [lo]. During the certification campaign for the freshwater, participants used ion chromatography, spectrophotometry and differential pulse polarography. They pointed out the need for an alternative technique, such as isotope dilution mass spectrometry (IDMS) [ 111, since the latter has been identified by the ComitC Consultatif pour la QuantitC de Mat&e (CCQM) as a primary method of measurement [12,13]. Based on previous work [14,15], an isotope dilution mass spectrometric measurement procedure - combined with negative thermal ionisation of nitrate via NOz-, formed during the decomposition of nitronnitrate - has been developed by Wolff et al. [ 161. This was done on a thermionic quadrupole mass spectrometer (THQ, Finnigan MAT, Bremen, Germany) which had a limited dynamic range for the measurement of isotope-amount ratios. In that work [16], the Institute for Reference Materials and Measurement’s (IRMM’s) 64.5% “N-enriched nitrate spike, IRMM-

Chimica Acta 346 (1997) 3-15

627, was used to carry out the IDMS experiments. To improve the detection limit of the method, a higherenriched isotopic spike material (IRMM-629) is needed. Measuring such large (>20) isotope-amount ratios for nitrate cannot be done on the THQ instrument, because of high electson background when measuring negative ions in the 40-50 mass range. In order to certify the isotopic composition of IRMM629, the mass spectrometric measurement procedure had to be adapted for performing measurements on a single-focusing magnetic-field-modified NBS-type mass spectrometer. Mass spectrometric measurement procedures, both on the quadrupole and the sector instrument, are compared. Isotope dilution experiments were carried out using the 99.4% “N-enriched nitrate spike, IRMM-629. It is shown how the use of IDMS yields SI-traceable values for nitrate in the analysed waters along an unbroken chain of a transparent procedure. Uncertainty propagation law was used to make a complete and fully orthodox uncertainty budget.

2. Experimental 2.1. Instrumentation Nitrate isotopic measurements were performed either using a commercially available quadrupole thermionic mass spectrometer (THQ, Finnigan MAT, Bremen, Germany) or a modified NBS-type, single-focusing magnetic-field mass spectrometer(deflection 90”, radius 12”). Ion currents were measured using a secondary electron multiplier (SEM). The THQ was equipped with a 90” off-axis, mounted SEM (AF 140, ETP Pty Ltd, Ermington, NSW, Australia), which was coupled to an ion counter (Universal counter/timer 6006, Kontron Messtechnik GmbH, Eching, Germany). The ion source for the NBS-type mass spectrometer was designed as it has been described by Inghram and Chupka [ 171. The instrument was equipped with highvoltage power supplies (ion source, SEM, magnetic field) from FUG (FUG Elektronik GmbH, Rosenheim, Germany) and a magnetic field controller from Bruker (B-H15, Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany). Detection of ions occurred either at a retractable single Faraday cup (not used in

J.-C. Wolff et al./Analytica

this work) or at an SEM (AF 150 (M34), ETP Pty Ltd) placed in line with the ion beam. The SEM was coupled to an amplifier (Model SR445, SRS, Stanford, USA) and an ion counter (universal counter 53131A, Hewlett-Packard, Fort Collins, CO, USA). Piloting of the instrument and collection of data were done using the TIMS Associated Software developed at IRMM in co-operation with IAEA (International Atomic Energy Agency) and LANL (Los Alamos National Laboratory) [18]. High vacuum, i.e., about lo-‘Pa, was obtained using a turbomolecular drag pump (Balzers-Pfeiffer GmbH, ABlar, Germany), cryocooling and an ion-getter pump (Thermionics Laboratory Inc., St. Hayward, CA, USA). Ionisation-filament temperatures were measured using a calibrated linear pyrometer (IRE, Stuttgart, Germany). 2.2. Sample preparation Sample preparations were carried out in the IRMM’s ultra clean chemical laboratory (UCCL,
Chimica Acta 346 (1997) 3-15

5

0.00618f0.00050 [20]. The uncertainties given are expanded uncertainties with a coverage factor k=2. Throughout this paper, the isotope amount ratios are expressed as n( 14N0s-)/n(‘5NOs-). 2.3. Mass spectrometry 2.3.1. Filament loading Filaments were made of rhenium (H. Cross Co., USA; thickness 0.025 mm, width 0.7 mm) and had previously been outgassed at 3.5 A for 20 min. On the NBS-type mass spectrometer, a triple-filament arrangement was used. The ionisation (central) filament was loaded with 5 ug of barium in the form of barium hydroxide octahydrate in 0.5% acetic acid. The filament was shortly brought to red-hot in order to transform the barium into its oxide. Barium, as the electron emitter, served to enhance ionisation. Nitrate samples in the form of nitron-nitrate in ethanol were loaded on the side-filaments, while passing a current of 0.5 A through them to avoid outspreading of the sample due to the low surface tension of ethanol. The mass of nitrate loaded was approximately 0.5 pg on each side-filament. The distance between the two sidefilaments was 2 mm. The filament loading procedure on the THQ instrument was similar, excepted that a double-filament arrangement - with ionisation and evaporation filaments at a distance of 3 mm from each other - was used. The amounts loaded were larger, i.e. 10 ug of barium, and 50 ug of nitrate. 2.3.2.

Heating procedure, data collection and processing Only the ionisation filament was heated. Evaporation-decomposition of nitron-nitrate to form NO*ions occurred by indirect heat transfer. For the measurements on the NBS-type mass spectrometer, the ionisation temperature was raised up to 730°C and a 10 min delay period was applied before the temperature was increased to 780°C for 5 min. A small ion beam at the mass position of the most abundant isotope started appearing. Heating was continued up to 850°C. The filament was kept at this temperature for 10-15 min, while the electrostatic lenses of the ion source were focused. The acceleration voltage used was 3.5 kV. After the ion beam and the vacuum in the ion source (at approximately

6

J.-C. Wolff et al./Analytica

Chimica Acta 346 (1997) 3-15

Certified Spike Isotopic Reference

I

Water sample (we&hi@

Material lRMM-629 (weighing)

I

I heating 1 Reduction of s/hlbll volume

1

Filtration to eliminate carbonates fbrmed in strongly mineralised water Addition of nitron acetate solution (10%)

slightheating Precipitation of nitron-nitrate

I Filtration on PTFE membrane filter

Drying of the precipitate in a vacuum oven at 60 “C for 2 hours

I Redissotion

in ethanol

1

I

Loading onto the filamentfix mass spectrometric measurement Fig. 1. Sample preparation procedure for nitrate determination by

5x 1O-6 Pa) was stabilised, the temperature was increased up to 900-1000°C in order to obtain a sufficient ion beam of 2 x 105-4x lo5 ions per second. Again the ion beam was allowed to stabilise.

IDMS.

Collection of data started ca. 45-55 min after the beginning of the filament heating. The isotope-amount ratios were measured in the peak jumping mode. Integration time of the ion beam at mass to charge

J.-C. Wolff et al./Analytica

position 46 (*4N02-) and 47 (tsN02-) was 4 s, and a delay time of 8 s was set between each peak jump. The same integration time was used for baseline measurement at -0.5 and +OS, at the low- and high mass side of each peak. Typical background noise was 20-150 ions per second. Data collection lasted approximately I h and about 100 isotope-amount ratios were measured. Data were stored in a raw data output file and were evaluated via a spreadsheet template. Peak intensities were then corrected for multiplier dead time, which was 15 ns. On the THQ mass spectrometer, the heating of the ionisation filament was more straight forward [ 161, i.e. in 12 min the temperature was raised to ca. 12001250°C. Ion-beam intensities of 3 x lo511 x 10’ ions per second were reached. Focusing of the electrostatic lenses took place during the heat up, so that collection of the data could start immediately because of the decrease in the ion-beam intensity. Sixty isotope mount-ratios were measured in the peak jumping mode during 35 min. The integration time was 10 s and the delay time was 4 s. Background noise, which was reduced to 1000-2000 ions per second by placing small magnets on the outside of the flange holding the ion source, was monitored at mass-to-charge position 48.5. Data collection and treatment was done by the THQ associated software. The ion source of the NBS instrument was cleaned thoroughly each time samples having significantly different isotope amount ratios were loaded, in order to avoid isotopic cross-contamination. On the THQ, this was achieved by baking out the ion source and mass analyser. 2.4. Ion chromatography Ion chromatographic analyses were performed using an isocratic HPLC pump (Knauer, Berlin, Germany), an AG12A guard column and an AS12A analytical column (Dionex, Sunnyvale, CA, USA). At least five replicates of 1 ml aliquots of the water samples were injected via a 20~1 injection loop (Knauer) using disposable 2 ml plastic syringes. Anions were eiuted by a c~bonate~ic~bonate buffer (Na2C03 2.7 mM/NaHC03 0.3 mM) at a flow rate of 1.5 ml min.-’ and were detected using suppressed conductivity (ASRS-I self-regenerating suppressor and CD20 conductivity detector from Dionex). Chro-

Chimica Acta 346 (1997) 3-15

-I

matograms were recorded on a Chromatopac C-R6A integrator (Shimadzu, Kyoto, Japan). The calibration solution (made from Merck Multi~Anion II, Darmstadt, Germany) was injected before and after the water samples to correct for any instrumental drift (external calibration).

3. Results and discussion 3.1. Comparison of the two mass spectrometric measurement procedures used Table 1 summ~ses and compares the main features of the mass spectrometric measurement procedure on both the quadrupole and the sector magneticfield mass spectrometers. Besides the large dynamic range of the NBS-type mass spectrometer, the sensitivity and stability of the measurements were highly improved compared to that of the THQ. The amount of NOs- loaded was decreased by a factor of 50, which rendered the loading much easier. The NOs- concentration in the ethanol suspension could be lowered, and hence the deposition onto the filament became more reproducible. Spreading of the sample during the loading of the filament was reduced, and the nitron-nitrate layer produced was more homogeneous. This homogeneity favourably influenced the between-filament reproducibility of the measurements. The use of a triplefilament ~angement, for which the ion source of the NBS was designed, permitted a more homogeneous evaporation of nitron-nitrate, thus increasing the stability of the ion beam. The stability was also favoured by the fact that the ionisation filament, i.e. the central filament, was arranged, so that desorptionionisation of the NO*- formed could take place in line with the lenses of the ion source and the flight tube of the mass spectrometer. In the ion source of the THQ, both evaporation and ionisation filaments were placed face to face; thus, the surface of ionisation was parallel to the ion beam. This double-filament a~angement was modified, so that the ionisation filament was placed orthogonal to the evaporation filament, as in the NBS instrument. But the ion-beam stability was not improved. When loading 50 pg of NOa- for the measurements on the NBS, the ionisation temperature (i.e. ca. 730-

8 Table 1 Comparison

J.-C. Wolff et al. /Analytica

of the parameters

of the mass spectrometric

Chimica

measurement

Acta 346 (1997)

procedure

3-15

on the THQ and the NBS mass spectrometers

THQ

NBS

Filament arrangement Distance between filaments

Double 3mm

Ionisation aid Mass of NO,- loaded Ionisation temperature Ion-beam stability

10 ug Ba as Ba(OH)s in CHsCOaH 30-80 ug 12OCL1250°C Poor (decreasing gradually with time)

Background noise Dynamic range Internal relative standard deviation for one measurement External relative standard deviation (between filaments)

1000-2000 ions per second 2-3 orders of magnitude -0.5%-1.5%

Triple ca. l-l.5 mm between evaporation filaments and the ionisation filament, 2 mm between both evaporation filaments 5 pg Ba as Ba(OH)s in CHsCOaH 0.5-l pg 900-1000°C Good (stable for 2-3 h at 2.5x 10s ions per second or more) 20-150 ions per second 6 orders of magnitude -0.2%4X3%

-OS%-2.5%


750°C)

was lowered by ca. 500°C with regard to the temperature needed for ionisation on the THQ. The ion beam lasted for several hours when the ionisationfilament temperature was gradually increased by 50°C every l-2 h. This showed that the evaporation-decomposition of nitron-nitrate took place in layers. The use of 0.5 ug of NOs- on each side-filament, i.e. 1 ug of NOs- in total, involved an ionisation temperature of 900-100°C. The higher ionisation temperature was mainly due to the distance of the nitron-nitrate to the ionisation filament, as evaporation was reached by indirect heat transfer from the ionisation filament. More sample meant a thicker nitron-nitrate layer, thus increased closeness to the ionisation filament, i.e. faster evaporation at lower ionisation-filament temperature. Even if the optimal ionisation temperature was not reached, it was compensated by the fast evaporation of large amounts of nitron-nitrate. A total loading of 1 ug of NOs- permitted to obtain a sufficiently stable ion beam for at least 2-3 h, without any significant fractionation effect observable within the standard uncertainty of ca. 0.5%. Further reducing the loaded amount would probably have increased the fractionation effect and rendered blank contributions more critical. Moreover, the detection limit and the possible number of measurements was determined by the amount of NOa- needed for precipitating it as nitron-nitrate [16]. For reasons of

handling the precipitate, approximately 0.4 mg of N03- were needed. The background noise increased with increase in temperature and amount of substance loaded. So, a total loading of 1 ug and an ionisation temperature of 950°C constituted a good compromise to reach a background level of ca. 20-150 ions per second. This low background when compared with the 1000-2000 ions per second measured on the THQ - the ion source of which was equipped with small magnets to trap electrons - was due to a better elimination of secondary electrons and possibly small fragments from nitron by the sector magnetic field. A specially-designed copper RF-shielding of the SEM connections to the amplifier of the ion counter helped to reduce the background noise as well. The latter also permitted the measurement of large isotope amount ratios. For the measurements on the NBS, the removal of barium as the ionisation aid raised the ionisation temperature (~1200’C) of NO2 by ca. 200°C. The amount of barium did not have a significant influence on the ionisation efficiency. The heating procedure did not appear as critical on the THQ as on the NBS. On the latter, a too quick heating pattern influenced the measurement considerably. Instability of the ion-beam and changing isotopeamount ratios were the consequence of a too fast heating pattern. Ion beam and vacuum had to stabilise at the different temperature levels. Due to this strict

J.-C. Wolff et al./Analytica

Chimica Acta 346 (1997) 3-15

heating procedure for ca. 50 min (15 min on the THQ, for comparison) and due to the fact that the ion source of the NBS held only one filament, the sample throughput per day on the NBS was two, as compared to ten on the THQ which was equipped with a 13filament turret. The higher sensitivity, the improved ion-beam stability and the lower background of the NBS instrument reduced the measurement uncertainty, i.e. the relative standard deviation on l-filament (internal relative standard deviation) and the between filament reproducibility (external relative standard deviation) (Tables 1 and 2). 3.2. Analyses

Fig. 2. Uncertainty magnification factor (M) and isotope-amount ratio of the blend (R) obtained for different sample-to-spike ratios using either IRMM-627 or IRMM-629.

of water samples

The highly 15N-enriched nitrate-species-specific IRM, i.e. IRMM-629, was used to carry out the IDMS experiments. In fact, in their previous work, Wolff et al. [16] showed that the detection limit for nitrate on the THQ was limited by the amount of precipitate needed as well as by the enrichment of the spike-IRM used (Fig. 2). This limitation was also valid for measurements on the NBS mass spectrometer, as the range of ratios available for the spikings, when using IRMM627 (64.5% enriched spike), was restrained in the range 0X5-10.0. The upper limit was given by the nitrate concentration in the water, and hence by the amount of precipitate needed. When using IRMM-

Table 2 Input data for the waters analysed Blend (for the waters analysed) Rosport sparkling 960622A 960622B

9

Mass of water (g)

and observed

(i70-corrected)

Mass of IRMM-629 (g)

629, the measurement range could easily be expanded, i.e. from 0.01 to 10.0 (Fig. 2). Moreover, the range of small-uncertainty magnification increased considerably using IRMM-629 compared to that when using IRMM-627 (Fig. 2). To demonstrate the better precision (due to the smaller-uncertainty magnification factor for a large range of ratios) and the better detection limit obtainable, water samples having relatively low nitrate content were analysed. The water samples consisted of tap water from the laboratory, bottled sparkling mineral water (Rosport, Luxembourg), river water from Kaylbach (Luxembourg) and simulated rain-

isotope-amount Observed ratio (“O-corrected) on NBS

ratios on the NBS and the THQ Relative standard uncertainty

Observed ratio (“O-corrected) on THQ

Relative standard uncertainty

0.0452 (15) not measured

0.0332 _

mineral water 39.396 (10) 137.9 (1)

1.4926 (10) 1.6239 (10)

0.0447 (11) 0.1273 (11)

0.0255 0.0084

19.900 (10) 54.296 (10)

2.5571 (10) 1.9670 (10)

0.068525 (41) 0.22240 (71)

0.0006 0.0032

0.07013 (80) 0.2318 (45)

0.0139 0.0195

30.312 (10)

2.5353 (10)

0.43104 (65)

0.0015

0.4576 (75)

0.0163

20.166 (5) 19.866 (5)

2.3629 (5) 1.5374 (5)

0.1876 (15) 0.2800 (20)

0.0079 0.0072

0.1899 (37) 0.2904 (31)

0.0193 0.0107

Tap water 960627A 960627B Kaylbach 960629A

river water

Simulated 960628E 960628F

rainwater

10

J.-C. Wolff et al./Analytica

water. The mineralisation and biological life of these water samples were very different. The river-water sample had to be preserved by the addition of few ml of a 10% (w/w) phenyl mercury chloride solution to prevent plant growth and microbiological degradation of nitrate. This water sample also contained some nitrite which was carefully destroyed using amidosulfuric acid. The sparkling mineral water was degassed with helium. Except for the river-water sample, two spikings to a different ratio were undertaken. Prior to IDMS analysis, the nitrate content of the water samples was estimated by ion chromatographic analysis in order to meet the requirements for good spiking conditions, i.e. combining a minimal uncertainty magnification (Fig. 2) with an easy-to-measure isotope-amount ratio. This holds especially for the THQ, where ratios ~0.1 were difficult to measure. Furthermore, THQ measurements also required more precipitate, i.e. more nitrate, and thus more water. It was assumed that nitrate nitrogen in the waters had natural isotopic composition (i.e. IUPAC tabulated value) and that isotopic variations were small and thus within the uncertainty of the IUPAC tabulated value. The measured isotope-amount ratios were corrected for ‘70-contribution, as the nitrate was measured as NOz-. The correction consisted of subtracting the contribution of 14N’60’70 and “N’70’60 from the ion-beam intensity, at mlz=41. Assuming that oxygen had natural isotopic composition according to IUPAC tabulated values, the probability of occurrence (p) of the isotopically-different species were calculated so that p(‘4N160170) and p(‘4N’70160) was subtracted from the observed ion-beam intensity at m/z=47[ 161. The ‘70-corrected isotope-amount ratios were thus calculated according to Ro-corrected= (1 - 0.000;~;~;

&&ed)

The amounts of water and spike-IRMM-629 investigated, and the observed isotope amount ratios corrected for “O-contribution are given in Table 2. The blend 960622B for the sparkling mineral water could not be measured on the THQ because the amount of precipitate was too small. During the evaporation step of the sample preparation procedure (Fig. l), a large amount of carbonate precipitate appeared due to the

Chimica Acta 346 (1997) 3-15

strong mineralisation of this water. This precipitate had to be filtered-off twice, and some nitrate probably got lost by adsorption or co-precipitation during this stage. When the nitron-acetate solution was added, only a minimal amount of nitron-nitrate precipitated, which served for the measurements on the NBS instrument. This non-quantitative recovery did not have an influence on the determination of the nitrate concentration, as the comparison of the results for blend 960622A with those for blend 960622B showed (Table 3). The loss of nitrate by adsorption or coprecipitation did not lead to significant isotope fractionation. Thus, in IDMS experiments, the preparative chemistry need not be “fully” quantitative, once isotopic homogeneity has been reached after spiking. To avoid the problem of precipitation of carbonate, nitrate could be isolated from the matrix using anion exchange resins and subsequent precipitation with nitron. This could also result in a pre-concentration of nitrate, and thus in a lower detection limit for the method. To account for fractionation effects due to the sample preparation procedure, the measurement process (i.e. during ion production, the lighter isotopic species tends to evaporate more likely at lower temperature) and the instrumental parameters of both mass spectrometers, a mass fractionation factor (K) was determined. The K-factor, which is defined as the ratio between the “true” isotope-amount ratio and the observed one, was determined by measuring the certified isotopic reference material IRMM-627. On the THQ, the mean observed isotope-amount ratio of IRMM-627 (five replicates) was 0.5567f0.0047, which gave a Kfactor of 0.984f0.014. On the NBS, an isotopeamount ratio of 0.5520f0.0066 was measured for IRMM-627 (six replicates), and K calculated was 0.993f0.016. The rather large uncertainty on both K-factors (1.5%) was mainly due to the uncertainty on the certified isotope-amount ratio of IRMM-627, which was certified with a relative expanded uncertainty (k= 1) of 1.1%. The measurement uncertainty of the ratio of IRMM-627 (i.e. the reproducibility between filaments) was also rather high (ca. 1%) on both the THQ and NBS instruments. Generally, this reproducibility was much better on the NBS, i.e.
J.-C. Wolff et al./Analytica Table 3 Isotope-amount investigated

ratios, corrected

Blend (for the waters analysed) Rosport sparkling 960622A 960622B

for mass fractionation

Chimica Acta 346 (1997) 3-15

(K), and calculated

K-corrected isotope amount ratio on NBS

c(N03-)/ mmol kg-’

0.0443 (13) 0.1264 (23)

nitrate concentration

11

of the waters for each individual

blend

K-corrected isotope-amount ratio on THQ

UHQ)

0.00561 (20) 0.00549 (I 1)

0.0445 (16)

0.00562 (24)

0.0680 (11) 0.2208 (36)

0.03081 (56) 0.03016 (51)

0.0690 (13) 0.2281 (87)

0.03129 (64) 0.03118 (77)

0.4298 (70)

0.1369 (23)

0.4503 (98)

0.1442 (32)

0.1862 (33) 0.2780 (49)

0.0818 (15) 0.0816 (15)

0.1870 (45) 0.2857 (51)

0.0821 (21) 0.0839 (15)

(NBS)

c(NO,-)/ mmol kg-’

mineral water

Tap water 960627A 960627B Kaylbach 960629A

river water

Simulated 960628E 960628F

rainwater

The slightly different mass fractionation observed on the THQ and the NBS originated from several parameters, but the ionisation-filament temperature certainly played a significant role. The higher ionisation temperature on the THQ led to a larger fractionation which is in accordance with theoretical fractionation models, e.g. Rayleigh distillation [21]. The K-corrected isotope-amount ratios and the corresponding nitrate concentration, calculated using the IDMS equation [22]

Ry-K.RRf3 .- l+Rx my CX=i&'CY'~.~;-~X I+R~ for each of the blends of the different water samples, are given in Table 3. The subscript Y refers to the spike, i.e. the mass of spike investigated (my), the concentration (cv) and isotope-amount ratio (RY)of IRMM-629, whereas the subscript X represents the different water samples (mass, concentration and ratio), and B refers to the blend (observed ratio R'B) between the water and IRMM-629. The values obtained from the THQ were slightly higher than those from the NBS mass spectrometer. However, when considering the uncertainties, both ranges did overlap. Uncertainties were determined according to IS0 [23] and EURACHEM [24] guidelines. The combined

uncertainty (u,) on the nitrate concentration in the different water samples was calculated using uncertainty propagation law accounting for both type A (based on statistical methods for data treatment) and type B (based on scientific judgement using the relevant information available) uncertainties. A typical uncertainty budget, exemplified for the measurement of simulated rainwater (blend 960628E on NBS), is given in Table 4. The major contribution to the total combined uncertainty came from the measurement of the blend (RB)(especially on the THQ, c.f. Table 2) and from the uncertainty on the K-factor. As the ratio of the blend was located in the favourable range from the point of view of uncertainty magnification (Fig. 2), the high uncertainty on the isotopic composition of IRMM-629 did not contribute very much to the total combined uncertainty. This high uncertainty came from the variability between ampoules of IRMM629, which was mainly due to the blank contribution introduced during the sample preparation. Different treatments of IRMM-629 during sample preparation revealed a blank contribution of 0.04 nmol of NOsabsolute. As for the water samples, the volume investigated was about 20 ml, the blank contribution to the nitrate concentration would be ca. 2 nmol kg-‘, i.e. for the sparkling mineral water, less than 0.04%. Hence, blank contributions were neglected.

12

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Chimica Acta 346 (1997) 3-15

Table 4 Uncertainty budget for the blend 960628E measured on NBS (type A uncertainties being based on statistical methods for data treating and type B uncertainties being based on scientific judgement using the relevant information available) Type (of n,)

Relative combined uncertainty

0.0058 0.0005 6.7 0.00025 0.0015

A B A B B A

0.00025 0.0015 0.00021 0.0246 0.0405 0.0080

0.016

A+B

0.016 0.0188

Source of uncertainty

Typical value

Combined uncertainty

Mass of rainwater, nx (g) Nitrate concentration of IRMM-629, cy (mmol kg-‘) Mass of IRMM-629, my (g) Isotope amount ratio of rainwater (IUPAC value), Rx Isotope amount ratio of IRMM-629, Ry Measured isotope-amount ratio of the blend, R’B (repeatability of at least three replicates) Mass fractionation factor, K Total relative combined uncertainty (calculated from uncertainty

20.166 3.8849 2.3629 272.2 0.00618 0.1876

0.005

(u,)

0.993 propagation

A similar uncertainty budget (Table 5) was made for the nitrate concentration, determined by highpressure ion chromatography (HPIC). The possible matrix effects, which could lead to a suppression or enhancement of the nitrate peak, were quantified arbitrarily by introducing a relative uncertainty of 0.5%. The uncertainty contribution for the non-linearity of the detector’s response was considered to be small (O.l%), as the external calibration solution had nearly the same concentration as the water to be analysed. The results obtained by the different methods (Table 6) compared well within their uncertainty range (expanded uncertainties with a coverage factor k=2). In previous work [ 161, the method was validated by the measurement of a certified reference material, i.e. CRM 409 (from the Community Bureau of Reference). Close agreement was found between the certi-

Table 5 Typical uncertainty

budget for the determination

law)

fied (c(NOs-)=78.lfl.O pmol kg-‘) and the determined value (c(NOs-)=78.8&3.7 umol kg-‘). Using IRMM-629, nitrate concentrations ~0.1 mmol kg-’ could be measured with expanded uncertainties (k=2) 4%, and generally ca. 2.5% when using the NBS mass spectrometer. The higher uncertainty observed on the THQ was due to the fact that the isotope-amount ratios of the prepared blends were measured with a relative standard uncertainty of 2%. Ratios close to 1 would have led to a better precision, but the amount of water investigated would have been much larger to obtain enough precipitate. On the NBS, isotope-amount ratios down to the ratio of IRMM-629, i.e. 0.0062, could easily be measured. Moreover, due to the small amounts loaded onto the filament, “micro” amounts of precipitate (<0.6 mg) as it was possible to wash the filter with ethanol to redissolve “micro” amounts of nitron-nitrate

of the nitrate concentration

using HPIC, e.g. simulated

rainwater

Source of uncertainty

Type of uncertainty

Relative combined uncertainty

Repeatability of the measurement of rainwater (5 replicates) Repeatability of the measurement of the calibration solution (6 replicates) Concentration of the calibration solution (Merck Multi-Anion 11) Dilution of the calibration solution Non-linearity of the conductivity detector Estimated influence of matrix effects

A A B A B B

0.0105 0.0054 0.0020 0.0035 0.0010 0.0050

Total relative combined uncertainty (square root of the sum of the squares of the relative uncertainties)

A+B

0.0135

J.-C. Wolff et al./Adytica Table 6 Comparison c(N03-)/

of the nitrate concentrations

mmol kg-’

Using IRMM-629 Using IRMM-629 Using HPIC

on NBS on THQ

and their corresponding

Chimica Acta 346 (1997) 3-15

expanded

uncertainty

13

(k=2) determined

by the different methods

Rosport sparkling mineral water

Tap water

Kaylbach river water

Simulated rainwater

0.00555 (22) 0.00562 (48) 0.00552 (22)

0.03049 (76) 0.0312 (11) 0.03054 (94)

0.1369 (45) 0.1442 (63) 0.1403 (64)

0.0815 (21) 0.0828 (36) 0.0816 (22)

(co.1 mg of NOs-) - were sufficient for IDMS measurements. Hence, when determining the blank contribution, it was found that the detection limit of the mass spectrometric method on the NBS using IRMM629 was about 1 nmol kg-‘. In fact, this nitrate concentration induced a change of ca. 25% in the isotopeamount ratio for IRMM-629. Theoretically, this detection limit would also be valid for the THQ, but the small dynamic range, the large amounts of nitrate loaded and the poor sensitivity did not permit to reach such low detection limits. Actually, it was very difficult to quantify this parameter, but it could be reasonably said that samples having a nitrate content of 1 pmol kg-’ could be measured. 3.3. Traceability of the measurement by IDMS

results obtained

The CCQM defined primary methods of measurement as: “methods whose operation can be completely described and understood, for which a complete uncertainty statement can be written down in terms of SI units, and whose results are, therefore, accepted without reference to a standard of the quantity being measured” [25]. The whole IDMS-measurement procedure is carried out in terms of SI units, as only ratios of amount of substance are measured (Table 7). No external calibration is needed. An amount of substance is compared to another one, so that it can be said that actually the mass spectrometer acts as a “balance” or “comparator”. Considering the uncertainty budget for an IDMS measurement, written down in terms of SI units (Table 4), it appears that there is no empirical correction to be applied. What is actually measured, i.e. isotope-amount ratios and masses, can be used straightforward in the IDMS equation to obtain the measurement result. One could argue that the mass

fractionation factor K constitutes an empirical correction. But K is not necessarily needed in an IDMS experiment. When combining Eqs. (l)-(5) given in Table 7, it results in the following equation where K cancels out if Rx is equal to Rz (which generally is the case). RY - R,(X/Y)

n(NO” ‘) =RB(X/Y) .-.1 +Rx 1 +RZ

RB(Y/Z)

-

Rz

- Rx . Ry - Rs(Y/Z) m(N03,

Z)

M(NO,,

Z)

For convenience, a mass fractionation factor is determined as the ratio of two isotope-amount ratios, one established gravimetrically (in this work, the certified IRM, IRMM-627) and the other one measured on the mass spectrometer. Knowing K, it is not necessary to carry out a “double” IDMS for determining the nitrate content of unknown water samples. The spike-IRMM629 is the end-point of the reverse IDMS against a characterised pure substance having natural isotopic composition, IRMM-628, and the starting point of the normal IDMS, which leads to the nitrate concentration. Once IRMM-629 is perfectly characterised, the reverse IDMS experiment, based on the preparation of synthetic isotope mixtures, is unnecessary. The nitrate content of the water samples is directly related to a pure substance characterised for its nitrate content (IRMM-628) along an unbroken “traceability chain” (Table 7). IRMM-628 was prepared gravimetrically from a pure substance, carefully characterised, with an orthodox uncertainty budget in terms of SI units. IRMM-628 thus acts as a primary reference material w-ith a close link to the SI system, as the characterisation was done in terms of SI units for amount of substance and mass. As a traceability chain exists between the nitrate content of the water samples

a Increasing

downwards.

Known amount of NOs- in IRMM-628 (Z) (natural isotopic composition)

Measurement of isotope amount ratios of IRhIM-628 and IRMh&629 (Y) (isotopically enriched material, spike)

Known amount of isotopes I4 and “NO 3- m IRMM-628 (ZI?

Measurement of isotope amount ratio in the blend (IRMM-628+IRMM-629)

Known amount of NOsin IRMW629

Measurement of isotope amount ratio of water (X) (natural isotopic composition) ’

=

n(“NO;,

n(14NO;,

X)

X) = n(14NO;,

’

Z) =

n(t5NO;,

= n(‘4NO;,

n(NO;,

R Y) ’

Y)

(1 +Ry)

Z)

Y)

Z) &

z

t Z)

z

Y) + n(NO,-, Z)

Y) + n(14NO;,

Y)

X) +n(NO;,

Y)

(1 +Rx)/Rx

X) + n(14NO;,

X)

n(“NO;,

Z)

Rz = n(t4NO;> Z)

Z) = n(NO;,

n(14NO;,

“(“NO;,

n(14NO;,

n(N0; Z) = l+R

=

@NO;,

RB(Y/Z)

X)

Y) = n(“NOj,

@NO;,

= u(14NOS> X)

n(NO;,

R

RB(X/Y)

n(14NO;,

Amount of isotope 14NOsin water (X)

Measurement of isotope-amount ratio in the blend (water (X)+IRMM-629 (Y))

n(NO;,

to SI system a

Traceability

of nitrate using IDMS

Amount of NOs- in water X (natural isotope composition)

chain for amount measurements

Table 7 Traceability

mol

mol

mol

mol -mol

mol

mol

mol

_mol mol

mol

mol

Y) =

RZ

n(t4NOj,

X) = n(‘5NO;,

n(NO;, Z) = m(NO;, Z) M(NO;,Z)

.&

'RY

Z

-RB(Y/Z)

- Rz

&(X/Y)-&

RY--B(X/Y)

Z) R,(Y/Z)

Y) Rx

X) = n(14NO;, X) (I +Rx)/Rx

~I(‘~NO;, Z) = n(NO;, Z)

@NO;,

n(14NOj,

n(NO;,

(3

(1)

J.-C. Wolff et al. /Analytica

analysed and IRMM-628 (Table 7) and thus to the SI system, the nitrate values determined in this work are thus traceable to the SI system. Consequently, IDMS has the potential to act as a reference method by linking its results to the SI system.

4. Conclusion It was shown how traceable and accurate values for nitrate could be obtained using isotope dilution in combination with negative thermal-ionisation mass spectrometry. Due to its higher sensitivity, the magnetic-field NBS-type mass spectrometer improved the detection limit of the method considerably, i.e. from 2 pmol kg-’ [16] to 1 nmol kg-’ (this work). The use of IRMM-629 instead of IRMM-627 did not substantially lower the detection limit on the THQ, because of the small dynamic range of this instrument. The precision obtainable with the two spikes was comparable. However, the easy-to-handle, small, low-cost THQ allowed a much higher sample throughput so that isotope dilution experiments could find larger use; for instance, preparing an “in house” reference material for nitrate, in the matrix of interest. Such a material, together with a complete unce~ainty budget and thus traceable to a higher ranking nitrate reference material, could then be used to calibrate other instrumentation In this way, nitrate values could be linked to SI-traceable reference materials. The NBS instrument could be used to produce high ranking nitrate reference materials in terms of traceability to SI 1121.

Acknowledgements The authors wish to thank the Luxembourg “Minis&e de 1’Education Nationale et de la Formation Professionnelle, Recherche Scientifique et Recherche Appliquee” for the financial support given to one of the authors (J.-C. Wolff) which permitted to carry out this work at the IRMM of the European Commission in GEEL (Belgium).

Chinica Acta 346 (1997) 3-1.5

15

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