Certified reference materials (CRMs 398 and 399) for the quality control of major element determination in freshwater

Mikrochim. Acta 108, 195-204 (1992) Mikrochimica Acta

�9 by Springer-Verlag 1992 Printed in Austria

Certified Reference Materials (CRMs 398 and 399) for the Quality Control of Major Element Determination in Freshwater

Philippe Quevauviller*, Kristien Vercoutere, and Bernard Griepink

Community Bureau of Reference (BCR), Commission of the European Communities, Rue de la Loi 200, B-1049 Brussels, Belgium

Abstract. Analyses of freshwater are routinely performed by a number of organ- isations to monitor the levels of major elements. In order to improve and control the quality of such determinations, the Community Bureau of Reference (BCR) has organised a certification campaign to produce two reference materials (CRM 398, low element content and CRM 399, high element content) certified for their contents of A1, Ca, C1, Fe, Mg, Mn, K, Na, P and S. These materials were carefully prepared (addition of the elements mentioned in the form of ammonium salts or nitrates to silica free deionised water) and their homogeneity and long term stability were verified. This paper presents the certification work performed.

Key words: certified reference material, freshwater, analytical quality control, major elements.

During the last years a number of Directives, concerned with the quality of several types of water, have been issued dealing with the quality of drinking (75/440/CEE, 79/869/CEE, 80/778/CEE) or underground water (80/68/CEE) or with the protec- tion of fish life (78/659/CEE). They prescribe the determination of e.g. A1, Ca, C1, Fe, Mg, Mn, P, K, Na and S. Other types of water (e.g. rain water) are frequently analysed in the member states for the same elements without the support of a directive.

It has however been demonstrated that a large number of errors may occur in the analysis of water. The need for improvement of the quality of these determinations has led the Community Bureau of Reference (BCR) of the Commission of the European Communities to produce as part of a larger project two certified reference materials (CRMs) of freshwater, one with a low mineral content (CRM 398) and one with a higher mineral content (CRM 399).

* To whom correspondence should be addressed

196 P. Quevauviller et al.

C e r t i f i c a t i o n P r o c e d u r e

The user of analytical results requires proof of the accuracy of the measurements. The only possibility for any laboratory to ensure that a good accuracy is achieved is to verify the analytical procedure by means of a so-called matrix reference material certified in a reliable manner. The use of certified reference materials (CRMs) is a good means to link the user's results to basic units and therefore to these of the international community. The laboratory which measures such a CRM by its own procedure and finds a value in disagreement with the certified value is thus warned that its measurement includes an error, the source of which must be identified [1].

The following steps were undertaken in order to ensure that no substantial systematic errors were left undetected in the certification:

(a) special care to avoid contamination, losses, incomplete digestion, manipulation errors, etc.;

(b) application of good QC-principles, e.g. control of calibration, interferences, peak overlap, background corrections, and laboratories well under control (e.g. per- formance, experience, clean areas, training and motivation of staff, application of QA system, etc.);

(c) comparison of different methods applied in different laboratories for the detec- tion of sources of error.

P a r t i c i p a t i n g L a b o r a t o r i e s

The preparation of the reference materials of freshwater, as well as the homogeneity and stability studies were performed by the Anglian Water Authority in Cambridge (UK).

The analyses for certification were performed by the following laboratories:

- E.C.N. Netherlands Energy Research Centre, Petten (NL) Fondazione Clinica del Lavoro, Pavia (I)

- GSF, Forschungszentrum ffir Umwelt und Gesundheit, Neuherberg (D) - Istituto Italiano di Idrobiologia CNR, Pallanza (I) - Istituto di Ricerca sulle Acque, Milano (I) - Instituut voor Nucleaire Wetenschappen, Rijksuniversiteit, Gent (B) - Central Laboratory of Water Producers, Nieuwegein (NL) - Minist~re des Affaires Economiques, Brussels (B) - Portsmouth Polytechnic, Dept of Chemistry, Portsmouth (UK) - Risoe National Laboratory, Roskilde (DK) - Service Central d'Analyse CNRS, Vernaison (F) - Universidad Complutense, Facultad de Quimica, Madrid (E) - Water Quality Institute, Hoersholm (DK)

P r e p a r a t i o n

A 260 1 polypropylene tank was used for the homogenisation; this tank was thor- oughly cleaned by filling it close to the brim with filtered acid tap water acidified with nitric acid to a final concentration of 0.1 mol/1, which was circulated in the tank by pumping continuously for several days. The tank was rinsed with deionised water and refilled with 0.1 mol/1 nitric acid (subboiling quality) deionised water.

Certified Reference Materials (CRMs 398 and 399) 197

After soaking for one week the tank was emptied and rinsed again with deionised water. Immediately before the bottling run of each CRM the tank was cleaned again using acidified deionised water.

Polypropylene bottles were studied for possible contamination or losses through evaporation. The test consisted of filling ten bottles with freshly prepared 0.1 mol/1 nitric, screwing on the cap and leaving the bottle at room temperature for a period of 1 to 7 days, before analysing the solutions for each of the 10 determinands. On the day of analysis a fresh batch of 0.1 tool/1 nitric acid was prepared and the solutions from the bottles analysed in the same run with the fresh solution as references. The results did not reveal any significant contamination with the bottles selected. The bottles were carefully cleaned by soaking for two hours in a diluted detergent solution and rinsed six times inside and outside with deionised water. The bottle caps were treated in a similar way.

For each of the reference materials 1000 bottles were conditioned for 7 days by leaving them in contact (alternating top/bottom positions) with the solution they would contain. For the higher level of mineral matter there was no detectable change of analyte content in 10 bottles tested. For the solution with lower levels however the conditioning had to be repeated for 7 additional days.

The two reference materials were prepared from silica free deionised water (0.1 tool/1 nitric acid), to which the compounds of interest were added in the form of acidic aqueous solutions. Sulphate, phosphate and chloride were added as ammonium salts, A1, Ca, Fe, K, Mg and Na as nitrates whereas the solution of Mn was obtained by dissolution of the pure metal in HNO3. The concentrations expected upon spiking are given in Table 1.

Homogenisation was carried out in the polypropylene tank covered with a close fitting polyethylene lid. A centrifugal pump connected to the tank with polyethylene piping ensured constant recirculation of the solution. The pump had no metal parts in contact with the water. An additional Cover of heavy gauge polyethylene was fitted over the tank and the pipework to prevent ingress of dust. It was established by separate experiments that mixing for 15 rain was sufficient to achieve a good sample homogenisation.

Table 1. Concentrations expected upon spiking

CRM 398 CRM 399

A1 30 200 ng/g Ca 30 80 #g/g C1 10 50 pg/g Fe 30 200 ng/g Mg 5 15 pg/g Mn 29.9 199.2 ng/g K 1.0 3.0 pg/g Na 5 30 /ag/g P 0.1 1.0 #g/g S 3.3 8.3 pg/g


The bottling was done manually through pre-rinsed polymer tubing avoiding any contact with metals.

After the final filling, each bottle was put in a tin plated steel can which was subsequently sealed and stored at ambient temperature. The can was selected to prevent any change in concentrat ion due to evaporation during storage or to exchange of atmospheric gases.

Homogeneity Study The contents of ten bottles taken evenly throughout the entire bottling run of each of the two batches, were analysed to verify the between-bottle homogeneity. The method variability was assessed by 10 determinations on one bottle assuming that the content of each bottle was homogeneous. All elements to be certified were determined.

The analytical methods used were visible light or U.V. spectrophotometry (Fe, C1, P), ETAAS (A1, Mn), FAES (K, Na), FAAS (Mg, Ca) and flow injection turbidimetry measurement (S as sulphate). The samples were analysed in a ran- domised order in the most repeatable manner (e.g. automated equipment where possible, same day, same equipment, same technician, calibrant solutions immediately prior and after each determination, etc). Such determinations alternating on calibrant and on sample solution enabled a correction to be made for the slight drift of sensitivity that occurs during an analytical run, thus increasing the obtained accuracy.

Table 2 gives the results of the homogeneity tests as well as the coefficient of variation of the method.

The statistical analysis-of-variance (F test) at the 95% confidence level applied on these results did not reveal any significant between bottle variability. It was concluded that both batches were homogeneous.

Table 2. Results of the homogeneity study

CRM 398 CRM 399

Element Unit CV ~o meth.* CV ~o b** CV ~o meth.* CV ~/o b •*

Fe gg/kg 1.33 1.33 0.16 0.175 A1 /~g/kg 1.42 1.79 1.36 1.36 Mn #g/kg 1.13 1.13 1.81 2.26 K mg/kg 0.6 0.7 0.58 0.77 Na mg/kg 1.36 1.36 1.10 1.10 C1 mg/kg 0.16 0.17 0.34 0.34 Mg mg/kg 1.07 1.43 0.82 0.82 Ca mg/kg 0.71 0.74 2.16 2.16 S mg/kg 0.32 0.36 0.96 0.96 P mg/kg 0.5 0.6 0.79 0.79

* Determined by 10 replicate determinations on one solution ** Determined by a single determination in each of 10 different bottles


Stability Study The stability test was performed by analysing 5 randomly selected samples, after 1, 3, 6 and 12 months of storage at ambient temperature. Determinations of A1, Fe, Mn and P were made in four-fold using the same analytical procedures as for the study of homogeneity. There are no data for the P content after 1 months' storage because of an instrumental failure. The stability over 12 months was verified for K, Na, C1, Mg, Ca and S as well. The analyses were performed in duplicate on 10 randomly selected samples.

The results are given in the Fig. 1 as the relative variations observed, Rt = Xt/Xo, where (Rt) is the ratio of the mean values (-~t) of four measurements made after a period t, and the mean (,~o) obtained by measurements immediately after bottling (results of the homogeneity study).

The uncertainty U t on the ratio R t has been obtained from the coefficient of variation of each set of measurements:

1.05

1.00

Fig. la.

I I I f I I I I

I 3 6 12

t ime (months)

- 4 ~ C R M 3 9 9 4~. C R M 3 9 8

1.0Z

1.00

Fig. lb.

I 3 6 12

t ime (months)


1 . 0 6

1.00

0 . 9 5

Fig. lc.

4a- C R M 3 9 9 - - -o- C R M 3 9 8

i , 4 , , . . . . I

I 3 6 12

t ime (months)

C R M 3 9 9 - - - I - C R M 3 9 8

1 . 0 4 '

1 .00

0 . 9 7

, I I I I I I I I

I 3 6 12

t ime (months)

Fig. ld.

Fig. 1. Results of the stability study for: a aluminium, b iron, c manganese, d phosphorus

U t = (CVt 2 + CVo2)t/2" Rt/100

In the case of ideal stability or absence of additional changes, Rt should be 1. In practice however there are some random variations due to the uncertainty on the measurement. In almost all the cases the value 1 is comprised between R t - U t and Rt + Ut. The uncertainty on the CV of the method can account for the deviations observed for some of the elements.

On the basis of these results, it was concluded that no instability could be demonstrated and the materials were considered suitable for certification. The materials will be monitored further at regular intervals.

Certified Reference Materials (CRMs 398 and 399)

Table 3. Summary of techniques of final determination

201

Element Techniques

A1 Ca C1 Fe Mg Mn P K Na S

DCPAES, ETAAS, GS of 28A1, ICPAES, ICPMS of 27A1, ZETAAS DCPAES, FAAS, ICPAES, ICPMS of '*'*Ca, TITR CSV, GS of 38C1, ICPMS of 35C1, TITR DCPAES, DPP, ETAAS, GS of 59Fe, ICPAES DCPAES, FAAS, ICPAES, ICPMS of 26Mg, IDMS of masses 24, 25 and 26 DCPAES, ETAAS, GS of 56Mn, ICPAES, ICPMS of 55Mn DPP, ICPAES, SPEC DCPAES, FAAS, FAES, ICPAES DCPAES, FAAS, FAES, GS of 24Na, ICPAES, ICPMS of 2'*Na IC, ICPAES, IDMS of masses 32 and 34, NEPH

CSV DCPAES DPP ETAAS FAAS FAES GS IC ICPMS ICPAES IDMS NEPH RNAA SPEC TITR ZETAAS

Cathodic stripping voltammetry Direct current plasma atomic emission spectrometry Differential pulse polarography Electrothermal atomic absorption spectrometry Flame atomic absorption spectrometry Flame atomic emission spectrometry Gamma spectrometry Ion chromatography Inductively coupled plasma mass spectrometry Inductively coupled plasma atomic emission spectrometry Isotope dilution mass spectrometry Nephelometry Neutron activation analysis with radiochemical separation Visible light or U.V. spectrometry Potentiometric titration ETAAS with Zeeman background correction

Techniques used for Certification

Thirteen laboratories from 8 European countries participated in the certification (see section analyses). All proved that they were working under a good quality control system. Table 3 summarizes the different techniques of final determination used by these laboratories for the different elements.

Technical Evaluation of the Results

The results were discussed in a technical meeting to confirm the methods of analysis and the values and traceability of the results submitted. Some observations were made for the following elements:

Chlorine

The relatively high standard deviation of the NAA- techniques (laboratories 6 and 12) could be explained from counting statistics. There was no reason to suspect poor accuracy of the results.


Sulphur

Nephelometry gives rather large s tandard deviations; this is inherent to the technique and there was no reason to suspect poor accuracy of the method.

Comparison of the Methods

In the certification exercise of some elements three or more laboratories applied the

same technique of final determination. In such a case it was possible to compare the results per technique. A grand mean of the means of all the three or more laboratories applying the same technique of final determinat ion was calculated. The obtained grand means were then compared to investigate whether a particular bias could be at tr ibuted to any method. Table 4 presents the results of the evaluation

respectively for CRM 398 and 399. As shown in the table, the CVs within one method are systematically larger than

those between different techniques. Consequently, it cannot be infered that the results of one technique do not agree with those of other techniques, for all elements

certified.

Certified Values

The accepted results and methods were discussed amongst all participants and various statistical tests were applied which are described in the certification report

Table 4. Results of the evaluation of consistency of the methods used

CV(%) between Techniques of means of lab. with the Number of sets

Element final determination same techniques of results CV(%) between means of different technique

CRM 398 Ca FAAS 4.8 5 0.3

ICP 1.5 4 Fe ETAAS 6.4 5 4.4

ICP 5.8 3 Mg FAAS 1.3 4 0.5

ICP 1.2 4 Mn ETAAS i.1 3 0.5

ICP 0.6 3 Na FAAS 3.2 3 0.08

ICP 1.4 3 CRM 399 Ca FAAS 1.4 3 0.6

ICP 1.1 3 Mg FAAS 2.1 4 1.0

ICP 0.7 3 P SPEC 2.2 6 0.3

ICP 2.8 3


[2]. The certified values are p resen ted in Tab le 5 a long with their ca lcula ted

uncertaint ies . It can be observed tha t the values expected u p o n spiking (Table 1) are compr i sed in the range of uncer ta in ty of the certified values which conf i rms (i) tha t no losses n o r c o n t a m i n a t i o n occur red dur ing the p r e p a r a t i o n of the samples and (ii) tha t no systemat ic e r rors cou ld be suspected in the de t e rmina t ions of the different elements.

Indicative Values

A high spread in the results of P in C R M 398 and S in C R M 399 was a t t r ibu ted to the presence of sys temat ic errors. There fo re the conten ts of these e lements cou ld no t be certified and are given as indicat ive values (Table 6).

Table 5. Certified contents in CRMs 398 and 399

Element Certified value a Uncertainty b Units P

CRM 398 A1 36.3 4.3 ng/g 7 Ca 30.0 0.7 #g/g 12 C1 10.3 0.4 #g/g 7 Fe 29.3 1.4 ng/g 9 Mg 5.03 0.06 #g/g 11 Mn 29.8 0.3 ng/g 9 K 1.03 0.04 #g/g 10 Na 5.07 0.08 #g/g 10 S 3.39 0.14 #g/g 5 CRM 399 A1 207 9 ng/g 8 Ca 79.2 0.9 #g/g 9 C1 50.5 0.9 #g/g 6 Fe 202 3 ng/g 7 Mg 15.1 0.2 #g/g 9 Mn 199 3 ng/g 8 K 2.99 0.12 #g/g 8 Na 30.4 0.7 #g/g 12 P 1.01 0.03 #g/g 7

a This value is the unweighted mean of p accepted sets of results o b The uncertainty is taken as the halfwidth ofthe 95% confidence

interval of the mean of p accepted sets of results

Table 6. Indicative values (p: number of accepted sets of results)

CRM Element Indicative value S.D. Unit p

398 P 104.8 5.0 ng/g 6 399 S 8.7 0.7 #g/g 5

204 Certified Reference Materials (CRMs 398 and 399)

Availability

The reference materials are available from the BCR, Commission of the European Communities, Rue de la Loi 200, B-1049 Brussels. Each bottle is accompanied by a certificate and a report describing the work performed (preparation, homogeneity and stability studies, individual results).

Conclusions

The production of the two freshwater CRMs presented in this study is the first step of a programme for the improvement of the quality of inorganic analysis of water. This series will be soon completed by the certification of other types of water, e.g. trace metals in seawater (CRM 403, [3]) and maj or elements and acidity of rainwater (CRMs 408 and 409, [-4]). The feasibility of preparation of estuarine and ground water candidate CRMs is currently investigated.

Acknowledgements. The collaboration of Dr. Croll and Mr. Bousfield for the preparation of the samples and the drafting of the certification report, as well as the analytical work performed by the participants in the exercise are gratefully acknowledged.

References

[1] B. Griepink, Fresenius Z. Anal. Chem. 1990, 337, 812. [2] Ph. Quevauviller, K. Vercoutere, D. Bousfield, B. Griepink, Report EUR (in press). [3] Ph. Quevauviller, K. J. M. Kramer, K. Vercoutere, B. Griepink, Report EUR (in press). 1-4] Ph. Quevauviller, H. Reijnders, D. Vanrenterghem, H. Van der Jagt, B. Griepink, Report EUR (in preparation).

Received October 21, 1991. Revision January 15, 1992.

Documents

Certified reference materials (CRMs 398 and 399) for the quality control of major element determination in freshwater