Development of a ed Extraction Procedure and Certification of a Sediment Reference Material

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Development of a harmonised phosphorus extraction procedure and

certi®cation of a sediment reference material

V. Ruban,a J. F. LoÂpez-SaÂnchez,b P. Pardo,b G. Rauret,b H. Muntauc and Ph. Quevauviller*d

aLaboratoire Central des Ponts et ChausseÂes, Division Eau, BP 19, F-44340 Bouguenais,

FrancebUniversitat de Barcelona, Departament de QuõÂmica AnalõÂtica, Marti i FranqueÁs, 1-11, E-

08028 Barcelona, SpaincEuropean Commission, Environment Institute, I-21020 Ispra (VA), Italyd European Commission, Standards, Measurements and Testing Programme, 200 rue de la Loi,

B-1049 Brussels, Belgium

Received 13th July 2000, Accepted 25th October 2000First published as an Advance Article on the web 30th November 2000

A harmonised procedure for the determination of the forms of phosphorus in freshwater sediments, developed

in the frame of the European Programme, Standards, Measurements and Testing (SMT) has been used for a

certi®cation campaign for a reference material. This operationally de®ned scheme is a good compromisebetween method performance and reproducibility. Furthermore, the method is rather simple to implement and

could be used by water managers on a routine basis. A homogeneous and stable sediment reference material

has been prepared and will be available before mid 2001. The so-called SMT protocol, together with the

reference material, are useful tools in the ®eld of water management, especially at a time when quality

assurance is of paramount importance in laboratory analyses. Knowledge of the bioavailable forms of

phosphorus is important not only for analysis of sediments but also for sludge and soils. Therefore, the SMT

protocol could be extended to these materials.

Introduction

Single or sequential extraction procedures are often used in

environmental studies in order to assess the mobility and the

bioavailability of a given element.1±3 The determination of speci®c chemical species is dif®cult and often hardly possible.

Therefore, determination of broader forms or phases de®nedby their function can be a reasonable compromise, e.g.,

``bioavailable'' forms can give suf®cient information to achieve

a sound environmental policy.4 The proposed methods areoperationally de®ned, related to speci®c reagents and proce-

dures, i.e., results are interpreted as being related to a speci®c

phase of the sediment (although sensus stricto they are relatedsolely to a chemical procedure).

Regarding phosphorus (P), sequential extraction schemeswere ®rst developed for soils and then extended to sediments.5

Many operationally de®ned schemes are available, allowing the

fractionation of the following forms: exchangeable P;6±7 the

fraction associated with Al, Fe and Mn oxides and hydroxides;8

and the fraction in Ca-bound compounds often referred to as

apatite P.9±13 The lack of uniformity in the procedures used did

not allow the results to be compared world-wide or the methodto be validated (since the results are operationally de®ned).

There is also considerable interest in the certi®cation of

reference materials for environmental analysis. However, theusefulness of a certi®ed reference material (CRM) in the

validation of an analytical methodology depends on how wellthe certi®ed values are established.14

In order to improve this situation, the European Commis-sion through the Standards, Measurements and Testing (SMT)

programme has launched a collaborative project (SEPHOS,

sequential extraction of phosphorus in freshwater sediment),

which aimed to: (i) design a harmonised extraction scheme, (ii)test the selected scheme in interlaboratory studies involving

expert European laboratories, and (iii) certify the extractable

phosphorus content of a sediment CRM.15 The so-called SMT

scheme allows the de®nition of the following forms: NaOH-

extractable P (NaOH-P; P bound to Al, Fe, Mn oxides or

hydroxides), HCl-extractable P (HCl-P; Ca-bound P), organic

P (OP), inorganic P (IP), concentrated-HCl P (conc. HCl-P;

total P). This paper presents the results of the certi®cation

campaign as well as the homogeneity and stability tests carried

out on CRM 684, a freshwater sediment.

Origin and preparation of the CRM

The CRM was collected in the Po River (Italy), a large river

in¯uenced by agriculture and industries, large settlement, and

extended rural run-off. The collection site is situated at the

lower Po River, close to the city of Gorino. The sediment was

collected at a depth of 2±3 m, by means of an INOX grab

sampler.

The sample was passed through a 2 mm INOX sieve and thefraction less than 2 mm was collected. The sediment was then

air-dried at room temperature. The drying process was

completed in a drying oven with circulating air, not exceeding

60 ³C in order to preserve the organic phosphorus compounds.

The dried material was passed through a Retsch jaw crusher set

at its smallest opening (1 mm) and ground in a Retsch hammer

mill equipped with tungsten carbide blades. The ground

material was passed through a 90 mm INOX sieve and the

fraction less than 90 mm was collected in a specially designed

mixing drum. Table 1 gives the composition of the sample. The

material was homogenised for 2 weeks. In order to test the bulk

homogeneity, ten sub-samples of about 10 g each were taken

from the mixing drum and bottled. From each bottle, a pellet

was prepared and analysed for a number of major, minor andtrace elements. The interbottle and intrabottle variability were

measured, special attention was drawn on total P. The

homogeneity was good (see results below).

DOI: 10.1039/b005672n J. Environ. Monit., 2001, 3, 121±125 121

This journal is# The Royal Society of Chemistry 2001



Analytical techniquesThe determination of major and trace elements was carried out

by X-ray ¯uorescence spectrometry using a Siemens sequentialspectrometer. Organic carbon was determined by applyingboth C, H, N elemental analysis and WaÈ sthoff combustionanalysis. Calibration was performed using CRMs.

The SMT protocol is described in the Appendix, furtherdetails are given elsewhere.15 This operationally de®ned schemecomprises ®ve steps i.e., NaOH-P (#P bound to Al, Fe and Mnoxides and hydroxides), HCl-P (#P associated with Ca), OP,

IP, and conc. HCl-P (#total P). In this procedure, NaOH andHCl are used as extractants and an intake of 200 mg of

sediment is necessary for the extraction. For each form of P, alllaboratories made ®ve independent replicates on two different

days and from two different bottles. All 15 laboratories usedspectrophotometry, based on the Murphy and Riley spectro-photometric method16 as the ®nal detection method for thephosphorus extracted by the SMT procedure.

Although the Williams scheme is an operationally de®ned

extraction scheme, biotests were carried out (on Scenedesmusquadricauda) in order to test the bioavailability of NAIP (non-apatite inorganic phosphorus), the fraction bound to ironoxides and which is called NaOH-P in the SMT protocol.

Therefore, the SMT protocol can be considered as a valuabletool in the estimation of the available P fraction in a sediment.

All the reagents used were of analytical reagent grade.Suprapure KH2PO4 was used to prepare standard solutionsand suprapure NaOH and HCl were used for calibration. The

glassware and plasticware were soaked in 0.3% HCl and rinsedwith de-ionised water. A quality control procedure was appliedthroughout the different steps from sampling, to preparationand analysis.

The data were treated using TEDI software (Tractament

Estadistic de Dades Interlaboratori, Statistical Treatment of Intercomparison Data) supplied by the University of Barcelonaand designed by A. Padro. This treatment was used to calculatethe mean values and the standard deviations of each of the

laboratory sets, which were submitted to a series of statisticaltests, e.g., the Nalimov test to detect outlying values in thepopulation of results and the Cochran test to detect outlyingvalues in the laboratory variances.

Homogeneity study

One of the prime conditions for acceptance of a candidate

reference material is for it to be homogeneous. The between-bottle homogeneity of the extractable P content was veri®ed bythe application of the extraction procedure on sub-samplestaken from 20 bottles selected at random from the total set. The

within-bottle homogeneity was assessed by 10 replicatedeterminations on the well mixed content of one bottle.17

The relative standard deviation (RSD) and the total uncer-

tainty (U RSD) are presented in Table 2. Between-bottle RSDsare low, ranging from 1.5% for conc. HCl-P to 2.7% for HCl-P. For the within-bottle homogeneity, RSDs are also very low.

The lowest values are for IP (1±1.6%), the highest for HCl-P

(1.9±3.8%) and NaOH-P (1.7±4.2%).RSDs obtained for the between-bottle homogeneity are

sometimes lower than those for the within-bottle homogeneity

but this is not systematic. Furthermore, the values obtained

when applying the two-tailed F -test at a signi®cance level of

0.05 (Table 3) showed no difference between the within- and

between-bottle homogeneity variances.

Stability study

The stability of the extractable phosphorus content was tested

to determine the suitability of this kind of material as a

candidate reference material. Sample bottles were stored atz4,

z20 and z40 ³C during a period of 12 months starting in

December 1998 and the extractable phosphorus contents weredetermined (in six replicates) after 1, 3, 6, and 12 months. Any

change in the content of an analyte with time indicates an

instability provided that a good long-term analytical reprodu-

cibility is obtained. Instability would be detected by comparing

the contents of different analytes in samples stored at different

temperatures with those stored at a low temperature, at the

different stages of analysis.

The samples stored atz4 ³C were used as the references for

the samples stored atz20 andz40 ³C. Table 4 gives the ratios

(RT , where T denotes the temperature of storage in ³C) of the

mean values (X T ) of six measurements made at both z20 and

z40 ³C, and the mean value (X 4) from six determinations made

at the same time on samples stored at a temperature of z4 ³C:

RT ~X T =X 4

The uncertainty U T has been obtained from the RSD of six

measurements made at each temperature:

U T ~(RSD2T zRSD4)1=2

.RT =100

For ideal stability, the ratios RT should be 1. In practice,

however, there are some random variations due to the error on

the measurement.As can be seen from Table 4, the ratio RT is close to 1 for all

the forms of P extracted ; at z20 ³C 0.94vRT v1.03 and at

z40 ³C 0.96vRT v1.02. The uncertainty U T is small, generally

less than 0.03. The variations of RT with time are slightly higher

for HCl-P. Note that the stability of another sample, rich inorganic matter, had previously been tested using the SMT

protocol and proved satisfactory.18

Table 1 Composition of CRM 684

Si Al Ca Fe P Org. C

Mean (%) 23.7 8.1 5.6 4.8 0.126 2.7sa (%) 0.30 0.11 0.06 0.07 0.002 0.04RSDb (%) 1.27 1.36 1.07 1.46 1.59 1.48as~standard deviation. bRSD~relative standard deviation.

Table 2 Within- and between-bottle variances resulting from thehomogeneity study (between-bottle, n~20; within-bottle, n~10)

Between-bottle RSD (%)

Within-bottle RSD (%)

1 2 3 4

NaOH-P 2.5 2.2 4.2 1.7 1.8HCl-P 2.7 2.7 3.8 3.3 1.9IP 1.6 1.6 1.4 1.5 1.0OP 1.9 2.9 2.6 3.1 2.0

Conc. HCl-P 1.5 2.1 1.6 1.7 2.0

Table 3 F -test used for the homogeneity study

F experimental (F -test for comparison of variances)

B±W1 B±W2 B±W3 B±W4

NaOH-P 1.303 2.769 2.113 1.887HCl-P 1.046 2.095 1.567 1.919IP 1.025 1.361 1.107 2.813OP 2.204 1.782 2.570 1.042Conc. HCl-P 1.991 1.277 1.308 1.877

Critical values for a two-tailed test (P ~0.05): F (19.9)~3.68;F (9.19)~2.88.

122 J. Environ. Monit., 2001, 3, 121±125



Certi®cation campaign

The sets of results were submitted to the following statistical

tests: (i) a Kolmogorov±Smirnov±Lilliefors test, to assess the

conformity of the distributions of individual results and of

laboratory means to normal distributions; (ii) a Nalimov test,

to detect ``outlying'' values in the population of individual

results and in the population of laboratory means; (iii) a Barlett

test to assess the overall consistency of the variance values

obtained by the participating laboratories; (iv) a Cochran test

to detect outlying values in the laboratory variances (s2i ); and

(v) a one-way analysis of variance (ANOVA; F -test) to

compare and estimate the between- and within-laboratory

components of the overall variance of all individual results.A summary of the statistical data, as obtained from

computing (HOSTAN software, SMT), is given in Table 5.

The sets of results found acceptable on statistical grounds were

represented in the form of bar charts in which the length of a

bar corresponded to the 95% con®dence interval of the mean of

laboratory means. The certi®ed values were calculated as the

arithmetic means of laboratory means (taking into account the

number of sets accepted for certi®cation after both statistical

and technical scrutiny). This value is featured as a vertical

dotted line on the bar graphs; its uncertainty is given by the

half-width of the 95% con®dence interval of the mean of

laboratory means. Fig. 1 gives an example of a bar graph for

IP.

Discussion

As can be seen from Table 3, no difference was detected

between the within- and the between-homogeneity variances

using an F -test. Therefore, the material was considered to be

homogeneous. There was also no signi®cant difference in the

RT values, the ratio being close to 1, which showed the stability

of the material both at 20 and 40 ³C. The CRM is

homogeneous and stable and can be used for the certi®cation

campaign.Before starting the statistical discussion a technical discus-

sion was carried out to make sure that all laboratories strictly

followed the SMT extraction protocol. A critical point that was

stressed was the necessary calibration using extracting solu-

tions (or external calibration with cross-check of calibrants in

the extracting solutions).

All laboratories used colorimetry as the ®nal method of

determination. A wavelength of 880 nm was originally

speci®ed in the protocol. However, two laboratories used a

wavelength of 700 nm and did not observe any difference

with the other results based on a 800 nm wavelength.

Although it is recognised that the choice of wavelength

(700 or 880 nm) has an effect on the performance character-

istics of the colorimetric method, in particular, its sensitivity,it had no detectable effect on the between-laboratory

agreement (i.e., no detectable bias was observed for one

particular wavelength). Therefore it was decided to accept

both wavelengths in the analytical protocol.The temperature of extraction was also discussed. A

temperature of 21¡1 ³C was required in the original protocol.

It was pointed out that several laboratories had lower or higher

temperatures without their results being affected. Therefore,

the revised protocol was made more ¯exible and now stipulates

a temperature of 21¡3 ³C.Regarding the certi®cation campaign, the estimates of the

within-laboratory standard deviation (sW) and the between-

laboratory deviation (sB), as derived from the one-way

ANOVA, demonstrated that the sB was not signi®cant. Forreasons of uniformity, it was decided to base the certi®cation

on the laboratory means rather than on all individual results.

The half-width of the 95% CI of the mean of the data set means

was adopted as the uncertainty.

For Cochran and Nalimov tests, a value is called an

``outlier'' when the hypothesis that it belongs to the population

of results considered can be rejected with a risk of error of 0.01.

The criterion was adopted that an outlier of variance would be

eliminated only if the standard error of the mean (si /dni ) of the

set exceeded the standard deviation of the distribution of all

laboratory means.The statistical evaluation of the results was carried out in

order to ensure that the population of results accepted for

Table 4 Stability tests on sediment CRM 684

Fraction Time/month R20¡U 20 R40¡U 40

NaOH-P 1 1.03¡0.02 1.01¡0.023 1.01¡0.01 1.00¡0.036 0.98¡0.02 1.00¡0.01

12 0.98¡0.01 0.94¡0.01

HCl-P 1 0.98¡0.03 1.01¡0.033 1.00¡0.02 1.02¡0.02

6 0.94¡0.02 0.97¡0.0412 0.99¡0.01 1.01¡0.01

IP 1 0.98¡0.01 0.97¡0.023 1.01¡0.01 1.01¡0.016 0.98¡0.01 0.99¡0.01

12 1.02¡0.01 1.01¡0.01

OP 1 0.97¡0.03 0.98¡0.033 1.00¡0.03 1.00¡0.026 0.98¡0.01 0.96¡0.01

12 0.98¡0.02 0.93¡0.02

Conc. HCl-P 1 1.00¡0.02 1.00¡0.023 1.01¡0.01 1.00¡0.016 1.01¡0.01 1.01¡0.01

12 1.03¡0.02 1.00¡0.02

Table 5 Summary of statistical data for extracts of CRM 684 (mg kg21)a

NaOH-P HCl-P IP OP Conc. HCl-P

Number of data sets 12 14 15 14 15Number of accepted replicates 60 70 75 70 75All data sets compatible two by two? (Scheffe's multiple t-test) Yes Yes Yes Yes YesOutlying data sets? (Dixon test, Nalimov t-test and Grubbs test) No No No No NoOutlying variances? (Cochran test) No No No No No

Means of means 550 536 1113 209 1373sW 17 17 26 6 27sB 32 47 41 14 61Between-data s signi®cant? (Snedecor test) No No No No NoVariances homogeneous No Yes No Yes No

s of means 33 48 43 15 62Data sets means normally distributed? (Kolmogorov±Smirnov±Lilliefors test) Yes Yes Yes Yes Yes95% CI of the mean of means 21 28 24 8 34aCI, con®dence interval; sW within-laboratory and sB between-laboratory standard deviations, respectively.

J. Environ. Monit., 2001, 3, 121±125 123



certi®cation had a normal distribution before the 95% CI wascalculated. This was true in all cases (Kolmogorov±Smirnov± Lilliefors tests). In addition there were no outlying values(Nalimov). The set of variances was often not homogeneous(three cases of ®ve), which was due to the different repeatability

and reproducibility of the method as applied by the differentlaboratories.

A few laboratories were withdrawn: for NaOH-P, 3 labs; for

HCl-P, 1 lab; and for OP, 1 lab. All the labs were accepted forIP and conc. HCl-P.

Certi®ed values

The certi®ed values (unweighted mean of P accepted sets of

results) and their uncertainties (half-width of the 95% CIs)are given in Table 6 as mass fractions of the respectiveextracts obtained at the different steps (in mg kg21 dry

mass).

Follow-up

A homogeneous and stable sediment reference material for thecerti®cation of extractable phosphorus has been prepared andwill be available before the middle of 2001 under the reference

number CRM 684. This will allow laboratories to check andimprove their results in this ®eld. The harmonised SMTprotocol, together with the reference material are useful tools inthe ®eld of water management, especially at a time when

quality assurance is of paramount importance in laboratoryanalytical work.

Finally, the results of the certi®cation campaign carried outin the frame of the SEPHOS European project (EC ContractSMT4-CT96-2087) are promising. Since the knowledge of thebioavailable forms of P are important not only for the analysis

of sediments but also for sludge and soils, the SMT protocolcould be extended to these materials. New research will belaunched in this direction.

Availability of CRM 684

CRM 684 will be available by the middle of 2001 from the

Institute for Reference Materials and Measurements(IRMM), Retieseweg, B-2440 Geel, Belgium (Fax: z32 14

590406; E-mail: [email protected]). Further informationon other available CRMs can be obtained from the IRMM

Website at http://www.irmm.jrc.be/mrm.html.

Acknowledgement

This project was carried out under EC contract no. SMT4-CT96-2087 and was co-ordinated by the Laboratoire Central

des Ponts et ChausseÂes. The following laboratories pa rtici-pated in the interlaboratory studies: Bundesamt undForschungszentrum fuÈ r Landwirtschaft, Vienna (Austria);CEMAGREF (France); Geological Survey of Finland,

Espoo (Finland); Institut National de Recherche Agrono-mique, Villenave d'Ornon (France); Joint Research Centre,Environment Institute, Ispra (Italy); Laboratoire Centraldes Ponts et ChausseÂes, Bouguenais (France); Macaulay

Institute for Land Use Research, Aberdeen (UK); Uni-versitat de Barcelona, Departament de QuõÂmica AnalõÂtica

(Spain); Universidad de Cordoba (Spain); University of Gent (Belgium); UniversitaÈ t Hamburg (Germany); Univer-sidad de Huelva (Spain); University of Lisboa (Portugal);UniversiteÂ Montpellier I (France); University of Uppsala,

Erken laboratory (Sweden); University of Wageningen (TheNetherlands).

Appendix: SMT protocol

A NaOH-extractable P and HCl-extractable P

(1) Weigh 200 mg of dry sediment in a centrifuge tube. It isimportant to keep the sediment : volume ratio constant. 200 mgof sediment is the minimum required. (2) Add with a pipette,

20 ml of 1 M NaOH. (3) Cover the tube and stir overnight(16 h). Thorough mixing is necessary, the sediment must bekept in suspension (use, e.g., a magnetic stirrer, a shaker table).(4) Centrifuge at 2000 g for 15 min.a NaOH-P. (1) Collect the extract. (2) Set apart (with a

pipette) 10 ml of the extract in a test-tube. (3) Add 4 ml of

3.5 M HCl. (4) Stir energetically for 20 s and let standovernight (16 h). Cover the tube. (5) A brown precipitateappears and progressively settles. Centrifuge at 200 g for

15 min. (6) NaOH-P is determined in the supernatant.b HCl-P. (1) Wash the cake of the previous centrifugation(A-4) with 12 ml of 1 M NaCl. Stir for 5 min. (2) Centrifuge at2000 g for 15 min, discard the supernatant. (3) Repeat b-1 andb-2 once. (4) Add with a pipette, 20 ml of 1 M HCl. (5) Cover

the tube and stir overnight (16 h). (6) Centrifuge at 2000 g for15 min. (7) HCl-P is determined in the extract.

B Concentrated HCl-extractable P

(1) Weigh 200 mg of dry sediment in a porcelain crucible. (2)

Calcine at 450³

C for 3 h. (3) Pour the cool ash into a centrifugetube. (4) Add 20 ml of 3.5 M HCl with a pipette. HCl can beadded directly to the crucible to ease the transfer of the ash. (5)

Cover the tube and stir overnight (16 h). (6) Centrifuge at

Fig. 1 CRM 684. An example of a bar graph for IP.

Table 6 Certi®ed values of extractable contents of phosphorus in CRM684

Certi®edvalue/mg kg21

Uncertainty/mg kg21

P (number of data sets)

NaOH-P 550 21 12

HCl-P 536 28 14IP 1113 24 15OP 209 9 14Conc. HCl-P 1373 35 15

124 J. Environ. Monit., 2001, 3, 121±125



2000 g for 15 min. (7) Collect the extract in a test-tube for thedetermination of concentrated HCl-P

C Inorganic and organic P

a IP. (1) Weigh 200 mg of dry sediment in a centrifuge tube.(2) Add with a pipette, 20 ml of 1 M HCl. (3) Cover the tube

and stir overnight (16 h). (4) Centrifuge at 2000 g for 15 min. (5)Collect the extract in a test-tube for IP determination.b OP. (1) Add 12 ml demineralised water to wash the

residue. Stir for 5 min. (2) Centrifuge at 2000 g for 15 min,discard the supernatant. (3) Repeat a-1 and a-2 once. (4) Let

the residue dry (in the tubes) in a ventilated drying cupboard at80 ³C. Put the tubes in an ultrasonic bath for 10 s and transferto a porcelain crucible. (5) Calcine at 450 ³C for 3 h. (6) Pourthe cool ash into the centrifuge tube. (7) Add 20 ml of 1 M HCl

with a pipette. HCl can be added directly to the crucible to easethe transfer of the ash. (8) Cover the tube and stir overnight(16 h). (9) Centrifuge at 2000 g for 15 min. (10) Collect theextract in a test-tube for OP determination.

Calculation

The concentration, C , in mg g21 (dry weight) is:

C ~ SV 103m

()

with: S = P concentration in the extract (IP, OP, HCl-P) inmg l21; V = volume of reagent used for extraction (20 ml); andm = mass of the test sample (200 mg dry weight).

For NaOH-P:

C ~S 14V

104m()

References

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(A2)

J. Environ. Monit., 2001, 3, 121±125 125

Development of a ed Extraction Procedure and Certification of a Sediment Reference Material

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