Sample Filtration as a Source of Error in the Determination of Trace Metals in Marine Waters Michael Gardner* and Sean Comber WRc, Henley Road, Medmenham, Marlow, Buckinghamshire, UK SL7 2HD Adequate performance in interlaboratory proficiency tests using filtered, pre-treated or pre-digested test materials does not necessarily demonstrate that laboratories’ data are of adequate comparability. Sample handling can be an important source of error which is not examined by routine proficiency tests.This paper reports a study of sample filtration as a source error in the determination of trace metals in marine water samples. The results indicate that that current practice may need to be reviewed if important contamination errors are to be controlled. Keywords: Trace metals; quality control; contamination; filtration; sea-water The UK National Marine Analytical Quality Control (NMAQC) Scheme was established in 1991 with approximately 20 participating laboratories.1 Its primary aims are (i) to provide laboratories which submit data to the UK National Marine Monitoring Plan with a means of demonstrating that they are achieving the necessary (pre-defined) standards of analytical accuracy for trace metals and organic determinands in waters, sediments and biota and (ii) to assist laboratories to improve their accuracy, where necessary.Work undertaken by the Scheme falls into two categories: regular proficiency tests and special investigative exercises.This paper describes work of the second type in connection with the filtration of water samples for the determination of trace metals. The determination of trace constituents in environmental samples is subject to errors from a wide range of different sources. In common with many other interlaboratory quality programmes, the NMAQC scheme has concentrated on checking the accuracy of the measurement stage of the analytical process. However, it is clear that sampling and sample handling are stages where errors might arise and, therefore, where some form of check on accuracy is needed.Measures taken to address these aspects of analysis for the preparation and digestion of sediment samples have already been reported.2,3 For the determination of trace metals in water, routine checks have involved interlaboratory tests on filtered, homogenised, acidpreserved sea-water samples and standard solutions. Whilst this aspect of an external quality control programme is necessary, parts of the analytical process not addressed by such test materials are also likely to be important in determining the accuracy of trace analysis.This paper reports investigations of sample filtration as a potential source of error in the determination of trace metals in marine waters. The aim of the work was to determine the size and sources of any errors and to help to identify and promote good practice. Choice of Filtration Procedures Sample collection and sample handling (including filtration) are not always the responsibility of staff who undertake analyses in participating laboratories. This meant that there might not be a ‘routine’ procedure to be tested in all participating laboratories.Given this, the test was designed with the aim of providing an illustration that satisfactory filtration procedures could be applied. Participants were asked to choose a filtration procedure (to be examined in these tests) with two criteria in mind: (i) the procedure should be representative either of what is actually done for marine samples, or (ii) it should be sufficiently practicable to be applied to marine monitoring samples, if it proves satisfactory.Test Design Two principal types of analytical error might be introduced during filtration: contamination from the apparatus or handling procedure and adsorptive losses to the filter, filter support, etc., during filtration. The test was designed to assess both types of error.Five 1 l samples were provided for filtration: sample A was a de-ionised water sample; sample B was a filtered (0.2 mm) coastal sea-water; samples C and D were accurately measured (1 l ± 5 ml) portions of the same filtered sea-water; and sample E was a portion of sample B which had been spiked with the determinands of interest and which also contained added microcrystalline cellulose [BP grade (Merck, Poole, Dorset, UK)] at a concentration of 100 ± 5 mg l21.Participating laboratories were asked to filter samples A, B and E as received, using the method (or methods) chosen for the test. Samples C and D were to be spiked at the participating laboratory immediately before filtration by adding 500 ml of a corresponding concentrated spiking solution (also supplied) to the measured 1 l portion of water. The two spiking solutions were provided at a pH between 2.0 and 2.5 (to minimise adsorptive losses). A separate spiking solution was provided for chromium (as CrVI) because of the insolubility of lead chromate. The other metals were present in a mixed spiking solution.The spiked samples were mixed by shaking and filtered within 1 h. The portions of filtrate were put into the laboratory’s own bottles (the type normally used for filtered samples) and preserved by addition of 400 ml of 5.5 m hydrochloric acid (Aristar grade, Merck) (supplied) per 100 ml of filtrate. The labelled filtrates were returned to WRc.Participants were asked to carry out filtrations in duplicate. All samples were analysed at WRc. Determinations of Cd, Cu, Pb and Ni were made using a semi-micro chelation solvent extraction procedure.4 Chromium5 and zinc6 were determined by previously described methods. A summary of each laboratory’s filtration procedure is given in Table 1. Results Fig. 1 shows a comparison of analytical data for the filtrates supplied by each participating laboratory.The pair of results for each laboratory corresponds to the two filtrations carried out on each sample. Results are summarised for each metal, on the basis that sources of contamination or the tendency for adsorption are likely to be metal-related. Analyst, October 1997, Vol. 122 (1029–1032) 1029Discussion It is worth emphasising that the comparison shown in Fig. 1 relates to the effects of filtration (and subsequent sample storage), instead of the more familiar comparison of interlaboratory analytical performance.Differences between laboratories’ results can be regarded as arising from a combination of within-batch analytical variation and what might be termed ‘filtration errors’. Effort was made to make analytical variations as small and homogeneous as possible. Determinations for a given metal were carried out in a single batch of analysis to minimise the effects of between-batch analytical errors. The limits of detection (LOD) estimated from duplicate blank determinations (for a series of analytical batches) are as follows:7 Cd 0.05, Cr 0.01, Cu 0.03, Pb 0.04, Ni 0.05 and Zn 0.05 mg l21.Potential Filtration Problems Contamination Contamination can produce errors which could be categorised as both random and systematic. Consistent contamination will tend to produce a positive bias in metal concentrations; sporadic contamination will tend to produce differences between replicate filtrates (i.e., an increase in imprecision).Hence it is unwise to interpret the results for filtration error in distinct categories of precision and bias. Data for samples C and D were evaluated against a ‘spiked’ value calculated as the spiked concentration + a ‘background’ value determined at WRc. In all cases the background was small in relation to the added spike. For sample E, a mean of laboratories’ concentrations was used as a reference point, for reasons discussed below under Adsorption.Cadmium (Fig. 1). Results for the de-ionised water sample (A) indicated that at least two laboratories (4 and 19) are subject to serious contamination. This is confirmed for laboratory 4 by data for the unspiked sea-water (B). For samples B–E, results for laboratories 7, 8 and 16 showed some evidence of very small elevations in concentration or wider differences (than those for other laboratories) between replicate filtrations. The results showing excellent agreement within replicate filtrations and with one another including those for laboratories 3, 11, 12 and 17. Chromium.The results for the de-ionised water and the blank sea-water were too close to the limit of detection of the analytical method to allow clear conclusions to be drawn concerning the smaller observed variations. However, there was evidence of contamination of between 0.2 and 1 mg l21 for laboratory 19. At the higher concentration of samples C, D and E there is close agreement between concentrations measured in laboratories’ filtrates.Copper (Fig. 2). Several laboratories’ results indicated sources of copper contamination. A large effect was evident for laboratory 4 (contamination of > 1 mg/l21); other laboratories Table 1 Summary of filtration procedures Material in Laboratory contact with No. Apparatus* sample* Filter type 3 P/e funnels, P/c P/e: bottle, funnel Nuclepore, 47mm Swinloc P/c: membrane, silicone sealing ring 4 P/c Sartorius P/c: bottle, filter, Nuclepore membrane 7 Sartorius 500 ml P/c: glass Cellulose nitrate, measuring cylinder 50mm 8 P/c Swinloc P/c: bottle, filter, Nuclepore membrane 9 Becton Dickenson P/p Cellulose acetate, 60 ml syringe 47mm filter? 11 Becton Dickenson P/p and rubber: Cellulose acetate, 50 ml syringe syringe Acetate: 47mm Sartolab P filter filter holder 12 Gelman P/e: bottle Gelman, 633 cm Glass-fibre: filter ?syringe 15 Becton Dickenson — Cellulose nitrate, syringe, Sartolab 50mm P, 120 sample pot 17 P/s Nalgene P/s: bottle, filter, Cellulose nitrate, membrane 47mm 19 Millipore PTFE PTFE Whatman, 142mm 1.5 l apparatus cellulose nitrate Pore size/ Bottle mm Preparation† Storage preparation† 0.4 Overnight in 1% Prepared prior Detergent rinse, HNO3 50 ml 1% to use 1% HNO3 for 1 HNO3 50 ml DIW week, 0.1% HNO3 for 1 week, DIW before use 0.4 50 ml 5% HNO3 Prepared prior As filter except no 50 ml DIW to use sample rinse 50 ml sample (discarded) 0.45 Syringe and filter Prepared prior DIW, 10% HCl, rinsed with DIW to use 10% HNO3, 0.1% 10% HCl and HNO3 until used DIW 0.4 50 ml 1% HNO3 Prepared prior As filter 50 ml DIW to use 50 ml sample (discarded) 0.45 Syringe and filter None rinsed with 30 ml 1% HNO3, 30 ml sample 0.45 Rinsed once with Rinsed with sample sample 0.45 None Sealed plastic HNO3 wash, DIW bag 0.45 Syringe and filter — None rinsed with sample 0.45 10% HNO3, Prepared prior As filter DIW, sample to use (discarded) 0.45 10% HNO3 for Prepared prior Detergent, 24 h 2 h, DIW rinse to use 10% HNO3, DIW rinse * P/c = polycarbonate, P/e = polyethylene, P/p = polypropylene, P/s = polysulfone † DIW = de-ionised water. 1030 Analyst, October 1997, Vol. 122(8, 11 and 19) were subject to smaller biases which were mainly evident at the blank level. The data showing excellent agreement within replicate filtrations and with one another including those for laboratories 3, 7, 9, 12, 15 and 17. Lead. The majority of results for lead in blank filtrates were lower than the 0.04 mg l21 reporting threshold, indicating good control over contamination.Good comparability of filtrates was observed at higher concentrations (samples C, D and E). The range of results obtained was narrow and close to the expected spiked values. Nickel. Excellent comparability and accuracy, with respect to the spiked value, were achieved for the spiked samples. Only one serious bias was observed at the blank level, in laboratory 19 (this may arise from the same source of contamination as that already noted for chromium).Zinc (Fig. 3). The data for zinc showed the most obvious instances of contamination bias. Replicate results tended to be similar but were often displaced with respect to filtrate data from other laboratories. At both the blank and at higher levels, laboratories 4, 8 and 9 showed clear bias. In the first two laboratories this bias was small (1–2 mg l21); in the last, bias was around 10 mg l21.Blank data from three other laboratories (7, 11 and 15) might be subject to small bias of approximately 1 mg l21. This was not borne out by the higher concentration samples (except in the case of laboratory 15). Adsorption There is no clear evidence of a bias between the ‘spiked’ value and observed values for samples C and D, for any of the metals. This indicates that adsorptive losses during filtration are probably not important. Sample E, containing 100 mg l21 of microcrystalline cellulose, was included with two aims in mind: (i) To test the hypothesis of increased adsorption for increased contact time during filtration: if adsorptive losses had been significant for filtration of samples C and D (no suspended solids, so a relatively short filtration time), it might be expected that the longer contact time for a sample containing solid matter would result in larger adsorption. Sample E could have provided a guide to the additional adsorptive losses which might apply to real samples.The test data show that adsorption during filtration was not important (or at least was not detectable in relation to the observed random errors). In freshwater samples, such adsorption can lead to significant losses of trace metals.8 The lack of adsorption in sea-water might be ascribed to the high ionic strength of the matrix and consequent competition for adsorption sites from major ion constituents of the sample (e.g., Mg2+ and Ca2+).(ii) To provide a check on adsorption in the sample bottle: sample E was prepared in bulk (with solids) and spiked at the same concentration as sample D. The bulk sample was equilibrated for 1 week to allow partitioning between the solid and dissolved phases. This was mixed thoroughly and then dispensed into the sample bottles. Markedly lower concentrations in the dissolved metal concentration of sample E in relation to sample D (given that samples D and C were not subject to large negative bias) might be taken as evidence of adsorptive losses of determinand to the solids or to the inner surface of the sample bottle.Fig. 1–3 illustrate the important comparisons between the mean of laboratories’ filtrate concentrations for sample E and the spiked + residual value for sample D (the two should be the Fig. 1 Comparison of metal concentrations in participants’ filtrates: cadmium. +, LOD; –5–, spike; and –x–, mean of laboratories. Fig. 2 Comparison of metal concentrations in participants’ filtrates: copper.Symbols as in Fig. 1. Analyst, October 1997, Vol. 122 1031same, if there is no adsorption and other sources of bias are small). The values were found to be similar for Cd, Cr, Ni and Zn, if the instances of contamination (applicable to both samples, but not necessarily consistent) are ignored. However, for Cu the concentration in sample D is markedly higher than that in sample E [3.7 mg l21 (ignoring data from laboratory 4) versus 2.7 mg l21]. This is statistically significant (p = 0.05), indicating losses of copper.It is likely that adsorption on the cellulose was more important than loss to the bottle, given the affinity of the metal for oxygen-containing organic matter. The nominal surface areas of the solids and the bottle were similar, estimated to be approximately 500 cm2 each. The results for Pb also show a small decrease between samples D and E. Conclusions The test demonstrates that sample filtration can be a source of important error in the determination of trace metals in sea-water samples.It is worth noting that the laboratories taking part in this exercise are relatively experienced, both in trace analysis and in the application of quality control techniques. Given this, it might be assumed that sample handling practices used in these laboratories are typical of or better than the current norm. The finding that several approaches are not suitable indicates that sample handling may require more attention in many laboratories. The choice of filtration procedure is critical.It is accepted widely that acid-washed plastic (e.g., low-density polyethylene, polypropylene, polycarbonate) is suitable for use at metal concentrations found in coastal waters. Other materials, in particular rubber and high-density plastics (e.g., PVC and highdensity polyethylene), can be sources of contamination which are difficult to eliminate, even by acid washing.The use of plastic ware and filter materials ‘as received from the manufacturer’, i.e., without acid washing and rinsing with de-ionised water, is clearly inadvisable. Filtration procedures which, on the evidence of their description, appear to be appropriate to the task in hand have also been shown to be subject to serious contamination. This is a further illustration of the principle that the method alone does not determine performance; its mode of use is also crucial.Adsorption on the filtration apparatus appears not to be such an important source of error as contamination, for marine samples. It is likely that the high ionic strength of sea-water protects against the adsorptive losses encountered during filtration of some freshwater samples. The test has confirmed the conclusions of other work on adsorption during sample storage, i.e., that of the six metals of interest, copper and lead are likely to be most prone to adsorptive interactions with suspended matter. Recommendations A programme of periodic checks on filtration blanks is recommended as the only means of establishing control over contamination.Unless suitable test data are obtained to demonstrate fitness for purpose, it cannot be assumed that contamination during sample filtration is adequately controlled. Continuing checks of filtration blanks are also a necessary illustration that control is maintained during routine analysis.The analysis of spiked (pre-filtered) samples, although not as important as the use of blanks, can be used as an initial confirmation that adsorption to the filtration apparatus is not responsible for important losses of the determinand of interest. The authors acknowledge the cooperation and assistance of the following organisations which are participants in the NMAQC programme: the Environment Agency of England and Wales, the Scottish Environment Protection Agency, the Ministry of Agriculture, Fisheries and Food, the Scottish Office Environment, Agriculture and Fisheries Department, the Department of the Environment (Northern Ireland) and the Department of Agriculture (Northern Ireland).References 1 Dobson, J. E., Gardner, M. J., Griffiths, A. H., Jessep, M. A., and Ravenscroft, J. E., Accred. Quality Assur., in the press. 2 Cook, J. M., Gardner, M. J., Griffiths, A. H., Jessep, M. A., Ravenscroft, J. E., and Yates, R., Mar. Pollut.Bull., in the press. 3 Dixon, E. M., Gardner, M. J., and Hudson, R., Chemosphere, in the press. 4 Apte, S. C., and Gunn, A. M., Anal. Chim. Acta., 1987, 193, 147. 5 Gardner, M. J., and Ravenscroft, J. E., Fresenius’ J. Anal. Chem., 1996, 354, 602. 6 Bird, P., Comber, S. D. W., Gardner, M. J., and Ravenscroft, J. E., Sci. Total Environ., 1996, 181, 257. 7 Analytical Methods Committee, Analyst, 1987, 112, 199. 8 Gardner, M. J., and Hunt, D. T. E., Analyst, 1981, 106, 471. Paper 7/04527A Received June 27, 1997 Accepted July 16, 1997 Fig. 3 Comparison of metal concentrations in participants’ filtrates: zinc. Symbols as in Fig. 1. 1032 Analyst, October 1997, Vol. 122 Sample Filtration as a Source of Error in the Determination of Trace Metals in Marine Waters Michael Gardner* and Sean Comber WRc, Henley Road, Medmenham, Marlow, Buckinghamshire, UK SL7 2HD Adequate performance in interlaboratory proficiency tests using filtered, pre-treated or pre-digested test materials does not necessarily demonstrate that laboratories’ data are of adequate comparability.Sample handling can be an important source of error which is not examined by routine proficiency tests. This paper reports a study of sample filtration as a source error in the determination of trace metals in marine water samples. The results indicate that that current practice may need to be reviewed if important contamination errors are to be controlled. Keywords: Trace metals; quality control; contamination; filtration; sea-water The UK National Marine Analytical Quality Control (NMAQC) Scheme was established in 1991 with approximately 20 participating laboratories.1 Its primary aims are (i) to provide laboratories which submit data to the UK National Marine Monitoring Plan with a means of demonstrating that they are achieving the necessary (pre-defined) standards of analytical accuracy for trace metals and organic determinands in waters, sediments and biota and (ii) to assist laboratories to improve their accuracy, where necessary.Work undertaken by the Scheme falls into two categories: regular proficiency tests and special investigative exercises. This paper describes work of the second type in connection with the filtration of water samples for the determination of trace metals. The determination of trace constituents in environmental samples is subject to errors from a wide range of different sources.In common with many other interlaboratory quality programmes, the NMAQC scheme has concentrated on checking the accuracy of the measurement stage of the analytical process. However, it is clear that sampling and sample handling are stages where errors might arise and, therefore, where some form of check on accuracy is needed. Measures taken to address these aspects of analysis for the preparation and digestion of sediment samples have already been reported.2,3 For the determination of trace metals in water, routine checks have involved interlaboratory tests on filtered, homogenised, acidpreserved sea-water samples and standard solutions.Whilst this aspect of an external quality control programme is necessary, parts of the analytical process not addressed by such test materials are also likely to be important in determining the accuracy of trace analysis. This paper reports investigations of sample filtration as a potential source of error in the determination of trace metals in marine waters.The aim of the work was to determine the size and sources of any errors and to help to identify and promote good practice. Choice of Filtration Procedures Sample collection and sample handling (including filtration) are not always the responsibility of staff who undertake analyses in participating laboratories. This meant that there might not be a ‘routine’ procedure to be tested in all participating laboratories. Given this, the test was designed with the aim of providing an illustration that satisfactory filtration procedures could be applied.Participants were asked to choose a filtration procedure (to be examined in these tests) with two criteria in mind: (i) the procedure should be representative either of what is actually done for marine samples, or (ii) it should be sufficiently practicable to be applied to marine monitoring samples, if it proves satisfactory. Test Design Two principal types of analytical error might be introduced during filtration: contamination from the apparatus or handling procedure and adsorptive losses to the filter, filter support, etc., during filtration.The test was designed to assess both types of error. Five 1 l samples were provided for filtration: sample A was a de-ionised water sample; sample B was a filtered (0.2 mm) coastal sea-water; samples C and D were accurately measured (1 l ± 5 ml) portions of the same filtered sea-water; and sample E was a portion of sample B which had been spiked with the determinands of interest and which also contained added microcrystalline cellulose [BP grade (Merck, Poole, Dorset, UK)] at a concentration of 100 ± 5 mg l21.Participating laboratories were asked to filter samples A, B and E as received, using the method (or methods) chosen for the test. Samples C and D were to be spiked at the participating laboratory immediately before filtration by adding 500 ml of a corresponding concentrated spiking solution (also supplied) to the measured 1 l portion of water.The two spiking solutions were provided at a pH between 2.0 and 2.5 (to minimise adsorptive losses). A separate spiking solution was provided for chromium (as CrVI) because of the insolubility of lead chromate. The other metals were present in a mixed spiking solution. The spiked samples were mixed by shaking and filtered within 1 h. The portions of filtrate were put into the laboratory’s own bottles (the type normally used for filtered samples) and preserved by addition of 400 ml of 5.5 m hydrochloric acid (Aristar grade, Merck) (supplied) per 100 ml of filtrate.The labelled filtrates were returned to WRc. Participants were asked to carry out filtrations in duplicate. All samples were analysed at WRc. Determinations of Cd, Cu, Pb and Ni were made using a semi-micro chelation solvent extraction procedure.4 Chromium5 and zinc6 were determined by previously described methods.A summary of each laboratory’s filtration procedure is given in Table 1. Results Fig. 1 shows a comparison of analytical data for the filtrates supplied by each participating laboratory. The pair of results for each laboratory corresponds to the two filtrations carried out on each sample. Results are summarised for each metal, on the basis that sources of contamination or the tendency for adsorption are likely to be metal-related. Analyst, October 1997, Vol. 122 (1029–1032) 1029Discussion It is worth emphasising that the comparison shown in Fig. 1 relates to the effects of filtration (and subsequent sample storage), instead of the more familiar comparison of interlaboratory analytical performance. Differences between laboratories’ results can be regarded as arising from a combination of within-batch analytical variation and what might be termed ‘filtration errors’. Effort was made to make analytical variations as small and homogeneous as possible.Determinations for a given metal were carried out in a single batch of analysis to minimise the effects of between-batch analytical errors. The limits of detection (LOD) estimated from duplicate blank determinations (for a series of analytical batches) are as follows:7 Cd 0.05, Cr 0.01, Cu 0.03, Pb 0.04, Ni 0.05 and Zn 0.05 mg l21. Potential Filtration Problems Contamination Contamination can produce errors which could be categorised as both random and systematic.Consistent contamination will tend to produce a positive bias in metal concentrations; sporadic contamination will tend to produce differences between replicate filtrates (i.e., an increase in imprecision). Hence it is unwise to interpret the results for filtration error in distinct categories of precision and bias. Data for samples C and D were evaluated against a ‘spiked’ value calculated as the spiked concentration + a ‘background’ value determined at WRc. In all cases the background was small in relation to the added spike.For sample E, a mean of laboratories’ concentrations was used as a reference point, for reasons discussed below under Adsorption. Cadmium (Fig. 1). Results for the de-ionised water sample (A) indicated that at least two laboratories (4 and 19) are subject to serious contamination. This is confirmed for laboratory 4 by data for the unspiked sea-water (B). For samples B–E, results for laboratories 7, 8 and 16 showed some evidence of very small elevations in concentration or wider differences (than those for other laboratories) between replicate filtrations.The results showing excellent agreement within replicate filtrations and with one another including those for laboratories 3, 11, 12 and 17. Chromium. The results for the de-ionised water and the blank sea-water were too close to the limit of detection of the analytical method to allow clear conclusions to be drawn concerning the smaller observed variations.However, there was evidence of contamination of between 0.2 and 1 mg l21 for laboratory 19. At the higher concentration of samples C, D and E there is close agreement between concentrations measured in laboratories’ filtrates. Copper (Fig. 2). Several laboratories’ results indicated sources of copper contamination. A large effect was evident for laboratory 4 (contamination of > 1 mg/l21); other laboratories Table 1 Summary of filtration procedures Material in Laboratory contact with No.Apparatus* sample* Filter type 3 P/e funnels, P/c P/e: bottle, funnel Nuclepore, 47mm Swinloc P/c: membrane, silicone sealing ring 4 P/c Sartorius P/c: bottle, filter, Nuclepore membrane 7 Sartorius 500 ml P/c: glass Cellulose nitrate, measuring cylinder 50mm 8 P/c Swinloc P/c: bottle, filter, Nuclepore membrane 9 Becton Dickenson P/p Cellulose acetate, 60 ml syringe 47mm filter? 11 Becton Dickenson P/p and rubber: Cellulose acetate, 50 ml syringe syringe Acetate: 47mm Sartolab P filter filter holder 12 Gelman P/e: bottle Gelman, 633 cm Glass-fibre: filter ?syringe 15 Becton Dickenson — Cellulose nitrate, syringe, Sartolab 50mm P, 120 sample pot 17 P/s Nalgene P/s: bottle, filter, Cellulose nitrate, membrane 47mm 19 Millipore PTFE PTFE Whatman, 142mm 1.5 l apparatus cellulose nitrate Pore size/ Bottle mm Preparation† Storage preparation† 0.4 Overnight in 1% Prepared prior Detergent rinse, HNO3 50 ml 1% to use 1% HNO3 for 1 HNO3 50 ml DIW week, 0.1% HNO3 for 1 week, DIW before use 0.4 50 ml 5% HNO3 Prepared prior As filter except no 50 ml DIW to use sample rinse 50 ml sample (discarded) 0.45 Syringe and filter Prepared prior DIW, 10% HCl, rinsed with DIW to use 10% HNO3, 0.1% 10% HCl and HNO3 until used DIW 0.4 50 ml 1% HNO3 Prepared prior As filter 50 ml DIW to use 50 ml sample (discarded) 0.45 Syringe and filter None rinsed with 30 ml 1% HNO3, 30 ml sample 0.45 Rinsed once with Rinsed with sample sample 0.45 None Sealed plastic HNO3 wash, DIW bag 0.45 Syringe and filter — None rinsed with sample 0.45 10% HNO3, Prepared prior As filter DIW, sample to use (discarded) 0.45 10% HNO3 for Prepared prior Detergent, 24 h 2 h, DIW rinse to use 10% HNO3, DIW rinse * P/c = polycarbonate, P/e = polyethylene, P/p = polypropylene, P/s = polysulfone † DIW = de-ionised water. 1030 Analyst, October 1997, Vol. 122(8, 11 and 19) were subject to smaller biases which were mainly evident at the blank level.The data showing excellent agreement within replicate filtrations and with one another including those for laboratories 3, 7, 9, 12, 15 and 17. Lead. The majority of results for lead in blank filtrates were lower than the 0.04 mg l21 reporting threshold, indicating good control over contamination. Good comparability of filtrates was observed at higher concentrations (samples C, D and E). The range of results obtained was narrow and close to the expected spiked values.Nickel. Excellent comparability and accuracy, with respect to the spiked value, were achieved for the spiked samples. Only one serious bias was observed at the blank level, in laboratory 19 (this may arise from the same source of contamination as that already noted for chromium). Zinc (Fig. 3). The data for zinc showed the most obvious instances of contamination bias. Replicate results tended to be similar but were often displaced with respect to filtrate data from other laboratories.At both the blank and at higher levels, laboratories 4, 8 and 9 showed clear bias. In the first two laboratories this bias was small (1–2 mg l21); in the last, bias was around 10 mg l21. Blank data from three other laboratories (7, 11 and 15) might be subject to small bias of approximately 1 mg l21. This was not borne out by the higher concentration samples (except in the case of laboratory 15). Adsorption There is no clear evidence of a bias between the ‘spiked’ value and observed values for samples C and D, for any of the metals.This indicates that adsorptive losses during filtration are probably not important. Sample E, containing 100 mg l21 of microcrystalline cellulose, was included with two aims in mind: (i) To test the hypothesis of increased adsorption for increased contact time during filtration: if adsorptive losses had been significant for filtration of samples C and D (no suspended solids, so a relatively short filtration time), it might be expected that the longer contact time for a sample containing solid matter would result in larger adsorption.Sample E could have provided a guide to the additional adsorptive losses which might apply to real samples. The test data show that adsorption during filtration was not important (or at least was not detectable in relation to the observed random errors). In freshwater samples, such adsorption can lead to significant losses of trace metals.8 The lack of adsorption in sea-water might be ascribed to the high ionic strength of the matrix and consequent competition for adsorption sites from major ion constituents of the sample (e.g., Mg2+ and Ca2+).(ii) To provide a check on adsorption in the sample bottle: sample E was prepared in bulk (with solids) and spiked at the same concentration as sample D. The bulk sample was equilibrated for 1 week to allow partitioning between the solid and dissolved phases. This was mixed thoroughly and then dispensed into the sample bottles.Markedly lower concentrations in the dissolved metal concentration of sample E in relation to sample D (given that samples D and C were not subject to large negative bias) might be taken as evidence of adsorptive losses of determinand to the solids or to the inner surface of the sample bottle. Fig. 1–3 illustrate the important comparisons between the mean of laboratories’ filtrate concentrations for sample E and the spiked + residual value for sample D (the two should be the Fig. 1 Comparison of metal concentrations in participants’ filtrates: cadmium. +, LOD; –5–, spike; and –x–, mean of laboratories. Fig. 2 Comparison of metal concentrations in participants’ filtrates: copper. Symbols as in Fig. 1. Analyst, October 1997, Vol. 122 1031same, if there is no adsorption and other sources of bias are small). The values were found to be similar for Cd, Cr, Ni and Zn, if the instances of contamination (applicable to both samples, but not necessarily consistent) are ignored.However, for Cu the concentration in sample D is markedly higher than that in sample E [3.7 mg l21 (ignoring data from laboratory 4) versus 2.7 mg l21]. This is statistically significant (p = 0.05), indicating losses of copper. It is likely that adsorption on the cellulose was more important than loss to the bottle, given the affinity of the metal for oxygen-containing organic matter. The nominal surface areas of the solids and the bottle were similar, estimated to be approximately 500 cm2 each.The results for Pb also show a small decrease between samples D and E. Conclusions The test demonstrates that sample filtration can be a source of important error in the determination of trace metals in sea-water samples. It is worth noting that the laboratories taking part in this exercise are relatively experienced, both in trace analysis and in the application of quality control techniques. Given this, it might be assumed that sample handling practices used in these laboratories are typical of or better than the current norm.The finding that several approaches are not suitable indicates that sample handling may require more attention in many laboratories. The choice of filtration procedure is critical. It is accepted widely that acid-washed plastic (e.g., low-density polyethylene, polypropylene, polycarbonate) is suitable for use at metal concentrations found in coastal waters.Other materials, in particular rubber and high-density plastics (e.g., PVC and highdensity polyethylene), can be sources of contamination which are difficult to eliminate, even by acid washing. The use of plastic ware and filter materials ‘as received from the manufacturer’, i.e., without acid washing and rinsing with de-ionised water, is clearly inadvisable. Filtration procedures which, on the evidence of their description, appear to be appropriate to the task in hand have also been shown to be subject to serious contamination.This is a further illustration of the principle that the method alone does not determine performance; its mode of use is also crucial. Adsorption on the filtration apparatus appears not to be such an important source of error as contamination, for marine samples. It is likely that the high ionic strength of sea-water protects against the adsorptive losses encountered during filtration of some freshwater samples.The test has confirmed the conclusions of other work on adsorption during sample storage, i.e., that of the six metals of interest, copper and lead are likely to be most prone to adsorptive interactions with suspended matter. Recommendations A programme of periodic checks on filtration blanks is recommended as the only means of establishing control over contamination. Unless suitable test data are obtained to demonstrate fitness for purpose, it cannot be assumed that contamination during sample filtration is adequately controlled. Continuing checks of filtration blanks are also a necessary illustration that control is maintained during routine analysis. The analysis of spiked (pre-filtered) samples, although not as important as the use of blanks, can be used as an initial confirmation that adsorption to the filtration apparatus is not responsible for important losses of the determinand of interest. The authors acknowledge the cooperation and assistance of the following organisations which are participants in the NMAQC programme: the Environment Agency of England and Wales, the Scottish Environment Protection Agency, the Ministry of Agriculture, Fisheries and Food, the Scottish Office Environment, Agriculture and Fisheries Department, the Department of the Environment (Northern Ireland) and the Department of Agriculture (Northern Ireland). References 1 Dobson, J. E., Gardner, M. J., Griffiths, A. H., Jessep, M. A., and Ravenscroft, J. E., Accred. Quality Assur., in the press. 2 Cook, J. M., Gardner, M. J., Griffiths, A. H., Jessep, M. A., Ravenscroft, J. E., and Yates, R., Mar. Pollut. Bull., in the press. 3 Dixon, E. M., Gardner, M. J., and Hudson, R., Chemosphere, in the press. 4 Apte, S. C., and Gunn, A. M., Anal. Chim. Acta., 1987, 193, 147. 5 Gardner, M. J., and Ravenscroft, J. E., Fresenius’ J. Anal. Chem., 1996, 354, 602. 6 Bird, P., Comber, S. D. W., Gardner, M. J., and Ravenscroft, J. E., Sci. Total Environ., 1996, 181, 257. 7 Analytical Methods Committee, Analyst, 1987, 112, 199. 8 Gardner, M. J., and Hunt, D. T. E., Analyst, 1981, 106, 471. Paper 7/04527A Received June 27, 1997 Accepted July 16, 1997 Fig. 3 Comparison of metal concentrations in participants’ filtrates: zinc. Symbols as in Fig. 1. 1032 Analyst, October 1997, Vol. 122