Phase I:  Evaluation of Bottle Type, Bottle Cleaning, Filter and Preservation Technique

EXECUTIVE SUMMARY

 Background

Guidelines and criteria levels for metals and metalloids in surface waters (CCME, BC, Ontario, EPA) are the subject of much discussion currently and there is a movement to (a) lower many of them from ppb to ppt levels and (b) define element species (rather than total) with respect to level. Their measurement at ppt levels is made possible by the establishment of ICP-MS (inductively coupled plasma mass spectrometry) as a routine analytical technique. However, the various sampling and preservation protocols in effect for measurement at the higher ppb levels have not been verified (in the literature) at the lower ppt concentration levels. Some protocols (e.g. Environmental Protection Agency (EPA) Method 1631 for Hg) call for the exclusive use of expensive Teflon bottles for sample collection and storage; these protocols were published when alternative plasticware development was in its infancy. The ‘dissolved’ fraction of metals is operationally defined as that which passes a 0.45 μm filter. Often filter types (tortuous path, sieve-like), filter systems, and cleaning thereof, are not specified (e.g. in EPA Methods 200.7, 200.8, CLP Method 6020), though the analytical procedure is extremely detailed.

The objective of the work described herein was to recommend the most cost-effective and efficient procedure by which to sample, filter and preserve surface water for the accurate determination of Al, Ag, As, Cd, Co, Cr, Cu, Hg, Fe, Mn, Mo, Ni, Pb, Sb, Se, Tl and Zn to ‘consensus’ water quality guidelines. The work is focussed on the filtered (0.45 μm) water sample, not on ‘total recoverable’. Furthermore, it does not enter into speciation of the ‘dissolved’ fraction but rather the analytical methods used here are designed to measure the ‘total dissolved’ fraction (with the exception of Se where only inorganic species are measured).

All elements except Hg and Se were measured by conventional nebulisation ICP-MS which provided detection limits (DL) of: 0.03 ppb for Al; 1.7 ppb for Fe; 20 ppt for Cu and Ni; and ≤10 ppt for the other 11 elements. These DLs are well under the consensus criteria levels for waters (information taken from Environment Canada, BC and Ontario). Hydride or vapour generation ICP-MS was used for Hg (Hg^o) and Se (SeH₂) to achieve the required sensitivity of measurement, down to 1 and 4 ppt, respectively. All experimental work prior to analysis was carried out in a Class 100 Cleanroom.

Results

Test-tubes 

The test-tubes recommended for use in the analysis of waters at low analyte concentrations are the Fisherbrand polypropylene centrifuge tubes. These should be soaked in 1% HNO₃ for 24 hours and rinsed with water. Their blue polystyrene caps are to be avoided when analysing for Al and Zn, unless these undergo vigorous cleaning. These test-tubes do not require cleaning for the determination of Hg and Se at levels of 1 ppt or greater.

Bottles

The bottles studied comprise: Teflon (FEP, Nalge #1600); high density polyethylene (HDPE, Nalge #2007); polyethylene terephthalate copolyester (PETG, Nalge #2019); polypropylene (PP, Nalge #2006); and precleaned HDPE^P (‘Superfund-Analyzed’ to meet or exceed EPA specifications). Two cleaning methods were investigated: a modified EPA Method 1638; and one promoted by the State of Virginia. These procedures are similar in that they focus on the use of HNO₃ but the EPA method employs concentrated (12M) acid whereas the Virginian method uses a much lower concentration of 5% (v/v) with a subsequent step employing only 0.5% HNO₃. The EPA method incorporates an initial wash with a soap solution. A third method was tested, that of simply rinsing each bottle with deionised water three times prior to filling. Each group of five bottles, cleaned in three different ways, was filled to 125 ml with 0.4% HNO₃ (usual concentration of acid as preservative employed by the Geological Survey of Canada (GSC) and others) and a charge of 1 ml of 8M HNO₃ was added to each of the alternate group of five. After six days, they were analysed for all elements save Hg which required a completely separate test as different preservatives are needed for this element.

The approach practised by some laboratories – to add a charge of concentrated (≥8M) HNO₃ reagent to the bottle hours or days before water collection – is not acceptable. This strategy causes much higher levels of contamination for all bottles than is the case when acidifying during or after sample collection. Cleaning does not remove ‘available’ elements as prolonged contact with HNO₃ leaches out significant quantities.

The least expensive bottle (ca $Cdn 0.90), made of high density polyethylene (HDPE), shows the best characteristics and is highly recommended. It could be used without rigorous cleaning (only rinsing with deionised water) if batches are checked but a rinse with weak HNO₃ (5%) is probably advisable.

HDPE bottles purchased precleaned are an unnecessary expense (ca $Cdn 2.30) as they are inferior to their uncleaned counterparts and show a startling increase in Zn contamination, to 739±195 ppt (cf 7±4 ppt). Higher levels of Al, Cr, Ni and Pb are also evident.

Comparable in cost to the HDPE is the polypropylene bottle (PP) which does require cleaning if Al is of concern (contamination level of 594±40 ppt). Cleaning by either method reduces this consistent level of contamination to insignificance. Considerably more expensive at $Cdn 2.60, the polyethylene terephthalate copolyester (PETG) bottle must also be cleaned, but by the 5% HNO₃ method.

The extremely expensive ($28) Teflon (FEP) is not recommended. It was, by far, the dirtiest bottle. Both methods of cleaning adequately reduce contamination by Cr, Fe, Co, Ni, Cu, Zn, Mo and Pb for environmental projects but concern remains for some of these elements (e.g. Cr and Ni) if geochemical mapping is the focus.

Overall, results indicate that the less costly method of cleaning, as outlined by the State of Virginia, is preferable to the EPA method. This method could probably be shortened by eliminating the second 24-hour stage of contact with 0.5% HNO₃.

All bottle types – FEP, HDPE, PETG and PP – can be used without any cleaning (i.e. rinsing only) for the determination of Hg in waters down to levels of 1-2 ppt. None of the commonly used preservation media for Hg – 0.5% BrCl, 2% HCl or 0.04% K2Cr₂O₇ in 0.1% HNO₃ – appears to leach out detectable concentrations of these elements from the bottle material. Thus, the elaborate cleaning methods, EPA 1631 and 1638, can be avoided for Hg.

Filter systems

The majority of the twelve 0.45 μm and two 5 μm filters tested (of syringe, in-line and vacuum type) were from two leading manufacturers, Gelman and Millipore. The objective of this project was twofold in that the expected contamination levels for both an ordinary water sample (e.g. stream) and an acidic sample (e.g. end-of-pipe) were desired. The test media were Type I deionised water and 0.4% HNO₃.

Overall, optimum performance in terms of contamination and ease of use was achieved with the ion chromatography Acrodisc syringe filter with Supor membrane, from Gelman and the Sterivex syringe filter capsule with Durapore membrane, from Millipore. Nylon membranes should be avoided as they are slow and do not show superior contaminant characteristics. The Millex LS 5μm syringe prefilter is recommended for samples high in particulate matter. Also acceptable for environmental monitoring are: the Millicup bottle top with Durapore membrane (vacuum system); the in-line Gelman AquaPrep with Thermopor membrane; and the AquaPrep 250 with Supor membrane. For the filtration of acidic samples, the following systems should be avoided: the Gelman syringe nylon Acrodisc; the Millipore all-glass vacuum; the Gelman groundwater capsule; and the Gelman AquaPrep 250.

These filters were investigated further – for their propensity to retain elements which are present as colloids which should pass through a 0.45 μm pore size. A bulk control sample of Ottawa River water and a synthetic spiked water sample were employed for this purpose. Although the Acrodisc Supor membrane from Gelman was recommended for its low contamination level, recoveries of certain analytes in Ottawa River water are significantly lower than those obtained using the Millipore systems which incorporate the Durapore membrane. This indicates partial retention of the elements Al, Cr, Fe, Mn, Co, Zn and Pb present in colloidal form. The high and consistent recoveries for all 17 elements found in the Ottawa River control with the Millipore systems advocate recommendation of the Sterivex capsule system or Durapore-based alternatives if the goal is to measure that fraction of an element present at #0.45 μm. Lowest recoveries, and hence maximum retention of colloidal species, were found with Gelman’s Supor membrane-based systems.

Further assessment of the sorption of Hg, in its free ion form, by different filter systems is required. Unlike the other 16 elements, spikes of Hg added to deionised water samples were not fully recovered 12 through all filter systems. Minimum loss is evident using the Millipore systems, particularly the Millicup bottle top model with Durapore membrane.

Stability

Four samples of different matrix – Ottawa, Rideau and Gatineau River waters and a spiked water sample – were used to verify that preservation of the 16 analytes (i.e. all except Hg) in a medium of 0.4% HNO₃ was adequate for prolonged storage (one month).

Acidification to 0.4% in HNO₃ should maintain elements Al, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se, Tl and Zn in water samples for at least a month at room temperature. Stability over this period for Ag at concentrations of several hundred ppt is questionable and matrix-dependent. Stability of these elements is independent of container material.

The best preservation reagent for Hg is 0.5% BrCl: it maintains Hg at ppt levels in solution for at least one month. Preservation in 2% HCl or 0.04% K₂Cr₂O₇ may be inadequate, especially for the former medium. Stability of Hg was independent of the container material. Diffusion of Hg from the atmosphere into the sample was not in evidence over the 28-day trial period for any of the bottle types.

AETE