4.1 Selecting a laboratory
Analytical laboratories must be selected on the basis of their experience and ability to carry out the selected analyses to the required standard. This suitability can be verified in a number of ways including:
- accreditation by bodies such as IANZ 5 or NATA 6 to NZS/ISO/IEC 7Guide 17025
- an audit of the laboratory, or by reviewing the results of external auditing by some other party
- past experience of the type of work undertaken at the laboratory
- participation by the laboratory in inter-laboratory comparison programmes.
Accreditation by an independent third-party auditing body such as IANZ or NATA provides formal recognition that the laboratory meets the minimum standards of Guide 17025. To achieve accreditation a laboratory must prove that they have suitable technical expertise, facilities, instrumentation and quality management systems in place to carry out the testing involved. Documentation of staff training, test methods, quality procedures, equipment calibration and maintenance, document control, response to laboratory client queries, corrective and preventive actions, and ongoing auditing are required. Personnel from both the auditing agency and independent technical assessors carry out the audit leading to accreditation.
Note that a laboratory's being accredited does not imply that all test methods used in the laboratory are accredited. To achieve accreditation for an individual test method the laboratory must demonstrate to an independent technical assessor that they have a documented test method procedure, have validated the method (see section 4.5.2), have suitable equipment, and have staff with the knowledge, experience and competence to carry out the test as documented. The laboratory must also be using the test method on a regular basis, and for this reason some rarely used methods may not be specifically accredited.
4.2 Sample handling
Effective site investigations require planning, and the analytical test requirements should be discussed with the laboratory before you do any sample collection. This discussion with the laboratory should cover:
- the matrices to be sampled
- required analytes
- analysis method
- method detection limits (MDL), or practical quantitation limits (PQL)
- sample collection containers
- preservation requirements
- storage and transport conditions
- provision of trip blanks
- required turnaround for results (a non-routine 'priority' turnaround may need special organisation with the laboratory)
- compositing of samples
- dealing with non-homogeneous samples
- sample retention after testing.
The chain of custody form (see Section 3.7.5 and Appendix E) must accompany samples to the laboratory. It details the links in the transfer of samples from collection to arrival. The chain of custody must contain at least the following information:
- time and date the samples are collected
- name of person transferring the samples
- time and date the samples are received at the laboratory
- name of person receiving the samples
- name and contact details of who to report to
- urgency of analysis (routine or priority turnaround)
- consignment identifier or job reference.
For each sample there must be a record of:
- unique identifier (which must match those on the containers)
- matrix (eg, soil)
- the tests required, with minimum detection limit (DL)/PQL
- whether specific test methods are required (these should be discussed with the laboratory beforehand).
Other useful information you can supply to the laboratory includes:
- how the laboratory results are to be reported (eg, any combination of hard copy, fax, phone, electronic)
- an indication of possible levels of contaminants in the sample, especially if high (this is very useful for the laboratory, because high levels of analytes may contaminate laboratory equipment, cause cross-contamination of other samples, and require re-analysis using smaller sample amounts, or dilutions, which slows turnaround)
- a laboratory quote or reference number if required for pre-arranged work
- the name, address and contact details of another laboratory if split samples are to be forwarded for analysis and reported/invoiced direct to the person submitting the samples.
4.2.3 Receipt at the laboratory
Each consignment of samples should be given a unique identification reference by the laboratory, and each sample in the consignment should also be individually identifiable. All samples must be able to be tracked through every stage of analysis in the laboratory.
Upon receipt at the laboratory, all samples should be unpacked, checked against the chain of custody and placed in appropriate storage as soon as possible. The chain of custody should be completed with the date and time of receipt, laboratory identifier, the name of the laboratory representative responsible for the samples, and any comments if necessary (eg, names on chain of custody not matching those on the containers, containers missing or broken, sample temperature or temperature of the sample container). The completed chain of custody should be faxed to the indicated contact person to confirm sample receipt.
4.2.4 Sample holding times
Recommended sample containers and guideline sample holding times before analysis are given in Appendix D. Holding times are not standards and are useful for reference only, as times may vary depending on the particular sample matrix. Once a sample has been collected, the nature of the analytes present may change as the result of:
- loss by volatilisation
- degradation by exposure to light
- degradation be exposure to oxygen or other chemicals
- degradation by living organisms.
The rate of sample degradation or loss will depend on the analyte, matrix and other factors present (eg, oxygen, light, soil microbes, moisture, temperature), and the site conditions. These changes can be minimised by collecting samples in appropriate containers, using preservatives (if appropriate), keeping samples chilled, cold or frozen and undertaking analysis as soon as possible. Sample preservation methods should be documented.
Example: The recommended holding time before extraction of polycyclic aromatic hydrocarbons (PAHs) is 14 days, although there is unlikely to be any significant change in PAH concentrations after sampling where contamination occurred several years ago, even over a period of several months. However, PAHs collected from a deep excavation, where the environment was anoxic, may undergo rapid changes on exposure to light and oxygen.
Guideline holding times before analysis should be taken into consideration when setting the DQOs, and should take account of:
- required turnaround
- regulatory (legal) requirements
- location and transport considerations
- number of samples and laboratory capacity.
4.2.5 Sample retention after analysis
Samples can be retained at the laboratory for a length of time after the tests have been carried out in case further tests are requested or there are queries regarding the results. The time for which samples are held will depend on the analysis (eg, microbiology samples would not be retained, while samples for metals can be stored almost indefinitely), matrix, storage conditions and space considerations. Any special requirements should be discussed with the laboratory in advance.
The nature of the analytes and possible loss/degradation should be taken into consideration when requesting further analyses from retained samples.
4.3 Hazardous samples
It should be standard practice for laboratories to treat all samples as 'potentially hazardous' and to use appropriate protective clothing, such as laboratory coats, gloves and safety glasses, as required. The site investigation health, safety and environment plan (HSEP) should identify any chemical, biological or radiation hazards and the laboratory should be informed of these (see section 3.11.1).
Samples known to be particularly hazardous should be clearly identified on the container and may need special packaging and transport to the laboratory. An example would be free hydrocarbon product being sent for identification. Transportation of these samples requires consideration of the Land Transport Act.
Laboratories should have a procedure in place for identifying, labelling, storing and disposing of hazardous samples and waste. Any hazardous samples and hazardous waste generated by the laboratory analysis should be stored in a dedicated area and removed by hazardous waste contractors. In some situations this may include returning the samples to the waste generator for disposal/treatment with the other material on site.
4.4 Sample preparation methods
4.4.1 Non-homogeneous samples
All soil samples received at a laboratory should be treated as inhomogeneous and should be homogenised before a sub-sample is removed for analysis, although (as outlined in section 4.4.3) this must not be done in a manner to cause loss of analytes. Samples for volatile analyses must remain as undisturbed as possible. Homogenisation is the process by which a sample is mixed to obtain consistency throughout the sample prior to analysis. Unrepresentative material such as twigs, leaves and stones are often removed by the laboratory, if requested to do so. Larger cobbles etc. may be removed in the field (see section 2.3.2). The particle size of the sample is often reduced to ensure uniformity of the sample, and this can be done by crushing and grinding.
Certain soils samples such as fill material, clays, shingly soils and very oily soils can be difficult to mix and may require special treatment. The method for dealing with non-homogeneous samples should be discussed with the laboratory ahead of sample receipt at the laboratory. The options for dealing with non-homogeneous samples will depend on the DQOs.
Practices that can be used by the laboratory to deal with inhomogeneity include the following.
- Sieve the sample using a 2 mm sieve, and record the proportion of the fractions separated. Analyse the sub-2 mm fraction.
- Reduce the particle size by crushing and grinding to pulverise the sample so that the whole sample is included in the analysis.
- Sub-sample (see section 4.4.2) by separating the material of interest and analyse (eg, visible hydrocarbon contamination coating gravels). The free-phase hydrocarbons can be separated analysed for product identification and a visual estimate of the amount of free phase given.
In the first instance the sub-2 mm fraction may make up only a very small portion of the whole sample (by weight or by volume) and this will bias high the results if they are applied to the original sample. To overcome this, the proportion of the under and over 2 mm fractions will need to be determined so a correction factor can be applied. The second approach will give a more correct value for the overall sample, assuming the analytes are stable to the grinding process, but may not reflect the DQOs requirements.
4.4.2 Sub-sampling in the laboratory
Sub-sampling in the laboratory is necessary to reduce the sample size for analysis. Containers of up to 1 kg are typically supplied to the laboratory for analysis and a number of analyses are usually carried out from the sample, but only a few grams of material are used for individual analyses.
The sub-sampling procedure must be carried out after the sample has been homogenised by the laboratory, and must be undertaken in an unbiased manner to ensure that the sub-sample is truly representative of the original sample. It is essential that the sub-sampling procedure does not alter the overall nature of the sample, or cause loss of target analytes for any reason.
The method of sub-sampling will depend on both the analytes to be determined, and the sample. Methods of sub-sampling include the following.
- Long-pile method - the sample is laid out in a long pile during the unloading process, the pile is separated into two equal piles by using a shovel and placing alternate shovel loads to either side to form two mounds. Then one mound is randomly selected and the process continued to reduce the sample size.
- Cone and quarter method - the sample is piled into a cone shape with a flattened top, and the cone divided into quarters. The opposite quarters are discarded and the remaining quarters mixed together to form a second cone. The process is repeated until the desired sample size is reached
- Riffle methods - a riffle is a trough divided into a number of compartments, with doors that open on alternate sides. On each pass through the riffle, soil samples are separated and the sample size is halved.
Sub-samples for analysis of volatiles (volatile organic compounds, BTEX 8 and total petroleum hydrocarbons) should be taken using a technique such as coring, which minimises losses and gives a reasonably representative sub-sample.
Example of sample preparation: if only metals and non-volatile organics (eg, PCBs) are to be determined, the sample may be spread out on a tray, thoroughly mixed, quartered, and opposite quarters returned to the original container for retention as 'field moist' samples. The rest of the sample is air dried at 35°C overnight before lightly grinding in a mortar and pestle and passing through a 2 mm sieve. The dried, sieved sample is then further sub-sampled as part of the analytical procedure (eg, only 0.5-1.0 g is used for metals analysis).
Compositing in the laboratory involves mixing together equal quantities of individual samples to make one composite sample for analysis. This is often done to enable more cost-effective investigations to be undertaken. (Further details on the collection of samples for compositing are provided in section 3.6.4.)
Samples for analysis of volatile and semi volatile constituents such as polycyclic aromatic hydrocarbons and total petroleum hydrocarbons must not be composited owing to the potential for the loss of volatiles, leading to under-reporting of the concentrations in the sample.
All samples should be thoroughly homogenised before compositing; for example, by spreading on a tray, mixing, quartering, returning opposite quarters to the original container and using the remaining quarters in the composite. Homogenising and compositing of individual samples must not compromise the integrity of the target analytes.
The remaining homogenised constituent samples should be retained so they can be reanalysed separately at a later date if further individual analysis is required.
4.5 Analytical methods
4.5.1 Selecting an analytical method
Analytical methods must meet the requirements of the DQOs. Factors to consider when selecting a method include:
- the required detection limits (eg, screening methods for initial investigations, specific methods to trace levels for final clean-up validation)
- the required turnaround time for results - lower detection limits usually require more work in the laboratory, which takes more time
- the required technique (eg, is the extraction method appropriate for comparison with the guidelines?).
There is almost always a trade-off between turnaround, detection limit and cost.
A number of different common instrumental methods can be used for analysing substances in soils, and methods for metals and organics are summarised in Appendix G. Screening test methods are generally less rigorous than 'reference' procedures. They may be suitable for monitoring the progress of a site remediation, although the precision may not be acceptable for a site validation.
Any method can be used provided the laboratory has validated the method for the analytes and matrix under investigation. In practice, most laboratories base their methods on a standard from a body such as US EPA, ASTM 9 or APHA/AWWA/WEF 10. Further guidance on obtaining copies of US EPA methods is available on the US EPA website (www.epa.gov/region01/oarm/testmeth.pdf). An individual laboratory may modify the method to use different equipment or new innovations. Any modification must be fully validated by the laboratory.
This allows the introduction of methods using new technology or different techniques, provided the method is fully validated first. Validation data must be available on request.
4.5.2 Validating analytical methods
The laboratory should be able to provide a validation report for any methods used. This must include:
- specificity for the compounds being analysed
- analytical range
- recovery efficiency from the matrix
- method detection limit (the level of quantification can then be calculated, as outlined in Appendix F)
- precision, both within batch and between batches.
Where possible the validation report should include:
- results of inter-laboratory comparison programmes
- results for certified reference materials, if these are available
- stability of the analytes in the matrix
- stability of the analytes in any extract/digest
- comparison with other methods for the same analyte.
The validation report should also include acceptable ranges for laboratory QC analyses such as blanks, spikes, replicates and QC samples.
4.5.3 Inter-laboratory comparison programmes and certified reference materials
Laboratories should validate the analytical methods against appropriate certified reference materials, where available. Certified reference materials are not available for all analytes and are normally used as part of a method validation (due to expense), rather than as part of the routine laboratory QC samples.
Inter-laboratory comparison programmes can be used to demonstrate the ability of a laboratory to undertake analyses on specific sample matrices, and performance results in the programmes can also be used in method validation. Ongoing participation and monitoring of the results of comparison programme performances should form part of a laboratory QA programme.
4.5.4 Metals and metalloids
The choice of analytes and metallic elements of interest will be dependent on the site history, and previous and proposed land uses (see section 2.2.1). In general, low levels of trace metals occur naturally in the environment, but elevated concentrations of metals may indicate land where hazardous materials have been used. The metallic parameters that are toxic and harmful to human health and that are commonly analysed in soil include arsenic, boron, cadmium, chromium, copper, mercury, nickel, lead and zinc. This group, with the exclusion of boron, are commonly referred to as 'heavy metals', although arsenic is not strictly a metal but a metalloid. For specific sites, other metals (eg, silver) or metalloids (eg, antimony) may be of interest, depending on the activities undertaken at the site.
Metals in soils are present in a number of different forms, including soluble ions and complexes, metal hydroxides, sulphides, precipitates, and insoluble complexes. The soils can be analysed for total metals or extractable metals, and must first be dried to ensure the results can be presented on a weight per weight (often mg/kg dry weight) basis. The extraction method used will determine the fraction of the metals analysed. If required by the nature of the site, speciation of metals such as chromium VI or arsenic III may be requested. These require specific test methods and cannot be analysed from the total recoverable digest described below.
For detailed site investigations the most common fraction to analyse (US EPA) is total recoverable metals, being the fraction of the metals that is likely to be extracted or leached from the sample under normal environmental conditions, not the total material bound to the soil silicate matrix. Preparing the sample involves drying and grinding the sample, passing it through a 2 mm sieve to produce a homogeneous sample, and then taking a sub-sample of the soil material for digestion.
US EPA Method 200.2 for total recoverable metals involves digestion in nitric and hydrochloric acids. This digestion method does not totally destroy the silica matrix and does not fully extract strongly interstitially held metals, but represents the readily extractable fraction of the metals present. Analysis of the digest, using matrix-matched standards for calibration, can be carried out by any suitably validated instrumental technique (see Appendix G).
If required, total metals can be determined using x-ray fluorescence spectroscopy, and field instrumentation is available for this purpose (see section 3.5), or by carrying out a hydrofluoric/aqua regia digestion before instrumental analysis (see Appendix G).
4.5.5 Semi-volatile organic compounds (SVOCs)
Analytical techniques for organic compounds follow the general steps of preparing the sample, extracting the compounds of interest from the soil matrix, clean-up, and analysis of the extract. A separate sub-sample is dried and the moisture content determined, and results are corrected to mg/kg dry weight. Semi-volatile organic compounds (SVOCs) are compounds which are extractable into a non-polar solvent (eg, hexane, dichloromethane or supercritical carbon dioxide) and are thermally stable under the conditions of analysis (usually GC-MS, vaporisation at about 320°C, temperature programme from around 50°C to 350°C).
The extraction may be modified by extracting at high pH (the 'base-neutral extractables') and again at low pH (the 'acid extractables'). These extracts may then be combined to give the base-neutral and acid extractables, sometimes referred to as BNA analysis but referred to as SVOC in New Zealand.
Analysis for SVOC is usually undertaken as a screening test for soil investigations. A very large number of compounds fall within this definition, including organochlorine pesticides, other pesticides, polycyclic aromatic hydrocarbons, phthalates, phenols, some hydrocarbons, polychlorinated biphenyls (PCBs) and industrial chemicals.
The SVOC screen method should be calibrated for a number of these compounds (typically 80 or more), which are determined by specific testing requirements, availability of standard mixes and practicality. The calibrated compounds are selected from the extensive lists given in the US EPA method SW-846, which covers the main toxic compounds in solid waste. Non-calibrated compounds, with semi-quantitative data, may also be reported by identifying peaks in the chromatogram by comparison with database library mass spectra (a library search report).
Note that large concentrations of any one compound (or compounds), especially hydrocarbons, will result in higher than normal detection limits being reported.
Analytical methods are also available to analyse specific SVOC compounds, including the organochlorine pesticides, PCB congeners, polycyclic aromatic hydrocarbons and dioxins. The analysis is undertaken on the specific groups if the target parameters are known. The advantages of using specific organic analyses, as opposed to the SVOC screen method, are that the method provides lower detection limits, suffers from less interference, and is more accurate than a screen analysis. The disadvantages of using specific tests are the time and costs for analysis compared to the screening method.
4.5.6 Total petroleum hydrocarbons (TPHs)
The total petroleum hydrocarbon (TPH) test is a non-specific test based on extracting compounds from the soil matrix into an organic solvent, and measuring the concentration of compounds dissolved in the solvent, usually using a gas chromatography flame ionisation detector. It is a very useful test for site investigations, and the shape of the chromatogram can be used to help identify the type of contamination present. The TPH test will determine all compounds that are soluble in the solvent, including petroleum hydrocarbon as well as other organic compounds.
The TPH analysis is a misnomer when applied to sites where hazardous substances are present, as the results could also include many compounds that are not petroleum related, such as naturally occurring compounds (eg, terpenes) or other industrial chemicals (eg, solvents). Hydrocarbons are defined as compounds containing only hydrogen and carbon atoms, but many other types of compound will also be extracted and separated, such as chlorinated (PCBs, organochlorine pesticides) and oxygenated (phthalates, triglycerides, sterols) molecules.
The solvent extraction method can involve a loss of volatiles. The typical carbon chain length extracted by a TPH method is C7-C36. Figure 5 identifies the chain lengths for some typical hydrocarbons. A purge-and-trap GC or headspace method must be specified if volatile fractions (eg, benzene) are required. TPH methods that use purge and trap or headspace analysis will usually start at C6, so volatile hydrocarbons such as n-hexane and benzene will be included in the C6-C9 band. If concentrations of individual compounds are required, such as the aromatics benzene, toluene, ethylbenzene and xylenes (BTEX) or polycyclic aromatic hydrocarbons, they should be requested separately, as the TPH test is non-specific.
TPH methods that do not use purge-and-trap or headspace analysis (eg, the New Zealand oil industry method) will not cover benzene or n-hexane, etc., because the extraction methods can involve loss of volatiles. The limitations of the analytical method should therefore be considered when interpreting the analytical results.
|Product||Minimum Carbon number||Maximum Carbon number|
|Aviation turbine fuel||8||16|
|Gas oil, fuel oil||11||High|
The TPH test results are grouped into a series of bands corresponding to chain length (eg,C7-C9, C10-C14, C15-C36, and a total), and a chromatogram should be supplied with the results for all samples over the detection limit.
4.5.7 Volatile organic compounds (VOCs)
Volatile organic compounds (VOCs) are compounds whose boiling point or sublimation temperature is such that they exist at a significant concentration in the gaseous phase under ambient conditions. Compounds in this group include solvents (oxygenated and chlorinated), hydrocarbons, halogenated hydrocarbons (eg, trihalomethanes) and monocyclic aromatics such as BTEX.
Analysis for VOCs is used as a screening test, and the method should be calibrated for a number of these compounds determined by the specific testing requirements, availability of standard mixes and practicality. The calibrated compounds are usually selected from the extensive lists given in the US EPA methods and will cover the main toxic compounds.
Non-calibrated compounds, with semi-quantitative data, may also be reported by identifying peaks in the chromatogram by comparison with database library mass spectra (a library search report). Note that large concentrations of any one compound (or compounds), especially hydrocarbons, will result in higher than normal detection limits being reported.
Volatile hydrocarbons, except for the monocyclic aromatic hydrocarbons (eg, BTEX and the trimethylbenzenes), are not usually included in VOC analysis.
VOCs in soil samples
The soil sample should be sub-sampled as soon as possible after receipt in the laboratory. This should be carried out while the sample is cool or cold, and is preferably done by using a cork borer to take the sub-sample, which is immediately transferred to a pre-weighed extraction vial containing methanol. The sample weight is determined and the volatile compounds extracted into the methanol using tumbling or ultrasonic extraction.
The methanol extract can then be analysed using headspace or purge-and-trap gas chromatography - mass spectrometry (GC-MS) (see Appendix G).
Benzene, toluene, ethylbenzene and xylenes (BTEX) in soil samples
BTEX are a sub-set of the VOCs, so they are generally analysed using the same GC-MS method, with only the BTEX compounds reported. A simpler technique using GC with a photo-ionisation detector may also be used. The GC-MS method gives the certainty of absolute compound identification and should be used if complicated hydrocarbon matrices are anticipated.
4.5.8 Soil leaching procedure
Analytical tests to determine the leaching characteristics of soils are used to determine the potential for contaminants to mobilise from the soil phase to the water phase. A leaching test is a procedure in which soil contaminants are extracted into a liquid phase, and the resultant extract - the leachate - can be analysed for the parameters of interest. Two types of leaching tests commonly used are:
- toxicity characteristic leaching procedure, which is the US EPA Method 1311 used for evaluating whether a waste material is hazardous or non-hazardous under municipal landfill conditions
- synthetic precipitation leaching procedure, which is the US EPA Method 1312 designed to mimic the effect of acidic rainfall on wastes and soils, and thus the possible leaching of contaminants into ground or surface waters. The extractant fluid used is generally water, but this must be specified, and operationally it is very similar to the toxicity characteristic procedure.
The limitations of the leaching tests are that they cannot be used for all types of chemicals, and are generally used for metals and certain organic parameters. VOCs require a special type of extraction apparatus (a zero headspace extractor) in order to minimise loss of the volatile compounds during the extraction procedure, and the final filtering is done via the extractor under gas pressure. The extract is collected directly into a VOC vial ready for analysis.
Complications associated with the procedure include problems with obtaining representative samples (particularly for waste material), and maintaining the exacting conditions during the extraction, giving rise to poor precision in inter-laboratory comparisons.
4.5.9 Other tests not specifically covered
Other tests may be required, depending on the activities that have been carried out at the site under investigation. The principles of proper method validation and quality control procedures should apply when selecting a suitable laboratory to undertake the analyses.
4.6 Laboratory QA/QC
Laboratories selected for analysis should be accredited and so must be able to demonstrate the procedures and checks in place to ensure accurate testing and reporting of analyses. As a minimum, every batch of analyses should include:
- calibrating standards
- a laboratory 'blank'
- replicates, at an appropriate frequency (usually 1:10 or 1:20) - this is a test of both sample homogeneity and laboratory precision.
Where possible, every batch of analyses should include:
- 'spiked' samples - these are difficult to prepare in such a way that the spike is in the same form as in the native soil, because an analyte added to a soil sample will be adsorbed on the outside of the soil particles, but in the soil itself the analyte may be right throughout the particles
- laboratory QC samples - for soils these are usually well-homogenised samples, which the laboratory has analysed many times to determine mean and standard deviation for the analytes.
It is not possible to prepare stable QC samples for some analytes, such as VOCs.
Only data that have passed the internal laboratory QC tests will be valid for reporting from the laboratory. If the laboratory has a 'QC failure' - such as duplicates not matching within the limits determined in the validation report, contaminated blanks, or poor spike or surrogate recoveries - then the analysis must not be approved for reporting and the whole batch will need to be repeated. Occasionally, the analyst may decide that there is an obvious valid reason for the failure (most commonly matrix interference), and the data would be reported with appropriate comments.
For this reason, any laboratory QC data that are reported to the client should always fall within the acceptable limits as determined during method validation, so a laboratory QC report can prove that the laboratory has, in fact, carried out the QC work.
An example of typical QC data and limits is given in Appendix H.
4.7 Data reporting
The laboratory report should include the following information:
- client company and contact name
- batch identifier or job reference
- date received.
For each sample there should be:
- the sample identity, usually as written by the sampler on the container
- the result for each analyte, including specific definition (eg, 'total copper', not just 'copper'); dry matter percentage should be requested separately if required, because it is not always determined as part of the test method (eg, testing for metals only does not need a dry matter percentage)
- appropriate units, which should be specific (eg, 'mg/kg dry weight' or 'mg/kg as received', not just 'mg/kg'); specific comments should also be included if the sample received is not in the normal form of 'field moist' for soils (eg, 'already dried and sieved', 'freeze-dried', etc.)
- a description of the test method used, including any extraction/digestion procedure, and the source reference if appropriate
- the accreditation status of the method
- the method detection limit or level of quantification LOQ (some method detection limits may not be achievable in certain samples due to matrix interferences, or limited sample size).
Laboratory QA/QC data are used by the laboratory to ensure that results are acceptable for reporting. No results should be reported if the laboratory QA/QC does not meet the set criteria, unless a specific comment is added to the report. The field investigator should request to see the laboratory QA/QC criteria, and data must be made available on request, and provided if required as part of the results package.
An indication of the uncertainty of measurement for each test result must be made available on request, and be provided if required as part of the results package.
4.8 Uncertainty of measurement
For every numerical result reported by a laboratory there will be an associated uncertainty.
NZS/ISO/IEC 17025-1999 requires that a laboratory estimate the uncertainty of measurement in such a way that "the estimate is reasonable, must not give a wrong impression of the measurement and must take into account all sources of that uncertainty". This uncertainty is due to a large number of factors including, but not restricted to:
- sub-sampling variations
- incomplete homogeneity of the sample
- concentration of the analytes (instrument noise is relatively higher at low contaminant concentrations)
- purity of calibrating standards (there is no such thing as 100% pure)
- inherent uncertainty in balances, volumetric glassware, etc.
- uncertainty in visual estimations (eg, reading the meniscus in a burette)
- variations in extractability of analytes
- variations in instrument response
- uncertainty in the final reading (eg, absorbance using a spectrophotometer, 'counts' in a GC-MS).
Precision (ie, repeatability) is only one component of the overall uncertainty in a measurement.
All analytical results have an uncertainty. This may vary from a few percent for simple one- or two-step procedures, such as a weighing or titration, to 100% (or more) for a complex organic analytical analysis involving extraction, concentration, clean-up, derivatisation, concentration and chromatographic determination at close to detection limits.
Knowledge of the measurement uncertainty is essential for interpreting the results against the DQOs.
Back to footnote reference 5 International Accreditation New Zealand.
Back to footnote reference 6 National Association of Testing Authorities, Australia.
Back to footnote reference 7 New Zealand Standard / International Organisation for Standardisation / International Electrotechnical Commission.
Back to footnote reference 8 Benzene, toluene, ethylbenzene and xylene(s).
Back to footnote reference 9 American Society for Testing and Materials.
Back to footnote reference 10 American Public Health Association, American Water Works Association, Water Environment Federation.