Field and Continuous Measurements

Making Sure We Make the Best Use of Them

The specification and use of online/field instruments and test kits can be a complicated business. Making the wrong choice could prove expensive.

Tim White explores the range of International Standards and related guidance which can help when specifying equipment and validating the data against required performance set by standard methods advocated by regulatory bodies.

Introduction

Many facets of modern economies require rigid compliance with regulatory or self imposed real time data norms. None is more rigorously scrutinised and then promptly overlooked than those regimes designed to control environmental impact. Data collection is often swiftly implemented and because it is acquired as a constant, and often reassuring stream, the means of collection are often forgotten, or ignored, until the data stops arriving, or proves to be inappropriate.

The demand for the information, usually at significant cost, often drives those charged with procuring it to use field test equipment or on-line monitoring to reduce expenditure burdens. The cost and time savings achieved can be rapidly lost or become superfluous if best practice is not applied in the selection, use of the equipment and the interpretation of the data determined with it.

The principles of good practice discussed here apply equally to the use of data from field instruments and test kits. Such equipment is used to capture information where samples are time sensitive, and those applications where instruments are fitted in situ for collection of continuous data.

The value of such data is not being underestimated but sometimes the key to its use and the purpose to which it is put is. From an economic and social perspective the most important uses are those that provide assurance of compliance where the consequences of failure are punitive; and those required for the maintenance of product quality such as the production of drinking water.

In both of these cases there are numerous circumstances where regulatory or health based standards need to be met. In all cases where regulators are involved the issue of compliance will inevitably, unless negotiated otherwise, require that their data demonstrates compliance. This data is usually laboratory derived and generated using established methods of analysis which have proven performance characteristics. In all cases of dispute the regulator will inevitably claim supremacy of the validity of its data. Challenging this is costly and ultimately demonstrates a failure in the relationship required in a modern regulated environment.

In order to avoid disputes and misinterpretation of such information some basic principles need to be applied.

Is the use of field equipment imperative?

Before the decision to use field testing or online equipment is made the potential to obtain data whose integrity is equally assured by the use of laboratory methods should be formally ruled out in order to make the choice of technique more robust.

The need to reject conventional methods may be a simple choice of safety or logistical grounds. For example sampling could be in an inhospitable location not conducive to the collection and transporting of large numbers of samples in pre- prepared containers. For in-line sensors the choice is often very simple in that there is a need for control data available at a rate faster than even a dedicated laboratory or bench test facility can provide it. The decision making process can be assisted with the use of guidance to help the data user and analyst alike. The first issue to examine is the preservation requirements of the parameter of interest in terms of maintaining the sampled condition in a representative manner until the time of measurement.

The best source of general guidance in this respect for water samples is:

• ISO 5667-3:2003 , Water quality. Sampling. Guidance on the preservation and handling of water samples

For sludges and sediments the guidance comes in the form of:

• ISO 5667-15:1999 , Water quality. Sampling. Guidance on preservation and handling of sludge and sediment samples

Sludges are an important consideration in terms of environmental monitoring needs but they are not normally associated with use of field test equipment specific to the media. However, sludges do present certain challenges to field and online equipment in terms of fouling of probes and localised effects which can bias data. The collection of samples for laboratory analysis may therefore on many occasions be the preferred option

These guidance documents do not offer a panacea of choices but they do represent a good consolidation of accepted practice. Of importance in any specific situation there is nearly always advice on preservation and handling of samples provided with authoritative published methods of analysis used by the laboratories. The choice of preservation technique for any laboratory based method should always be directed by those analysing the sample.

Ensuring the relevance of data to be collected

Many modern field instruments are now the primary accepted method of determination, for example: dissolved oxygen, pH, and electrical conductivity are often required to be measured at the time of sampling. In these cases, as a matter of routine, the imperative for acceptance is to be able to demonstrate adequate calibration records for the equipment used. However, as the growth of the knowledge base and acceptance that real differences need to be understood, the use of such equipment should be proved as a viable equivalent determination.

This can be done by replicate analysis testing using a set of samples for which the value is determined in a laboratory as well as by the field equipment or on line instrumentation. Often manufacturers may be able to provide illustrative examples of such equivalence testing against a range of established laboratory methods. However, for the most robust defense of any data, site and method specific information will be required.

This type of validation will almost certainly be required if third party accreditation of the field equipment or on-line instrumentation is required.

There are established statistical techniques for making assessments of the differences between paired sets of data. In this case they can be applied to that which is measured by the field equipment being tested, compared with determination by laboratory instruments. It is not the place of this article to discuss the merits of the various statistical techniques detailed and formal international guidance can be found in:

• ISO 3301-1975 , Guide to statistical interpretation of data. Comparison of two means in the case of paired observations

Once such a comparison has been completed and equivalence has been demonstrated, there is a need for continued quality control of the field measurements. This is not an easy task as the essence of field measurement is avoiding the pitfalls of the instability of the parameters of interest in the samples in transit. The production and storage of viable quality control samples is therefore problematic. In order to assess the stability of the equipment in use it is therefore necessary to keep rigorous records of compliance with any instrument set up procedures in the field.

In addition, paired testing of results can be carried out by taking separate samples and measuring the test parameter using the field equipment and the conventional technique under controlled conditions in the laboratory. Here again there are established procedures for determining the significance of such paired data. Although not immediately obvious in its application, useful advice is contained in guidance on sampling quality control:

• ISO 5667-14:1998 , Water quality. Sampling. Guidance on quality assurance of environmental water sampling and handling

The choice of equipment

In many instances there are no relevant field instruments on the market. In such circumstances many practitioners turn to the numerous field kits available. These kits are immensely valuable in circumstances where transport difficulties and a need for real time data are important for the monitoring task in hand. There is still a need to develop a procedure whereby the data generated is equivalence tested against a laboratory method. This is particularly important if reliance is being placed on the data for compliance monitoring. Guidance on the choice and deployment of such test kits is available in British Standard:

• BS 1427:1993, Guide to field and on-site test methods for the analysis of waters

For those parameters where instrumentation is available there is firm direction available in the form of the performance standard:

• ISO 15839:2003 , Water quality. On-line sensors/analysing equipment for water. Specifications and performance tests

This guidance does not cover the selection and specification in absolute terms for a finite list of parameters. However, it does set out procedures that reinforce the design of performance criteria that allow specification and purchase to become more robust. Thus the user of the data can be more confident of the fitness for the purpose for which it was gathered.

Use of the monitoring data

Irrespective of the need for monitoring there will be an interest in, or an imperative requirement, to evaluate time series information against data targets. In order to make the best possible judgments about the meaning of the data, application of the basic statistical principles set out in the standards listed below is recommended:

• BS 2846-2:1981, ISO 2602- 1980 Guide to statistical interpretation of data. Estimation of the mean: confidence interval
• BS 2846- 4:1976, ISO 2854-1976 Guide to statistical interpretation of data. Techniques of estimation and tests relating to means and variances
• BS 2846-5:1977, ISO 3494-1976 Guide to statistical interpretation of data. Power of tests to means and variances
• BS 2846-7:1997, ISO 5479:1997 Guide to statistical interpretation of data. Tests for departure from normality
• BS 2846-8:2001, ISO 16269-7:2001 Guide to statistical interpretation of data. Median. Estimation and confidence intervals

Conclusion

The current market proliferation of time saving and logistically practical equipment can produce robust data for compliance monitoring provided the performance of the instruments or kits is properly understood. The use of recognised statistical and operational quality assurance protocols provides the most effective method for making judgments about the equivalence of data. The equivalence of data is only of relevance if the test species determined by the field or on-line equipment is the same. The purchase and use of this equipment should not be entered into lightly or in an unplanned manner.

The old adage of ‘look before you leap’ has a lot going for it in terms of the application of these technologies. It is important to focus on the fact that just because the instrument or test kit is ‘low cost’ the data, and the use to which it is put, still has the same monitory value in terms of the protection from punitive action or environmental benefit achieved. It is imperative therefore that method performance is understood.

Published: 01st Mar 2005 in AWE International