Independent Consultants in Environmental and Forensic Chemistry
Volume 5, Issue 1, Spring 2002
Background - What is it?
One of the most ignored environmental samples is the background sample. What exactly is a background sample? It is not a quality control sample. It is not a blank sample such as a field blank or method blank. When the objective in an environmental program is to determine if man=s activities have increased a hazardous substance(s) in excess of what occurs naturally at the site, then background samples become important.
For example, chromium is an EPA listed hazardous substance, and stainless steel is 17 percent chromium. A superfund site in central New York State contained a pile of scrap stainless steel. An expert consultant/witness explained to the court that stainless steel will corrode, and, thus, chromium from the stainless steel will contaminate the soil at the site. All soils contain some amount of chromium. The contractors for the EPA did not have a single valid background result for chromium in or near this community. How do you argue that the soil at the site is no different from background unless there is some background data? The U.S. Geological Survey (USGS) and a few states such as New Jersey have gathered background data for metals/elements in soils. These can be used for background ranges for elements in soil. However, the cross-examiner will ask if these data are relevant to this site.
At a pesticide wholesale and spraying facility, sampling, analyzing, and finding man-made pesticides is easy. However, if the facility is in the middle of a farm belt, regional contamination from farm pesticide use may exist. Where does the facility=s contamination stop and the regional contamination begin? Background samples would help define the distribution.
Sometimes one or two background samples are taken. These results are easily dismissed as not being statistically valid and not representative of the background concentrations of the analyte at the site. How many background samples does one need? In order to be statistically valid and defensible in court, one should have as many background samples as possible. The exact number will depend on the desired confidence level of the estimated range of concentrations. If only two background samples are collected, the interval for a 95% confidence interval would be almost thirteen times the calculated standard deviation for the two measurements and may be too wide to differentiate background from site samples. This probably means that you really do not know what your background concentrations are. However, most clients faced with a background problem do not want to spend money on a large number of background samples. This is a major mistake.
Background samples should be incorporated into all initial sampling and analysis plans. They can be obtained at a later time, but, in those cases, the site conditions must not change. Risk assessments, remediations, allocations, and litigation outcomes can and do depend on data determining whether contamination at an area of concern is greater than background. Background samples are always important and should be a routine consideration in every site investigation.
How Important Are Reference Materials?
When any chemical analyses are conducted, qualitative and/or quantitative data are expected to be produced. The first concern is whether the method of analysis is sensitive to the compound(s) of concern, i.e., can the compound be observed by the method? Second, can the amount of the compound in the sample be measured?
A simple manner to determine if a response can be obtained for the compound of interest is to challenge the method using an authentic sample of the compound (a reference material). To verify that the authentic sample is, in fact, the compound of interest, the material should be obtained from a reliable source and should be traceable to a source such as the National Institutes for Science and Technology (NIST). While this concept may seem too trivial for discussion to some people, many analyses have been discounted due to the lack of any verification that the method was sensitive to the compound of interest. Also, depending on the analysis being conducted, the response obtained from the instrument may not be unique to the compound of interest. In gas chromatographic (GC) analyses, various compounds are identified by their retention times. However, if an unreliable and/or impure reference material is used, the largest peak in the chromatogram is often labeled as the compound of interest when, in fact, it may not be.
Measurements obtained from an analytical instrument are strictly responses to the amount of material present in the sample. Without a reference point or points, the quantity of compound in the sample cannot be determined. In order to determine the dependency of the response on the quantity of material present, the instrument must be calibrated with known amounts of an authentic reference material. Once again, the quantity of compound present in the laboratory=s reference material must be traceable to a reliable source such as NIST. Any errors in the purity of the reference material will be transferred to diluted calibration standards and then to the estimate of the concentration in a field sample. Such an error coupled with other inherent errors and imprecision in the analytical method may be the difference between complying with or exceeding a regulatory standard.
Another service provided by NIST is the collection, preparation, and analyses of contaminated samples in various matrices. These samples are available to laboratories as standard reference materials (SRMs) for specified contaminants. The certified analytical results, along with an error term, accompanying the SRMs are usually provided with more significant figures than most people care to know. A laboratory should purchase an SRM and analyze it as their laboratory control sample (LCS) along with the field samples. An LCS is a quality control sample containing a known quantity of an analyte of concern in a matrix similar to the field samples being analyzed and should be as independent from the laboratory as possible. If a laboratory creates an LCS from the same reference material as the calibration standards, any errors in the reference material or in the preparation of solutions may still be consistent with the calibration standards and may provide a false sense of security in the results. Obtaining results from an SRM within the NIST specifications would serve as another check on the accuracy of the results and would add considerable credibility to the results obtained for the field samples.
The importance of reliable reference materials to a laboratory cannot be overemphasized. They form the basis for identification and quantification of contaminants in environmental samples. Without them, analytical results can be, and are, highly questionable. How important are reference materials? How important are your analyses?
Did Your Data Die?
Samples have been collected and have been sent to the laboratory for analyses. The Areduced@ deliverables data package which includes the sample results and summary forms of quality control results has been received. Years later, although not originally intended, the project is now in litigation, and the data must be defended. Is everything available to substantiate the analytical results? No, but this is not considered a problem. A simple telephone call to the laboratory can be made to request all of the documentation supporting these data. However, in the meantime, the laboratory that performed the analyses has gone out of business. The data have suffered from Adata death.@
This scenario occurs more often than one may think. As laboratories are bought and sold, or simply disappear, data and supporting documentation from past analyses become almost impossible to retrieve. Data are often lost during the changes in ownership or control. If data can be found, laboratory owners and managers are reluctant to provide the time and personnel to recover old data, especially if the analyses were done by someone else. Attempting to recover data can be costly and time consuming, and the best attempts may be futile. The data have died.
A change in laboratory ownership or control is not the only cause of irretrievable data. The amount of information generated for each analysis can be enormous, and most laboratories cannot retain large amounts of information indefinitely. Data are usually retained and readily available for a stipulated time period and then placed in the archives and eventually purged. Once data have been placed in the archives, they may not be easy to retrieve. Therefore, at the onset of the project, the laboratory=s policy on storing data (i.e., length of time, types of data, format, etc.) should be known.
Another common cause of data death comes about with changes in software, storage media, and storage equipment. Changes in these facets may prevent access to older files. Who still has a magnetic or streamer tape drive? Does anyone remember five and a quarter floppies? What changes will the future bring?
What is the best defense against data death? Request a full data package before the samples arrive at the laboratory. The data package should include all of the sample raw data as well as the supporting documentation (i.e., calibrations, quality control samples, extraction logs, chain of custody records, etc.). Having all of this information may seem like a waste of storage space now, but it could save your case, or a part of your anatomy, in the future.
What are Surrogate Analyses?
What is a surrogate? A surrogate is a substance with chemical and/or physical properties similar to the target analytes being determined by an analytical method. The surrogate is not, and cannot be, an environmental contaminant.
The surrogate substance is added to calibration standards, quality control samples, and field samples and is analyzed using the same techniques and methods used to isolate, identify, and quantify the target analytes. The amount of surrogate determined by the method is expressed in terms of percent recovery which is 100 times the amount found in the sample divided by the amount of the surrogate added to the sample. Surrogate recoveries give the data user an estimate of how well the method performed for the target analytes and are especially important when no target analytes are detected by the analysis. If the percent recovery of the surrogate is zero or very low, one can conclude that the method failed to analyze the target analytes. A good percent recovery of the surrogates (usually between 80 and 120%) would indicate that the method was conducted satisfactorily, and target analytes were not present in the sample.
To be a useful quality control (QC) parameter, the path of the surrogates through the analytical procedures must mimic, as closely as possible, the path of the target analytes in the sample. Therefore, the surrogate must be added to the sample as early as possible in the analytical scheme and be given the same opportunity as the target analytes to be exposed to the remaining procedures in the analytical method. If surrogates are added at a later time in the analytical method, they would not share the same experiences as the target analytes.
Surrogates are generally added to field samples just prior to sample extraction. When this is done with reasonable care, surrogate recoveries tend to be within acceptable limits, and the analytical method appears to be adequate for conducting the intended analyses. However, these results may not truly represent the ability of the laboratory or analytical method to extract and measure the target analytes in the sample.
For example, in method 8081 for the analysis of polychlorinated biphenyls (PCBs), two surrogate compounds, tetrachloro-m-xylene and decachlorobiphenyl, are used. Most laboratories add these compounds to a soil sample immediately prior to the extraction step of the analysis. However, prior to the extraction step, the sample is weighed; water is removed by rigorous stirring with anhydrous sodium sulfate; and the dried sample is transferred to the extraction vessel/equipment. During these steps, ample opportunity is available for the loss of target analytes. However, these losses will not be experienced by the surrogates since the surrogates have not yet been added. The surrogates need to be added immediately after the weighing procedure is complete and prior to any further handling of the sample.
James S. Smith