Blank contamination

Sudden shifts to lower or higher values in the T plot are generally operator related. Different operators may use slightly different procedures that lead to bias. New reagent lots may introduce systematic error through blank contamination or different potency. Sudden undocumented environmental events may... 

The current study improves PFC methods by reducing the above-mentioned sources of blank contamination. 

Most often, trip blank contamination originates in the laboratory, either from common airborne laboratory contaminants (methylene chloride, acetone) or from laboratory water containing VOCs, typically methylene chloride, acetone, and toluene or water disinfection byproducts (chloroform, dichlorobromomethane, chlorodibromomethane, bromoform). Rare, but well documented sources of trip blank and associated field samples contamination are insufficiently clean sample... 

Conversely, on occasion, invalid data may still be used for project decisions. In DQA Step 4, the chemist made decisions related to the effects of trip and equipment blank contamination on sample data by applying the logic demonstrated in Example 5.8. In DQA Step 6, this decision logic will be extended further to compare the contaminants of concern concentrations in the samples to the action levels as demonstrated in Example 5.12 on the next page. 

Field blanks are samples that go through all of the procedures as actual field samples but are designed to have none of the target analyte present, so that if the analyte is found in the blank, contamination may have occurred at some step on the way. Spikes are samples into which known amounts of the target analyte have been introduced if, upon analysis, the spikes do not exhibit the expected concentrations within statistical norms, the integrity of the field samples may also be in question. 

Review of 10% samples for all the listed elements review holding times, blank contamination, precision, accuracy, error determination, detection limits, and confirmed identification for the remaining 90%. 

Artefacts may arise from sample and blank contamination, reactions and breakdown products within the chromatographic system. Examples of typical artefacts include acetal, silicone compounds, squalene, phthalate esters and other plasticisers and anti-oxidants. 

In order to limit filtration blank contamination, a clean PFA filter unit was developed to separate the oil fi-actions by precipitation of asphaltenes (heavy fi"action) in n-Heptane. Membrane filters of size 0.45 pm in regenerated cellulose were used with the filter unit. One gram of crude oil was dissolved in 50 ml of n-Heptane (in a 250 ml PFA balloon). The solution was then heated for 20 min at 60°C under magnetic stirring and finally filtered under vacuum. Asphaltenes were accumulated on the filter and stocked in 50 ml polypropylene tubes at room temperature in the dark for further analysis. Maltenes, which are soluble in n-Heptane, were collected in a polyethylene Erlenmeyer and evaporated under pure nitrogen flow in 50 ml polypropylene tubes. 

United States Environmental Protection Agency, Eilter blank contamination in asbestos abatement monitoring procedures, in Proceedings of a Peer Review Workshop, Cincinnati, OH, 1986. Webber, J.S., Carhart, L.J., and Czuhanich, A.C., Performance of membrane filters used for TEM analysis of asbestos, /. Occup. Environ. Hyg., 4(9), 2007 (in press). 

The types and amount of quality control used during the field component of a study can vary depending on the data requirements of the stu. At a minimum, field blanks should be used to identify any contamination either through direct contact or airborne exposure of the sample. Other quality control samples to be considered include equipment blanks, if the same sampling equipment is repetitively used, trip blanks (contaminant-free water samples which accompany the field collected samples from the field to the laboratory but are not exposed to the air), and positive control... 

Experience at NBS in the certification of high-purity Standard Reference Materials has illustrated the point that on most real samples, trace element analysis below the ppm level gives values of questionable accuracy. All trace methods of analysis except activation analysis must deal realistically and directly with problems of blank contamination, i.e., contamination introduced into the sample through preliminary handling and/or subsequent processing in the analytical procedure. 

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