Common Laboratory Difficulties with TBT Analysis
The following text discusses problems encountered during analysis for tributyltin (TBT). It is specific to analytical procedures that use extraction with an organic solvent (i.e., hexane, toluene, or methylene chloride), derivatization with a Grignard reagent, and gas chromatograph detection using either flame photometric or mass spectrometric detection (See Figure 1). Similar difficulties may be encountered using other procedures (i.e., hydride derivatization/graphite furnace), but these issues relate specifically to the procedure mentioned above, which is used on the U.S. EPA-required environmental monitoring program. Figure 1 presents a schematic of a typical analytical procedure.
Achieving Low Parts Per Trillion Detection Limits
The first area that causes difficulty is that the level of concern in water samples is very low; the U.S. EPA chronic marine water quality criterion for TBT is 10 ng/L and some State standards are even lower. This requires that a large volume (at least two liters is required) water sample be concentrated down to ~100 �L (a concentration factor of 20,000) and causes any contaminant or interferant to be magnified by a similar factor.
Problems with the analytical method are generally of two types: contamination and interferences.
Contamination with butyltins generally comes from one of two sources: 1) incompletely cleaned glassware, or 2) reagents containing low levels of the target analytes. Common sources of contamination are listed below:
To give an example of how contamination can occur, one laboratory noted contamination of water samples because a staff member was painting a wooden deck with a preservative paint containing butyltins. Sufficient butyltins were being transferred on feet or clothing to contaminate water samples.
Incompletely cleaned glassware can also cause contamination of samples. Typical cleaning procedures for glassware have been found to be insufficient to remove residual traces of butyltins. Baking glassware in a muffle furnace, or similar oven, can alleviate carryover problems. Prior to baking, glassware is cleaned using a detergent, and rinsed with an organic solvent.
Interferences generally occur with sediment samples due to the presence of sulfur compounds that co-elute with the butyltins or the internal standards. Methods, such as USEPA Method 3660 (1986), that utilize activated copper to bind and remove sulfur compounds are effective, but unfortunately can also remove quantities of the internal standards and possibly the target analytes. Careful dilution of samples to remove/dilute the interference to acceptable levels or selection of internal standards that do not co-elute with the interference are potential solutions. However, even with a careful selection of procedures, it may be impossible to analyze some sediment samples.
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