Contaminant concentrations in biota can be measured and reported on either a wet, dry or lipid weight basis, but must be converted to a common basis for statistical analysis.
The following formulae are used to convert a concentration measurement between bases
\(C_w = C_d \frac {\text{% dry-weight}} {100}\)
\(C_w = C_l \frac {\text{% lipid-weight}} {100}\)
\(C_d = C_l \frac {\text{% lipid-weight}} {\text{% dry-weight}}\)
where \(C_w\), \(C_d\) and \(C_l\) are the concentrations on a wet, dry and lipid weight basis and % dry-weight and % lipid-weight are the percentages of the sample that are dry and lipid respectively. For example, if \(C_d\) = 10 mg kg\(^{-1}\) dw and if 20% of the sample is dry, then \(C_w\) = 2 mg kg\(^{-1}\) ww.
To see how these formulae come about, consider a sample that contains an amount (burden) \(B\) of a contaminant. Let \(W_w\) and \(W_d\) be the wet and dry weight of the sample. Assuming that all of the contaminant is in the dry matter, then:
\(C_w = \frac B {W_w}\)
\(C_d = \frac B {W_d}\)
so
\(C_w \times W_w = B = C_d \times W_d\)
giving
\(C_w = C_d \frac {W_d} {W_w} = C_d \frac {\text{% dry-weight}} {100}\)
Concentration measurements are converted using the % dry-weight or % lipid-weight measured in the same sample. If conversion is necessary and the % dry-weight or % lipid-weight are not reported, then the concentration measurement is discarded. Some Assessment Concentrations also need to be converted to the basis used for statistical analysis, in which case estimates are used of the typical % dry-weight and % lipid-weight for the species and tissue combination of the time series being assessed.
Tributyl tin (TBT) derivatives are organo-metallic compounds with the general formula (n-C4H9)3Sn-X. The TBT cation consists of a tin (Sn) atom connected to three butyl groups (CH3CH2CH2CH2-). The TBT cation will be linked to an anion (X in the general formula), such as Cl, F, OH or CO3.
Degradation of TBT usually leads to dibutyl tin (DBT) and then monobuytyl tin (MBT), i.e. by loss of the butyl groups with a steep decrease in toxicity followed by each debutylation.
Sn has an atomic weight of 118.71 and each butyl group has a molecular weight of 57.115, so the whole TBT cation weighs 118.71 + 3 × 57.115 = 290.06. The CAS number for the TBT cation is 36643-28-4. The structure of tributyl tin can be viewed here.
There are two TBT-related ICES parameter codes:
| Code | Description | Recommended unit |
|---|---|---|
| TBSN+ | tributyl tin cation | \(\mu\)g kg\(^{-1}\) (or ng kg\(^{-1}\) etc.) |
| TBTIN | tributyl tin (TBT) - group/mixture - legacy code | \(\mu\)g kg\(^{-1}\) (or ng kg\(^{-1}\) etc.) |
TBTIN is a ‘group’ parameter (oxides, chlorides, hydroxides) and was the original parameter used for reporting (established around 1990). However, as a ‘group’ parameter, it was not clear what was actually being reported. The Marine Chemistry Working Group (MCWG) therefore recommended that all organotins be reported as the cation concentration. For TBT this would be using parameter TBSN+. Other ions (DBSN+, DOSN+, DPSN+ , MBSN+, MOSN+, MPSN+, TCTIN+, TPSN+) have also been added to PARAM, so all organotins can be submitted as the cation concentration.
Following the MCWG advice, MIME recommends that tributyl tin data should be submitted as the TBT cation concentration using code TBSN+ and units of \(\mu\)g kg-1 (or ng kg-1 etc.). Similarly, dibutyl tin and monobuytyl tin should be reported as the DBT and MBT cation concentrations using codes DBSN+ and MBSN+ and units of \(\mu\)g kg-1.
The units \(\mu\)g Sn kg-1 (or ng Sn kg-1 etc.), which have sometimes been used to report TBT in the past, are no longer allowed.
If reporting laboratories are provided with TBT data in the form of tin ion concentrations, the data can be converted to TBT cation concentrations by multiplying by 2.44 (since the molecular weight of a TBT-cation (290.06) is 2.44 times that of a tin ion (molecular weight = 118.71)). Other conversion factors include 1.96 (DBT), 1.48 (MBT), 2.95 (TPT), 2.30 (DPT) and 1.65 (MPT).