Organic Lead

TML, TEL, and mixed methyl-ethyl analogs have been used extensively as anti-knock additives in gasoline. Currently their use is restricted in most countries worldwide due to the increasing accumulation of lead (Pb) along roadsides and its toxicity.

In California, total organic lead is determined by atomic absorption after extracting the sample with xylene (HML Method 939-M). The organic solvent extracts the organic lead (essentially TML and TEL) and separates it from forms of inorganic lead.

Tetramethyllead

(TML, Me₄Pb)

Tetraethyllead

(TEL, Et₄Pb)

CA HML Method 939-M

ICP-MS Analysis

In 1989, WCAS developed a more modern approach using ICP-MS. While it is possible to analyze organic solvents by ICP and ICP-MS, changing back and forth between organic solvents and aqueous acids is time consuming. Residual carbon from the organic solvents forms ArC in the argon (Ar) plasma which is an interference for chromium (Cr). So while it is necessary to extract a sample with an organic solvent to remove and separate the organic lead from inorganic forms, it is also desirable to convert the extract into form that is compatible with aqueous acids, i.e. to inorganic Pb soluble in dilute nitric acid.

Several attempts were made with various organic solvents to find one that could easily digest or dissolve in nitric acid. Most solvents react violently with hot nitric acid during the digestion procedure. Pentane was found to

evaporate rapidly as the nitric acid is heated before the oxidation process began. Therefore the procedure that was developed uses pentane as the extraction solvent, followed by digestion of a portion of the extract in concentrated nitric acid, and finally dilution of the digestate with water. In this way converting the ICP-MS for organic solvents and introducing contamination is avoided.

While WCAS had been accredited by CA ELAP for Organic Lead for many years, ELAP eventually adopted more stringent requirements for approval using the results of round robin test programs conducted by RTC. Our ICPMS procedure was specifically approved as an alternative to the HML method by CA ELAP in 2004 based on successful completion of round robin of testing. The detection limit for organic Pb is ~0.02 ug/g (ppm) in soil.

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Speciation of Organic Lead:
GC-ICPMS Analysis

With our new Agilent GC-ICPMS, it's possible to analyze samples for the specific organic lead compounds, TML and TEL without interferences from hydrocarbons. With the ICPMS monitoring only the Pb isotopes, hydrocarbons and organic solvents are not detected and do not interfere.

If you think that your samples are biased high due to interferences from hydrocarbons or anything else, GC-ICPMS should be your next step.

The chromatogram below shows a 100 ppb standard of TML (3.39 min) and TEL (5.42 min) in hexane. Traces of the mixed methyl-ethyl analogs can also be seen.

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ICP-MS Analysis

Speciation of Organic Lead: GC-ICPMS Analysis

Speciation of Organic Lead:
GC-ICPMS Analysis