Interpretation of qualitative results
As can be seen from Table 4.3 some volatile substances give rise to volatile metabolites which may themselves be substances associated with volatile substance abuse or accidental exposure, and this possibility must be considered when a volatile substance is detected. For example, acetone is a metabolite of propan-2-ol and ethanol is a metabolite of ethylacetate. Detection of a volatile compound in blood does not always indicate VSA or occupational/environmental exposure to solvent vapour. Some medical conditions can give rise to volatile substances in blood. Acetone and some of its homologues may occur in high concentrations in ketotic patients. Large amounts of acetone and butanone may also occur in blood and urine of children with acetoacetylcoenzyme A thiolase deficiency for example, and may indicate the diagnosis.
Possible contamination via sampling procedures or clinical treatments (as discussed under sample collection, transport and storage) should be considered when evaluating data, although it should be recognised that such problems are best avoided by the use of sampling procedures and equipment designed to eliminate the possibility of contamination.
It is well known that ethanol may be both produced and metabolised by microbial action in biological specimens as may other low molecular-weight alcohols such as propanols and butanols, so this possibility must be evaluated if these substances are detected. ‘Congener alcohols’, such as methanol, propan-1-ol, butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2 methylbutan-1-ol, and 3-methylbutan-1-ol (and ketone metabolites of secondary alcohols) may arise from the ingestion of alcoholic drinks (Bonte 2000). Butyraldehyde, dimethyl disulfide, is valeraldehyde and valeraldehyde may arise from putrefaction. Small amounts of hexanal may arise from degradation of fatty acids in blood on long-term storage, evenat -5to -20C.
This compound may co-elute with toluene on some GC systems and hence give rise to a false positive for toluene, particularly if GC-FID is use for analysis rather than GC-MS.
The likelihood of detecting exposure to volatile substances by headspace GC of blood is influenced by the dose and duration of expo sure, the time of sampling in relation to the time elapsed since exposure, and the precautions taken in the collection and storage of the specimen. In a suspected VSA- or anaesthetic-related fatality, analysis of tissues (especially fatty tissues such as brain) may prove useful since high concentrations of volatile compounds may be present even if very little is detectable in blood.
Analysis of metabolites in urine may extend the time after which exposure may be detected but, of the compounds commonly abused, only toluene, the xylenes and some chlorinated solvents, notably trichloroethylene, have suit able metabolites (Table 4.3). The alkyl nitrites that can be abused by inhalation (isobutyl nitrite, isopentyl nitrite) are extremely unstable and break down rapidly in vivo to the corresponding alcohols and usually also contain other isomers (butyl nitrite, pentyl nitrite). Any products submitted for analysis usually also contain the corresponding alcohols as well as the nitrites. The profound methaemoglobinaemia that often arises after ingestion of these compounds can be detected easily.
