Tentatively identified compounds

Petty, J.D. Gale,R.W. Huckins, J.N. Cranor, W.L. A1 varez, D. A. Clark, R.C. 2000,Development and Application of Techniques for Sampling Bioavailable Airborne Contaminants-Tentatively Identified Compounds by Gas Chromatography/Mass Spectrometry. USGS Colirmbia Environmental Research Center Colirmbia, MO Unpublished report to U.S. EPA National Exposure Assessment Laboratory Las Vegas, NV. 

Chemical/Physical. Ozonolysis of acrylonitrile in the liquid phase yielded formaldehyde and the tentatively identified compound glyoxal, an epoxide of acrylonitrile and acetamide. The reported rate constant for the reaction of acrylonitrile and ozone in the gas phase is 1.38 x lO cm moFsec (Munshi et al., 1989). In the gas phase, cyanoethylene oxide was reported as an ozonolysis product... 

Soil A reversible equilibrium is quickly established when aniline covalently bonds with humates in soils forming imine linkages. These quinoidal structures may oxidize to give nitrogen-substituted quinoid rings. The average second-order rate constant for this reaction in a pH 7 buffer at 30 °C is 9.47 x 10 L/g-h (Parris, 1980). In sterile soil, aniline partially degraded to azobenzene, phenazine, formanilide, and acetanilide and the tentatively identified compounds nitrobenzene and jD-benzoquinone (Pillai et ah, 1982). 

Soil Under aerobic conditions, indigenous microbes in contaminated soil produced pentachlorocyclohexane. However, under methanogenic conditions, a-BHC was converted to chlorobenzene, 3,5-dichlorophenol, and the tentatively identified compound 2,4,5-trichlorophenol (Bachmann et al., 1988). Manonmani et al. (2000) isolated a microbial consortium from sewage and soil that could completely mineralize a-BHC in 14 d at 30 °C. The acclimated consortium could degrade up to 100 mg/L of a-BHC within 72 h at a degradation rate of 58 mg/L-day. 

Biological. In anoxic groundwater near Bemidji, MI, 1,2,4-trimethylbenzene anaerobically biodegraded to the intermediate 3,4-dimethylbenzoic acid and the tentatively identified compounds 2,4- and/or 2,5-dimethylbenzoic acid (Cozzarelli et al, 1990). 

Figure 6.15 presents the LC-UV chromatogram of the hydrolysis products from the first step of simulated waste water treatment of Remazol Black 5 (RB5), a commercially important textile dye, while Figure 6.16 shows a series of stop-flow LC-NMR spectra acquired in an LC-NMR-MS run. The NMR and MS data of the tentatively identified compounds are shown in Tables 6.6 and 6.7, respectively. These are only by- or degradation products which elute earlier than the hydrolysed Remazol Black. Peaks which elute later consist of coeluting dye components which have not yet been identified. 

The capability to identify compounds outside the target analyte list is one of the greatest advantages offered by GC/MS. Searching the EPA/NIST spectral library, the computer finds the best matching library spectra for an unknown compound and tentatively identifies it using the best fitting spectrum. The retention time information is not available for this compound as it is not part of the calibration standard, hence only tentative identification. These tentatively identified compounds (TICs) can be also quantified to provide an estimated concentration value. 

Request tentative identification of unknown individual compound peaks with GC/MS, particularly when they are sizable or recurring. (Exceptions are tentatively identified compounds in petroleum fuels, which are fuel constituents, such as aliphatic and aromatic hydrocarbons.)... 

Tentatively identified compounds for each sample, if requested S S ... 

Internal standard areas and acceptance limits Raw data for each sample, blank, spike, duplicate, and standard (quantitation reports, reconstructed ion chromatograms) Raw and background subtracted mass spectra for each target analyte found Tentatively identified compounds mass spectra with library spectra of 5 best-fit matches Sample preparation bench sheets Gel permeation chromatography clean-up logs S S S S S S S ... 

Qualitative analysis may be extended to provide semi-quantitative determination of concentrations. This may be carried out by addition of internal standards at known concentrations. Quantitation for each analyte detected is achieved by ratio of the total ion current for the analyte to that of the closest eluting internal standard. However, as the response of different compounds to mass selective detectors can vary considerably, this approach can lead to significant errors. A complementary approach that has found favour and can reduce the uncertainty involves the analysis of standards of tentatively identified compounds, in order to calculate response factors to the nearest internal standard. Standards are usually analysed several times over a period of time to calculate a mean response factor. This enables a semi-quantitative analysis of reasonable accuracy to be performed by reference to an in-house database of response factors. This eliminates the need for extensive calibrations for every batch and also reduces the errors from widely varying response factors. 

Relative concentrations of positively identified compounds were determined using their MS response factors compared with the internal standard. Response factors were determined by analyzing. standard compounds at tliree levels under identical GC/MS conditions. The relative concentration of each tentatively identified compound wa.s estimated from its peak area relative to the internal standard. 

Library search A technique in which an unknown mass spectrum of a compound is compared to the mass spectra of compounds contained in a computer library in an effort to identify the compound. Compounds identified in this manner are referred to as tentatively identified compounds (TICs). 

Tentatively identified compounds (TICs) Compounds detected in samples that are not target compounds, internal standards, system monitoring compounds, or surrogates. TICs usually consist of up to 30 peaks that are greater than 10% of the peak areas, or heights, of the nearest internal standard. They are subjected to mass spectral library searches for tentative identification. A client may specify the number of unknown peaks in its samples it wishes the laboratory to tentatively identify. 

Photolytic. The reported photolysis half-lives of chlorsulfuron in distilled water, methanol and natural creek water at X >290 nm were 18, 92 and 18 hours, respectively. In all cases, 2-chlorobenzene sulfonamide, 2-methoxy-4-methyl-6-amino-l,3,5-triazine and trace amounts of the tentatively identified compound nitroso-2-chlorophenylsulfone formed as photoproducts (Herrmann et al., 1985). 

In relation to quantification, standard addition methodology is usually used due to the presence of matrix effect [75]. For that, spiking samples with different amounts of standard compounds were prepared, in order to obtain a concentration range in agreement with the levels observed in assayed samples. Tentatively identified compounds (with no standard available) were quantified as equivalents of the most similar compound, taking into account their molecular weight [94]. In the other cases, triphenyl phosphate (TPP) was used as an internal standard for quantification purposes... 

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