Individual compound analysis

52 Petroleum group type analysis (not suitable for risk estimation) Group type analyses are performed to measure the amounts of individual petroleum classes of hydrocarbons (e.g. saturates, aromatics, polars). This type of measurement is typically used for heavier fractions and can be used to interpret or identify the type of oil (Fig. 6.5). 

Various methods can be employed, but the most common semi-quantitative method is a modified thin layer chromatography (TLC) technique. In addition, gravimetric group type analysis based upon column clean-up is used as the basis of speciated TPH (see later). 

For more detailed and generally more relevant analysis, individual compounds such as BTEX or individual PAHs can be determined by target compound analysis. This is generally performed by GC-MS. This precise identification and quantification is usually required for risk assessments. 

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Of all the physical techniques, infrared (IR) spectroscopy gives the most valuable information about the constitution of organic materials. Indeed, qualitative information about specific structural and functional elements can often be deduced even though the spectra are too complex for individual compound analysis. With regard to quantitative evaluation of the constituents of coal by infrared... 

In individual compound analysis with the FID, analyte identification relies solely on retention time. As a result, single compound determinations (for example, individual phenols or PAHs) are usually gravely affected by interferences that often render the results unreliable or unusable. Because the FID methods for individual compound analysis are so susceptible to false positive results, they should be used with caution and only for interference-free matrices. 

Was second column confirmation, if required by the method, performed for individual compound analysis with GC methods ... 

A relatively new methodology caEed aroma dEution analysis (ada), which combines aroma dEution and gas chromatography-olfactometry to gain a better understanding of the relative importance of aroma compounds, was recently done for coffee. In a roasted Colombian coffee brew, 41 impact compounds were found with flavor dEution threshold factors (FD) greater than 25, and 26 compounds had FD factors of 100 or above. WhEe the technique permits assessment of the impact of individual compounds, it does not evaluate synergistic effects among compounds (13). 

Many individual compound reports contain infrared spectral information, but there is only one in which detailed analysis appears. The 3-hydroxytriazolopyri-dine 125 used as a catalyst for peptide coupling (Section IV.J) has been studied in the solid and in solution, in association with a crystallographic study, and shown to exist as a dimer in solution (99MI1). 

Although SFE and SFC share several common features, including the use of a superaitical fluid as the solvent and similar instrumentation, their goals are quite distinct. While SFE is used mainly for the sample preparation step (extraction), SFC is employed to isolate (chr-omatography) individual compounds present in complex samples (11 -15). Both techniques can be used in two different approaches off-line, in which the analytes and the solvent are either vented after analysis (SFC) or collected (SFE), or on-line coupled with a second technique, thus providing a multidimensional approach. Off-line methods are slow and susceptible to solute losses and contamination the on-line coupled system makes possible a deaease in the detection limits, with an improvement in quantification, while the use of valves for automation results in faster and more reproducible analyses (16). The off-line... 

An alternative elution technique is to transfer the powder (e.g. for bromophenol blue) to a glass column fitted with a glass-wool plug or glass sinter, and elute the dye with ethanol containing a little ammonia. The eluted solution, made up to a fixed volume in a small graduated flask, may be used for colorimetric/ spectrophotometric analysis of the recovered dye (see Chapter 17). A calibration curve must, of course, be constructed for each of the individual compounds. 

It is crucial in quantitative GC to obtain a good separation of the components of interest. Although this is not critical when a mass spectrometer is used as the detector (because ions for identification can be mass selected), it is nevertheless good practice. If the GC effluent is split between the mass spectrometer and FID detector, either detector can be used for quantitation. Because the response for any individual compound will differ, it is necessary to obtain relative response factors for those compounds for which quantitation is needed. Care should be taken to prevent contamination of the sample with the reference standards. This is a major source of error in trace quantitative analysis. To prevent such contamination, a method blank should be run, following all steps in the method of preparation of a sample except the addition of the sample. To ensure that there is no contamination or carryover in the GC column or the ion source, the method blank should be run prior to each sample. 

However, since fluorometric methods require sophisticated instrumentation, their applicabihty is limited because of cost. In conclusion, spectroscopic methods usually enable crude estimates of chlorophylls in an extract, but in most cases accurate and detailed analysis of a specific composition requires separation of the mixture into individual compounds using methods such as HPLC. 

The aim of all the foregoing methods of factor analysis is to decompose a data-set into physically meaningful factors, for instance pure spectra from a HPLC-DAD data-set. After those factors have been obtained, quantitation should be possible by calculating the contribution of each factor in the rows of the data matrix. By ITTFA (see Section 34.2.6) for example, one estimates the elution profiles of each individual compound. However, for quantitation the peak areas have to be correlated to the concentration by a calibration step. This is particularly important when using a diode array detector because the response factors (absorptivity) may considerably vary with the compound considered. Some methods of factor analysis require the presence of a pure variable for each factor. In that case quantitation becomes straightforward and does not need a multivariate approach because full selectivity is available. 

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