Unknown peaks, identifying

Provided that operating conditions remain constant and are reproducible, the retention times of the components of a sample can be compared directly with those of known materials and synthetic mixtures. An unfamiliar peak can sometimes be identified by spiking a sample with a pure substance whose presence is suspected. An increase in the size of the unknown peak is good evidence for it being the substance added. As two materials may have the same retention time for a given stationary phase, this method is not infallible. It is advisable, therefore, to run unknowns on two different stationary phases. 

Unknown 11. Identify the compounds on the basis of their isotopic peaks. The spectra are represented as m/z value (intensity to the base peak in the spectrum, %). The molecular peak is the first in the row. 

The TC detector and the FID are usually within a factor of two of the same response for an unknown organic compound as for other peaks in the run. This is a desirable property. Not only does it allow estimates to be made for the amount of unknown peaks, but it insures that there are no major volatile impurities if a chromatogram does not show them. This allows manufacturers of chemical products to specify the level of organic volatile impurities without identifying them in each case. 

The unknown peaks are identified by comparing their retention times with those of their respective standard steroids, as the TMSI derivatives. The average retention time for the last compound, 11-g-hydroxyetiocholanolone, is 60 min and for the internal standard 5-g-cholestan-3-a-ol, 30 min (43). 

Gas chromatography can be applied to both qualitative and quantitative analyses. There are two general methods for qualitative identification of eluted compounds. A compound may be identified by retention time or by peak enhancement. If you have some idea of the identity of an unknown peak or if you know it is one of three or four compounds, then you can inject the known compounds into the GC under conditions identical to those for the unknown sample. Compounds with the same retention times ( 2 or 3%) can generally be considered identical. Alternatively, you can add pure sample of a suspected component to the unknown sample and inject the mixture into the GC. If a recorder peak is increased in size, this also provides evidence for the identity of an unknown. The latter technique is called peak enhancement or spiking. 

The analytes are identified from their mass spectra and retention times. The retention time must fall within 3 sec of the retention time of the known compound. The mass spectra of the unknown peak must have the primary and all secondary characteristic ions (and often the molecular ion). 

Figure 4. GC/IR spectra of 2,6-dimethylpyrazine (top) and 2,5-dimethylpyrazine (middle) identified in a spectrum of an unknown peak (Rt 19.685 min., bottom) from charbroiled chicken.

The mass spectrometer is the most powerful GLC detector used so far. However, the GLC-MS combination is expensive for routine and repetitive analyses but is invaluable for rapid identification of unknown peaks. It has been widely applied in steroid analysis (B16, H23), identification of drugs (L4) and for the identification and quantitation of unusual metabolites in some inborn errors of metabolism (C23). The high sensitivity and specificity of the GLC-MS combination enables all metabolites in a urine to be unequivocally identified. Inexpensive mass spectrometers, with a limited range of m/e values, are becoming available and could become useful for repeated analyses of similar samples for a limited range of constituents. 

The ester compositions of the JCME are given in Table 3. Three main fatty acid esters were identified in the biodiesel fuel, namely the methyl esters of palmitic acid (17%), oleic acid (17%) and linoleic acid (43%). The biodiesel also showed an unknown peak with retention time of 79.5 minutes. Other components were as described in the literature, although the relative amounts differed. ... 

In the analysis of complex mixtures, further information may be gathered by co-running pure samples in order to identify unknown peaks. Identity should be confirmed using a different column support. 

Several unknown peaks were still observed in chromatogram (A) (Fig. 1). Peroxyoxalate-chemiluminescence detection is highly sensitive and selective for aminopolycyclic aromatic hydrocarbons which are amino derivatives of NPAHs. Therefore, these unknown peaks (a - f), which were detected only after reduction, might have originated from NPAHs. We are presently attempting to identify these unknown peaks. 

MSD gives even better detection limits (1-3 ng/m ) by selected ion monitoring. It was also possible to identify unknown peaks as decomposition products of dichlofluanid and propiconazole which came out with less than 2 % of the original WPA air concentration. 

A more convenient and expeditious means of mass measurement with either design is to interface an electronic detector with an on-line computer that acquires and stores all the data, both m/e values and intensity data, while the spectrum is being scanned. After identifying the m/c ratios of the mass standard, the computer calculates the exact masses of all the unknown peaks from the scanning time between standard and unknown and, within a few minutes, prints on a teletype the exact masses and intensities of all the peaks in the mass spectrum. This is possibly the most elegant technique in mass spectrometry, for it provides the analyst with exact masses which can be used to determine the elemental compositions of all peaks in a mass spectrum. 

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. 

---