Unknown peak, presence

Many unknown peaks Presence of oxidized cholesterol analogs Perform GC measurement as recommended... 

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. 

For isolated cases, the first indication of the presence of a volatile substance may be the presence of an unknown peak when a sample is analyzed for ethanol and other volatile alcohols (see the previous section). In these instances, method development focuses on the compound encountered and frequently involves modification of the ethanol analysis procedure. Numerous procedures for single VOCs or classes of VOCs have been developed and summarized in review articles. HS-GC remains the most frequently used... 

Mass spectrometry can be used for peak identification, with fragmentation patterns showing the presence of specific chemical groups. Infrared and NMR detection are useful for this because they also give information about the chemical functionaHty of the peaks detected. This can be used either to determine the stmc-ture of unknown peaks or to monitor specific chemical structures such as methyl groups, a carbonyl group, or an ether Hnkage. 

The most efficient identification procedure involves the determination in rapid succession of retention data (retention volumes, or preferably retention factors) for the components of a mixture and a series of authentic compounds. This method permits straightforward identification of components known to be present in the mixture, and it can also rule out the presence of other possible components. However, it cannot positively identify the presence of a given, isolated component. It is recommended that such determinations be carried out using similar amounts of compounds, and that a number of spike analyses be performed, where small amounts of authentic compounds are added to the mixture in order to approximately double the sizes of unknown peaks. Comparison of... 

A new protein of unknown structure has been purified. Gel filtration chromatography reveals that the native protein has a molecular weight of 240,000. Chromatography in the presence of 6 M guanidine hydrochloride yields only a peak for a protein of M, 60,000. Chromatography in the presence of 6 M guanidine hydrochloride and 10 mM /3-mercaptoethanol yields peaks for proteins of M, 34,000 and 26,000. Explain what can be determined about the structure of this protein from these data. 

Therefore, a mass analyzer capable of resolving powers of 26,000 or better is required to reveal the presence of two ions (representing two distinct compounds). If an analyzer with RP< 26,000 is used, the presence of a second compound would be overlooked. Additionally, attempts to measure the exact m/z value of the peak will not provide a true value, but rather some average value (e.g., m/z 937.5072, assuming the compounds produce signals of comparable intensity) that will be of little use in identifying either unknown compound. Ambiguous results such as these only complicate qualitative analysis efforts. 

Less than half a century after Balboa had stood silent on a peak in Darien, facing the unknown ocean, a famous Italian scholar and poet, Julius Caesar Scaliger, or della Scala, recorded the presence there of an unknown noble metal. In 1557 he made what is probably the first definite allusion to platinum. Girolamo Cardano (1501—1576), in his well-known work On Subtlety, had defined a metal as a substance which... 

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