Retention time matching

Obtain a chromatogram of your unknown. The peaks will be smaller since the concentrations are not 100%, as they were for the pure liquids. You may have to adjust the attenuation to get large, well-defined peaks. Measure the retention times for each peak, and compare these to your data for the pure liquids. Those liquids that have retention times matching a peak in your unknown are contained in your unknown. 

Prepare and filter standard solutions of several of the components shown on the label. The concentrations should be reasonable guesses of what might match what is in the sample. Inject each individually and observe the retention times for each component. Check to see if one of these retention times matches the retention time for a resolved peak from the sample. 

Just because the retention times match does not necessarily mean that you have identified the peak. To be sure, change the stationary phase and inject again. If a sample peak is again resolved from the others (and it is the same size), and if the retention times of the same standard peak match the resolved peak, you can be sure that you have identified the peak and your quantitation in step 5 is valid. 

To reduce the possibility of false positives, the intensities of one to three selected ions are compared to the intensity of a unique target ion of the same spectrum. The sample ratios are compared to the ratios of a standard. If the sample ratios fall within a certain range of the standard, and the retention time matches the standard within specifications, the analyte is considered present. Quantification is performed by integrating the response of the target ion only. 

Species identiFcation D the retention time matching that is usually employed requires the availability of standards. When standards are not available, the use of a molecule-speciFc detection technique is mandatory. 

Retention time matching can be validated to establish identity, but is insufficiently specific for general use. Retention times are not necessarily unique for chemically distinct species even isomers may or may not co-elute. A change in synthetic process may introduce a different impurity profile any of these new process impurities has a non-zero chance of co-elution with... 

The second gc technique determined the individual n-alkanes and 1-alkenes in the pyrolyzed sample. A 100 m wall-coated glass capillary gave the required resolution and the n-alkanes and 1-alkenes stood out as distinct, well resolved peaks. OV-101 or OV-17 wall coatings provide adequate separation. A carrier gas flow of one cc/min was combined with an inlet split ratio of 50 1 and a 310°C injector temperature. The column temperature was raised to 250°C at 4°/min after an 8.0 min initial hold at 80°C. Peak identification was based on retention time matching with n-alkane and 1-alkene standards. 

Volatile analytes cryogenically preconcentrated at —78°C on the U trap species identified by retention time matching, elemental isotopic fingerprints, and ion trap MSn analysis Batch HG to generate hydride derivatives cryogenic preconcentration at — 196°C on the U-trap thermally desorbed into an ICP-MS identification of species based on comparison with standards or calculation of boiling point... 

Additional certainty in using fj values to assign peak identity is obtained when retention times match on two columns with significantly different stationary phases, or a single planar stationary phase eluted by orthogonal, sequential application of two mobile phases. This is the principle and application of 2D chromatography discussed earlier in Section... 

M -CHiOH), 111 (24%, M -C3H7), 93 (17%, Nf.H2O-C3H7), 69 (90%, CsH9, and 41 (100%, 3115 ), indicative of a monoterpene alcohol. The con )ound was tentatively identified as the monoterpene alcohol lavandulol (1) by conq)arison with database spectra. The identification was verified with retention time matches on 3 GC capillary columns (DB-5, DB-17, and DB-WAX), and the mass spectrum matched t t of an authentic standard prepared by hydrolysis of commercial lavandulyl acetate (TCI America). 

At present, ion chromatography (IC) with conductivity detection is the recommended method for analysis of perchlorate in drinking water, in which identification is based on retention-time matching. IC is extremely sensitive (lod of 4 ppb for perchlorate in relatively pure water), but exhibits some problems for analysis of environmental samples. They include the following ... 

Finally, qualitative determination based solely on retention-time matching is dangerous. This is true of any chromatographic technique that does not have a specific detector. 

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