Detectors library search

These data for successive scans are then stored for subsequent manipulation. The reconstructed total ion current trace, equivalent to that obtained from a flame ionisation detector in gas chromatography, shows the variation of total ion current with time and allows spectra of interest to be identified. A typical example is shown in Fig. 6A. The background may be subtracted to give clean spectra, and their identification may be attempted using libraries of standard spectra. If a composite spectrum is obtained from two unresolved peaks, complex subtraction routines may be used to obtain a pure spectrum of each of the components. These may be separately submitted for library searching. The spectra may then be plotted or obtained as a mass versus intensity listing. 

At the same time, there are some limitations in utilizing CE for library search routines. CE utilizes very small amounts of sample, nanoliter volumes, and thus very low levels of possible recognition elements. Because detection is usually through the capillary, except for MS, detection limits are very high (poor), making this a generally insensitive technique for trace analysis. There are some recent improvements in detector cell design, bubble cells, Z cells,... 

After injection of the prepared urine sample, saturation of the detector can occur rapidly with some peaks. The resulting mass spectra are less suitable for a library search, as the ratios of the individual masses to each other differ from those in the library, in which the spectra are measured at lower concentrations, and false results of the automatic library search can be obtained. Poorly separated peaks can also be a result of substance concentrations that are too high. A practical example is illustrated in Fig. 8-9. This urine sample was obtained from a patient who had been operated on a few hours earlier and who had received 250 pg fentanyl, as well as other substances, during the operation. 

Infrared detector (IRD) 1 X lO- gofastrong infrared absorber 1 X 10 Universal for compounds with mid-infrared active functionality A costly and temperamental instrument that requires high purity carrier gas, a nitrogen purge of optical components (purified air will, in general, not be adequate) Must be isolated from vibrations Presence of carbon dioxide is a typical impurity band at 2200 cm" to 2300 cm Requires frequent cleaning and optics maintenance Amenable to library searching for qualitative identification... 

Table 2 shows an example of a hit list generated for identifying a benzodiazepine drug separated by high-performance hquid chromatography using a library search of UV spectra recorded by a diode-array detector (DAD). 

Earlier in the chapter, you learned that poisonous carbon monoxide can form during incomplete hydrocarbon combustion. The use of carbon monoxide detectors in homes and businesses has reduced the number of deaths due to carbon monoxide poisoning. Are all carbon monoxide detectors the same Telephone your local fire department, go to a library, or search the Internet to find out about carbon monoxide detectors. 

An alternative approach has been pioneered by Lewitt et al. [26], This uses an array of coulometric detectors following HPLC separation to detect redox-active compounds in a complex mixture such as in CSF or tissue extracts. Although this approach does not identify compounds directly, the combination of retention time and redox properties can serve as a basis for database searching of libraries of standard compounds. The separation output can also be directed to a mass spectrometer for additional identification experiments. 

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