Indirect detection using system peaks

Typical probes for the analysis of ionic solutes include 3-hydroxy-L-tyrosine (DOPA)24 and naphthalene-2-sulfonate,26 whereas those for use with uncharged solutes include nicotinamide,27 theophylline,28 and anthracene 29 Indirect detection is nonspecific and less suitable for the analysis of complex or impure samples, because unpurified biological samples, such as urine, contain a large number of hydrophilic solutes that will give problems such as extra system peaks. However, analyses of pharmaceutical products and quantification of impurities in substances are typical of applications.23... 

Note that it is a common practice to dissolve the analytes to be separated in the mobile phase. In fact, the injection of the sample in a solution whose composition differs from that of the eluent would create a disturbance at the top of the column that will migrate down the column. The lack of detectable eluent component in the injected solution would create a negative system, perturbation, or ghost peak because the concentration of that component is lower compared to the initial conditions under which the baseline was recorded. Obviously the retention time of the system peak is that of the component under the experimental conditions used in the chromatographic run [28,29]. System peaks are also very useful for indirect detection [30], as will be explained in Chapter 13. 

The experiments based on proton detection of rare spin nuclei are usually the most sensitive methods of determining NMR parameters of magnetically diluted spin systems. Unfortunately, recording of 2D correlation maps is usually also a time consuming experiment, especially if wide spectral bands have to be covered in the indirectly detected dimension. The most frequently encountered situation is that only one or a few peaks are expected within a narrow spectral band. However, the position of this band is not known. Several attempts have been made to reduce the experimental time needed to perform such experiments. One approach would be to record a highly truncated data set and use the linear prediction [86,87] to reduce the effect of the data truncation on the appearance of the spectrum. This technique is now available with most... 

Fig. 3-67. Separation of anions derived from weak inorganic acids using a strongly basic eluent. — Separator column Wescan 269-029 eluent 0.004 mol/L NaOH + 0.0005 mol/ L sodium benzoate flow rate 1.5 mL/min detection indirect conductivity injection volume 100 pL solute concentrations 5 ppm borate (as B), 10 ppm silicate (as Si02), 10 ppm formate and sulfide, 20 ppm chloride and cyanide (system peak appears after 28 min.) (taken from [70]).

As discussed earlier, a chromatogram containing n -b 1 peaks is recorded upon injection of a sample containing n nondetectable components in a chromatographic system using a binary mobile phase with a detectable additive [8,18,20]. Of these peaks, the n analyte system peaks, appear at the retention times of the nondetectable analytes. The extra system peak is characteristic of the additive. Equation 13.22ab shows that the area of an analyte system peak is proportional to the size of the perturbation, ie., to the amoimt of the corresponding component in the sample injected. The response factor in the case of an additive used for indirect detection can be derived from Eq. 13.22b. In indirect detection, the detector responds to... 

Indirect detection is an alternative to derivatization for the detection of analytes with a weak detector response. It is commonly used in ion exchange (particularly ion chromatography) and ion-pair chromatography with absorbance, fluorescence or amperometric detection [168,254,255]. This requires the selection of an eluent ion with favorable detection properties to regulate the separation process and provide a constant detector signal. Detector transparent analyte ions cause displacement of eluent ions from the eluted band and a decrease in the detector response compared with the steady state signal for the mobile phase. The detected ion concentration is coupled to the retention mechanism, which can result in the appearance of additional system peaks in the chromatogram (section 4.3.3.2). These applications should be... 

Since reproducibility of the flow system is critical to obtaining reproducibility, one approach has been to substitute lower-performance columns (50-to 100-p packings) operated at higher temperatures.1 Often, improvements in detection and data reduction can substitute for resolution. Chemometric principles are a way to sacrifice chromatographic efficiency but still obtain the desired chemical information. An example of how meaningful information can be derived indirectly from chromatographic separation is the use of system or vacancy peaks to monitor chemical reactions such as the titration of aniline and the hydrolysis of aspirin to salicylic acid.18... 

Quantification of impurities can be conducted through a number of different methods. The two most often used are the mass balance approach and the standardization approach. The preferred quantification method is standardization, due to its greater accuracy. In this approach, the analyte sample is compared to a sample having a known quantity of a known standard substance. This standard can be either internal or external, or direct or indirect. The mass balance approach adds all of the peaks found in a chromatogram and calculates a percentage of total detected area. This is a much quicker and less costly approach than the standardization approach, but is susceptible to systemic bias, on account of which its accuracy suffers. 

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