Polar compounds, analytical problems

Polar compounds present the most problems because of their low breakthrough volumes with common sorbents. In the last few years, highly crosslinked polymers have become commercially available which involve higher retention capacities for the more polar analytes (37, 38). Polymers have also been chemically modified with polar groups in order to increase the retention of the compounds previously mentioned (35, 37). 

Online LC-MS is a good solution for separation, identification, and quantification because it permits the confirmation of polar and nonvolatile compounds without need for derivatization.4 The use of LC-MS for biological sample detection and data analysis has grown rapidly during the past few years. Many reliable and easy to use LC-MS systems are commercially available and have been adapted for solving analytical problems by scientists in proteomics research, metabolic study, complex natural product separation and characterization, and drug discovery. 

Derivatization After Desorption. Alkanolamines, highly polar basic compounds, present a difficult analytical problem. Although direct gas chromatographic separations can be achieved, this technique is not applicable to trace analysis due to sorption problems at trace concentrations. A derivatization/gas chromatographic procedure has been developed for the determination of alkanolamines in air as low as 100 ppb (54,55). The samples are collected on activated alumina and desorbed with an aqueous solution of 1-octanesulfonic acid. The... 

Compound Obtain information on the test article solubility, purity, polarity and stability in order to avoid analytical problems due to compound precipitation or compound decomposition. One should also estimate, which LOQ level will be required for the assay and it should be estimated which calibration range is desirable (the calibration range should reflect the expected sample concentration range)... 

Cf-FAB was widely used to solve analytical problems concerning highly polar and/or ionic compounds, e.g., carotenoids [51], acylcamitine in the urine of a medium-chain acyl-CoA dehydrogenase deficient patient [52]. 

Even the simplest bifunctional compounds of these classes being analyzed on nonpolar phases indicate broad nonsymmetrical peaks on chromatograms. This leads to poor detection limits and reproducibility of retention indices (the position of peaks maxima depends on the quantity of analytes) compared with nonpolar compounds. The general way to avoid these problems is based on the conversion of hydroxy compounds to thermally stable volatile derivatives. This task is a most important purpose of derivatization (see the entry Derivatization of Analytes in Chromatography, General Aspects). This chemical treatment may be used not only for nonvolatile compounds but also for volatile substances. The less polar products typically yield narrower chromatographic peaks that provide the better signal-to-noise ratio and, hence, lower detection limits. Nonpolar derivatives have much better interlaboratory reproducibility of retention indices compared with this parameter for initially polar compounds. 

The Stringfellow Superfund site in California poses analytical problems similar to those encountered with most waste sites across the United States and that may be best addressed via LC/MS based methods. Most of the organic compounds in aqueous leachates from this site cannot be characterized by GC/MS based methods. Analysis of Stringfellow bedrock groundwater shows that only 0.78% of the total dissolved organic materials are identifiable via purge and trap analysis (IQ). These are compounds such as acetone, trichloroethylene etc, whose physical properties are ideally suited for GC/MS separation and confirmation. Another 33% of the dissolved organic matter is characterized as "unknown", i.e., not extractable from the aqueous samples under any pH conditions and thus not analyzed via GC. Another 66% is 4-chlorobenzene sulfonic acid (PCBSA), an extremely polar and water soluble compound that is also not suitable for GC analysis. This compound, a waste product from DDT manufacture, is known to occur at this site because of the history of disposal of "sulfuric acid waste from industrial DDT synthesis. 

In spite of this critical note, the potential of SFC in analytical and preparative-scale enan-tioseparations has been already illustrated. Technical development in this field may open even more challenges for this technique. The advantage of SFC for preparative separations is that the high-pressure liquid carbon dioxide used as mobile phase can easily be removed from the product. In addition, carbon dioxide is non-hazardous and relatively inexpensive. On the other hand, this mobile phase creates the following problems the solubility of polar compounds is limited, and alcohols or other polar modifiers have to be used. Although this makes the technical advantage of SFC questionable, the method may offer some advantages for chiral compounds that may dissolve in SFC mobile phases. Selected examples of preparative SFC enantioseparations are summarized in Table 10 [168-171]. 

Since only PDMS coating is available as an extraction phase, SBSE has been used predominantly for low-polarity analytes, but the problems of extraction of polar compounds may be solved by in situ derivatiziation. It is clear that further developments in stir bar coatings and designs could extend the applicability of the method. The main drawback of this method is the duration of extraction, typically 30-150 min. For this reason SBSE may be impractical for routine high-throughput laboratories. 

The last statement can be illustrated by the following example. The analytical problem is the characterization of 2,4,4-trimethyl-2-pentene (diisobutylene, I) by RI values on packed column with polar inorganic sorbent Silipor-600 (LaChema, Czech Republic) in temperature programming regime from 50 up to 220°C (ramp 8 deg/ min). Raw retention times of (/) and reference n-alkanes (min) are (/)—20.16 Ce—12.70 Cj—15.94 Cg—18.73 Cg—none Cio—23.74 fo 1.01. The results of RI calculation with various sets of n-alkanes illustrate the maximal reproducibility of just lin-log RIs in the absence of the influence of the choice of reference compounds. The negative value of parameter q is... 

Monitoring the concentrations of various pollutants in industrial wastewater, as well as surface and ground water, with precision and accuracy is a difficult task. Wastewater is a complex matrix that normally contains organic pollutants of non-polar to polar nature in addition to bulk inorganic contaminants. Additionally, interferences are likely when handling complex mixtures of compounds. Under these circumstances, chromatography has played an important role and has provided solutions to complex analytical problems. It is extremely important to have an excellent analytical technique that can accurately... 

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