Chromatographic coelution

For example, assume that Compounds I and II are poorly absorbed, while Compound III is well absorbed following an oral cassette dose (Fig. 2c). There-fore, the plasma concentrations of Compounds I and II are expected to significantly lower compared to those of Compound III. If these three compounds chromatographically coelute then it is likely that plasma concentrations of Compound III are overestimated due to lack of competition for ionization from the other analytes. This quantitation error can be generally avoided through chromatographic separation of the analytes. 

Mass spectrometric detection has a great advantage that all known ergot alkaloids can potentially be determined in one run and chromatographic coelution is not a problem for compounds having different MS/MS transitions. Furthermore, analogs not available as reference standards can be tentatively identified. 

When a purity search does not yield satisfactory results, it may be worthwhile to try another search based on fit. If we obtain a result with a high probability value, it may indicate a chromatographic coelution that was not suspected due to the Gaussian aspect of the peak. 

FIGURE 8.4 Chromatographic coelution evidenced by database search in fit mode. 

A liquid chromatographic experiment resulted in the same retention time for the electroactive compounds A and B. Which electrochemical detection scheme would offer a selective detection of the two coeluting analytes Explain your selection. (E = +0.43 V Eg = +0.77 V.)... 

This concept assumes that each fraction (peak) collected in the first dimension further separates in the second dimension with regular spacing and that the entire 2D separation space is evenly covered by eluting peaks. More realistically, the peaks would be distributed randomly over the 2D separation space some peaks are likely to coelute, while some area will remain vacant of peaks. Therefore, Equationl2.1 represents an idealized peak capacity estimate although the real number of resolved peaks is lower. Most importantly, the peak capacity proposed by Equation 12.1 is achievable when the chromatographic modes used for separation are completely orthogonal. 

As has been pointed out, both entropic and enthalpic interactions affect the chromatographic behavior of macromolecules. They are adjusted to the required type of separation by selecting appropriate stationary and mobile phases. In a third mode of liquid chromatography of polymers, liquid chromatography at the critical condition (LCCC) (Entelis etal., 1985,1986 Pasch, 1997), the adsorptive interactions are fully compensated by entropic interactions. This mode is also referred to as liquid chromatography at the critical point of adsorption. Hence, TAS is equal to AH and therefore, AG becomes zero. K is 1 irrespective of molar mass and, consequently, homopolymer molecules of different molar masses coelute in one chromatographic... 

Two-dimensional GC can be used to separate complex mixtures of polyaromatic compounds, and MS used to subsequently identify the compounds. In this method, the original sample is injected into a gas chromatograph with one type of column. As the components exit the first GC, they are fed into a second GC, with a different column, for further separation and finally into a mass spectrometer. In this way, compounds that coeluted from the first column are separated on the second. Focant et al. [19] were able to separate polychlorinated dibenzo-p-dioxin (PCDD), polychlorinated dibenzofuran (PCDF), and coplanar polychlorinated biphenyl (cPCB) using this type of analytical procedure, including isotope dilution TOF-MS. These compounds are frequently found as contaminants in soils surrounding industrial settings thus, the ability to separate and identify them is extremely important [6,12,19],... 

Reference values of this approach are not different from those for other amino acid analyses. An example of a mass chromatogram, representing the plasma of a PKU patient, is shown in Fig. 2.1.1. When evaluating the results of MS/MS amino acid analyses, one has to reahze that the hquid chromatographic separation is by far less efficient that the AAA separation. For this reason, any amino acid may (partly) coelute with other amino acid(s), which potentially interferes with its mass spectromet-ric behavior. This effect is known as quenching. In order to overcome this as much as possible, stable-isotope-labeled internal standards (as many as possible) should be used. However, this matrix effect of ion suppression is the major pitfall in the MS/MS analysis of amino acids. Consequently, the MS/MS analysis of amino acids cannot be regarded as a reference method, similar to all other amino acid analytical methods. 

Fig. 3.5.4a-c Chromatographic separation of glycolate, chloride, sulfate, oxalate, phosphate, and citrate, a Standard mixture b nondiseased urine C urine from a patient with primary hyperoxaluria type I. In this system, glycolate coelutes with fluoride... 

Artifacts from the resin can interfere with the chromatographic analysis of the XAD resin extract. For example, the artifact may be a pollutant being studied, or coelution of the resin artifacts and compounds of interest may occur during capillary gas chromatographic (GC) analysis. Artifacts can also take part in competitive adsorption during sampling. This situation can cause sample breakthrough because certain compounds are preferentially collected. 

Figure 2. Weight percent (peak mass I total mass X 100%) of each peak for an 80% 1016/20% 1254 standard Aroclor mixture and for the top section office Twelve Mile Creek—Lake Hartwell sediment cores. Sample location distances from the PCB source and weight-percent summaries for peaks 1-22 and 25-64 are given. Some peaks are not quantified because of chromatographic interferences (GC peaks 24, 27, and 64) or lack of analytical sensitivity (GC peaks 45, 48, 55, 56, 59, and 62). Peak 15 coelutes with peak 14. Peak 23 is the internal standard aldrin.

Determination of the odor character and intensity of enantiomers relies heavily on complete separation of the components of the sample where there is no coelution and baseline separation of enantiomers is seen (see Fig. Gl.4.4). If these ideal conditions are not met, considerable errors will be incurred in making odor measurements, particularly in cases where both enantiomers have similar odors, or where one is odorless. Traces of odorants coeluting with analytes under investigation, tailing of peaks, and low resolution all seriously affect chromatographic odor data. If the retention times of two enantiomers differ by <1 min, quantitative odor data may be inaccurate. 

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