Coeluting peaks

Peak purity analysis software allows users to sample spectra at equidistant points across an HPLC peak. In general, the poorer the resolution between potentially coeluting peaks is, the more desirable it is to use greater numbers of data points to detect the impurity. Traditionally, spectra have been sampled upslope, at the apex, and downslope of the eluted peak. This selection pattern may overlook the presence of impurities near the peak extremities. However, acquisition of many spectra may increase calculation and display time without adding significant information. 

Voltammetric analysis is performed by scanning the potential or by applying triangular potential wave form to the electrode. Coeluting peaks are distinguished if their voltagrams are significantly different. 

In a technique referred to as GC X GC or two-dimensional GC, direct coupling of two columns with different selectivities allows for a continuous separation of closely eluting or coeluting peaks therefore, it readily separates PAH isomers and many other species that coelute under ordinary capillary column conditions. A GC X GC-MS system has been described where the first separation was on a 20 m X 0.25 mm i.d. 5% phenyl dimethylpolysiloxane column with 0.25 /rm film thickness. Eluate from the first column passed through a cryogenic modulator onto a... 

Using SIM, coeluting peaks can be separated and quantitated without difficulty. Figure III shows separation of six trichothecene standards that were derivatized separately and combined before injection. The molecular ion was monitored for each compound in this demonstration. Even though, under the conditions used here, NSL and DAS are not completely resolved, they can easily be detected independently with SIM. 

Method was compared with that obtained at the collaborative trial [39], The results were encouraging for the performance of most laboratories except for sample six. The participants quantified dc-GTX2,3, dc-STX, and GTX5 in this sample, but were not able to quantify GTX-6, Cl,2, C3,4, and dc-NEO owing to the lack of standards and coeluting peaks. The performance between laboratories (RSDr%) for some PSP toxins was similar or better than the one obtained in the Lawrence s collaborative study [39]. 

Coeluting peaks can occur as a result of a deteriorated column. As discussed earlier, the retention time of a compound can shift and, when this occurs, peaks may appear to coelute partially. The interaction between a basic compound and the exposed active negative sites on the column can give the appearance of a peak tail (see previous discussion). This can normally be rectified by changing the column. 

There is a certain risk of system peaks (Section 19.11) with columnswitching methods. System peaks can be invisible but they alter the shape of coeluted peaks. ... 

NCI-MS eUminated chlorinated interferences but presented different brominated interferences well resolved with the EI-MS approach. When PBBs and PBDEs were simultaneously determined, some important coeluting peaks appear. One critical chromatographic pair is BDE-I54 and PBB-I53 which coelute in many cases [54]. PBB-I53 and TBBPA can also coelute with BDE-... 

Gas chromatography-mass spectrometry Advantages of GC-MS over traditional methods that use multiple element selective detectors include (1) simultaneous quantitation and confirmation of detected analytes (2) detection of a wide range of analytes independent of elemental composition (3) possibility to spectrometrically resolve (deconvolute) coeluting peaks and, as a consequence, (4) reduced personnel, laboratory space, and instrumentation needs, and (5) increased sample throughput. On the other hand, GC-MS instruments are still more expensive than GC instruments with element selective detectors, and sometimes, in very complex matrices, element selective detectors may permit detection of lower concentrations of residues masked by matrix interferences in GC-MS. 

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