Dual-column confirmation

Fig. 2.6. Chromatogram showing the simultaneous analysis of 16 PAHs (A), 7 chlorobenzenes (B), 10 organochlorine pesticides (C), and 7 PCBs (D). Column RapidMS (10 ml x 530 jim i.d., 0.12 pm df, Varian, The Netherlands. GC oven 50°C (lmin) to 190°C (3 min) at 30°C/min to 280°C at 40°C/min. MS time-selected SIM mode with dual ion confirmation. Analyte concentration levels were between 50 and 175 ppb [38].

For separation, high resolution capillary columns are used, and in most circumstances each extract is examined by a dual column technique. This is to add a degree of confirmation into the method. 

Confirmation In GC, an unknown compound in a sample is identified on the basis of its retention time on a specific chromatographic column. Because several compounds may exhibit that exact same retention time on a given column, a secondary confirmation on a different column or detector is often recommended for additional confidence in the compound identification. This additional confirmation is often referred to as dualcolumn or dual-detector confirmation. 

The standard method for analyzing paraquat is by ion-exchange cleanup followed by reduction to the radical cation, which is colored and read spect rophotometrically(5). The labor requirements and somewhat limited detectability of this method led us to develop a GC method based upon reduction with sodium borohydride to a mixture of the monoene and diene tertiary amines - both of which are amenable to GC on a deactivated column using an N-selective detector( ). The chromatogram shows both peaks (ratios may vary somewhat) which provides built-in confirmation of paraquat because of this dual response pattern. But the reduction-GC method is still a laborious one with a throughput of perhaps 8-10 samples/day under good conditions. It took nearly one man-year to develop and fully Validate. 

We believe that many other classes of compounds will be shown to be suitable analytes for these approaches, and that MDLs will be, as they have been for most of the above compounds, in the low ppb ranges (1-50 ppb or below). In those Instances where particular analytes are already amenable to oxidative LCEC, without post-column photolysis, confirmative evidence for their presence can be obtained by HPLC-hv-EC. That is, the dual electrode response ratios for these compounds with and without irradiation are almost always very different, depending on the particular starting analyte. Also, the optimum working electrode potentials with and without irradiation post-column are also very different, providing further confirmation possibilities. We have demonstrated for many compounds additional confirmation via the dual electrode response ratios possible with and without irradiation, using the same electrode working potentials or different sets of these. 

The column splitter is used on the end of a GC column in order to direct the effluent to two different detectors. This is used for dual confirmation analyses on a single irtjection. 

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