Single detector issues

The T junctions and corners of the layout currently used in synchronized cyclic CE contribute more to zone broadening than do the channel connections at the corners used in single-pass chips. In addition, a significant percentage of analyte is lost on each cycle as it passes by T junctions and the detector. These issues are under investigation. 

The second issue is to interface two detectors to a single GC. Since both the MS and the matrix isolation interface are destructive detectors, the sample is split in a 1 1 effluent splitter and half the sample is routed to each detector via a specially-designed open split interface (18). 

Figure 9.16 Optical path from the exit of the monochromator to the detector for two double beam instruments, (a model with two rotating mirrors and a model with a semi-transparent mirror). The arrangement of the apparatus with rotating mirrors is similar to that of IR spectrophotometers apart from the fact that the light beam issuing from the source passes first through the monochromator before it hits the sample. In this way the photolytic reactions which could occur owing to an overexposure to the total radiation issued from the source are minimized. A more compact and simple optical assembly with a single beam associated with two detectors. A semi-transparent and fixed mirror replaces the delicate mechanism of synchronized, rotating mirrors.

A detailed description of detectors and their performanee in TCSPC applications is given under Chap. 6, page 213. Deteetor problems related to special TCSPC applications are also discussed under Chap. 5, page 61. The following paragraph deals with some practical issues related to single photon detectors and their use in TCSPC. 

The fundamental problem with determining a detection limit for a method applied to trace quantities of an analyte extracted from a complex matrix is that many variables may affect the result on any given day. These include (but are not limited to) variability in matrix coextractives, variations in the source or the detector response, cleanliness of the detector cell, and variations between instruments if more than one instrument is available for use in the laboratory (particularly an issue for LC-MS and LC-MS/MS). The many issues that render determination of a detection limit more complex than it may initially appear are discussed in the lUPAC guidance on method validation in a single laboratory, where it is also suggested that for analytical systems where the validation range does not include or approach it, the detection limit does not need to be part of a validation. However, if a detection limit specification is critical to a regulatory result, it becomes important to include a sample fortified at the detection limit in each analytical run to demonstrate that in the set of samples analyzed in that run, detection of a positive result at the claimed detection limit was achieved. To demonstrate that a detection limit is realistic, 19 of 20 typical samples fortified at that concentration should be detected as positive. 

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