Effluent stream splitting to two different detectors

Abstract

Several papers (e.g. [l-91 have discussed the advantages of dividing the effluent stream from a gas chromatographic column between two different detectors and various stream splitters have been proposed. A recent paper [lo] evaluated the use of a "zero dead volume" tee (Valco Instrument Co., Houston,Texas, USA) for this purpose, but severe tailing was observed. Our initial results using thisdevice were disappointing,due towhatwediagnosed as cold trapping in the tee, whose thermal mass is appreciable. By mounting the tee in a small aluminum block contained within the GC oven and heated by a variable transformer, this difficulty was solved and excellent chromatographic results were obtained.Initial attempts employed a Varian 3700 with the Varian split injector; because of discrimination problems, a more linear splitter (J & W Scientific, Inc., Rancho Cordova, California, USA) was retrofitted.Thiswasoperatedatatemperature of2500Candasplit ratio of 1 : 60. The tee was mounted in a 6 X 9 X 2.5 cm aluminum block, containing a 250 W cartridge heater; a variable transformer was used to power the heater and the unit was fitted with a thermocouple readout. The block was initially heated to lOOOC and the temperature then slowly increased to 25OoC, paralleling the temperature of the GC oven. The fused silica column (30 m X 0.25mm,witha0.25~mfilmofDB-5),wasconnectedtothecentral port of the tee, using the standard polyimide ferrule. The relative amounts of the eluting stream directed to the two detectors was controlled by using different lengths (cut from the column) between the tee and the two detectors; 34.5cm was used on the FID side, 9.5 cm on the ECD side; this would indicate a division of ca. 3.6 to 1 in favor of the FID. Great care was taken in cutting the column and tubing ends to ensure square cuts and the absence of polyimide and fused silica debris in the flow path. Table 1 shows the test compounds employed; because of solubility differences, two test solutions were required. One microliter of each solution was drawn into the syringe and the two samples were co-injected. Polynuclear aromatic hydrocarbons were dissolved in dichloromethane; all other solutes were dissolved in hexane. The concentrations of pesticides were all 1 ng/pL; all other solutes were 50 nglpL.

Consecutive gas chromatograms from a parallel columns using a single injection and a common detector

“…where tR is the retention time of the eluate and k is the corresponding capacity factor. If, for example, one column is twice as long as the other, the carrier velocity will be half and the retention 4 times longer.…”

“…As shown in eq 4, k is directly proportional to the partition coefficient, K, the inside radius, rc and the liquid film thickness, dt (18). k = 2Kdf/rc (4) Since in the present work all other variables except column length and film thickness are kept constant, the retention time will depend on d¡ according to the relationship tR = A + Bd( (5) where A and B are constants.…”

“…But because of the possibility that more than one compound may have the same or sufficiently similar retention index on a given liquid phase, a confirmatory procedure is often necessary. Aside from the method of two-dimensional gas chromatography, i.e., confirming the identification by obtaining a second set of retention times on a dissimilar liquid phase (1,2), other techniques include using a selective detector, splitting the effluent into separate detectors, and others (3)(4)(5). The most definitive of these is GC/MS in which the retention data are combined with mass spectral fragmentation patterns and molecular ion masses (5).…”

Instrumental Aspects of Gas Chromatography

“…Ideally, an effluent splitter should provide a fixed split ratio that is independent of flow rate, temperature and sample volatility. Effluent splitters for columns have been fabricated from glass-lined stainless steel [226,230,231], platinum/iridium microtubing [232,233], glass capillary tubing [234][235][236] and zero-dead volume metal tees [237]. For packed columns, T-splitters with fixed or valve-adjustable split ratios are generally used [227][228][229].…”

“…Dead volume effects are not usually a problem at packed column flow rates, although changes in the split ratio with temperature and the constancy of the split ratio with samples of different volatility may be. The appreciable thermal mass of the commercially available zero-dead volume tees can cause problems due to cold trapping, and again, separate thermostating at a temperature above the column oven temperature is required [237]. Effluent splitters for columns have been fabricated from glass-lined stainless steel [226,230,231], platinum/iridium microtubing [232,233], glass capillary tubing [234][235][236] and zero-dead volume metal tees [237].…”