Linear range, chromatographic detectors

Final sample concentrations ranged from 0.05% to 1.0% (by weight), depending on latex particle size, In order to generate chromatographic signals in the linear range of the detector. Diluted samples were placed in a low power ultrasonic bath (Ultrasonics, Plainview, New York) for 60 seconds just prior to injection. Peak areas were determined for each injection. Each sample was injected until three consecutive areas differed by less than two percent from one value to the next. 

However, NP or RP methods represent the most powerful and common chromatographic techniques allowing PG analysis with very good reproducibility, sensitivity, wide linear ranges for quantitative analysis, and a broad spectrum of detectors. 

Each laboratory used its own optimised procedure for the sample preparation, clean-up, method of injection, calibration and gas chromatographic conditions [33]. Calibrants were obtained as pure crystalline certified materials from BCR [32] in addition, CBs 105, 128, 149, 156, 163 and 170 were obtained from BCR after characterisation in an independent laboratory of the identity of the compound by elemental analysis, NMR and melting point determination, and of the purity of the crystalline materials by GC-MS, GC-ECD [32] these pure compounds were used to prepare calibration solutions for these congeners. Each laboratory prepared separate calibration solutions in iso-octane of the appropriate concentration to calibrate the detector within the approximately linear range. Calibration procedures used by the participants are described in details in the certification report [32]. 

Determination of solubility by headspace analysis offers several advantages over spectrophotometric techniques. First, because of the selectivity of chromatographic analysis, compound purity is not a critical factor second, absolute calibration of the gas chromatographic detector is not necessary if the response is linearly related with concentration over the range necessary for the measurements and finally, this method does not require the preparation of saturated solutions, since a partition coefficient, not a solubility, is actually measured. However, headspace methodology would probably not be applicable for determining PAH solubilities for three reasons. First, there is little data in the literature on the vapor pressures of PAHs. Second, the aqueous solubilities of most PAHs are too low to be measured by this procedure. Finally, adsorptive losses of PAHs to glass surfaces from the vapor phase would cause errors. 

Figure 1.18. Methods for calculating the linear response range for chromatographic detectors.

The PID is nondestructive, relatively inexpensive, of rugged construction and easy to operate. The linear range is approximately 10. For favorable compounds the PID is 5 to 50 times more sensitive than the FID [280,286]. In other cases it may not respond at all or respond poorly determined by the ionization potential of the compound and the photon energy and flux. On an individual compound basis relative detector response factors vary over a wide range allowing the PID to be used as a selective detector for some applications. Major applications of the PID are the analysis of volatile organic compounds from environmental samples and in field-portable gas chromatographs [292]. 

Figures 2 and 3 show the toluene concentration In the feed, permeate and process tank, respectively, as functions of time. The data scatter Is more pronounced at higher toluene concentrations because the Internal standard method of calculation used the relative response factors which were developed at dilute toluene concentrations (l.e., toluene concentration In permeate). Also, this data scatter could Indicate that the gas chromatograph detector response Is not linear over the wide range of toluene concentrations examined.

The gas chromatographic separation should be carried out following the advice given in this and other chromatographic treatises some objectives are good resolution of all peaks, symmetrical peaks, low noise levels, short analysis times, sample sizes in the linear range of the detector, etc. 

Sulfur compounds play a major role in determining the flavor and odor characteristics of many food substances. Often sulfur compounds are present in trace levels in foods making their isolation and quantification very difficult for chromatographers. This study compares three gas chromatographic detectors the flame photometric detector, sulfur chemiluminescence detector and the atomic emission detector, for the analysis of volatile sulfur compounds in foods. The atomic emission detector showed the most linearity in its response to sulfur the upper limit of the linear dynamic range for the atomic emission detector was 6 to 8 times greater than the other two detectors. The atomic emission detector had the greatest sensitivity to the sulfur compounds with minimum detectable levels as low as 1 pg. 

The flame ionization detector (FID) is the most popular detector because of its high sensitivity and wide linear dynamic range [1]. The FID is an ionization detector that exhibits a nearly universal response to all organic compounds. The sensitivity, stability, excellent linear range, ease of operation and maintenance, along with wide applicability and low cost has made this detector the most popular gas chromatographic detector in use today [2]. 

---