Detector/system linearity calculation

Experiments 10-27 are designed to check the autosampler injection precision, pump repeatability and detector/system linearity. One programs the system to automatically inject multiple replicate volumes of a certified test standard. One typically injects 6-10 replicates per volume. The standard component s peak areas are used for calculated injection precision (reproducibility) and system linearity whereas, the retention times are used to calculate pump repeatability. 

The significant intrinsic limitation of SEC is the dependence of retention volumes of polymer species on their molecular sizes in solution and thus only indirectly on their molar masses. As known (Sections 16.2.2 and 16.3.2), the size of macromolecnles dissolved in certain solvent depends not only on their molar masses but also on their chemical structure and physical architecture. Consequently, the Vr values of polymer species directly reflect their molar masses only for linear homopolymers and this holds only in absence of side effects within SEC column (Sections 16.4.1 and 16.4.2). In other words, macromolecnles of different molar masses, compositions and architectures may co-elute and in that case the molar mass values directly calculated from the SEC chromatograms would be wrong. This is schematically depicted in Figure 16.10. The problem of simultaneous effects of two or more molecular characteristics on the retention volumes of complex polymer systems is further amplifled by the detection problems (Section 16.9.1) the detector response may not reflect the actual sample concentration. This is the reason why the molar masses of complex polymers directly determined by SEC are only semi-quantitative, reflecting the tendencies rather than the absolute values. To obtain the quantitative molar mass data of complex polymer systems, the coupled (Section 16.5) and two (or multi-) dimensional (Section 16.7) polymer HPLC techniques must be engaged. 

A transfer function, defined as the Laplace transfer of the impulse response of a linear system, can be obtained from the model. This can be very useful, because with a transfer function the influence of extra-column effects (detector, amplifier, filter) on the peak shape can be easily calculated. The transfer function is ... 

A Varian 3400 gas chromatograph equipped with a flame ionization detector and a nonpolar fused silica capillary column (60 m x 0.25 mm i.d. 0.25 pm thickness, SPB-1, Supelco, Inc.) was used to analyze the volatile compounds from the model systems. The injector temperature was 250°C, and the detector temperature was 260°C. The flow rate of the helium carrier gas was 1 mL/min and the split ratio was 50 1. The temperature program consisted of a 10 min isothermal period at 35°C, temperature increases of 2°C/min from 35°C to 120°C and of 4°C/min from 120°C to 235°C, and a 40 min. isothermal period at 235°C. The chromatograms were plotted and integrated on a Varian 4270 integrator. Linear retention indices for the volatile compounds were calculated using n-paraffin standards (C6-C25 Alltech Associates) as references according to the method of Majlat and co-workers (5). 

One of the keys for generating reproducible data is to consider a system s lower detection limit and the upper limits for linear detector response (Beer s law). For instance, coulometric moisture analysis is accurate for samples containing 10 (Xg of H20. Injection of a particularly dry sample may deliver <10 xg of H20,in which case the amount of water measured may not be accurate. Similarly, injecting a dilute sample aliquot may not allow the detection of an impurity present at low levels, so a more concentrated sample preparation may be required. The analysis of a sample preparation more concentrated than those routinely prepared may give detector area count responses beyond the linear response for that compound, making the calculated concentration lower than the true concentration. The injection of a highly concentrated sample may also saturate the detector... 

In the test, a sample aliquot diluted with a viscosity-reducing solvent is introduced into the gas chromatographic system, which uses a nonpolar open tubular capillary gas chromatographic column for eluting the hydrocarbon components of the sample in the order of increasing boiling point. The column oven temperature is raised at a reproducible linear rate to effect separation of the hydrocarbons. Quantitation is achieved with a flame ionization detector. The sample retention times are compared with those of known hydrocarbon mixtures, and the cumulative corrected area of the sample determined to the 371°C (700°F) retention time is used to calculate the percentage of oil volatilized at 371°C (700°F). 

---