Detector response chromatography

Potentiometry is the measurement of the potential at an electrode or membrane electrode, so the detector response is in units of volts. The potentio-metric response tends to be slow, so potentiometry is used infrequently in analysis.47 One example is the use of a polymeric membrane impregnated with ionophores for the selective detection of potassium, sodium, ammonium, and calcium 48 In process chromatography, potentiometry may be used to monitor selected ions or pH as these values change over the course of the gradient. 

Kordorouba, V. and Pelletier, M., Ion chromatography using an electrochemical detector response to non-electroactive anions, /. Liq. Chromatogr., 11, 2271, 1988. 

Thuren [1] determined phthalates in sediment using solvent extraction (acetonitrile, petroleum ether), clean-up with deactivated Florisil, and quantitative analysis by gas chromatography. The detector response was linear between 0.5 and lOOng. The detection limit (signahnoise ratio 2 1) was O.lng for dimethylphthalate, dibutylphthalate and di(2-ethylhexyl) phthalate, and 0.05ng for benzoylbutylphthalate. Recovery was between 30% and 130% depending on the ester. Low recovery for dimethylphthalate (30%) was probably due to pyrolysis in the detector (detector temperature was 320°C). 

The HPLC pump draws the mobile phase from the reservoir via vacuum action. In the process, air dissolved in the mobile phase may withdraw from the liquid and form bubbles in the flow stream unless such air is removed from the liquid in advance. Air in the flow stream is undesirable because it can cause a wide variety of problems, such as poor pump performance or poor detector response. Removing air from the mobile phase, called degassing, in advance of the chromatography is a routine matter, however, and can be done in one of several ways 1) helium sparging, 2) ultrasonic agitation, 3) drawing a vacuum over the surface of the liquid, or 4) a combination of numbers 2 and 3. 

Generally, different components possess different response factors, application of which not only compensates for different detector response for different components but also take into consideration the other factors inherent with the procedure. However, these factors may be calculated by preparing a synthetic mixture absolutely identical to what is expected in the sample, and subsequently carrying out the gas-chromatography of this mixture exactly under idential experimental parameters as described in the method of analysis. Thus, we have ... 

To illustrate consider gas chromatography. Figure 8.16 shows an idealized plot of detector response vs. time. Here to is the time lapse between injection and elution of inert material, and ti and t2 are retention times for the isotopomers of interest. For difficult separations ti t2 to. The resolution, R, is related to the band widths, R = (t2 — ti)/(2w), and the number of plates (assumed to be the same for both isotopomers) is, n = 4(t/w)2. The separation factor is a = t2/ti, and the... 

Solutes eluted from a chromatography column are observed with various detectors described in later chapters. A chromatogram is a graph showing the detector response as a function of elution time. Figure 23-7 shows what might be observed when a mixture of octane, nonane, and an unknown is separated by gas chromatography, which is described in Chapter 24. The... 

The ELS detector was previously also referred to as a mass detector, pointing to the fact that the response is (mainly) determined by the mass of the sample rather than by its chemical structure. Van der Meeren et al., though, demonstrated that the ELSD calibration curves of phospholipid classes were also dependent on the fatty acid composition (52). The dependence on the fatty acid composition is, however, completely different in nature and much less pronounced than for UV detection. The reason for this behavior is to be found in the partial resolution of molecular species, even during normal-phase chromatography. Thus, the peak shape depends not only on the chromatographic system but also on the fatty acid composition and molecular species distribution of the PL sample (47). Because it was shown before, based on both theoretical considerations and practical experiments, that the ELS detector response is generally inversely proportional to peak width (62,104), it follows that the molecular species distribution of the PL standards used should be similar to the sample components to be quantified. It was shown that up to 20% error may be induced if an inappropriate standard is used (52). 

Several papers have reported use of a weak acid/crown ether eluent in ion-exclusion/cation exchange chromatography with conductimetric detection on a weakly acidic cation-exchange column to effect the simultaneous determination of both cations and anions. Resolution was significantly improved when 18-crown-6 was present in the eluent. Detector response was positive for anions and negative for cations. [76-80] A sample chromatogram is shown in Figure 9. 

For gas chromatography volatile derivatives such as the methyl ester of ABA (27, 29, 30) or trimethylsilylated ABA (20, 28) must be prepared. ABA is a molecule with a high electron affinity so that the methyl ester can be measured with a gas chromatograph equipped with an electron capture detector. Injections of as little as 5 pg of Me-ABA cause a detector response (27). Metabolites of ABA such as phaseic and dihydro-phaseic acid can also be measured by this method (33). 

G-25 columns but with a 10-fold increase in efficiency A linear detector response to amounts of water derived fulvic acids separated on the gel permeation chromatography 100 column was obtained enabling direct measmement of amounts of fulvic acid in water samples. The detection limits were 40ng for copper EDTA and 12 gg for fulvic acid. 

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