Baseline stability

The solvent delivery system is one of the most important components of the liquid chromatograph since its performance directly affects retention time reproducibility d detector baseline stability [1,2,39,40]. As well as the pump, other components such as check valves, flow controllers, mixing... 

A most powerful detection mode under actual investigation in CE is CL. Because CL detection does not require a light source for excitation—the required energy being produced by a suitable chemical reaction—problems in baseline stability limiting detection limits are overcome, providing excellent sensitivities due to the low background noise. Recently, some review articles have been produced in this field [46, 65-68],... 

Van Staden reported a rapid, reliable automated method for direct measurement of the chloride content in milk based on the principles of flow injection analysis and the use of a dialyser to remove interferents. Dialysed chloride was measured by means of a coated tubular chloride ion-selective electrode. Potential changes arising from the interference of casein were thus avoided and baseline stability ensured. The results obtained for chloride in milk compared well with those provided by standard recommended methods. The linear range for chloride was 250-5000 pg/mL for 30 pL of sample, and the coefficient of variation was better than 0.5%. The throughput was ca. 120 samples/h [132],... 

Electrochemical detection places a number of restrictions on the mobile phase used in the chromatographic separation but is well suited to reversed-phase separations. Even so, the mobile phase must be oxygen-free, which requires bubbling an inert gas such as helium through the mobile phase reservoir as well as the sample. Additionally, both the mobile phase and the sample must be free of metals to ensure baseline stability during the readings. Despite these constraints, electrochemical... 

GPC techniques are applicable to a wide variety of solute materials, both low and high molecular weight, dissolved in solvents of varying polarity. The selection of column type, column pore size, solvent, and temperature must be appropriately made for each solute. Care must be taken to avoid reaction between the solute and the columns or other adsorption phenomena, especially when two solvents are used—one to dissolve the solute and one in the chromatograph. Changing from one solvent to another in the chromatograph can take 24 hr before the baseline stabilizes. 

Operation of the Waters Associates model 100 instrument at eler vated temperatures, necessitated by dissolution of polyethylene, requires several modifications in order to improve baseline stability and sensitivity (I). Usually the first modification is to reduce the unacceptably high temperature gradients in the oven which may be as much as 20 °C from... 

Column equilibration ( 10 column vol. recommended) ensures baseline stability, good peak shape, and reproducible retention times. For the specific column chosen in this unit for isoflavone analysis, at least 18 ml total is needed to equilibrate the system with mobile phase. The common practice is to purge the pump system and connect the inlet end of the column to the injector outlet. The initial pump flow should be set at 0.1 ml/min and increased to 0.6 ml/min in 0.1 ml/min increments. Once a steady backpressure and baseline have been achieved, the column is ready to use. Before injecting samples, it is suggested to run a blank gradient first to clean the column and help check for the possibility of impurity peaks. 

The separation of safflower oil (SFO)-linseed oil (LSO) methyl esters is shown in Fig. 16. Free fatty acid methyl ester elution reproducibility, resolution, and baseline stability were maintained at sample sizes of 17-170 /zg, although capacity factors (k) decreased approximately 25% between the 17- and 170-/zg sample sizes. The trend of longer retention times with smaller sample sizes was consistent throughout their studies. Peak distortion, such as observed when gas chromatographic columns are overloaded, was not observed in their system. Perhaps larger FAME samples compete for silver ion sites the same way the ACN cosolvent competes for those sites. Excellent peak shapes were obtained, even with sample elution times of 1.5-2.0 h. 

A major problem in practical applications of electrochemical sensor systems stems from the adsorption of surface-active materials onto the working electrode. This adsorption causes electrode fouling, which can have a very negative impact on sensor characteristics like baseline stability and response slope. Such surface fouling... 

Subsequent experiments using dodecane (6) to inhibit evaporative cooling at the spin fluid/air Interface have demonstrated even more improvement in baseline stability and run-to-run reproducibility when using the EGM. 

Lamkin et al. [276] studied in detail the GC analysis of silylated methylthiohydantoins of all protein amino acids. They effected the silylation with BSA-acetonitrile (1 3) at 100°C for 10 min. They separated the products in a simple column packed with 2% of OV-17 on Gas-Chrom Q at 145—230°C, and Fig. 5.20 illustrates the results. The authors used a flame photometric detector, sensitive to sulphur-containing compounds, in order to ensure sensitive and selective detection. Minor incidental peaks that were often noticed during the analysis of the samples obtained by the Edman degradation of proteins with the use of an FID did not appear and the peak of the solvent was not detected. The baseline stability was good and the response was linear over a range of two orders of magnitude of concentration. Asn and Phe were the only unresolved pair Arg, as in previous instances, did not form a volatile derivative. 

Efficiency Separation Factor (a) Specificity (Detector) Retention Quantitative Time Reproducibility Reproducibility Sensitivity Linearity Dynamic Range Throughput (Preparative) Speed Baseline Stability Total... 

Detection is another aspect of IE CEC that has received attention, since only a few inorganic anions absorb UV radiation. Indirect UV detection was employed in some cases and it was found that it interfered with baseline stability... 

Some highly efficient liquid systems use a heated column a 1°C rise in temperature can alter the retention time by 2-3%. The viscosity of liquids decreases as temperature rises, so solvents are easier to pump and faster flow rates are possible. However, precise temperature control, preferably to 0.2°C (K14), is necessary for reproducible performance and to secure baseline stability by maintaining a constant solubility between the stationary and mobile phases. With GLC, it is possible to alter the retention time 1000-fold by increasing the temperature. The analyst is then faced with the conflicting requirements of temperature stability, to ensure a reproducible retention time, and the ability to vary it rapidly according to a predetermined program in order to elute the material it is required to analyze. 

The principle of conductivity suppression is the reduction of background conductivity by converting the eluent to a less conductive medium (H2O) through acid-base neutralization while the analyte ions conductivity is increased, by converting them to a more conductive medium Anions are converted to their acid forms and cations to their hydroxide forms. These reactions lead to higher S/N ratios, thus significantly improving baseline stability and detection limits. 

Direct detection, which is useful when the repetition rate of the transient signal is greater than about 10 per second. A superimposed very low frequency field modulation sometimes yields improved baseline stability. 

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