Baseline noise, reducing

Besides CZE and NACE, micellar electrokinetic chromatography (MEKC) is also widely used, and ionic micelles are used as a pseudo-stationary phase. MEKC can therefore separate both ionic and neutral species (see Chapter 2). Hyphenating MEKC with ESI/MS is problematic due to the non-volatility of micelles, which contaminate the ionization source and the MS detector, resulting in increased baseline noise and reduced sensitivity. However, MEKC—ESI/MS was applied by Mol et al. for identifying drug impurities in galantamine samples. Despite the presence of non-volatile SDS, all impurities were detected with submicrogram per milliliter sensitivity and could be further characterized by MS/MS. 

The low frequency baseline noise of the viscometer can be substantially reduced by careful filtration of samples and regular checking and maintenance of column end fittings and fractional sections of tubing in the system. Figure 5 shows the effect of column screen replacement on the stability of the baseline signal at a flow rate of 1.0 ml/min. 

Very often baseline problems are related to detector problems. Many detectors are available for HPLC systems. The most common are fixed and variable wavelength ultraviolet spectrophotometers, refractive index, and conductivity detectors. Electrochemical and fluorescence detectors are less frequently used, as they are more selective. Detector problems fall into two categories electrical and mechanical/optical. The instrument manufacturer should correct electrical problems. Mechanical or optical problems can usually be traced to the flow cell however, improvements in detector cell technology have made them more durable and easier to use. Detector-related problems include leaks, air bubbles, and cell contamination. These usually produce spikes or baseline noise on the chromatograms or decreased sensitivity. Some cells, especially those used in refractive index detectors, are sensitive to flow and pressure variations. Flow rates or backpressures that exceed the manufacturer s recommendation will break the cell window. Old or defective source lamps, as well as incorrect detector rise time, gain, or attenuation settings will reduce sensitivity and peak height. Faulty or reversed cable connections can also be the source of problems. 

Gradient shifts can be reduced by reducing the concentrations of the mobile phase additives (i.e., from 0.5-01% TFA to 0.03-0.05%) and by balancing (matching) the absorbances of MPA and MPB. See example in Chapter 8, case study 2. This would also reduce the short-term baseline noise caused by pump blending. 

In an effort to improve signal quality by reducing baseline noise, a water-cooled particle impinger bar was installed immediately upstream of the path control probes. This bar directs a portion of the particulates around the beam path and reduces the interaction of particles with the beam. In this configuration, the TDL and extractive CO measurements agreed to within approximately 2% CO over a range of approximately 0 to 15% CO. The output of both instruments contains a noise band of approximately +1% CO. 

In addition to flow precision, flow smoothness is also essential. Flow smoothness results in a very stable, low-noise baseline signal from the RI detector. Detector sensitivity is governed by the ratio of the signal that is produced to the baseline noise. With lower noise it is possible to reduce the scale of operation and still obtain a good signal for quantitation. Reducing... 

This also has an impact on the required maintenance efforts. To keep the baseline noise of a UV/visible detector within the manufacturers specification will require frequent cleaning of the flow cell, it will require ultra-pure mobile phases and the lamp may have to be changed frequently. For applications not requiring Ihe greatest sensitivity this effort can be reduced if the acceptance limit for the baseline noise is set higher. 

Because ginsenosides do not strongly absorb above 205 nm, HPLC methods based on low wavelength UV detection suffer from baseline noise and low sensitivity that limits the choice of solvents and mobile-phase additives for improved separation. With ELSD, interferences are reduced and baseline slopes are not as pronounced or as variable, maldng peak area determination more reliable [114,115]. 

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