Baseline problems

Replace the lamp even if the energy at e.g. 254 nm is still acceptable. Replace the flow cell windows. Check if the mobile phase is UV transparent and clean. Is the solvent degassed properly Use a degasser. 

Noise is still too high after UV lamp replacement  

The reasons can be warming-up, incomplete mixing or incomplete degassing. It is recommendable to use a degasser and a column oven. 

Often the problem disappears by being more patient with regard to flushing. Compounds from earlier injections can still be within the column which need to be eluted with suitable mobile phases. 

See the proposals given above under Pressure Fluctuations . If the drift occurs only in the low UV, it is well possible that the pump and/or detector are not suited. Use a pump with low pulsation and a detector with a flow cell which is insensitive to refractive index changes and is suitable for gradient separations. 

---

The intercept is an average in certain circumstances. It is an important term because the average response is not normally achieved when the factors are at their average values. Only in certain circumstances (e.g. spectroscopy if it is known there are no baseline problems or interferents) can this term be ignored. 

Short-term noise, long-term noise, and drift. The causes of the baseline problems and their remedies are discussed in Table 3. 

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. 

Baseline Problems (Chromatogram) Common baseline problems are illustrated with several diagnostic examples shown in Figures 10.6 and 10.7. The description of each symptom and its possible remedial action is as follows. 

Figure 10.7. Examples of various baseline problems and possible causes.

Small amounts of impurities in carrier gases can cause baseline problems (oil), reaction with the stationary phase (oxygen), and loss of sensitivity for several detectors (water), and hydrocarbons. 

Impurities in the gas (oil, water, oxygen, hydrocarbons) cause baseline problems. 

In summary, dirt in the UV detection cell causes inferior peak-to-noise ratio, spikes and baseline problems. 

For these sample types, the choice of solvent is important because choosing a solvent system that closely matches the separation solvent minimises baseline problems and other unwanted separation effects. 

Baseline problems in isocratic systems can usually be spotted due to spikes or to waves. Spikes usually occur as a result of air trapped in the detector cell and waves usually occur as a result of lamp deterioration or contamination. A good place to start with any baseline issues is to switch off the pump and continue to monitor the baseline. 

Does not require Baselines, Problem Reporting, Change Review, Configuration Status accounting. Media Selection/ Refreshing/ Duplication and Release. 

---