Troubleshooting Detector Problems; Gas Saturation - PerkinElmer 200 UV/VIS Series User Manual

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Troubleshooting Detector Problems

After your Series 200 UV/VIS LC Detector has passed all baseline drift/noise tests and

exhibits a quiet, steady baseline when operating alone, it is ready for use. Sometimes, how-

ever, problems may still be encountered.

Most flowcell bubbles are caused by air leaking into the solvent line or by improperly

degassed mobile phases. The presence of dissolved gases in mobile phase solvents can

change both the UV cutoff for a given solvent, as well as the slope of the absorbance curve for

that solvent. When working at a wavelength well above the cutoff value, dissolved gases can

cause drift. As the cutoff value is approached or more sensitive output settings are used, the

apparent drift becomes more pronounced (see the troubleshooting guide at the end of this

section to eliminate the possibility of a detector malfunction as the cause of baseline drift).

Gases tend to stay in solution in the mobile phase between the HPLC pump and column out-

let because of the relatively high pressures in that area of the chromatographic system.

However, these gases may bubble out of solution after leaving the column when the pressure

drops to a few psi, and they may subsequently become lodged in the flowcell light path. Con-

nect a 50 psi backpressure device to the exit side of the flowcell (PerkinElmer

PN 0990-7126). This causes the bubble to redissolve and flush from the flowcell. There is lit-

tle danger of damage if backpressure is applied since PerkinElmer flowcells are rated to 500

psi.

As soon as a gas bubble begins its transit across the flowcell lightpath, the absorbance will

change dramatically and erratically, causing large swings in the signal output. Overcoming

the problem of gas saturation is best done before the mobile phase even enters the HPLC

pump by degassing or sparging.

Bubbles may be observed by dialing up a visible

wavelength (wear safety glasses). If no bubbles

are present, a bright circle of light can be seen.

if bubbles are present, however, the circle of

light will be eclipsed, and various configura-

tions will be observed instead of a simple circle

of light.

Maintenance

7 - 15

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