Sample loops, high-speed

The different forms of dispersion profiles that are obtained from various types of connecting tubes used in LC are shown in figure 9. These dispersion curves were obtained using a low dispersion UV detector (cell volume, 1.4 pi) in conjunction with a sample valve with a 1 pi internal loop. All tubes were of the same length and a flow rate of 2 ml/min was employed. The peaks were recorded on a high speed... 

Example—Consider a sample point located 100 ft away from an analyzer requiring 200 cm min of sample. Using standard conditions and 0.19 in. inside diameter tubing, a lag time of 75 min can be calculated. By increasing the sample flow to 2200 cm min and splitting the excess sample to a high-speed loop, the lag time decreases to 7.5 min. The sample pressure should be reduced at the analyzer. 

A normally closed high pressure solenoid valve is included in the speed loop to stop the sample flow under alarm conditions. 

Instabilities in the feedback system are detrimental to resolution and include poor piezoelectric positioner response, cross talk, and noise. When high aspect ratio features are imaged, the pipette may retract too quickly in the Z direction which can result in lag of the feedback loop. Cross talk can be observed between topographic and current images of a substrate with both conductive and nonconductive features are generated. As the probe is scanned across a sample surface that transitions from nonconductive to conductive, the feedback mechanism assumes the tip is closer to the surface than the set point, which results in a decreased ion current signal. The opposite occurs when the probe is scanned from conductive to nonconductive regions. These effects can be minimized with decreased scan speed and optimized scan parameters. 

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