Choosing The Load-Ahead And Loop-Offline Options; Reducing Carryover - Waters ACQUITY UPLC Overview And Maintenance Manual

result, the gradient can fail to remove all sample from the needle, resulting in low sample
recovery and an increased risk of carryover.

2.33 Choosing the load-ahead and loop-offline options

To choose the load-ahead and loop-offline options:
1. In the instrument method editor, click the ACQ-FTN tab, and then click the General tab.
2. Select the check boxes for the load-ahead and loop-offline options.
3. Specify an interval for the loop-offline option.
Tip:
A programmed gradient typically flows through all parts of the instrument that contact
the sample. If you initiate the needle and extension-loop offline option before the gradient
reaches its final conditions, the highly organic portion of the gradient does not pass through
the needle. As a result, the gradient may fail to remove all sample from the needle,
resulting in low sample recovery and an increased risk of carryover.

2.34 Reducing carryover

In a chromatographic system, any substance that creates unwanted peaks or excessive
background noise is contamination. Carryover, a specific type of contamination, occurs when
sample material remaining in the system after an injection appears as peaks in subsequent
injections, compromising quantification. To optimize system performance, carryover must be
minimized and held to an acceptable level (often below the limits of detection).
Carryover can result from incorrectly installed tubing, fittings, or other hardware or by ineffective
wash solvents. Take these actions to reduce carryover:
• Restrict extension-loop usage to one system.
• Ensure all tubing connections are properly seated. Tubing must seat properly (without internal
gaps) inside fittings before you tighten the fittings. Poorly seated connections create
reservoirs of unnecessary space that retain sample, increasing carryover.
• Inspect the needle guide for sample residue or debris, which can cause carryover. If
necessary, clean or replace the guide.
• Avoid plate or vial sealing systems that use sticky substances, which can cause carryover.
• If you suspect sample interaction with the needle material, increase the strength of the wash
solvent, or increase the wash time. If carryover persists, replace the needle with one of a
different material composition, such as the Bio (MP35N) needle, which can reduce sample
interaction.
• Follow the guidelines that appear in
selecting wash solvents.
General purge and wash solvent guidelines
June 11, 2019, 715005051 Rev. D
Page 57
when