A common cause of carryover is inadequate washing of the system. Choosing an appropriate
wash solvent can minimize carryover for a particular analysis. The wash solvent must be strong
enough to dissolve any remaining sample, and the wash duration must be long enough to remove
the residue from the system.
Method conditions also affect carryover. Too short a hold-time at the final conditions of a gradient,
especially if the gradient is steep, can fail to remove all analytes from the system. It is important
to completely flush the system and re-equilibrate the column before proceeding to a subsequent
analysis. Use caution when choosing the load-ahead and loop-offline options. Initiating these
options before the highly organic part of the gradient reaches the needle can leave sample
residue in the system, and whatever time savings you gain can be lost in terms of inadequate
system cleaning.
The hydrophobicity and solubility of samples as well as cleanliness during sample preparation are
additional factors to consider when trying to minimize carryover, as is contamination from sample
preparation tools.
2.4 Cycle time (between injections)
The short run time of a UPLC separation requires efficient use of the time between analyses.
The sample manager has a load-ahead option that can help decrease cycle time. This option
instructs the sample manager to aspirate the next sample while the current sample is running.
The Loop Offline option on the sample manager reduces the impact of delay volume on cycle
time by taking the needle and extension loop offline before the gradient reaches the injection
valve and after the sample transfers to the injection port.
Setting an appropriate syringe draw rate can also help reduce cycle time. By default, the system
uses feedback information from a pressure transducer to optimize the syringe draw rate for
maximum throughput and performance.
2.5 Preventing leaks
Preventing leaks during an analysis ensures adequate flow pressure in the system and the
integrity of the sample.
Leaks can occur at any tubing connection, gasket, or seal but are most common at tubing
connections. Low pressure leaks (on the intake side of the solvent manager’s pump) cause
solvent loss and air introduction during the intake cycle. Leaks at high pressure fittings
(downstream of the "intelligent" intake valves) can leak solvent but do not introduce air.
To prevent leaks, follow Waters’ recommendations for the proper tightening of system fittings.
Note specifically that different techniques apply to re-tightening fittings versus installing them for
the first time.
August 8, 2016, 715005049 Rev. C
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