Carryover is measured when residual peaks from a previous active-containing injection appear in a subsequent blank solvent injection. There will be carryover between active injections, which may lead to erroneous results. The level of carryover is reported as the area of the peak in the blank solution expressed as a percentage of the area in the previous active injection. The Multisampler is optimized for lowest carryover by careful design of the flow path and use of materials in which sample adsorption is minimized. A carryover figure of 0.001 % should be achievable even when a triple quadrupole mass spectrometer is the detector. Operating settings of the Multisampler allow the user to set appropriate parameters to minimize carryover in any application involving compounds liable to stick in the system. The following functions of the Multisampler can be used to minimize carryover:
Internal needle wash
External needle wash
Needle seat backflush
Injection valve cleaning
The flow path, including the inside of the needle, is continuously flushed in normal operation, providing good elimination of carryover for most situations. Automated delay volume reduction (ADVR) reduces both the delay volume and the flushing of the standard Multisampler and should not be used with analytes where carryover might be a problem.
The outside of the needle can be washed using a wash vial in a specific location or the flush port. If a wash vial is chosen, then this vial should have no septum and should contain a solvent suitable for washing the sample from the needle. The septum is not used to avoid wiping contamination off the needle on the downstream only to re-apply it on the upstroke. The needle can be dipped into the vial multiple times. This will be effective in removing a small degree of carryover, but for more effective external needle wash, use the flush port.
The flush port is located above and behind the needle seat, and in the standard hardware configuration a peristaltic pump delivers the wash solvent. It has a volume of 0.68 mL and the peristaltic pump delivers 6 mL/min, which means the flush port volume is completely refilled with fresh solvent in 7 s.
If the flush port is selected, the user can set how long the outside of the needle is to be washed with fresh solvent. This can last from 2 – 3 s in routine situations where carryover is less of a problem, to 10 – 20 s for complete washing.
It is recommended that the external needle wash in the flush port should be standard procedure to avoid contaminating the needle seat. If the needle seat becomes contaminated, it must be back-flushed. In the standard setup, this task can be done manually by changing the flow connections, or it can be automated using the Flexible Cube (G4227A). In the Multiwash setup (G4757A), the needle seat back-flush is automated and can be performed with up to three different solvents.
The flush port and its solvent delivery pump and tubing should be regularly flushed to ensure the lowest carryover. For example, before using the system each day, prime the flush pump for three minutes with appropriate solvent.
When other measures have failed to eliminate carryover, it might be that analyte is adsorbing to the inner surfaces of the injection valve. In this case, the auto clean feature in the CDS can be activated, and the valve will make additional switching movements to clean out the flow path. If the problem compounds need a high percentage of organic phase for elution, it is recommended to switch the injection valve at the high percentage of organic phase after the last peak has eluted. It is also recommended to switch the injection valve again after the initial conditions for the mobile phase have stabilized. This ensures that the bypass groove in the rotor seal of the valve contains the gradient start conditions, which is especially important for flow rates below 0.5 mL/min. For samples where the outside of the needle cannot be cleaned sufficiently with water or alcohol from the flush pump, use wash vials with an appropriate solvent. With an injector program, several wash vials can be used for cleaning.
The optimum carryover performance of the Multisampler is achieved after a run-in period of new instruments or after the exchange of consumable parts (like needle, needle seat and valve parts). During injections in this period, surfaces of these parts adjust to each other. After this period, we recommend back-flushing the needle seat to get the sealing areas between needle and needle seat clean. Regular preventive maintenance service is recommended as the carryover performance of the autosampler depends on the integrity of these consumable parts.
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