Agilent Technologies agilent 7683B, Installation, Operation And Maintenance Manual

Page: 79 / 202

Introduction to Operation
3
Installation, Operation, and Maintenance 79
Part 2, Operation
Sample Carryover
Carryover is the presence of peaks from an earlier injection in the present
analysis.
The injector uses solvent washes, sample washes, and sample pumps to
control carryover. Each of these actions reduces the amount of sample left in
the syringe. The effectiveness of each depends on your application.
Solvent wash
The injector draws solvent into the syringe from either the solvent A, A2, A3 or
solvent B, B2, B3 position, then discards the syringe contents into one or more
waste bottles. Solvent washes can occur before taking a sample (pre-injection
solvent wash) or immediately after the injection (post-injection solvent wash).
The volume of the wash can be adjusted on a 6850 or 6890 GC.
Sample wash
During a sample wash, the injector draws the next sample into the syringe and
discards the contents into one or more waste bottles. Sample washes occur
before the injection. When sample is limited, you can use a solvent prewash to
wet the syringe before drawing sample. The volume of the wash can be
adjusted on a 6850 or 6890 GC.
Sample pump
During a sample pump, the injector draws sample into the syringe and returns
it to the sample vial. Pumps occur after sample washes and immediately
before the injection. Pumps serve to eliminate bubbles. If the needle contains
solvent from a previous wash, the pump may add a small amount of solvent
that mixes with the sample and can dilute a small volume.
Number and type of washes
Under ideal conditions, four default volume (80%) washes reduce the
carryover to one part in 10,000. The number and type of washes you need
depends on:
• The amount of carryover that you can accept
• The viscosity and solubility of the analyte(s)
• The viscosity and volatility of the solvent(s)
• The degree of wear in the syringe barrel
• The wash volume