Reproducibility and Solvent Degassing 168
C
The compressibility of a solvent depends on two factors:
• Intrinsic properties
• Dissolved gas
Intrinsic Compressibility
The intrinsic compressibility varies between solvents. Water, with its low compressibility,
accurately couples syringe displacement with sample withdrawal. Methylene chloride
exhibits the most compressibility.
Intrinsic compressibility of a solvent does not vary from injection to injection, and therefore
does not affect the reproducibility of an injection. However, always consider intrinsic
compressibility when you perform compression checks and set the compression limit (see
Section 4.5.3,
Performing a Compression Check).
Note: Always perform a compression check when changing solvents or switching to a new
bottle of solvent.
Dissolved Gas
The dissolved gas in the mobile phase affects its compressibility. Dissolved gas can be
highly variable, and it is the largest factor affecting reproducibility in chromatography.
Viscosity
If the sample is viscous, its response to syringe displacement is slower. When the syringe
is withdrawn, the mobile phase forms a partial pressure and fails to withdraw the
programmed amount of sample. The Waters 717plus Autosampler allows you to adjust
syringe speed for viscous samples (see Section 4.8.3,
Injecting Viscous Samples).
C.1.2 Instrument Factors Affecting Reproducibility
Fluid system factors that affect reproducibility are:
• Integrity of sample loop
• Volume of sample loop
Integrity of the Sample Loop
The sample loop must be sealed from the sample vial to the syringe and the vial should be
at atmospheric pressure. If there is leakage, air can displace some of the sample.
Test sample loop integrity by running the compression test at pressures greater than
30 psi (see Section 4.5.3,
Performing a Compression Check).
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