Developing Methods
Matrix Effects
72
To determine if equilibrium is reached, use Chain Methods (see “Chaining
Methods” on page 65) to increase the Vial Equilibration Time for each method
in the chain, or use Param Increment (see “Advanced Function Descriptions”
on page 44) to increase the Vial Equilibration Time for each successive sample
in the series. Then plot the peak area versus equilibration time for a certain
peak. The system is at equilibrium when this curve flattens out. See Figure 12.
Figure 12. Equilibration curve
Loop Fill Time
Normally, set the loop fill time to 10 seconds.
A very short loop–fill time (one or two seconds) may help increase sensitivity
in headspace analysis. The headspace gas purges the sample loop, but the vent
valve is closed before the sample vial falls to atmospheric pressure. The loop is
maintained at the vial’s still-elevated pressure. Because the loop pressure is
greater, the analyte will be more concentrated and, consequently, more sample
will be injected into the GC.
Matrix Effects
The composition of the sample matrix can affect the amount of analyte that
escapes into the headspace. Adding an inorganic salt to aqueous samples
increases the concentration of organic molecules in the headspace by making
them less soluble in the sample matrix, increasing sensitivity. For nonaqueous
[X]
G
∆[X]
G
≈ 0
Equilibrium
∆T
Heating Time
Degradation
Condition
Where:
X is the component of interest
[X]
G
is the concentration of X
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