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period. The CI storage Level is typically set to a value that is lower than the

mass of the reagent ion of interest. The CI Storage Level can be set to any

value from 5 to 150 m/z. The optimum CI storage level will depend on the

molecular ion of the analyte. Generally, one should use higher CI storage levels

for higher molecular ions, without raising the storage level to cause ejection of CI

reagent ions. For example, the molecular ion of the analyte is 352 m/z. Using

acetonitrile, raising the rf storage level to 25 m/z may give better sensitivity than

the default of 19 m/z. If the CI storage level is raised, the Ejection amplitude will

have to be increased. For acetonitrile, raising the rf storage level from 19 to 25

m/z, requires an increase in the ejection amplitude from 15 to 25 volts. These

values are determined empirically.

Ejection Amplitude (v)

Ejection Amplitude (v) is a voltage that corresponds to a low mass ejection cutoff

that is slightly higher than the mass of the largest reagent ion produced by the

selected reagent gas. This voltage actively ejects unwanted ions (i.e., not

reagent ions) that are produced during ionization. All ions with masses equal to

or greater than the level established by this voltage will be ejected prior to the CI

reaction. When this value is set to 0, active ejection is disabled; otherwise,

increasing the voltage decreases the ejection cutoff mass. Generally, higher CI

storage values require higher ejection voltages. The voltage should not be set so

high as to cause the ejection of CI reagent ions.

Background mass

Background mass is a mass value that is greater than or equal to the mass of the

largest reagent ion produced by the selected reagent gas. All ions with masses

less than this mass value are ejected from the ion trap after the reaction between

the reagent ions and sample molecules. The background mass level can be set

to any value from 10 to 300 m/z. For methane, ammonia, and isobutane, the

reagent ion ejection levels are 45, 45, and 65 m/z, respectively.

Target TIC Value

In CI Auto mode, the target TIC value determines how many ions are allowed in

the ion trap. A maximum value of 30000 can be set.

Maximum ionization time

Maximum ionization time is the maximum time that energetic electrons, emitted

from the filament, are allowed to interact with reagent gas molecules to form

reagent ions. In CI, the maximum ionization time can be set to any value from 10

to 2500 sec. For methane, ammonia, and isobutane, the maximum ionization

time is usually set to 2000 sec.

Maximum reaction time

Maximum reaction time is the maximum time that reagent gas ions are allowed to

react with sample molecules to form ions. The maximum reaction time can be set

to any value from 1 to 128 milliseconds. For methane, isobutane, and ammonia,

the maximum reaction time is typically set to 60 milliseconds.

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