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Mass Range Segment Breaks

Each EI scan is divided by default into four mass segments: 10 to 99 m/z, 100 to

249 m/z, 250 to 399 m/z and 400 to 650 m/z. For certain applications, the mass

ranges in these segments can be adjusted. Additional segments (maximum of 6

segments) can be added by clicking on the Insert button.

Ionization Storage Level

Each segment has its own ionization storage level. This corresponds to an rf

voltage that is used to hold ions in the trap during the ionization period. It affects

ion storage in two ways— the storage efficiency of higher mass ions increases

as the level increases, while lower mass ions are not stored if their mass falls

below the cutoff. With AGC on, the default storage level is set to 35 m/z, causing

all ions above 35 m/z to be stored. This value gives good storage efficiency for

ions up to 650 m/z, while not storing the 18 and 19 m/z ions resulting from water

vapor.

For certain analyses, the storage voltage may be increased to prevent the

storage of ions produced from the solvent. For example, to exclude ions

produced from methanol (mass 32 and below), the storage voltage may be set to

38 m/z for all segments. Keep the EI Background mass greater than or equal to

the segment ionization storage voltage.

Ionization Time Factor

The Ionization Time Factor is a secondary factor that determines how well the

Saturn GC/MS is tuned. The Ionization Time Factor, given as a percent, is a

number that is multiplied by the calculated ionization time (determined by the

AGC pre-scan pulse) to give the actual ionization time. The default value of the

time factor is 100%. Ionization Time Factor determines the actual ionization time

for each segment; thus, it determines the number of ions in the ion trap for that

segment

The calculated ionization time, determined by the AGC pre-scan pulse, is

generally the optimum value for segment 2. At lower masses (segment 1),

however, an actual ionization time of as low as 75% (and never lower than 50%)

of the calculated ionization time will sometimes provide a better tune. And for

higher masses (segments 3 and 4), an actual ionization time of up to 125% (and

never higher than 200%) of the calculated value sometimes provides better

results.

As the ionization time factor is increased (actual ionization time increased), the

peak height increases. However, the resolution of adjacent mass peaks may

decrease and the peak shape might distort.

Target TIC Value

In EI mode with AGC on, the target TIC value determines how many ions AGC

allows into the ion trap during the calculated ionization time. While increasing the

target TIC value will increase peak heights, increasing it too far will result in a

loss of mass resolution. This would be observed as mass misassignments and/or

incorrect isotope abundance ratios. For full-scan EI operation, a target TIC of

20,000 is a good starting point. Under special circumstances, e.g., MS/MS, a

much lower target TIC is required. As the m/z distribution of the stored ions

becomes smaller, fewer ions can be trapped without a loss of resolution.

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