Hardware manual

Technical Information 6
Agilent 7500 Series ICP-MS Hardware Manual 6-21
The scan line of a Q- pole is represented in Figure 135. Lower voltages of U and
V select lighter masses while higher voltages select heavier masses.
Resolution can be expressed as follows:
Resolutions ∝ f
2
L
2
/V
f : Frequency
L :Length of Q-Pole
V :Speed of Ion
So, higher ion speed, shorter Q- pole length or lower frequency results in
poorer resolution.
When the Q- pole becomes dirty, the peak shape will be poor even if a higher
pole bias is applied. In this case, the cleaning of the Q- pole pre- filter and/or
main filter will be required. Only an Agilent Technologies engineer should
perform the Q- pole cleaning.
Electron Multiplier (EM)
The EM used in the Agilent 7500 is a discrete dynode type manufactured by
ETP, operated in both pulse count and analog modes. A brief description of the
principle of operation follows: when an ion enters the EM, the ion hits the first
dynode and a shower of electrons is generated. The electrons hit the next
dynode generating more electrons. This small signal is amplified and a higher
pulse signal is obtained: electrons are multiplied about 10
6
times. Finally, the
pulse generated is detected by the collector. The pulse width is about 10 nsec.
At this moment, the output signal from the amplifier contains electrical noise
and the pulse signal. After the amplifier, a discriminator circuit eliminates the
electrical noise. Then, pulse signals higher than the discriminator voltage is
converted to an ideal pulse shape. This pulse is measured as one count
(Figure 136).
When the concentration of ions is too high, ions will not be measured
separately and result in only one pulse count. In general, this phenomenon
becomes more critical when the number of ions is greater than several million
counts/sec (cps). When the number of ions is high, the electrical current at the
collector becomes measurable by the analog mode. Since the signal of a high
count rate causes a shorter lifetime, the current for the analog mode is
collected at the halfway point of the dynodes. The current is converted to
log- voltage by the log amp. Then it is sampled by the A/D converter, and is