Page 193 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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prevent possible damage. Areas of concern include:

• plasma formation without melting the torch;

• reflected power;

• punching the plasma;

• operating pressures in the mass spectrometer.

B.7.4.3 Tuning for Maximum Signal
The initial aim is to optimize the system in order to obtain the maximum signal for a given
concentration of analyte. A typical tuning solution comprises Be, Mg, Co, In, Ba, Pb and U at a
concentration of 10 ng ml .
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B.7.4.4 Mass Calibration

The correct procedure for mass calibration will be demonstrated.

B.7.5 Optimization

Before performing the actual isotope dilution analysis it is important to optimize the instrument to
ensure precise and accurate data. The following aspects must be considered.

B.7.5.1 Data Acquisition Parameters.

The precision of any isotope ratio measurement depends to a great extent on the mass spectrometer
operating conditions. Because a quadrupole mass spectrometer is a rapid sequential analyser, the
frequency at which it switches between the masses to be ratioed and the amount of time spent collecting
data at each mass must be optimized.

Two main modes of operation are possible. In the scanning mode, the whole profile of each of the
masses is scanned. In the peak-hopping mode only several points (i.e. channels) over the peak profile
are included in the data acquisition. The latter method is more rapid so is generally preferred, and is the
method used here.

To optimize the instrument we will use a 10 ng ml solution of Cd which has been blended from natural
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Cd and the enriched 106 Cd standard, to give a Cd: Cd ratio of approximately 1. Prepare this solution
106
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as follows:
1 Accurately weigh 100 µl of a 10 µg g natural Cd solution into a polypropylene bottle.
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