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Troubleshooting
6.4.9 Negative peaks and carrier contamination
Negative peaks (Figure 62) are normal for some chemistries like hypochlorite, alkalinity,
etc. These methods are called inverse chemistries. On the other hand, direct chemistries
always produce positive peaks. If the carrier of a direct chemistry is spiked or
contaminated with the analyte being determined, all standards and samples that have a
lower concentration than that of the carrier will produce negative peaks. To correct the
problem, get a fresh carrier solution. The ISE fluoride method is a bipolar chemistry so
the calibration will produce positive and negative peaks that look like carrier
contamination. In fact, the carrier of this chemistry has been spiked with fluoride to
increase sensitivity.
6.5 Flow blockages
An easy way to find out if there are any flow obstructions is by lifting each of the
transmission lines (the Tygon tubing that is submerged in every reagent container) one at
a time so that some air is aspirated (Figure 63 on page 74).
Note: Make sure the pump speed is at 35. If air in the tubing begins to move towards the pump,
there is no total blockage or clog in that reagent stream. If no air is aspirated, a total clog is probably
present. Follow the procedure below to determine the location of the clog. The transmission line that
does not aspirate air is called the suspect line. Sometimes a Dye Test is necessary to determine the
presence of a clog or partial clog (refer to section 6.12 on page 77).
Figure 62 Negative peaks and carrier contamination
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