HP 5890 Series II Plus Reference Manual page 213

Chromatographic Troubleshooting
Peak symptoms
4. Top (apex) of the peak is split:
Figure 9-4.
FID/NPD Flameout, or TCD with H (in He Carrier)
Verify that this is not a merged peak situation: Reduce oven
temperature 30 C and repeat the run. If the split peak becomes
better resolved, it is probably a merged pair.
Gross overload of an FID may cause the top of the peak to invert,
giving appearance of a split peak. Check gas flows; overload is
more likely when flows are too low.
Dilute the sample by a factor of at least 10 and repeat the run:
If the split disappears, overload is the problem. It is advisable to
dilute samples even more, by 100 or 1000, to ensure the detector is
not close to its overload condition. Such dilution generally
improves linearity as well.
H peaks, analyzed with a TCD using He carrier, often shows a
split top.
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