Metrohm 746 VA Trace Analyzer - Manual (Page 546)

7.5 Vagaries of the chemistry
746 VA Trace Analyzer / 747 VA Stand
7-47
7.5.7 Peak on highly curved baseline
If peaks lie on a highly curved baseline, the first attempts at rectification should in-
volve chemical or measurement technique countermeasures to eliminate the ad-
verse effect on the peak evaluation due to the highly curved baseline. Such meas-
ures include longer purging times (if oxygen interferes, see section 7.5.2), changing
the pH value, changing the supporting electrolyte concentration, modifying or
changing the supporting electrolyte, use of complexing agents (see example in
section 7.5.6), longer enrichment times and changing the measurement technique.
If the curvature of the baseline can not or only partially be eliminated by the above
measures, the 746 VA Trace Analyzer offers the two following possibilities to ap-
proximate a curved baseline (see section 5.6.3):
Type = polynom for baselines with normal curvature
Type = expon. for baselines with exponential curvature
A further possibility to evaluate peaks on curved baselines involves the background
subtraction, in particular when the curved baseline can be clearly attributed to the
supporting electrolytes (see instruction BACKGDN , section 5.7.3).
7.5.8 Peak overlapping
The 746 VA Trace Analyzer offers the possibility to evaluate overlapping peaks (see
section 6.7.1). If an overlapping is found at the front or rear of the peak, this is
displayed on the dialog page ”RESULTS/Evaluation” by appropriate comments in
the Comments column (see section 5.5.2). Depending on the extent of the overlap-
ping, the following three cases are distinguished:
• Overlapping < 10%
Comment front overlapping or rear overlapping . This overlapping is
admissible and leads to no noticeable result falsifications.
• Overlapping 10 … …40%
Comment crit. front ovlp. or crit. rear ovlp. . Although the peak is
evaluated, the overlapping has reached a critical level at which the calcu-
lated peak height or peak area is falsified by the neighbouring peak. In this
case it is advisable to take the overlapping into account by a change in the
baseline calculation. For this, the parameter Scope of whole (calculation
for the entire curve) must be set to either f.double or r.double
(calculation for double peak) or to f.half or r.half (calculation only for
front or rear, see sections 5.6.3 and 6.7).
• Overlapping > 40%
Comment illeg. front ovlp. or illeg. rear ovlp. . The overlapping
is too large, the peak can no longer be evaluated. In this case chemical or
measurement technique countermeasures must be used to attempt to
separate these peaks better. Possible measures include changing the pH
value (see example below), changing the supporting electrolyte concen-
tration, changing the supporting electrolyte, use of complexing agents
(see example in section 7.5.6), longer enrichment times and modifying or
changing the measurement technique.