BUCHI ELS Detector C-650 - Manual (Page 42)

C-650 Operation Manual, Version B 6 Operation 42
6.4.5 Optimizing the noise filter
The Digital Filter (Fig 5.18) allows maximizing Signal-to-Noise ratio by filtering the noise. The filter
strength should be optimized according to the peak shape, and more specifically to the peak
width.
The following table proposes some Filter settings depending on peak width:
Peak width at 50% (second) Proposed filter (second)
<1 0
2 1
4 2
6 4
8 6
>10 8 and higher
These suggested values can be optimized depending on your specific chromatography, by de-
creasing Filter if peaks are poorly resolved (e.g. when Rs<1.5), or increasing Filter when optimiz-
ing Signal-to-Noise ratio.
Example: Comparison of digital filters using the SOP test (injection of 5 ppm caffeine at gain 12).
Peak width at half-height is 2.5 seconds.
Filter 0 s Filter 1 s Filter 2 s
Signal height 124 mV 122 mV 110 mV
Noise (ASTM) 3.2 mV 1.1 mV 0.7 mV
Peak width (at 50 %
height) 2.5 s 2.5 s 2.8 s
S/N 37 110 157
Signal-to-Noise ratio is multiplied by 3 when selecting Filter 1 second without any peak broaden-
ing effect. If Signal-to-Noise ratio is more important than resolution, a Filter 2 seconds or higher
can be set to improve sensitivity even better.
6.5 Powering down and shutting down the detector
If desired, some or all functions of the device can be powered down at the end of an automated
series of analyses. These power down features are described in detail in Fig 5.21.
To shut down the device:
• Turn off the pump.
• Allow the nebulization gas to flow through the detector for few minutes (30 min is recom-
mended) to drain the evaporation tube and detection chamber.
• Turn off the power to the detector (if desired).
If you are using a mobile phase which contains salts, acids or bases, pump few mL of water or
methanol through the system before switching off the detector to prevent any deposition of sub-
stances and possible corrosion of the device.
If ELSD is used as a second detector and is not being used for some time, it is recommended to
remove it from the liquid chromatography flow path in order to avoid any clogging of the nebulizer
or deposition of substances inside the detector.
Closing gas valve while the pump is still running may result in serious nebulizer damage.