Intermittent loss of data in 2-D mode with 2996 detector - WKB63213

SYMPTOMS

• Intermittent flat line chromatogram during 2-D data collection of a single wavelength 
• Problem occurs when a single wavelength is collected in the low UV range "200 nm"
• Wavelength at 273 nm is ok - No flat baseline
• 3D mode also works fine - No flat baseline

ENVIRONMENT

• 2996 PDA Detector 

CAUSE

Acquisition of a single wavelength  

FIX or WORKAROUND

Set up a two-wavelength acquisition method. 

ADDITIONAL INFORMATION

When collecting 3-D data, at the beginning of each run, the 2996 captures a reference spectrum and derives an appropriate exposure time based on the energy level seen in that reference spectrum. Because of this fact, it is always best to power-up the detector with flow established through the flow cell to avoid a possible air bubble in the cell.

During collection of 2-D data, the data used to derive an exposure time may be different from the description above. If two or more wavelengths are collected, a reference spectrum is captured between the highest and lowest programmed wavelengths. But if only one wavelength is programmed, the exposure time is derived not from a spectrum but from the energy level at one wavelength only. In the case of single-wavelength collection, the exposure time depends on stable activity at a single pixel.

If that single pixel is located on a sloped area of an absorbance spectrum, such as on the UV cutoff curve of the mobile phase in use (215 nm for methanol), a slight perturbation (chemical or thermal) could lead to a higher light level, causing saturation of the diode at the calculated exposure time in use. If the calculated exposure time causes the diode to saturate, no data is collected and a flat-line chromatogram is displayed.

This problem can be avoided by collecting a second wavelength. The second wavelength need not be processed; it simply needs to be collected along with the wavelength of interest. 

Select a wavelength roughly 30 nanometers higher than the wavelength of interest (depending on the mobile phase absorbance spectrum). This forces the system to collect a reference spectrum at run start, instead of depending on a single data point for exposure time calculation.