Chromatography Forum

In my uv detector I see opitions signal polarity and data rate .how does it effects the chromatography

Polarity refers to the output of the detector - do you want the peaks to go "up" or "down." This is a common option for refractive index, but not absorbance. Very few chromatographers use methods that generate negative peaks with an absorbance detector.

The data rate, or sampling rate, tells the detector how often to collect data (1 data point/second = 1 Hz). In general, you need to collect a minimum of 25 - 30 data points across your peak, to ensure that you get the best accuracy. So, determine the width (in seconds) of your most narrow peak, and set your data collection rate so that you make at least 25 measurements in that time. For example, if your peak is 15 seconds wide, you should collect data at about 2 Hz or faster.

Thanks a lot

If I do not collect more than 25 data point for one peak..I mean i collect only 10 data point for my 15 sec wide peak How it will effect the peak shape?

There are several problems that can happen when you do not collect enough data points. First, your accuracy will not be very good, because noise in one data point represents 10% of your peak data. The integration start and stop points will also affect the area and height. Now, these errors will not be 50%, but they could easily be 10% or more. So if it is an important analysis, I would recommend collecting more data.

Also, with only 10 data points, you can miss important information, like changes in peak shape, or the appearance of impurities or shoulder peaks.

Another consideration is how the software and hardware work together. On some systems, the data collection rate and detector time constant (response time) are linked together. A slower collection rate usually means a larger response time, which means that you could lose much peak information (including shape, width, height, etc.).

Essentially, when you do not collect enough data points, your peak will be distorted, and you loose resolution to neighboring peaks. This is the main issue about an insufficient sampling rate.