Measure low DOM absorbance values in a microplate reader. Is there a suitable deep well microplate with flat bottoms available? I would like to use a microplate reader to measure the absorbance of dissolved organic matter (DOM), but the measured values are close to the lower detection limit. To overcome this, I thought of deep well plates as the pathlength is longer then. Nevertheless, I have not found any deep well plates with flat bottoms, which might be best for absorbance measurements. I want to avoid measuring in a photometer with 5 cm pathlength because the available amount of sample is limited and the microplate would need much less.

Does anybody have experience with similar problems?

Can anyone suggest a suitable microplate?

What is your opinion on the influence of the material?
I am not aware of suitable microplates for this application.  Here are some alternative approaches.

1. Low-volume quartz cuvettes.

There are quartz cuvettes in "micro" formats that may be suitable for you.  Mostly commonly they work with 1 cm pathlength.  The volume of the cells is controlled by the width of the non-optical dimension.  This can be made extremely small.  

One example I found lists cuvettes that still have path lengths of 1 cm with total fill volumes as low as 15 &mu;L.  

2. Sample concentration

Details depend heavily on the details of your DOM and application, but you could concentrate your samples and then use existing microplates.  

3. Longer measurement times.

Most plate readers (and cuvette-based spectrophotometers) let you control the number of "shots" or of the total amount of time a measurement of light intensity is integrated over.  You could crank this number up to get better sensitivity.