I assume you are measuring the transmittance?!

Actually, the blank should not show any absorbance at all, as the only intensity losses due to the blank should be caused by reflectance at its different interfaces, otherwise I would not call it a blank (actually there are reference samples where the concentration is non-zero).

In any way, if you measure the blank, against what are you referencing it? Probably to the empty beam path? This is actually in any way not a good idea (see e.g. Article Employing Theories Far beyond Their Limits-The Case of the (...

Maybe you can provide us with some more information about the spectral range, the blank itself, what you actually measure (transmittance or reflectance) and the spectrometer-types, so that we can better understand the nature of the problem?

Are you sure that the baseline correction in all instrument is done properly? The baseline in different spectrometer will be definitely different.

Thanks Sir Thomas for your suggestion. Actually I am using SDDC method for determination of arsenic in water. For doing so, I need to plot a curve between absorbance & known concentration of arsenic. So that I finally can find the concentration of an unknown sample with the observed absorbance value.

I considered blank sample as double distilled water treated with SDDC method. My point of interest is 520 nm wavelength for which I used a UV-VIS spectrophotometer. And the interesting point is that SDDC is itself coloured which forms a red-complex with arsenic. So lesser the light absorbed, lesser is the amount of arsenic present in the solution.

I am attaching a pdf for reference.

Hello Tuhin, I am doing spectral studies with one instrument only. But my question is that though the blank sample has no arsenic present in it (if then very negligible in combination with zinc fillings) for experimental consideration, the samples with 10 ppb or 20 ppb shows absorbance very near to the blank absorbance value. I have found LOD & LOQ by taking average of 10 blank values. The blank absorbance value does not vary more than 10 % which is satisfactory. But question arises why increase in concentrations does not lead to an increase in absorbance value.

Thanks.

Ok, thanks for the pdf! Obvioulsly, this is a comparably complex method of determination due to the fact that SDDC is itself absorbing, with several different reference samples. Quite interesting! Can you share your calibration curve with us?

I can tell that the curves are showing R2 value greater than 95%, which is quite good. But it should have risen with the concentration values.

Another point is that though it can detect upto 1 ug, but maximum permissible limit is 20 ug for arsenic amount. The curves are of no use until there is a proper justification.

Hello, Thomas Sir, the curves are given.

Hello Mohanty, thanks for your suggestion. The standard SDDC method is done either at 510 or 535nm. But some refer to 520 as well.

So we took the middle value.