double beam spectrophotometer.. do u have to blank before reading? - or the reference serves the purpose? (Apr/03/2008 )

hii all..
i have a question on the use of a double beam spectrophotometer. here we have a reference cell where we keep the blanking solution the OD of which is subtracted from the sample OD. so blanking is done simultaneous to sample reading inthe double beam. then why do u have to blank before reading? that is use the blanking liquid in both R and S cells and blank first? if this is only for pathlength correction.. can it be done with any solution other than blanking solution?

and what exactly is meant by the pathlength correction.. cos i read it a lot.. and as i can understand.. the light path is the same for both the cells. what is the correction for? pls let me know!
thanksss..

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when you "blank" the machine with the reference solution in both cuvettes you are actually "zeroing" the machine so that you can work with the same range of units at any given time.

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so that can be done with any solution.. same solution in both the cuvettes right? not necessarily the blanking soluiton for the sample?

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you are correct but it is better to use the same buffer that your samples are in. that way there should be no offset for your readings.

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cud u pls elaborate? what offset are u talking about?
besides, how wud blanking with balnk buffer help that? i'm still not clear on this. thanks!!

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when you blank with the buffer that your sample is in, you are setting the reading to zero.

if you use something else to blank, like water, then your buffer may have a positive reading. this reading would be considered an offset. a standard curve, with this offset, would not pass through the origin. your determined values for your samples would have to be calculated with the offset (makes it a little more cumbersome).

you can avoid this by simply blanking with your buffer in both the reference and sample.

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