[ken3009]

How are these 2 different from a standard?

[discodermolide]

How are these 2 different from a standard?

A blank is something that should contain nothing of the substance you are testing for. A control should contain a known amount of the substance. A standard is a sample which contains an exactly known amount of the substance.

[ken3009]

but, just to let you know, i have used blanks that require some sample in it (it says so in the procedure). thats why im a bit confused, trying to get my head around it.

[discodermolide]

but, just to let you know, i have used blanks that require some sample in it (it says so in the procedure). thats why im a bit confused, trying to get my head around it.

Well you now have me confused as well. A blank is there to estimate any source of external contamination. Have a look here:
http://www.epa.gov/region3/esc/qa/pdf/blanks.pdf

[Arkcon]

I had heard of analytical procedure called system suitability (that is also a term for something else, but whatever) where blanks, samples and standards are spiked with a known, small amount of standard not sample.  The point of that being you normalize every result -- blank, standard and unknowns to take into account the addition.  So maybe you're misreading the procedure, or maybe that's just the way the procedure is written.

[Arkcon]

{sorry to double reply -- lots to write}

From your other post on this topic:  http://www.chemicalforums.com/index.php?topic=60128.msg214938#msg214938  You can see this happening, your "blank" has less sample than your real "sample"  How you're going to normalize I don't know.  But if the blank were spiked with standard, then it'd be simple.

Usually, system suitability is used for a chromatographic analysis, say for a drug sample.  For example, for testing for drugs of abuse, the blanks, standards, and samples are spiked with something like a cocaine metabolite from the standard. If the samples give the same amount as the blank, then you can say there's none.  If there's more, you can use the blank value to normalize and determine the amount from the spiked standard.  If no peaks show up in blank and standard, you've done it wrong, and the system will stop running to tell you so.  This is because you can't run a clean blank, an unspiked standard, and a clean sample, see a peak just in the sample, and say, "By the retention time, this peak in the sample is the same as the peak in the standard" -- that logic just won't fly in a court case. 

Of course, none of this usually applies to a UV sample, so I don't really know whats going on.  But try to ask people in charge is this sort of thing is what's going on.

[Babcock_Hall]

I wonder whether there might be differences in the how each branch of chemistry uses certain terms.  "Blank" in biochemistry usually means everything but the macromolecule (protein or DNA) of interest.  As such it is a kind of control.  For example if one is doing spectrophotometric enzyme assays, the no-enzyme blank might show a little bit of color change from nonenzymatic reaction of the substrate.

Controls in forensics might be either positive or negative.  No-DNA template controls are used in PCR based DNA profiling to look for contamination.  But one might also use positive controls to make sure that everything in the PCR reaction is working properly.

[ken3009]

Oh i think i get it now, read below:
so basically for the heavy metal test in water, the procedure (im sure its AOAC) asks to add 2ml sample to both blank (10ml DI water) and standard (10ml 1ppm Pb solution)..and just 12ml sample ONLY to test...so all tubes have 12ml added together..that seems more fair right? it got to be, but i cant explain it.

[voidSetup]

This might be a procedure similar to "spike recovery", which is usually a part of the validation process to show that a method is suitable for quantifying the analyte of interest. The blanks, standards, and samples are spiked with a known amount of sample to show that there are no interferences from matrix effects.