Please help me with this

[Gamewizard]

Estimate the %CV for the urea assay at two concentrations and comment on results?

 

Using the data in table 1 which is below.

 

(Table 1)

 

Vol urea stock = 2.5 ml

Vol of creatinine stock = 1.25ml

Diluent = 6.25 ml

total vol is 10 ml

 

(Table 2)

Absorbance (low) Urea concentration (low) Absorbance (high) Urea concentration (low)

0.25 0.67

0.252 0.72

0.26 0.70

0.24 0.65

0.238 0.68

0.245 0.715

0.251 0.63

0.161 0.645

0.25 0.68

0.245 0.75

 

I am told that I am supposed to use the concentration values to calculate CV% and not absorbances, but I have no idea how to get the concentration values ????

[ewmon]

It seems that you're missing the formula to convert absorbances into concentrations. Did the teacher provide it? Was it determined from previous work? I'm accustomed to using the four-parameter model (of the standard curve) to compute concentration from absorbances.

 

Also, please explain the column headings in Table 2.

Edited June 19, 2012 by ewmon

[Gamewizard]

It seems that you're missing the formula to convert absorbances into concentrations. Did the teacher provide it? Was it determined from previous work? I'm accustomed to using the four-parameter model (of the standard curve) to compute concentration from absorbances.

 

Also, please explain the column headings in Table 2.

 

Hi, sorry I forgot to add the absorbances for 'Absorbance' (high)

0.67

0.72

0.70

0.65

0.68

0.715

0.63

0.645

0.68

0.715

 

Thats what I am not sure about, we are not given any formula or any kind of explanation. I have looked back at the other questions but nothing relates to this, and the question only specifically says to use the data in table 2, which I have described above.

Edited June 20, 2012 by Gamewizard

[ewmon]

So, the fourth column should read: "Urea concentration (high)" instead of "... (low)"?

 

But no formula or model. That's odd. I think you're stuck.

[imatfaal]

Gamewizard

 

Your table two needs explanation. At present there are 4 headings, 3 columns, two seems to have same title, and two columns are indentical bar last entry?

 

If you have a smart phone take a picture and post it - and ensure that all the details of the question are included.

[Gamewizard]

So, the fourth column should read: "Urea concentration (high)" instead of "... (low)"?

 

But no formula or model. That's odd. I think you're stuck.

 

 

Oh yes it should say high sorry my stupid mistake

 

No formula or anything The only possibility I can see is plotting a graph from previous urea concentrations, against these absorbances and then deriving the concentrations like that. What do you think ?

 

So, the fourth column should read: "Urea concentration (high)" instead of "... (low)"?

 

But no formula or model. That's odd. I think you're stuck.

 

Gamewizard

 

Your table two needs explanation. At present there are 4 headings, 3 columns, two seems to have same title, and two columns are indentical bar last entry?

 

If you have a smart phone take a picture and post it - and ensure that all the details of the question are included.

 

Sorry, it gets muddled up when I post it on here. 4 columns in total, first is absorbances (low), then urea concentration (low) which is blank, then absorbance (high) and then last heading is urea concentration (high), which is also blank. Suppose to work out the concentrations myself somehow.

 

I dont have a smartphone

Edited June 21, 2012 by Gamewizard

[mississippichem]

Gamewizard,

 

Are you familiar with the Beer-Lambert law? If not google it.

 

[math] A= \varepsilon c \ell [/math]

 

You need a molar extinction coefficient [epsilon] to get concentration from absorbance though. You get that from the slope of the calibration curve prepared from solutions of known concentration. You also need the path length "l" of whatever type of spectrometer you are using. As far as I know, this can't be found with just the information you have posted.

[Gamewizard]

Gamewizard,

 

Are you familiar with the Beer-Lambert law? If not google it.

 

[math] A= \varepsilon c \ell [/math]

 

You need a molar extinction coefficient [epsilon] to get concentration from absorbance though. You get that from the slope of the calibration curve prepared from solutions of known concentration. You also need the path length "l" of whatever type of spectrometer you are using. As far as I know, this can't be found with just the information you have posted.

 

I have done it before yes. But I dont have the path length information, it doesnt say anything about spectrometer. On the previous page I checked, I am supposed to draw a calibration curve from diffrent urea concentrations and absorbances, and it says that the creatinine assay obeys the Beer-lambert law and a one-point linear calibration curve is generated, and then they have given us the creatinine concentration =1250umol/L and absorbance 1 = 0.627 and absorbance 2=0.623. I am not sure if this information could be related to this question in any way?

[imatfaal]

Mssssppchm can confirm but I think the only comment you could make from that data is regarding relative concentrations. Beer-Lambert says that Absorbance varies linearly with Concentration, Length of Path, and Extinction coefficient - you have two assays in which the length and epsilon are held constant; thus you could provide relative concentrations. Without knowledge of Epsilon and Length OR the absorbance of a known concentration sample (in the same set-up) then you are done.

Edited June 22, 2012 by imatfaal
Clarification