# Internal Standard Methodology

I understand the use of the internal standard methodology, however I am unsure which values I should use in the equation as although I have been given the internal standard solution I also have been given the second standard solution of both codeine and morphine?

The question is asking to find the content of morphine in the sample tablet (200mg) via the use of an internal standard solution of codeine (50mg) which was made up to 50mL ([codeine] = 1gL^-1). This was given as well as another standard solution of codeine (50mg) and morphine (50mg) made up to 50mL ([codeine] = 1gL^-1).

The equation I have been given is Drfx = (Ax / Ais) X (Cis / Cx)

x = unknown, is = internal standard, A = peak area, C = concentration

First we need to calculate the detector response factor (Drf) of the standard solution - but which standard solution?

Assuming that the standard solution conc. of both morphine and codeine are 1gL^-1, this gives a Drf for the standard solution of 0.722.

Using the calculated Drf value, the Drf equation can be rearranged to find the conc. of codeine in the sample tablet.

However my question is which values need to be used for the Ax and Ais - do they need to be both morphine or just the two values from the sample peak areas? * Values can be found in photo

Any help is appreciated

Jasmine

• Would you be able to type out a summary of the question in words (not as a .jpg) and also type out a very brief summary of your answer to it? I think this could be a great question, but having searchable text will help would-be answerers and also future questioners have far greater luck in finding this question. – Curt F. Jan 4 '16 at 13:26
• Have now edited as you mentioned. Thanks for the feedback :) – Jasmine Beer Jan 4 '16 at 13:55

However my question is which values need to be used for the Ax and Ais - do they need to be both morphine or just the two values from the sample peak areas? * Values can be found in photo

To answer just this part of the question, your $A_x = A_{unknown}$ is morphine peak area, your $C_x = C_{unknown}$ morphine concentration, $A_{IS} = A_{standard}$ codeine peak area, and $C_{IS} = C_{standard}$ codeine concentration.

Looking at your picture, it looks like you successfully calculated your $drf_{morphine}$ (though it's next to a misleading picture, as the peak area of codeine is larger than that of morphine when their concentrations are equal).

Moving down, you've successfully solved your $drf$ equation for unknown concentration, but the substitution is a little off. When you're dealing with the dissolved tablet solution, $A_{unknown}$ is indeed 17.6 but $A_{standard}$ would be the codeine area of 212.5, not the earlier morphine value.

If I had to guess, the confusion is between the terms "internal standard," a known spike of (usually) a single standard (here codeine) and "standard solution," which is an reference standard of known morphine and internal standard concentrations. Two common uses of reference standards are as initial calibrants and a good way to check for interference between analytes.

Getting back to the solution, once you have your unknown concentration, in units of $g/L = mg/mL$, you will simply multiply by the $50mL = 0.05L$ volume to have the morphine concentration of the solution and thus of the whole 200mg tablet. %w/w is also easy at that point.

• Thanks for your help! Your guess was right, I was getting confused about the difference between the internal standard and the standard solution. – Jasmine Beer Jan 10 '16 at 15:46