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I am little new to analytical chemistry, but after reading up on it, I realised that most analytical procedures need you to know what to test for beforehand.

May I know, given a sample of material G, is it possible to know its composition without knowing beforehand what to test for? ie. For a sample G, is there a procedure that can be followed so I can know that G has: 30% of compound X 20% of compound Y 50% of compound Z And that no other compounds exist in the sample. (But I do not know X, Y, Z are the compounds to test for beforehand.)

Or are all analytical chemistry procedures limited in the sense that you need to know what to test for first?

If yes, what is the closest possible procedure to follow to get what I want as outlined above?

Lastly, are analytical chemistry procedures also limited in the sense that you can never know for sure if that "no other compounds exist"?

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    $\begingroup$ You never start from zero. Some questions I would ask you before planning an analysis are "where did you get the sample?" and "why do you want to know the composition?" (this helping me assess exactly what it is about the composition that you really want to know, and therefore narrowing the experiments that need to be performed) $\endgroup$
    – Buck Thorn
    Jun 2 at 8:56
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    $\begingroup$ Analytical chemists are extremely reluctant to start from zero. Like Buck Thorn said, they'll ask you all kinds of weird questions. If you keep the mystery but continue to insist on analysis, they'll throw your sample at you and kick you downstairs. $\endgroup$ Jun 2 at 9:25
  • $\begingroup$ It depends on what you have in mind. In practice, see the comments above. In principle, yes you can. See also my comment to chemistry.stackexchange.com/questions/152296/… as it is related. But I like the way VDVH summarised the idea. First ask questions. $\endgroup$
    – Alchimista
    Jun 2 at 11:40
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It depends. Which instruments have you got (ISP-MS, IR, XRF, NMR or just pH metre)? What type of sample have you got (liquid, gas, powder, homogenous, heterogenous)? What results do you need (elemental composition, molecule composition)? What is the purpose of analysis? Do you need to analyse whole sample or just the surface.

Therefore you always have to ask questions and then find the appropriate available method. Generally, start with the cheap non-destructive methods and then decide what to do next.

I think you never know 100% composition. However does it really matter? E. g. the analyte is 99 % pure. Is it important to know the 1 % of impurities? Sometimes it is, sometimes it is not. Sometimes it is enough to say that there is 99 % of compound X and 1 % are impurities. Sometimes you need to say that there is 99 % of compoud X, 0.8 % of compound Y, 0.19 % of compound Z and 0.0009 % of compound A. The rest are impurities.

I would say that analytical chemistry answers questions. And the questions have to be asked first. And as you learn, you are able to answer more and more questions.

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