more questions

[bec]

hi, i just did a prac exam and i've got a few questions:

1. There are a number of chromatographic techniques available to the analytical chemist. Caffeine, C8H10N4O2, is an additive in some sports drinks. The best instrumental analysis to measure the caffeine content is
a) paper chromatography
b) tlc
c) gc
d) hplc

The answer is HPLC - why is that? i thought it would be (b), because paper chromatography isn't very accurate (ruling out a), caffeine isn't a gas (ruling out c), and HPLC is supposedly only for analysing organic compounds with molar mass over 1000....

2. Two pure solutions of thanol and ethanoic acid have lost their labels. The most appropriate instrumental analysis to use to distinguish and determine the concentrations of the organic molecules is
a) GC
B) H NMR spec
C) IR spec
D) AAS
The answer is (a)....why not (c)?

Thanks

[ed_saifa]

1. HPLC is usually used for organic compounds with a molar mass over 300.
2. IR doesn't give a quantitative analysis.

[bec]

I thought you could use IR for quantitative analysis though? in my notes i've written that you choose a significant peak on the spectrum (let's say CH3) and plot a series of samples with known CH3 concentration on a standard curve...

[ed_saifa]

The significant peaks suggest a strong absorption band. But it doesn't specifically tell how many types of a particular bond may be found.

[ice_blockie]

2. IR doesn't give a quantitative analysis.

This statement isn't totally correct. Quantative analysis in IR can be done.

If one peak in the spectrum is selected, the absorbance increases as the concentration of the molecule in the sample increases. This means IR spectroscopy can be used for quantitative analysis.

Just that it isn't easy.

Unfortunately calibrations are often significantly curved rather than linear.

[Mao]

its more about picking the best answer rather than possible solutions

in this case, GC will be the easiest and most efficient method, whereas IR would take longer time, more calibration, and possibly a lot more computational power.


as for the first question, when you vapourise the sample for GC, larger biological molecules could fragment [like in Mass Spec], which is undesired, hence why it is generally used for smaller molecules [but some larger ones can still be tested with GC, such as steroids]

HPLC is hence preferred for larger molecules, as fragments do not occur. However, appropriate solvent need to be chosen for the analytes in question

hence, as this is a fairly large biomolecule, HPLC is the preferred instrument.

[beezy4eva]

1. There are a number of chromatographic techniques available to the analytical chemist. Caffeine, C8H10N4O2, is an additive in some sports drinks. The best instrumental analysis to measure the caffeine content is
a) paper chromatography
b) tlc
c) gc
d) hplc

the fact that it says content instantly rules out a & b, and because caffeine is a large organic molecule you assume that hplc would be the best answer

2. Two pure solutions of thanol and ethanoic acid have lost their labels. The most appropriate instrumental analysis to use to distinguish and determine the concentrations of the organic molecules is
a) GC
B) H NMR spec
C) IR spec
D) AAS

As the others said, in terms of time and equipment usage, GC is much easier. Thats not to say that you can't use IR to determine them.

If you've got the heinemman workbook (not the textbook) theres a big table that shows all the different analytical techniques we learnt and what each one is most useful for.

[bec]

thanks for all that

another question, "In chromatography a particular component is adsorbed ontot he stationary phase very strongly. This means that the component will have a....high/low Rf value, and high/low Rt value.
I thought it would be high for both, but the solutions tell me it's low Rf value, high Rt value - why low Rf?

[ed_saifa]

Since its attracted strongly to the stationary phase, it doesn't move far in the mobile phase. If a component is attracted strongly to the mobile phase, it will have a higher rf value.

[bec]

just realised i was reading the equation wrong...
Rf = (Distance travelled by solute)/(Distance travelled by solvent) not the other way around
thank you though!

another Q:
"The compound C2H4O2 was tested by reacting it with a carbonate solution. Carbon dioxide bubbles were observed. List two mass to charge ratios you would expect to see in a mass spectrum of this compound." How do I work this out? And with the CO2 bubbles, what's the relevance - does that show us the way it's been broken up?

[Collin Li]

You should be able to recognise that it is ethanoic acid because the carbon dioxide produced from reaction with a carbonate indicates acidic behaviour. Hence, you will expect the parent peak, and definitely 15.

[bec]

yeah, this is something i definitely don't have a good understanding of...
i know it's ethanoic acid (from the previous part of this question), so now do i need to "guess" how it would fragment? how do i know the charge?

[Collin Li]

The charge will almost always be just +1

The fragments are usually between the C-C bonds, but sometimes in between the C-O bonds...

[bec]

alright, sounds good - i'll work on it...
another 2 Qs:

1. Which of the following instrumental techniques is not associated with the properties of isotopes of an element?
a) H NMR, b) 13C NMR, c) mass spec, d)IR
apart from mass spec, i don't really understand how any of these are associated with the properties of isotopes...

2. I'm a bit confused with zwitterions. We were taught that in acidic solution, they took on extra protons so there was an NH3+ group and a COOH group, but in solutions for a question i just did it said there was an NH2+[/b] group and COOH group... does it depend on the degree of acidity in the solution?

[Collin Li]

I'm a bit confused with zwitterions. We were taught that in acidic solution, they took on extra protons so there was an NH3+ group and a COOH group, but in solutions for a question i just did it said there was an NH2+[/b] group and COOH group... does it depend on the degree of acidity in the solution?

No, it doesn't depend. NH2+ is just blatantly wrong. Ignore it. It should be NH3+ or NH2.