Analysis of Mixtures & Compounds

Question 1

What are the 2 phases of chromatography?

Answer 1

Stationary phase & mobile phase

Question 2

What are the 3 types of chromatography?

Answer 2

1) Thin Layer Chromatography (TLC)
2) Gas Chromatography (GC)
3) High Performance Liquid Chromatography (HPLC)

Question 3

How does Thin Layer Chromatography (TLC) work?

Answer 3

1) Blank TLC plate with thin adsorbent layer, usually silica
2) Relies on principle that different components move at different speeds through at adsorbent
3) Compare standard (control) compounds to the sample, to see whether standard compounds are present in the sample

Question 4

What are 2 limitations of TLC?

Answer 4

1) Presumptive test: (only likely, not definitive)
2) Impossible to have every compound on the small TLC plate

Question 5

How does Gas Chromatography (GC) work? (5)

Answer 5

1) Sample: solution
2) Mobile phase: carrier gas
3) Column: steel tube coated with adsorbent (in oven)
4) Detector sends signal to recorder to record data
5) Get a graph: compare the retention time of standards and sample. Can measure the amount of components present in each mixture

Question 6

How does High Performance Liquid Chromatography (HPLC) work? (4)

Answer 6

1) reservoir stores liquid solvent (mobile phase)
2) injector: sample introduced to the instrument
3) Column: adsorbent material
4) Quantitative: area under each peak is proportional to the amount of that compound. Plotted against time it took for the compound to go through the column

Question 7

What are 5 shared advantages of HPLC and GC?

Answer 7

1) Efficient, highly selective, widely applicable
2) Only small sample required
3) May be non destructive of sample
4) Readily adapted to quantitative analysis
5) High resolution: able to distinct compounds that are structurally similar

Question 8

What is one advantage of HPLC?

Answer 8

Can accommodate non-volatile and thermally unstable samples

Question 9

What are 2 advantages of GC?

Answer 9

1) Simple and inexpensive equipment
2) Rapid

Question 10

What are 3 ways to identify the components of a mixture?

Answer 10

1) By chromatographic comparison with authentic samples (Still leaves room for doubt)
2) By spectroscopy (infra-red)
3) By spectrometry

Question 11

What is infra-red spectroscopy?

Answer 11

Identification of compounds by molecular vibrations (stretching & bending).

Question 12

How can we use infra-red spectroscopy to identify compounds? (4)

Answer 12

1) Different molecular features absorb infra-red and vibrate at different regions of the spectrum.
2) Lower frequencies (bigger number) is characteristic of the class of compounds, but not of the individual compound.
3) Higher frequencies (smaller number) is the fingerprint region (characteristic of a compound).
4) Take infra-red spectrum of unknown compound and compare it to known compounds to find matches

Question 13

What is an advantage of infra-red spectroscopy?

Answer 13

It gives a much stronger conclusion than GC

Question 14

For infra-red spectroscopy, how is stretching frequency determined?

Answer 14

1) mass: lighter atom will have higher frequencies
2) bond stiffness: stiffer bond = higher frequency

Question 15

What is mass spectrometry?

Answer 15

Identification of compounds by fragmentation

Question 16

How does mass spectrometry work? (7)

Answer 16

1) Sample will be vapourised by very high vacuum
2) Sample bombarded by electrons. Individual molecules converted into ions
3) High voltage applied - ions accelerated
4) Passed into the next instrument with strong magnetic field
5) Deflected from their path according to their mass (Lightweight ions more strongly deflected and vice versa). Deflection will give mass of ion
6) Most molecules will break up in the spectrometer (fragmentation). Fragments will be lighter and more strongly deflected
7) Since each isomer will give a different fragmentation pattern, fragment pattern: characteristic of a molecule

Question 17

Is molecular weight useful for identification?

Answer 17

Does not prove that it is a particular compound; one weight can have many isomers (Compounds with the same atoms but in a different arrangement)

Question 18

What is GC-MS?

Answer 18

1) using GC to separate mixtures
2) using MS to detect (deflecting & finding fragments)

Most widely accepted & used