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Chem 31 > chromatography > Flashcards

chromatography Flashcards

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1
Q

what is chromatography

A

it is a method by which a mixture is separated by distributing its components between two phases

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2
Q

what happens to the stationary phase

A

it remains fixed in place; it often acts as a constraint and slows down components in the mixture

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3
Q

what happens in the mobile phase

A

it carries components of the mixture through the medium, components that like this phase move through the medium quicker

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4
Q

what is retention time?

A

it is the time that passes between a sample injection and the maximum response for a solutes peak

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5
Q

how is retention time represented

A

TR= ts+tm

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6
Q

what is ts

A

the time spent in stationary phase

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7
Q

what is tm

A

the time spent in the mobile phase, it is also the retention time of an unretained compound that spends no time in the stationary phase

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8
Q

what is tR

A

the retention time

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9
Q

what is the retention factor?

A

it is k= (tR-tm)/tm

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10
Q

what is another way to think about k?

A

time in stationary/time in mobile

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11
Q

what is true of tm

A

tm is the fastest, it does not stop in the stationary phase at all

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12
Q

the amount of retention time increases as

A

the amount of stationary time increases

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12
Q

a larger retention time means what?

A

it means the substance favors the stationary phase

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13
Q

do all the substances spend the same amount of time in the mobile phase

A

yes, that is why we use tm because it is the same across all of the samples

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14
Q

what is the separation due to? why do all the compounds spend the same amount of time in the mobile phase

A

As all compounds have the same length to travel before they exit the column, they all spend the same amount of time in the mobile phase. Thus, separation is due to the varying amount of time spent in the stationary phase, or how long compounds adhere to the column coating

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15
Q

what is a separation factor

A

Selectivity is a relative measure of the retention of two solutes

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16
Q

what is the formula for the separation factor?

A

it is the larger partition coefficient divided by the smaller partition coefficient

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17
Q

what is relative retention?

A

it is given to us by the separation factor

18
Q

what does the selectivity factor compare?

A

the stationary phase of peak 2 over the stationary phase of peak 1 where the lowest factor is 1.00 which is when the two peaks overlap and have the same retention time

19
Q

what is the formula for the resolution

A

the change in retention times divided by the average peak width, it is arranged so Rs is always greater than 0

20
Q

what does the resolution tell us

A

how well separated something is

21
Q

do we want a larger resolution or smaller resolution

A

we want a resolution that is larger where 1 is the minimum and 1.50 is the best for quantitative analysis

22
Q

How is the concentration and response on the graph related

A

the higher the concentration, the higher the peak, this changes the area under the curve

23
Q

what does the retention time tell us

A

it tells us what the compound is

24
Q

what is the y axis on a chromatograph

A

instrument response

25
Q

in chromatography what is the x axis on the graph

A

it is the retention time

26
Q

why do we want narrow peaks?

A

when the peaks are narrow and high, its easier to separate because its more concentrated. As the analyte travels down the column, the peak gers smaller as the concentration becomes more diluted on the edges of the band

27
Q

what makes a better separation factor

A

if there is a greater difference in times, the peaks will be further separated from one another and will have a better separation factor

28
Q

where does the separation and stationary phase occur in an LC?

A

in the column

29
Q

what’s the main difference between liquid chromatography and gas chromatography?

A

liquid chromatography has a liquid mobile phase and a solid stationary phase

30
Q

how do LC and HPLC compare

A

LC involved low pressure and its elution is driven by gravity

it typically has low resolution (worse separation) and large particles

31
Q

What is different about HPLC and UHPLC

A

they involve higher pressue and smaller particles

32
Q

why are smaller particles better in LC?

A

they allow for better separation

33
Q

What is the normal mode of separation

A

it is where the stationary phase is polar and the mobile phase is nonpolar

34
Q

in a normal mode of separation, what types of analytes will elute first

A

in a normal mode of separation, the mobile phase is nonpolar so they nonpolar analytes will elute first

35
Q

In a normal mode of separation, what types of molecules will be retained

A

the stationary phase is polar so polar molecules will be retained

36
Q

In a reversed mode of separation, what analytes elute first

A

the polar analytes will favor the mobile phase

37
Q

describe the stationary and mobile phases of separation in a reversed mode of separation

A

the stationary phase is nonpolar

the mobile phase is polar with water and polar organic modifiers like methanol and acetonitrile

38
Q

what is the mobile phase composed of in a normal mode of separation

A

it is composed of hexane and polar modifiers: ethanol and acetone

39
Q

in separation, if the mobile and stationary phases become more similar, what happens to the retention times

A

the retention times decrease

40
Q

in separation, if the mobile and stationary phases become less similar, what happens to the retention times

A

the retention times increase

41
Q

in a gas chromatography, what is the mobile phase?

A

they are gaseous

42
Q

what drives the analyte separation in gas chromatography

A

higher temperatures force more gases into the mobile gas phase

43
Q
A

Chem 31 (14 decks)

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