chromatography

Question 1

what is chromatography

Answer 1

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

Question 2

what happens to the stationary phase

Answer 2

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

Question 3

what happens in the mobile phase

Answer 3

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

Question 4

what is retention time?

Answer 4

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

Question 5

how is retention time represented

Answer 5

TR= ts+tm

Question 6

what is ts

Answer 6

the time spent in stationary phase

Question 7

what is tm

Answer 7

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

Question 8

what is tR

Answer 8

the retention time

Question 9

what is the retention factor?

Answer 9

it is k= (tR-tm)/tm

Question 10

what is another way to think about k?

Answer 10

time in stationary/time in mobile

Question 11

what is true of tm

Answer 11

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

Question 12

the amount of retention time increases as

Answer 12

the amount of stationary time increases

Question 12

a larger retention time means what?

Answer 12

it means the substance favors the stationary phase

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

what is a separation factor

Answer 15

Selectivity is a relative measure of the retention of two solutes

Question 16

what is the formula for the separation factor?

Answer 16

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

Question 17

what is relative retention?

Answer 17

it is given to us by the separation factor

Question 18

what does the selectivity factor compare?

Answer 18

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

Question 19

what is the formula for the resolution

Answer 19

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

Question 20

what does the resolution tell us

Answer 20

how well separated something is

Question 21

do we want a larger resolution or smaller resolution

Answer 21

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

Question 22

How is the concentration and response on the graph related

Answer 22

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

Question 23

what does the retention time tell us

Answer 23

it tells us what the compound is

Question 24

what is the y axis on a chromatograph

Answer 24

instrument response

Question 25

in chromatography what is the x axis on the graph

Answer 25

it is the retention time

Question 26

why do we want narrow peaks?

Answer 26

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

Question 27

what makes a better separation factor

Answer 27

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

Question 28

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

Answer 28

in the column

Question 29

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

Answer 29

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

Question 30

how do LC and HPLC compare

Answer 30

LC involved low pressure and its elution is driven by gravity it typically has low resolution (worse separation) and large particles

Question 31

What is different about HPLC and UHPLC

Answer 31

they involve higher pressue and smaller particles

Question 32

why are smaller particles better in LC?

Answer 32

they allow for better separation

Question 33

What is the normal mode of separation

Answer 33

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

Question 34

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

Answer 34

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

Question 35

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

Answer 35

the stationary phase is polar so polar molecules will be retained

Question 36

In a reversed mode of separation, what analytes elute first

Answer 36

the polar analytes will favor the mobile phase

Question 37

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

Answer 37

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

Question 38

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

Answer 38

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

Question 39

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

Answer 39

the retention times decrease

Question 40

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

Answer 40

the retention times increase

Question 41

in a gas chromatography, what is the mobile phase?

Answer 41

they are gaseous

Question 42

what drives the analyte separation in gas chromatography

Answer 42

higher temperatures force more gases into the mobile gas phase