In LC, what is the effect on the retention time of a peak (increase, decrease, r

Question

In LC, what is the effect on the retention time of a peak (increase, decrease, remain same, become zero) of   (a) raising the column temperature (head pressure constant) (b) Decreasing the particle size in packed column HPLC (constant pressure) (c) Having a bubbleformin the column (d) Increasingthe column head pressure (e)Using an open tubular column of the same length w/ a diameter 5x the spacing between particles in a packed column at the samehead pressure (f) Increasing the polarity ofmobile phase in reversed-phaseHPLC   (g) Using less-polar stationary phase in normalphase HPLC (h) Using amore efficient column w/ twice as many plates w/ the same length, mobile and stationary phase compositions, and at the same flow rate

Explanation / Answer

a)

Raising the column temperature (head pressure constant):

Heating increases the kinetic energy. With increased energy molecules can get free from column and move faster.

Retention time will decrease.

b)

Decreasing the particle size in packed column HPLC:

Decreasing particle size enhances the resolution by increasing selectivity. It will also result in more time.

Retention time will increase.

c)

Having a bubble forming the column:

Bubbles can create baseline noise and give obscure peaks.

No effect on Retention time.

d)

Increasing the column head pressure

Molecules will move with more velocity.

Retention time will decrease.

e)

Using an open tubular column of the same length w/ a diameter 5x the spacing between particles in a packed column at the same head pressure:

In open column speed is greatly increased.

Retention time will decrease.

f)

Increasing the polarity of mobile phase in reversed-phase HPLC:

Hydrophilic analytes will move faster as water content is increased.

Retention time of polar compounds will decrease.

g)

Using less-polar stationary phase in normal phase HPLC:

Separation and speed of polar analytes will decrease.

Retention time of polar compounds will increase.

h)

Using a more efficient column w/ twice as many plates w/ the same length, mobile and stationary phase compositions, and at the same flow rate:

As the number of plate increases time will also increase.

Retention time will increase.

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