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4. Problems with the Chromatogram

Many problems in an LC system show up as changes in the chromatogram. Some of these can be solved by changes in the equipment; however, others require modification of the assay procedure. Selecting the proper column type and mobile phase are keys to "good chromatography."

A. Peak Tailing

Possible Cause	Solution

1. Blocked frit	1. a. Reverse flush column (if allowed)
	b. Replace inlet frit
	c. Replace column
2. Column void	2. Fill void
3. Interfering peak	3. a. Use longer column
	b. Change mobile-phase and/or column/
	selectivity
4. Wrong mobile-phase pH	4. a. Adjust pH
	b. For basic compounds, a lower pH
	usually provides more symmetric peaks
5. Sample reacting with active sites	5. a. Add ion pair reagent or volatile basic
	modifier b. Change column

B. Peak Fronting

Possible Cause	Solution

1. Low temperature	1. Increase column temperature
2. Wrong sample solvent	2. Use mobile phase for injection solvent
3. Sample overload	3. Decrease sample concentration
4. Bad column	4. See A.1. and A.2.

C. Split Peaks

Possible Cause	Solution

1. Contamination on guard or	1. a. Remove guard column and attempt
analytical column inlet	analysis
	b. Replace guard if necessary c. If analytical column is obstructed, reverse and flush d. If problem persists, column may be fouled with strongly retained contaminants e. Use appropriate restoration procedure f. If problem persists, inlet is probably plugged g. Change frit or replace column
2. Sample solvent incompatible with	2. Change solvent; whenever possible,
mobile phase	inject samples in mobile phase

D. Distortion of Larger Peaks

Possible Cause	Solution

1. Sample overload	1. Reduce sample size

E. Distortion of Early Peaks

Possible Cause	Solution

1. Wrong injection solvent	1. a. Reduce injection volume
	b. Use weaker injection solvent

F. Tailing, Early Peaks More Than Later Ones

Possible Cause	Solution

1. Extra-column effects	1. a. Replumb system (shorter, narrower
	tubing)
	b. Use smaller volume detector cell

G. Increased Tailing as k^' Increases

Possible Cause	Solution

1. Secondary retention effects,	1. a. Add triethylamine (basic samples)
reversed-phase mode	b. Add acetate (acidic samples)
	c. Add salt or buffer (ionic samples)
	d. Try a different column
2. Secondary retention effects,	2. a. Add triethylamine (basic compounds)
normal-phase mode	b. Add acetic acid
3. Secondary retention effects,	3. Add triethylamine (basic samples)
ion-pair

H. Acidic or Basic Peaks Tail

Possible Cause	Solution

1. Inadequate buffering	1. a. Use 50–100 mm buffer concentration
	b. Use buffer with pKa equal to pH of
	mobile phase

I. Extra Peaks

Possible Cause	Solution

1. Other components in sample	1. Normal
2. Late-eluting peak from previous	2. a. Increase run time or gradient slope
injection	b. Increase flow rate
3. Vacancy or ghost peaks	3. a. Check purity of mobile phase
	b. Use mobile phase as injection solvent
	c. Reduce injection volume

J. Retention Time Drifts

Possible Cause	Solution

1. Poor temperature control	1. Thermostat column
2. Mobile phase changing	2. Prevent change (evaporation, reaction,
	etc.)
3. Poor column equilibration	3. Allow more time for column equili-
	bration between runs

K. Abrupt Retention Time Changes

Possible Cause	Solution

1. Flow rate change	1. Reset flow rate
2. Air bubble in pump	2. Bleed air from pump
3. Improper mobile phase	3. a. Replace with proper mobile phase
	b. Set proper mobile phase mixture on
	controller

L. Baseline Drift

Possible Cause	Solution

1. Column temperature fluctuation	1. a. Control column and mobile phase
(even small changes cause cyclic	temperature
baseline rise and fall; most	b. Use heat exchanger before detector
often affects refractive index
and conductivity detectors, or
UV detectors at high sensitivity
or in direct photometric mode)
2. Nonhomogeneous mobile phase	2. a. Use HPLC-grade solvents, high-purity
(drift usually to higher	salts, and additives
absorbance, rather than cyclic	b. Degas mobile phase before use
pattern from temperature	c. Sparge with helium during use
fluctuation)
3. Contaminant or air buildup in	3. a. Flush cell with methanol or other
detector cell	strong solvent
	b. If necessary, clean cell with 1 n
	HNO₃ (never with HCl)
4. Plugged outlet line after detector	4. a. Unplug or replace line
(high-pressure cracks cell	b. Refer to detector manual to replace
window, producing noisy baseline)	window
5. Mobile-phase mixing problem or	5. a. Correct composition/flow rate
change in flow rate	b. To avoid, routinely monitor
	composition and flow rate
6. Slow column equilibration,	6. a. Flush with intermediate strength
especially when changing	solvent
mobile phase	b. Run 10–20 column volumes of new
	mobile phase before analysis
7. Mobile phase contaminated,	7. a. Check make-up of mobile phase
deteriorated, or prepared from	b. Use highest grade chemicals and HPLC
low-quality materials	solvents
8. Strongly retained materials in	8. a. Use guard column
sample (high k) can elute as very	b. If necessary, flush column with strong
broad peaks and appear to be	solvent between injections or periodi-
a problem	ically during analysis
9. Mobile phase recycled but	9. a. Reset baseline
detector not adjusted	b. Use new mobile phase when dynamic
	range of detector is exceeded
10. Detector (UV) not set at	10. Change wavelength to UV absorbance
absorbance maximum but at slope	maximum
of curve

M. Baseline Noise (Regular)

Possible Cause	Solution

1. Air in mobile phase, detector cell,	1. a. Degas mobile phase
or pump	b. Flush system to remove air from
	detector cell or pump
2. Leak	2. a. See Section 3
	b. Check system for loose fittings c. Check pump for leaks, salt build-up, unusual noises d. Change pump seals if necessary
3. Incomplete mobile phase mixing	3. Mix mobile phase by hand or use less
	viscous solvent
4. Temperature effect (column at	4. Reduce differential or add head
high temperature, detector	exchanger
unheated)
5. Other electronic equipment on	5. Isolate LC, detector, or recorder to
same line	determine if source of problem is
	external; correct as necessary
6. Pump pulsations	6. Incorporate pulse dampener into system

N. Baseline Noise (Irregular)

Possible Cause	Solution

1. Leak	1. a. See Section 3
	b. Check for loose fittings c. Check pump for leaks, salt build-up, unusual noises d. Change seals if necessary e. Check for detector cell leak
2. Mobile phase contaminated,	2. Check make-up of mobile phase
deteriorated, or prepared
from low-quality materials
3. Mobile phase solvents immiscible	3. Select and use only miscible solvents
4. Detector/recorder electronics	4. a. Isolate detector and recorder
	electronically
	b. Refer to instruction manual to correct
	problem
5. Air trapped in system	5. Flush system with strong solvent
6. Air bubbles in detector	6. a. Purge detector
	b. Install back-pressure device after
	detector
7. Detector cell contaminated (even	7. Clean cell by flushing with 1 n HNO₃
small amounts of contaminants	(never with HCl)
can cause noise)
8. Weak detector lamp	8. Replace lamp
9. Column leaking silica or packing	9. Replace column
material
10. Mobile phase mixer inadequate or	10. Repair or replace the mixer or mix off-
malfunctioning	line if isocratic

O. Broad Peaks

Possible Cause	Solution

1. Mobile-phase composition changed	1. Prepare new mobile phase
2. Mobile-phase flow rate too low	2. Adjust flow rate
3. Leaks (especially between column	3. a. See Section 3
and detector)	b. Check for loose fittings
	c. Check pump for leaks, salt build-up,
	and unusual noises
	d. Change seals if necessary
4. Detector settings incorrect	4. Adjust settings
5. Extra-column effects:	5. a. Inject smaller column (e.g., 10 µl vs.
a. Column overloaded	100 µl) or 1:10 and 1:100 dilutions of
b. Detector response time or cell	sample
volume too large	b. Reduce response time or use smaller
	cell
c. Tubing between column and	c. Use as short a piece of 0.007–0.010-
detector too long or ID	inch ID tubing as practical
too large
d. Recorder response time too	d. Reduce response time
high
6. Buffer concentration too low	6. Increase concentration
7. Guard column contaminated/worn	7. Replace guard column
out
8. Column contaminated/worn out;	8. a. Replace column with new one of same
low plate number	type
	b. If new column provides symmetrical
	peaks, flush old column with strong
	solvent
9. Void at column inlet	9. Open inlet end and fill void or replace
	column
10. Peak represents two or more	10. Change column type to improve
poorly resolved compounds	separation
11. Column temperature too low	11. Increase temperature; do not exceed
	75°C unless higher temperatures
	are acceptable to column
	manufacturer
12. Detector time constant too large	12. Use smaller time constant

P. Loss of Resolution

Possible Cause	Solution

1. Mobile phase contaminated/	1. Prepare new mobile phase
deteriorated (causing
retention time to change)
2. Obstructed guard or analytical	2. a. Remove guard column and attempt
column	analysis
	b. Replace guard if necessary c. If analytical column is obstructed, reverse and flush; if problem persists, column may be fouled with strongly retained contaminants d. Use appropriate restoration procedure; if problem persists, inlet is probably plugged e. Change frit or replace column

Q. All Peaks Too Small

Possible Cause	Solution

1. Detector attentuation too high	1. Reduce attenuation
2. Detector time constant too large	2. Use smaller time constant
3. Injection size too small	3. Use larger sample loop
4. Improper recorder connection	4. Use correct connection

R. All Peaks Too large

Possible Cause	Solution

1. Detector attenuation too low	1. Use larger attenuation
2. Injection size too large	2. Use smaller sample loop
3. Improper recorder connection	3. Use correct connection

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