Liquid chromatography-mass spectrometry

1. Liquid Chromatography-mass
Spectrometry(LC-MS)
GPRCP
S. Lakshmi narayana
reddy
M.PHARMACY
Pharmaceutical analysis
and
quality assurance.

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CONTENTS
• Hyphenated technique
• Introduction to LC-MS
• High performance liquid chromatography
• Mass spectrometry
• Instrumentation of LC-MS
• Sample preparation
• Ion sources
• Mass analyzers
• Applications
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3. Hyphenated Technique
The term “hyphenation” was first adapted by Hirschfeld in 1980. The technique
developed from the coupling of a separation technique and an on-line
spectroscopic detection technology is known as hyphenated technique.
What Is Hyphenated Technique ?
Advantages of hyphenated techniques:
1. Fast and accurate analysis.
2. Higher degree of automation.
3. Higher sample throughput.
4. Better reproducibility.
5. Reduction of contamination due to its closed system.
6. Separation and quantification achieved at same time.
List of Hyphenated Techniques :
1.GC-MS 2. LC-MS 3. LC-NMR 4. EC-MS 5. CE-MS
6. GC-IR 7. LC-MS-MS 8. GC-MS-MS 9. GC-GC-MS
10. GC-NMR
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INTRODUCTION TO LC-MS
• Liquid Chromatography/Mass Spectrometry (LC/MS) is a powerful
analytical technique that combines the resolving power of liquid
chromatography with the detection specificity of mass spectrometry.
• Liquid chromatography (LC) separates the sample components and then
introduces them to the mass spectrometer (MS). The MS creates and
detects charged ions.
• The LC/MS data may be used to provide information about the
molecular weight, structure, identity and quantity of specific sample
components.
• In LC-MS we are removing the detector from the column of LC and
fitting the column to interface of MS.
• In the most of the cases the interface used in LC-MS are ionization
source.
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High Pressure Liquid Chromatography
(HPLC)
• Liquid chromatography generally utilizes very small particles
packed and operating at relatively high pressure, and is referred to
as high performance liquid chromatography (HPLC).
• Modern LC-MS methods use HPLC instrumentation, essentially
exclusively, for sample introduction.
• In HPLC, the sample is forced by a liquid at high pressure (the mobile
phase) through a column that is packed with a stationary phase generally
composed of irregularly or spherically shaped particles chosen or
derivatized to accomplish particular types of separations.
• RP-LC is most often used as the means to introduce samples into the MS,
in LC-MS instrumentation.
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Mass Spectrometry
(MS)
• Mass spectrometry (MS) is an analytical technique that measures the
mass-to-charge ratio of charged particles.
• MS works by ionizing chemical compounds to generate charged
molecules or molecule fragments and measuring their mass-to-charge
ratios.
• In a typical MS procedure, a sample is loaded onto the MS instrument
and undergoes vaporization. The components of the sample are ionized
by one of a variety of methods (e.g., by impacting them with an electron
beam), which results in the formation of charged particles (ions). The
ions are separated according to their mass-to-charge ratio in an
analyzer by electromagnetic fields.
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Instrumentation of LC-MS
1)HPLC Constitutes The Lc Part:
a) Solvent System(mobile Phase)
b)Pumps
c)Mixer
d)Injector
e)Column
2) Mass spectrometer
A)Ion Sources
i)Electrospray ionization.
ii)Atmospheric Pressure
Chemical Ionization.
iii) Atmospheric pressure
photoionization
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B)Mass Analyzers
1) Quadrupole
2) Time-of-flight
3) Ion trap
4) Fourier transform-ion cyclotron resonance (FT-ICR)
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Sample preparation :
Sample preparation generally consists of concentrating the analyte
and removing compounds that can cause background ions or suppress
ionization. Examples of sample preparation include:
• On-column concentration to increase analyte concentration
• Desalting to reduce the sodium and potassium adduct formation that
commonly occurs in electrospray
• Filtration to separate a low molecular-weight drug from proteins in
plasma, milk, or tissue
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Ion Sources
• Ion Sources :
Electrospray ionization (ESI)
Atmospheric pressure chemical ionization (APCI)
Atmospheric pressure photoionization (APPI)
Electrospray ionization (ESI) :
• Process
API-ES is a process of ionization followed by evaporation. It
occurs in three basic steps:
 nebulization and charging;
 desolvation and;
 ion evaporation.
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Nebulization:
The HPLC effluent is pumped through a nebulizing needle
which is at ground potential. The spray goes through a semi-cylindrical
electrode which is at a high potential. The potential difference between the
needle and the electrode produces a strong electrical field. This field
charges the surface of the liquid and forms a spray of charged droplets.
There is a concentric flow of gas which assists in the nebulization process.
Desolvation :
The charged droplets are attracted toward the capillary sampling orifice.
There is counterflow of heated nitrogen drying gas which shrinks the
droplets and carries away the uncharged material.
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Ionization :
As the droplets shrink, they approach a point where the
electrostatic (coulombic) forces exceed the cohesive forces. This
process continues until the analyte ions are ultimately desorbed into
the gas phase. These gas-phase ions pass through the capillary
sampling orifice into the low pressure region of the ion source and the
mass analyzer.
Atmospheric Pressure Chemical Ionization :
Process :
APCI, a process of evaporation followed by ionization, is
complementary to API-ES.
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Nebulization and Desolvation :
APCI nebulization is similar to that in
API-ES. However, APCI nebulization occurs in a hot (typically 250°C–
400°C) vaporizer chamber. The heat rapidly evaporates the spray
droplets, resulting in gas-phase HPLC solvent and analyte molecules.
Ionization:
The gas-phase solvent molecules are ionized by the discharge
from a corona needle. In APCI there is a charge transfer from the
ionized solvent reagent ions to the analyte molecules in a way that is
similar to chemical ionization in GC/MS. These analyte ions then are
transported through the ion optics to the filter and detector.
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APCI - ION SOURCE 15

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Atmospheric pressure photoionization :
Atmospheric pressure photoionization
(APPI) for LC/MS is a relatively new technique. As in APCI, a vaporizer
converts the LC eluent to the gas phase. A discharge lamp generates
photons in a narrow range of ionization energies. The range of energies
is carefully chosen to ionize as many analyte molecules as possible
while minimizing the ionization of solvent molecules. The resulting ions
pass through a capillary sampling orifice into the mass analyzer.
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APPI- ION SOURCE
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Applications Of Ion Source
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Mass Analyzers
• Mass Analyzers :
Although in theory any type of mass analyzer
could be used for LC/MS, four types:
• Quadrupole
• Time-of-flight
• Ion trap
• Fourier transform-ion cyclotron resonance
(FT-ICR or FT-MS)
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Quadrupole :
A quadrupole mass analyzer consists of four parallel
rods arranged in a square. The analyte ions are directed down the
center of the square. Voltages applied to the rods generate
electromagnetic fields. These fields determine which mass-to-
charge ratio of ions can pass through the filter at a given time.
Quadrupoles tend to be the simplest and least expensive mass
analyzers.
Quadrupole mass analyzers can operate in two modes:
• Scanning (scan) mode.
• Selected ion monitoring (SIM) mode.
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•In scan mode, the mass analyzer monitors a range of mass-to-
charge ratios. In SIM mode, the mass analyzer monitors only a few
mass-to-charge ratios.
•SIM mode is significantly more sensitive than scan mode but
provides information about fewer ions. Scan mode is typically used
for qualitative analyses or for quantitation when all analyte masses
are not known in advance. SIM mode is used for quantitation and
monitoring of target compounds.
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Quadrupole Mass
Analyzer

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Time-of-flight :
In a time-of-flight (TOF) mass analyzer, a uniform
electromagnetic force is applied to all ions at the same time, causing
them to accelerate down a flight tube. Lighter ions travel faster and
arrive at the detector first, so the mass-to-charge ratios of the ions are
determined by their arrival times. Time-of-flight mass analyzers have a
wide mass range and can be very accurate in their mass
measurements.

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Time of flight mass analyzer 24

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Ion trap :
An ion trap mass analyzer consists of a circular ring
electrode plus two end caps that together form a chamber. Ions
entering the chamber are “trapped” there by electromagnetic
fields. Another field can be applied to selectively eject ions from
the trap. Ion traps have the advantage of being able to perform
multiple stages of mass spectrometry without additional mass
analyzers.
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Ion trap mass analyzer
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Fourier transform-ion cyclotron resonance (FT-ICR) :
An FT-ICR mass analyzer (also called FT-MS) is
another type of trapping analyzer. Ions entering a chamber are trapped
in circular orbits by powerful electrical and magnetic fields. When
excited by a radio-frequency (RF) electrical field, the ions generate a
time dependent current. This current is converted by Fourier transform
into orbital frequencies of the ions which correspond to their mass-to-
charge ratios.
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Collision-Induced Dissociation and Multiple-Stage MS :
The atmospheric pressure ionization techniques discussed are
all relatively “soft” techniques. They generate primarily:
• Molecular ions M+ or M–
• Protonated molecules [M + H]+
• Simple adduct ions [M + Na]+
• Ions representing simple losses such as the loss of a water
[M + H – H2O]+
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•The resulting molecular weight information is very valuable, but
complementary structural information is often needed.
•To obtain structural information, analyte ions are fragmented by
colliding them with neutral molecules in a process known as collision
induced dissociation (CID) or collisionally activated dissociation
(CAD). Voltages are applied to the analyte ions to add energy to the
collisions and create more fragmentation.
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Mass spectrum of sulfamethazine
acquired without collision-induced
dissociation exhibits little
fragmentation
Mass spectrum of sulfamethazine acquired
with collision-induced dissociation exhibits
more fragmentation and thus more
structural information
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Detectors :
once the ions have passed the mass analyser they have to
be detected and transformed into a usable signal. The detector is an
important element generating secondary electrons, which are
further amplified, or by inducing a current (generated by moving
charges). Ion detector fall into two main classes:
Point detectors: Ions are not spatially resolved and sequentially
impinge upon a detector situated at a single point within the
spectrometer geometry.
Array detectors: Ions are spatially resolved and all ions arrive
simultaneously (or near simultaneously) and are recorded along a
plane using a bank of detectors.
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Applications
• Molecular Weight Determination :
One fundamental application of LC/MS is the determination of
molecular weights. This information is key to determining identity.
• Differentiation of similar octapeptides :
Figure 1 shows the spectra of two peptides whose mass-to-charge
ratios differ by only 1 m/z. The only difference in the sequence is at
the C-terminus where one peptide has threonine and the other has
threonine amide. The smaller fragments are identical in the two
spectra, indicating that large portions of the two peptides are very
similar. The larger fragments contain the differentiating peptides.
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Mass spectra differentiating two very
similar octapeptides
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• Determining the molecular weight of green fluorescent protein :
Green fluorescent protein (GFP) is a 27,000- Dalton protein with 238
amino acids. It emits a green light when excited by ultraviolet light.
During electrospray ionization, GFP acquires multiple charges. This
allows it to be analyzed by a mass spectrometer with a relatively
limited mass (mass-to-charge) range. Mass deconvolution is then used
to determine the molecular weight of the protein.
Molecular weight
determination of green
fluorescent protein by
electrospray LC/MS
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Pharmaceutical Applications
Rapid chromatography of benzodiazepines
The information available in a mass spectrum allows some compounds
to be separated even though they are chromatographically unresolved.
In this example, a series of benzodiazepines was analyzed using both
UV and MS detectors. The UV trace could not be used for quantitation,
but the extracted ion chromatograms from the MS could be used.
MS identification and
quantification of individual
benzodiazepines from an
incompletely resolved mixture
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Clinical Applications
High Sensitivity Detection of Trimipramine and Thioridazine :
For some compounds, MS provides more sensitive detection.
Trimipramine is a tricyclic antidepressant with sedative
properties. Thioridazine is a tranquilizer. Figure 2 shows these
compounds in a urine extract at a level that could not be detected
by UV. To get the maximum sensitivity, the analysis was done by
selected ion monitoring.
Figure 2 shows
trimipramine and
thioridazine in urine extract
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• Food Applications :
Aflatoxins are toxic metabolites produced by certain fungi in foods.
Figure 3 shows the total ion chromatogram for four aflatoxins; each
could be uniquely identified by their mass spectra.
Figure 3 shows
Aflatoxins by API-
ES
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Environmental Applications
Carbamates by APCI-LC/MS
Carbamates are a class of pesticides usually analyzed by post-column
derivativation and fluorescence detection. The chromatogram in Figure
4 is that of a tomato extract that has been spiked with 11 carbamates.
Using APCI, these compounds are detected without derivitization. The
extracted ion chromatogram displays the peaks for the individual
compounds.
Detection of Phenylurea Herbicides
Many of the phenylurea herbicides are very similar and difficult to
distinguish with a UV detector (Figure 5).Monuron and diuron have
one ring and differ by a single chlorine. Chloroxuron has two chlorines
and a second benzene ring attached to the first by an oxygen.
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The UV-Vis spectra are similar for diuron and monuron, but different for
chloroxuron. Each of these compounds has a distinct mass spectrum. The
standards were run with an API-electrospray LC/MS.
Contd””
Carbamates by APCI(Fig. 4) Phenylurea herbicides by
API-ES (Fig. 5) 40

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REFERENCES :
https://en.wikipedia.org/wiki/Liquid_chromatography%E2%80%93m
ass_spectrometry .
http://www.agilent.com/cs/library/support/documents/a05296.pdf.
http://www.ecs.umass.edu/eve/background/methods/chemical/Openlit/
Chromacademy%20LCMS%20Intro.pdf.
http://ccc.chem.pitt.edu/wipf/Agilent%20LC-MS%20primer.pdf.
http://www.shimadzu.com/about/magazine/i7rr0a0000008xh6-
att/28_2.pdf.

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 W. Paul & H. Steinwedel; Zeitschrift für Naturforschung, 8A; 1953,
p448.
SPECTROMETRIC IDENTIFICATION OF ORGANIC
COMPOUNDS
BY ROBERT M. SILVERSTEIN AND FRANCIS X. WEBSTER page
no.2-6

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GPRCP

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