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Analytical Chemistry > LC/MS > Flashcards

LC/MS Flashcards

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1
Q

What is mass spectrometry?

A

Separation of ions by mass to charge ratio, the basis of mass spectrometry is the production of ions that are subsequently separated or filtered according to their mass to charge (m/z) ratio and detected. The resulting mass spectrum is a plot of the (relative) abundance of the produced ions as a function of the m/z ratio

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2
Q

Where does the molecular ion peak occur?

A

Molecular ion mass + 1

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3
Q

Sequence of LC/MS?

A 
LC (attached to central processing unit)
Interface 
Ions source
Mass analyser (attached to central processing unit) 
Detector
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4
Q

What does the interface do?

A

Must convert dissolved analyte into gas phase ions - how to transfer liquid to gas mobile phase gas reduce pressure and ionise

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5
Q

What are the LC/MS results?

A

Chromatograms
Mass spectra
Mass chromatograms

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6
Q

What is LC?

A

Separation of analytes by interactions with stationary phase

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7
Q

What is MS?

A

Separation of ions by mass to charge ratio

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8
Q

How to adapt HPLC method to LC/MS?

A

Increasing volatility of the buffer
Changing the pH
Adding adducts such as Na+

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9
Q

What are the requirements for the solvent in LC/MS?

A

Suitable viscosity, conductivity, surface tension and polarity
pH of the solvent eg the production of positive ions is favoured at acidic pH

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10
Q

What conversion processes are required for interfacing liquid chromatography with mass spectrometry?

A

State of matter –> Gas phase (evaporation)
Pressure –> High vacuum (pressure reduction)
Charge state –> Ionic (ionisation)
In charge state introduce charge on molecule - in reversed phase chromatography needs to introduce charge

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11
Q

Ionisation techniques?

A

Electrospray ionisation (ESI)
Atmospheric pressure chemical ionisation (APCI)
Matrix assisted laser desorption ionisation (MALDI)

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12
Q

Mass analysers?

A

Quadrupole Q
Quadrupole ion trap IT
Time of flight TOF

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13
Q

Tandem mass analysers?

A

Triple quadrupole QqQ

Quadrupole time of flight QTOF

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14
Q

What is ESI?

A

ESI is a soft ionisation technique that accomplishes the transfer of ions from solution to the gas (want to charge molecules but not fragment it, electron impact however fragments the molecule)

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15
Q

What happens in ESI?

A

Electrospray is produced by applying a strong electric field to a liquid passing through a capillary tube at a low flow, desolation by gas flow (N2) or gentle capillary heating (100-300 degrees C)

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16
Q

What are problems with ESI?

A

Prone to many issues high voltage high temperature to allow salvation of gas nitrogen pushes gas into mass analyser

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17
Q

What are the applications of ESI?

A

Ionic compounds that are intrinsically charged in solution
Neutral/polar compounds that may be protonated (for positive ion mass spectra) or deprotonated (for negative ion mass spectra) under the solution conditions employed eg appropriate pH
Non polar compounds that undergo oxidation (positive ion mass spectra) or reduction (negative mass spectra) at the electrospray capillary

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18
Q

What is the electrospray process susceptible to?

A

Competition and suppression effects (might not see charge on the molecule of interest), all polar/ionic species in the solution being sprayed will be potentially ionised

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19
Q

Advantages of ESI?

A

ESI is a soft technique which usually enables the molecule weight of the analyte under study to be determined
Ionisation occurs directly from solution and consequently allows ionic and thermally labile compounds to be studied
Virtually any ion in solution is amenable to analyse with psi including large macromolecules, quaternary salts etc
The production of multiple charge species especially proteins extends the effective range of a mass analyser to megadaltons

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20
Q

Disadvantages of ESI?

A

ESI is not applicable to non polar or low polarity compounds
Suppression effects may be observed and the direct analysis of mixtures is not always possible
ESI is a soft ionisation technique producing intact species and structural information might not be available - necessity to use cone voltage fragmentation of MS-MS
The mass spectrum produced from an analyte in terms of the m/z range of the ions obsessed and their reactive intensities depends on a number of factors and spectra obtained using different experimental conditions may therefore differ considerably in appearance

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21
Q

Five main characterises of an analyser?

A 
Mass limit
Transmission 
Resolving power 
Scan speed
Mass accuracy
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22
Q

What is the mass limit?

A

Determines the highest value of m/z ratio that can be measured - important for biomolecule analysis (how high we can go in mass to charge ratio, only need to worry about this for larger molecules eg proteins)

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23
Q

What is transmission?

A

The ratio between the number of ions reaching the detector and the number of ions produced in the source - important for quantification

24
Q

What is resolving power?

A

Ability to distinguish between two ions with a small mass difference - important for accurate mass analysis, critical for characterisation time of flight technology

25
Q

What is scan speed?

A

Rate at which the analyser detects ions over a particular mass range important in GC/MS and LC/MS - want Gaussian peaks so limited time when can identify and quantify molecules in each analysis, MS has to be quick enough to do this

26
Q

What is mass accuracy?

A

Difference between theoretical and observed mass - important in molecular formula determination, key in mass spectrometry how to decide what the molecule is

27
Q

Why is resolution necessary?

A

Resolution is necessary in accurate mass measurement to eliminate ions from mass analysis that have the same nominal masses but different elemental compositions

28
Q

Resolution and sensitive in GC and LC/MS?

A

In GC and LC/MS analyses, a compromise is stuck between sensitivity (ion transmission) and mass resolution

29
Q

What is the resolution of the quadrupole instrument?

A

In the quadrupole instrument the resolution is set the lowest possible value commensurate with resolving peak differing by 1 dalton (unit resolution)

30
Q

What is MS resolution?

A

The measurement of the analysers ability to separate one mass from adjacent mass

31
Q

What are sensitivity and detection limits?

A

The ability of the mass spectrometer to respond to a given amount of sample analyte at a given mass to charge ratio

32
Q

What is chromatographic selectivity?

A

Chromatographic selectivity allows physical separation of components in a mixture as they elute through the column (time dependent)

33
Q

What is mass selectivity?

A

Mass selectivity allows isolation (or selection) of specific components by virtue of their particular mass specific response albeit the different components may not be physically separated (time independent, separation based on mass specific response)

34
Q

What is chromatography isolation?

A

Physical separation in time, can make analysis lengthy

35
Q

What is MS isolation?

A

Separation is due to compound specific response, no physical separation, not time dependent, fast, may require high resolution or MS/MS

36
Q

What is chromatography interferences?

A

Is capable of complete isolation of analyte from interferences

37
Q

What is MS interferences?

A

Interferences may suppress or enhance response cannot separate and get rid of interferences

38
Q

What is a quadrupole?

A

Consists of four parallel rods

39
Q

How does the quadrupole work?

A

Two varying electrostatic fields, one direct current (DC) and one at varying radio frequency are applied at right angles to each other via the four rods (control conditions between four rods) of the quadrupole,, this creates a resonant frequency for each m/z value in a mass spectrum (mass to charge ratio), thus at any particular combination of DC and radio frequency, only ions of a corresponding m/z transit the quadrupole and reach the detector

40
Q

What is static mode in quadrupole?

A

Static mode keep radio frequency at particular positions so only one passes through, or vary frequency lets lots through

41
Q

What are the two modes the quadrupole mass analyser can be operated in?

A

Scan mode

Selected ion monitoring (SIM)

42
Q

What is scan mode?

A

The full mass range is scanned by varying the resonant frequency of the quadrupole such that ions of sequential m/z transits the analyser, it has lower sensitivity because most of the ions strike the rods during the scene, this mode is used to collect spectra for interpretation or a library search

43
Q

what is selected ion monitoring (SIM)?

A

Only ions of specified m/z can transmit the analyser this provides the greatest sensitivity and is used for quantitative applications, it is used when the analyst has prior knowledge of what ions to expect, used in quantitative measurement only want one compound of interest

44
Q

What is tandem mass spectrometry?

A

Mass analysers can be combined in a tandem arrangement to find additional information. Tandem mass spectrometry is the mode of operation that utilises multiple stages of mass analysis and provides the ability to mass analyse sample components sequentially in time or space in order to improve selectivity of the analyser or promote fragmentation and facilitate structural elucidation (couple quadrupole together to get a triple quadrupole)

45
Q

What are the modes of MS/MS?

A

Precursor ion scanning
Product ion scanning
Multiple reaction monitoring (MRM)

46
Q

What is precursor ion scanning?

A

MS2 is set to transmit a single m/z ratio (that of the product ion of interest) while MS1 is set to scan through the mass range of interest. The fragmentation of ions passing through mS1 is again carried out in collision cell

47
Q

What is product ion scanning?

A

MS1 is used to isolate an ion of interest, fragmentation of the ion takes place in collision cell, MS2 is scanned to provide a mass spectrum of the ions formed in the collision cell

48
Q

What is multiple reaction monitoring (MRM)?

A

Selected analyte precursor masses are continually introduced into the collision region by the first mass analyser and only selected product ions are monitored through the second mass analyser, the benefit is greatly enhanced selectivity and improves sensitivity especially for samples with a dirty or complex matrix

49
Q

Requirements for GC/MS interfaces?

A

Band broadening has to be minimised by a minimum dead volume (how to keep this as small as possible)
The carrier gas pressure has to be reduced at the column exit
There are no discrimination effects against thermally labile compounds owing to active sites or heating of the interface
There is efficient transfer of the entire sample to the ionisation chamber

50
Q

What are the direct connection of interfaces for GC/MS?

A

Direct connection of the column to the source
Small bore capillary tube (fused silica)
Open split

51
Q

Ionisation techniques?

A

Electron ionisation EI

Chemical ionisation CI

52
Q

Ionising agent, source pressure and uses of electron ionisation?

A

Allows fragments to occur in interface if intact molecules wanted use chemical ionisation technique, ionising agent of 50-70 eV electrons, 10-4 - 10-6 torr source pressure, uses of extensive fragmentation allows structure determination

53
Q

Ionising agent, source pressure and uses of chemical ionisation?

A

Gaseous ions as ionising agent, ~ 1 torr source pressure, uses of molecular mass determination

54
Q

What is electron ionisation EI?

A

Relatively high energy electrons (70eV) collied with sample molecules. These collisions produce (primarily) positive ions. Upon ionisation the molecules of a given substance fragment in fairly predictable patterns

55
Q

What is chemical ionisation CI?

A

In addition to the sample and carrier gas, reagent gas (ammonia, methane) is introduced into the ionisation chamber. The emitted electrons collide with reagent gas molecules, forming reagent ions which react with sample molecules

56
Q

Why is CI more gentle than EI?

A

CI ion formation involves much lower energy and is therefore much more gentle than electron ionisation. Because CI results in much less fragmentation CI spectra usually show high abundance of the molecular ion. For this reason CI is often used to determine the molecular weights of sample compounds

57
Q

What is the sequence of GC-MS?

A 
GC (attached to computer)
Interface
Ions source 
Mass analyse (attached to computer)
Detector (MS)

Analytical Chemistry (13 decks)

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