Introduction to Mass Spectrometer Gas Chromatography Fundamentals & Principle

Mass Spectrometer Gas Chromatography Fundamentals

Sample Inlet

Gas, liquid and solid compounds can be introduced into the ion source through specially designed inlets with controlled flow. This can be done a number of ways depending on the state of the sample. Several of the common types of sample inlets are:
• GC
• LC
• Direct Insertion Probe
• Batch

Ion Source

 The filament is the source of electrons. A repeller is used to help direct ions through a series of lenses in the ion source. Without these components, sample molecules would not be ionized and they would not reach the mass filter.

 The fragmentation process which occurs in the ion source is important because the results are ultimately used to identify species. The other parts of the mass spectrometer, the mass filter and detector, are used to select and count ions.

Two common types of ionization are electron ionization and chemical ionization.

The ion source is used to:

• Fragment and ionize sample molecules.
• Direct ions through a series of lenses into the mass filter.

Mass Filter

Once ionized, the ions enter the mass filter. The mass analyzer moves selected ions from the ion source to the detector. The mass analyzer filters and separates ions based on their mass to charge ratio. Only an ion of one mass is passed through the mass filter at a given time.
Several common mechanisms or methods are used to select and filter these ions. Four common types of mass analyzers which separate ions by their mass to charge ratio are:
• Magnetic Sector
• Quadrupole
• Ion Trap
• Time-of-Flight (TOF)

The mass filter is used to:

• Move ions from the ion source to the detector.
• Filter ions according to their mass to charge ratio.

Detector

Once an ionized sample has been filtered by mass-to-charge ratio in the mass filter, the abundance must be detected and reported to the data system. A detector is used to collect and count ions. Only one ion is detected at a given time, whether the mass filter is a time-of-flight, separation by space, quadrupole filter or ion trap.

Once inside the detector, the ions generate a signal which can be displayed by the data system.

The detector is used to:

• Count ions.
• Generate an electrical signal.

Vacuum System

The ion source, mass filter and detector are in a vacuum. The vacuum system makes it possible for ions to move from the ion source to the detector without colliding with other ions, sample molecules, air and water.

A vacuum is created by removing all species from the mass spectrometer. Minimizing collisions with other ions ensures that more ions will be counted by the detector.

The vacuum system is used to:

• Provide ions with a free path of travel from the ion source to the detector.
• Remove all species from the instrument.

Data System

The data system has three purposes:
• Controls the operational parameters of the ion source, mass filter and detector.
• Graphically displays the results of the sample compound as a mass spectrum.
• Manipulates data for identification and quantitation.

The data system is used to:

• Control the mass spectrometer hardware.
• Collect data and display it in a useful way.
• Integrate data to produce reports.
 

Sample Inlet

Scan mode

SIM/Scan

Of the nine sample introduction methods, the capillary direct inlet is the only inlet where the injected volume is routed directly to the ion source. A minimal amount of hardware is needed to get a capillary column in close proximity with the ion source. A capillary direct interface, in simple terms, is the capillary column itself. 

This allows a sample to be introduced into the vacuum system of the Mass Spectrometer from the atmosphere.

In a capillary system, all the flow of the column enters the Mass Spectrometer. This implies the ion source and vacuum system can handle the amount of flow. The optimal rate of total column flow is typically about 1 ml/min.

The capillary direct inlet consists of a piece of tubing which supports the capillary column as it leaves the GC oven and enters the Mass Spectrometer’s ion source. For chromatic graphic performance, the interface is heated.

Capillary Direct Advantages

1. simple

2. all sample goes to the Mass Spectrometer

3. best GC sensitivity

Capillary Direct Disadvantages

1. cross-linked columns only

2. retention time may be off due to vacuum effects

3. MS dependent

4. everything injected enters the ion source

Open-Split Inlet (Older Technology)

What is an Open-Split Inlet?

When a gas flow is too large, a hardware arrangement which allows a portion of the column flow to enter the ion source is used. A restrict or is positioned between a GC column and the vacuum  system, restricting the column flow. Two types of split inlet implementations which could be used are open-split and effluent split.

An open-split inlet has a purge flow which is typically the same as the carrier gas.This improves system performance, although additional purge hardware and a special interface are required.

Open-Split Inlet Advantages

1. accommodates wide-bore or narrow-bore columns 
2. no need to vent to change columns
3. no vacuum effects on chromatography or retention time

Open-Split Inlet Disadvantages

1. sample is split – only allows a small portion of the sample to enter the mass spectrometer
2. more flows to set
3. more places to leak
4. a plug can occur in the restrictor
5. split ratio is slightly temperature dependent

Effluent Splitter (Older Technology)

What is an Effluent Splitter?

An effluent splitter is a T-shaped piece with a port which provides a path for excess flow. An effluent splitter is used with a capillary direct interface. The column can deliver a large flow; the mass spectrometer will only use the flow that the restrictor will allow and the remainder of the flow is discarded.

Effluent Splitter Inlet Advantages

1. accommodates wide-bore or narrow-bore columns

2. no need to vent to change columns

3. no vacuum effects on chromatography or retention time

Effluent Splitter Inlet Disadvantages

1. sample is split – only allows a small portion of the sample to enter the mass spectrometer

2. more places to leak

3. a plug can occur in the restrictor

4. split ratio is slightly temperature dependent

Jet Separator

What is a Jet Separator?

The jet separator consists of two nozzles and an outlet to a vacuum pump. The incoming sample is either passed through the separator to the ion source or pumped away by the vacuum system.

The column effluent is forced through a fine jet into an evacuated enclosure. Opposite the first jet, and only a very small distance from it, is mounted a second jet. 

Under normal conditions, the sample will pass through the jets at high speeds. The heavy sample ions travel almost in a straight line from one jet to the other. The heavy ions have more momentum to carry them straight across to the receiving nozzle. Lighter molecules diffuse after leaving the inlet jet, and are pumped away by the jet separator’s vacuum system.

Jet Separator Advantages

• temperature independent

Jet Separator Disadvantages

• discriminates against low molecular weight compounds
• easily clogged
• easily mechanically misaligned, if mishandled

Direct Insertion Probe

What is a Direct Insertion Probe (DIP)?

The Direct Insertion Probe (DIP) is used to introduce less volatile samples into the ion source of the mass spectrometer. It consists of a long rod with a sample holder on the tip. A solid or liquid sample is placed in the holder, also called the nest, on the end of the probe. The probe enters the vacuum system through a series of vacuum interlocks, so the sample cup is nested against the ion source.

The probe is inserted past the first Teflon seal. The roughing valve is opened to evacuate the area between the first Teflon seal and the ball valve. After about 5 to 10 seconds, the probe is pushed past the second Teflon seal; then, the ball valve is slowly opened and the end of the probe that holds the sample is pushed into the high vacuum chamber until it rests in the source. Heating the probe causes the sample to volatilize into the source. A heater/sensor assembly allows the temperature of the probe tip to be carefully controlled or temperature-programmed.

Direct Insertion Probe Advantages

1. Allows spectra of relatively nonvolatile compounds to be obtained
2. minimum sample preparation

Direct Insertion Probe Disadvantages

1. samples must be relatively pure

2. requires a valving system

Batch Inlet

What is a Batch Inlet?

A batch inlet allows the sampling of the gaseous area above the liquid sample. This area is called the headspace. The needle valve controls the rate of introduction of the headspace, into the mass spectrometer. The batch inlet requires that a DIP port is mounted on the mass spectrometer.

Batch Inlet Advantages

1. allows spectra of more volatile compounds to be obtained

2. minimum sample preparation

Batch Inlet Disadvantages

1. samples must be relatively pure

2. requires a valving system

FAB Inlet

What is a Fast Atom Bombardment Gun?

The FAB Gun is a direct insertion probe that uses Fast Atom Bombardment to ionize the sample. It consists of a long rod with a sample stage and xenon gas source (gun) for ionization.

The FAB technique is a process which:

• Generates high energy ions and atoms.
• Focuses those ions and atoms through the gun onto a liquid surface.
• Induces secondary ions from the liquid surface.

FAB Advantages

• excellent technique for analysis of peptides
• good for nonvolatile compounds.

FAB Disadvantages

• static introduction of samples requires pure sample
• slow process which gives low sample throughput
• high noise background
• contaminates ion source quickly

DCI Inlet

What is a Desorption Chemical Ionization Inlet?

The Desorption Chemical Ionization probe is used to rapidly heat a sample, giving it the energy it needs to ionize under conventional chemical ionization conditions. This process increases theenergy of an analyte molecule to become a proton acceptor, i.e. <M+H>+. This is accomplished by heating a thin film of the analyte which has been deposited directly onto the probe’s filament.The chemical ionization conditions and current through the filament are controlled through external hardware.
This process differs from performing chemical ionization analysis using a DIP in the rate at which the sample is directly heated. By heating the sample rapidly, thermal degradation is minimized in DCI.

DCI Advantages

• soft ionization technique for samples not compatible with GC analysis
• sample less likely to degrade due to fast heating effects

DCI Disadvantages

• pure sample needed

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