Mass Spectrometry Notes

Instrument Technician: Mass Spectrometry

This module focuses on mass spectrometry, its principles, terminology, and applications in hydrocarbon processing plants. It emphasizes the importance of understanding this technology for optimizing production and maintaining a safe working environment.

Objective One: Principles of Mass Spectrometry

Mass Spectrometry Basics

Mass spectrometry is a technique used to measure the atomic and molecular masses of atoms and molecules. This is achieved by converting these particles into charged ions and then deflecting them using magnetic and electric fields.

The amount of deflection is directly related to the mass and charge of the ion.

Mass spectrometry measures:

  1. Mass (m) of each ion,
  2. Number of positive charges (z) on each ion,
  3. Number of ions.

Mass Spectrometers

Mass spectrometers analyze gas mixtures to determine their composition through qualitative (identifying compounds) and quantitative (finding concentrations) analyses.

The mass-to-charge ratio (m/z) identifies the original atom or molecule. For example, a nitrogen molecule (N2) with a single positive charge (z = 1) has the formula N2^{+1}. If the molecular mass (m) of N2 is 28 amu, then the m/z value of N2^{+1} is also 28 amu:

\frac{m}{z} = \frac{28 \text{ amu}}{1} = 28 \text{ amu}

The flow of positively charged ions is detected as a small electric current, from which the instrument calculates molecule concentrations.

Advantages of Short Measurement Times

Composition measurements via mass spectrometry are rapid, offering two key advantages:

  1. A single mass spectrometer can quickly analyze multiple streams.
  2. Reduced analysis lag times, ensuring data is available faster than process material changes composition, which is crucial for effective process control.

Mass Spectrometer Components and Function

Figure 1 illustrates the block diagram of a mass spectrometer, detailing the following steps:

  1. Sample Inlet System: Conditioned sample gas or vapor enters the inlet system, preparing it for the high vacuum system.
  2. Ion Source: A small volume of sample enters the high vacuum area and passes through the ion source, which imparts a positive charge to the gas molecules. This process may also break bonds, creating positively charged ion fragments.
  3. Mass Analyzer: This section separates ions and fragments according to their mass-to-charge (m/z) ratio.
  4. Ion Detection: Ion detectors detect the separated ions as weak electrical currents, which are then amplified. The detector signals are proportional to the number of ions detected.
  5. High Vacuum System: Maintains pressure inside the mass spectrometer below 10^{-6} mmHg to prevent interference from air molecules.
  6. Roughing Vacuum Pump: Reduces pressure from atmospheric to approximately 0.1 mmHg during instrument startup, before the high vacuum system takes over.
  7. Data Handling: A computer program identifies sample compounds from their ion m/z ratios and calculates their concentrations from detector signal currents.

All mass spectrometers contain these basic components, but variations occur in ion sources and mass analyzer sections.

Sample Inlet and Ion Source

Common ion sources include thermal ionization, electrospray ionization (ESI), electron ionization (EI), matrix-assisted laser desorption ionization (MALDI), and inductively coupled plasma (ICP).

Electron Ionization (EI)

Figure 2 illustrates electron ionization: A clean, dry sample flows past a sample leak (porous sintered metal disk or orifice) into the ion source's high vacuum.

A stream of energized electrons, originating from a heated filament and accelerated towards a positively charged plate, bombards the sample molecules.

This collision knocks off an electron from the sample molecule, resulting in a single positive charge. The electron beam's energy can also break chemical bonds, producing positively charged fragments with smaller masses.

The ions then pass through a series of negatively charged metal slits, accelerating towards the mass analyzer.

Mass Analyzers

The motion of ions through magnetic and electric fields is affected by their mass (m) and charge (z). Process mass spectrometers typically use an electron beam ion source that generates ions with a single positive charge (z = 1).

Several methods can separate ions by mass; common types of mass analyzers include:

  1. Fixed magnetic sector deflection,
  2. Variable magnetic sector deflection,
  3. Quadrupole mass filter, and
  4. Time-of-flight.
Fixed Magnetic Sector Deflection

When a moving beam of ions enters a magnetic field, it experiences a magnetic force, similar to how gravity affects a golf ball's path.

In a fixed magnetic sector analyzer, ions must have the same velocity. A velocity selector ensures that only ions with the same velocity pass through the magnetic field (Figure 3).

Ions entering the constant magnetic field are deflected, with heavier ions having more horizontal momentum and deflecting less.

All ions follow a curved path with the radius of curvature proportional to their mass. Detectors are precisely positioned to detect different mass fragments.

Fixed magnetic field mass spectrometers must be set up to detect specific ion masses, each requiring a separate detector. Variable magnetic field mass spectrometers overcome this limitation.

Variable Magnetic Sector Deflection

The mass-to-charge ratio (m/z) and the strength of the magnetic field affect the radius of an ion's curved path. For process mass spectrometers, the charge is usually +1, so only ion mass and magnet field strength affect curvature.

The magnetic field strength can be adjusted to make ions of different masses follow the same curved path (Figure 4). As the magnetic field increases, lighter ions are detected first, followed by heavier ions.

Variable field mass spectrometers require only a single detector that can detect any mass. The m/z ratio is determined by the magnetic field strength needed to bend the ions' paths to hit the detector.

Quadrupole Mass Filter

A quadrupole mass filter consists of two pairs of electrically connected parallel rods (Figure 5). Direct current (dc) and high-frequency alternating current (ac) voltages are applied between the rod pairs, generating a varying magnetic field at the center.

As the dc and ac voltages increase, the filter selects ions with increasing m/z ratios to pass through to the detector. Ions that are too light or too heavy are trapped by the rods.

Unlike fixed magnetic field mass spectrometers, quadrupole filters can detect ions of any mass within their operating range by varying the voltages. Like variable magnetic field instruments, they require only a single ion detector.

Quadrupole mass filter mass spectrometers are smaller and lighter than magnetic deflection instruments.

Time-of-Flight

A time-of-flight mass spectrometer (Figure 6) separates ions of different masses based on their transit time through a drift region. An electric field, produced by a pulsed voltage grid, accelerates all ions with an energy of q mem{V}, where q is the ion charge (typically +1) and V is the applied voltage.

Since kinetic energy (KE) is equal for each ion (KE = \frac{1}{2} m v^2), lighter ions have higher velocities and reach the detector faster.

Ion Detectors

Two types of ion detectors are used in process mass spectrometers:

  1. Faraday Cup: Measures process sample component concentrations in the range of 0.1% to 100%.
  2. Electron Multiplier: Detects low concentrations found in environmental samples, typically 0.1% to ppm.
Faraday Cup Detector

The Faraday cup (Figure 7) is the most common ion detector in process mass spectrometers. Positively charged ions enter a small metal cup, where their positive charge is neutralized by the cup's metal walls.

Electrons flow into the cup via a resistor connected to ground, resulting in a conventional current flow. An output voltage, proportional to the current, develops across the resistor and is then amplified.

Electron Multiplier Detector

In an electron multiplier detector (Figure 8), a single positive ion knocks electrons from its inside surface. The detector is coated with a conductive surface at high voltage (2 kV), negative at the entrance and positive at the exit.

The ejected electrons generate more electrons as they collide inside the tube, creating a large electric current. This amplified current is proportional to the number of positive ions entering the detector.

An electron multiplier detector is also called a continuous dynode particle multiplier.

High Vacuum Systems

To prevent contamination, mass spectrometers must maintain a high vacuum (pressure less than 10^{-6} mmHg) in the ion source, mass analyzer, and detector.

A high vacuum system includes:

  1. vacuum pumps and
  2. vacuum measurement gauges.
Vacuum Pumps

Mass spectrometers typically require two types of vacuum pumps:

  1. A roughing pump to remove high volumes of air during startup.
  2. A high vacuum pump to remove remaining traces of gas.

Roughing pumps, such as oil-sealed rotary vane vacuum pumps (Figure 9), quickly pump air from the mass spectrometer, reducing the pressure to about 0.1 mmHg. Spring-loaded vanes attached to a rotor draw in, trap, and compress the gas, which then opens an exhaust valve sealed under oil.

High vacuum pumps reduce the pressure to less than 10^{-6} mmHg. Two common types are:

  1. Turbomolecular pumps and
  2. Ion pumps.
Turbomolecular Pumps

A turbomolecular pump (Figure 10) has turbine blades that rotate at high speed, directing gas molecules through the pump. These pumps must exhaust into a backing pump, typically an oil-sealed rotary vane vacuum pump.

Ion Pumps

An ion pump (Figure 11) ionizes gas molecules and stores them on the metal walls of the pump. Metal cylinders, with a high voltage of 5000 Vdc applied, generate electrons. A strong magnet forces these electrons to move in spiral paths, colliding with neutral gas molecules and turning them into positive ions.

The ions are accelerated into metal plates, where they react chemically or bury themselves, reducing pressure to less than 10^{-6} mmHg.

The electrical current between the cylinder and plate indicates the vacuum level.

Vacuum Measurement Gauges

A mass spectrometer must continuously monitor and maintain a high vacuum using:

  1. Ionization gauges and
  2. Thermal vacuum gauges.
Ionization Vacuum Gauges

Ionization vacuum gauges (Figure 12) are more sensitive, monitoring high vacuum (less than 10^{-6} mmHg pressure). They measure pressure by ionizing neutral gas molecules and measuring the resulting current (Figure 12).

Electrons from a heated filament accelerate towards a positively charged electrode, colliding with neutral molecules to create positive ions. These ions move towards a negatively charged plate, causing current to flow through a connected ammeter. The current is proportional to the pressure in the mass spectrometer.

Thermal Vacuum Gauges

Thermal vacuum gauges sense the cooling effect of gases on a heated filament. The thermocouple vacuum gauge (Figure 13) is common. A constant voltage heats a metal filament, and its temperature depends on the heat conducted away by surrounding gas molecules.

As pressure decreases and vacuum increases, the filament temperature increases. A thermocouple measures the filament's temperature, and the readout is displayed in mmHg or Pa pressure units.

Thermal gauges monitor low vacuum from roughing vacuum pumps down to a lower limit of 10^{-3} mmHg pressure.

Objective Two: Applications of Mass Spectrometry

Qualitative Analysis Applications

Mass spectrometers are used to identify a broad range of compounds. The mass spectrum is a plot of detector output against the mass-to-charge ratio (m/z), acting as a unique fingerprint for identifying unknown compounds.

Figure 14 shows the mass spectrum of methane (CH_4). The largest peak, the base peak, is assigned a value of 100%, and other peaks are plotted as percentages of it.

The largest peak at m/z 16 amu corresponds to the molecular ion CH_4^{+1}. Smaller peaks result from the breaking of bonds to form lighter ion fragments.

Process Mass Spectrometry Applications

Mass spectrometers can analyze a sample stream for ten components in less than five seconds, providing rapid response to process changes and enabling the analysis of multiple streams. This offers faster composition data in response to process changes and allows one mass spectrometer to analyze multiple streams.

For instance, a mass spectrometer can analyze five streams, taking two seconds per stream. The time delay between sampling and analysis completion equals the number of streams multiplied by the analysis time. If you have 5 streams and it takes 10 seconds to analyze each, then the first stream get's analyzed every 50 seconds ( 5 \text{ streams} \times 10 \text{ seconds per stream} = 50 \text{ seconds}).

While mass spectrometers may be more expensive than gas chromatographs, using a single mass spectrometer for multiple streams can be more cost-effective than purchasing several gas chromatographs.

Stream Selectors

Two common stream selectors are:

  1. Manifold systems (typically select less than 20 sample streams) and
  2. Rotary stream selectors (typically select more than 20 streams).
Loop Manifold

Figure 16 shows a loop manifold. Conditioned gas streams connect to the stream selector, and a three-way solenoid valve selects one stream at a time, connecting it to a manifold loop of tubing.

The selected stream flows through the loop, purging it via a bypass tee before the mass spectrometer. Non-selected streams continue to flow to a waste vent or process return line, ensuring fresh gas is always ready.

Rotary Stream Selector

Figure 17 illustrates a rotary stream selector: Sample streams connect to ports on a circular body, and a rotating selector connects a selected port to the mass spectrometer.

Ports not selected connect to a common waste collection header, ensuring continuous flow and preventing delays. Calibrating the mass spectrometer with a gas of known concentration provides a calibration factor for calculating sample component concentrations.

Process Monitoring Applications

Mass spectrometers are suitable for processes requiring tight control for safety, efficiency, and product quality. They measure gas mixture components' concentrations in the range of 0.1% to 100%, typically using a Faraday cup detector.

Common applications include:

  1. Hydrocarbon processing,
  2. Iron and steel production, and
  3. Biotechnology.
Hydrocarbon Processing

Mass spectrometers can replace gas chromatographs in monitoring hydrocarbon and inorganic gases (carbon dioxide, oxygen, nitrogen, hydrogen sulfide) in natural gas processing plants, oil refineries, and petrochemical plants.

Examples include olefin (ethylene and propylene) production and ethylene oxide production.

Iron and Steel Production

Mass spectrometers monitor gases above molten iron and steel, providing data to control quality and measure hydrogen levels to prevent explosions.

Biotechnology Applications

Mass spectrometers measure the composition of gases above liquids in fermenters and bioreactors, with a single instrument monitoring over fifty vessels.

Environmental Applications

Environmental mass spectrometers measure gas mixture component concentrations in the range of 0.1% to 1 ppm, requiring an electron multiplier detector. These instruments can monitor volatile organic compounds (VOCs), such as benzene, to minimize their impact on plant workers.

A single mass spectrometer with a stream selector can sample over 100 points, placed near potential leak sources (valves, pipe connections, flanges, pump seals). The instrument can activate a local alarm near a leak, and maintenance workers can repair it.

Figure 18 illustrates the selection and introduction of air samples into an environmental mass spectrometer. Air samples must be conditioned to remove solid particles and moisture. Some instruments use a membrane selectively permeable to VOCs, increasing their concentration in the mass spectrometer.

Analyzer Safety Hazards and Procedures

Repair and maintenance of mass spectrometers require specialized training.

Be aware of the following safety hazards and recommended procedures:

  • High temperatures can cause skin burns.
  • Oil-sealed rotary pumps produce poisonous oil mist and must be exhausted into a vent.
  • Waste gas from the stream selector may be explosive or toxic and should be routed to a vent or process return line.
  • Lethal voltages are present; isolate the instrument from the power supply before service work.
  • Some parts may be constructed from poisonous substances, requiring personal protective equipment during handling.
  • Isolate and purge gas sample lines with an inert gas (nitrogen) before disassembling any part of the sample handling system to avoid exposure to toxic or explosive gas.
  • In areas with a risk of fire or explosion, use a purge and pressurization system to prevent flammable gas from entering the instrument enclosure, using clean air or an inert gas.