Key points
The equipment used for gas chromatography, liquid chromatography and mass spectrometry has specific performance requirements and must ensure accuracy. The complex equipment includes multiple subsystems that control the flow of sample analyte, carrier gas, and solvent fluids and depend on various types of valves that must handle different gases and liquids at a range of precise and repeatable flow rates. Leak-free operation and cleanliness are also critical considerations.
The following is a review of the valves used in the three types of analytical systems, as well as an explanation of their role in ensuring the highest levels of performance and productivity.
Valve Basics
The most common valve technologies used in analytical instruments are isolation valves, pinch valves, proportional valves and general-service valves.
Isolation valves prevent contamination of various liquids and/or gases in a system. A sealing mechanism keeps the fluid separate from the actuator and prevents it from coming into contact with the valve control surfaces, ensuring that the sensitive media within the valve body remains pure.
A variety of sealing techniques may be used in isolation valves, ranging from a simple diaphragm to more sophisticated flapper, bellows, lever or rocker seal designs. These sealing options make the valves versatile enough to handle and isolate virtually any type of media while protecting it from contamination.
In addition, these valves are designed to control and deliver very low internal fluid volumes and to be easy to clean for reuse. These technical advantages make isolation valves the primary choice for fluid handling in equipment used for research and analytical purposes.
Pinch valves are a type of isolation valve that is used to control the flow of a media being carried in a flexible tube that passes through the valve body. The valve actuator and body surround and compress, or “pinch,” the flexible tube to control media flow.
When open, pinch valves ensure a free flow throughout the tubing. Because of their hermetic separation and flow characteristics, pinch valves are ideal for handling all kinds of sensitive media, with the benefit of media non-contamination and fast tubing change-out.
Proportional valves, specially designed to respond precisely to the slightest changes in electrical input signals, are solenoid-driven valves that produce proportional movements of the actuator and make fine adjustments to the flow path for extremely accurate regulation of both gas flow and pressure. Compact proportional valves are ultra-reliable, offering precise results with high sensitivity and repeatability.
Because the actuator is designed to minimise friction and the valve body is able to handle varying input pressures, proportional valves are capable of regulating a wide range of flow rates, including the low flow rates (just a few millilitres per minute) required in gas-chromatography or mass spectrometry applications.
General-service valves typically do not offer the design characteristics for physical or hermetic separation of valve actuators from fluids they control. However, they can be built with biocompatible materials (e.g., O-rings, seals, valve bodies) that meet material compliance requirements for “wetted” surfaces. Therefore, they are quite versatile and are known for excellent stability and long cycle life under continuous duty.
How then are these four types of valves used in mass spectrometry, and gas and liquid chromatography?
Mass Spectrometry
Mass spectrometry is used for determining masses of particles and the elemental composition of a sample or molecule. It is a powerful analytical technique that measures the mass-to-charge ratio (m/z) of charged particles that are presented as a mass spectrum or a plot of intensity as a function of the mass-to-charge ratio.
When a sample is introduced into a mass spectrometer, it is first evaporated at high temperatures and then ionised to become either positively or negatively charged. Nitrogen gas is used to aid in solvent transportation and sample nebulisation and evaporation.
On/off or proportional valves provide proper flow control of the nitrogen gas, ensuring the stability and sensitivity of the mass spectrometry results. A low flow rate (around 0.2 liters per minute) is required for maximum accuracy.
Next, a “collision gas” such as argon is introduced into the collision cell where ions are broken into fragments as they collide with the argon molecules. Proportional valves control the argon supply into the collision cell.
All mass spectrometers operate at very low pressure (high vacuum). Therefore, the valve’s ability to achieve and maintain an effective vacuum is vital to the process. General-service valves with high tightness and vacuum operation capability allow the vacuum manifold to be vented and refilled with dry nitrogen or filtered air.
Gas and Liquid Chromatography
These two analytical techniques serve the same purpose: separating the components of any mixture. Gas chromatography separates and analyses the compounds in their volatile or gaseous phase. In contrast, the liquid chromatography technique separates the dissolved ions, particles or molecules in a liquid.
In gas chromatography, an inert gas is used to carry samples. Helium, nitrogen and hydrogen are commonly used as the carrier gas. The sample to be analysed is injected into the injection port and carried into the column. The sample is then separated inside the column, and different chemical compounds are detected by a specific type of detector.
Computerised or electronic pneumatic control (EPC) systems are widely used in modern gas chromatographs (GC). There are two main purposes for the EPC manifold: One is to control carrier gas flow, and the other is to control detector gas (air and hydrogen) for flame ionisation detection (FID).
In both functions, solenoid-driven proportional valves make fine adjustments of the flow path for extremely accurate gas flow control and are critical to ensure chromatography analysis reliability and repeatability.
These valves with small orifice sizes readily handle the low flow rates (just a few milliliters per minute) required in most gas-chromatography applications. Miniature general-service valves with compact size and high tightness will also be used to on/off control gas sampling, purge flow or vent the system.
Liquid chromatography also involves two distinct flow control operations. Upstream, the low-pressure-mixing system will use a proportioning valve to blend the solvents. Various proportioning valves open and close to allow the right proportion of each solvent (up to four) to enter the mixer.
Specific combinations of solvents (called gradients) are used depending on the sample being analysed. The proportioning accuracy with which each solvent is dispensed is critical because it affects the ultimate precision of the analysis and can reduce cycle time. Compact size, low internal volume and extremely short response time are other important features.
After the sample compound is injected into the solvent stream, it is pumped to the chromatography column, where it is separated into individual components. The constituent elements are then recognised and analysed by a detection unit. Two-way or three-way isolation valves are commonly used to control the wash solutions and waste drainage for system cleaning and waste disposal safety.
Conclusion
Designers of analytical instruments should keep several factors in mind when determining what type of valves to use in their gas and liquid chromatography and mass spectrometry equipment. Words like “reliable,” “repeatable,” “accurate” and “precise” should describe their performance.
Because this equipment has to perform at submolecular levels with absolute accuracy, it is also vital to evaluate the materials used in construction. And finally, the valves must ensure long working life, minimal deterioration, little maintenance, and infrequent replacement.
