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Gas chromatography, liquid chromatography, and mass spectroscopy flow control

Chromatography and spectroscopy are two major techniques used to perform quantitative and qualitative analysis of gas and liquid samples. Various sample analysers based on these methods have been developed, differentiated by features and specifications such as:

• Type(s) of detectors used
• Type(s) of phase chemical used
• Operating temperature and pressure conditions
• Ionization method (in mass spectroscopy)

In this article, we look at how Alicat mass flow controllers improve flow regulation, helping to design more valuable analyser systems.

Gas chromatography (GC)

Samples are separated in their volatile phases using carrier gases in gas chromatography, which then hits a detector for analysis. By using this technique, the individual components of a sample can be identified and characterized in more depth or simply separated out for further analysis, such as in gas chromatography mass spectrometry systems. Many variations of this technique exist to conduct diverse types of analysis while the fundamental principles described remain the same.

Using Alicat devices in GC

It is vital to keep consistent, low flow rates for mobile carriers. Using Alicat mass flow controllers adds value to gas chromatography systems by being more accurate, repeatable, and having automated control of mobile carrier/carrier gas flow rates than traditional control options like rotameters.

For gas chromatography, Alicat’s MC-Series and CODA KC-Series offer the following specifications and features:

• Full scale ranges down to 0.5 SCCM with a 0.01% – 100% controllable range for MC-Series; 40 g/h with a 2% – 100% of full-scale control range for CODA KC-Series
• MC-Series NIST-traceable accuracy of ±0.6% of reading or ±0.1% of full scale, whichever is greater; CODA KC-Series NIST-traceable gas accuracy of ±0.5% of reading or ±0.05% of full scale, whichever is greater
• Warm-up times less than 1 sec

Liquid chromatography (LC)

For high pressure liquid chromatography, a sample is separated into singular components that use a high-pressure liquid mobile carrier. In contrast to gas chromatography where the sample must be volatile to work with the inert carrier gas, liquid chromatography requires the sample to be soluble to work with the carrier liquid.

Just as in gas chromatography, in liquid chromatography the sample is sent through a heated chamber column, or stationary phase, resulting in the separation of different components which then hit a detector.

Using Alicat devices in LC

The CODA KC-Series allows for continuous, accurate, repeatable, mobile carrier flow control during liquid chromatography, as CODA devices are compatible with both gases and liquids. Alicat’s CODA KC-Series include the following additional features which are beneficial for liquid chromatography systems:

• Operating pressures up to 4000 PSIA
• Compatibility for various mobile carrier fluids with an aggressive or unknown composition
• NIST-traceable liquid accuracy of ±0.2% of reading or ±0.05% of full scale, whichever is greater
• Repeatability of ±0.05% of reading or ±0.025% of full scale, whichever is greater

Mass spectroscopy (MS)

Mass spectroscopy describes the process of determining the mass-to-charge ratio of ions, identifying the molecular weight of different components in a sample. This technique is often combined with gas or liquid chromatography, where the chromatography stage separates individual components of a sample and the mass spectrometry stage identifies those components, assessing the distribution, or weight, of different components in the sample mix.

Using Alicat devices in MS

Mass flow controllers such as Alicat’s MC-Series or CODA KC-Series can be used to control the ionisation of samples in mass spectroscopy, particularly via the electrospray ion source technique. This technique nebuliser gases are controlled by using the mass flow control, which is mixed with sample fluid such as the liquid exiting from a liquid chromatography system, forming an aerosol spray which is then vaporized by a drying gas that is also controlled using another mass flow controller.

Since the drying and nebulizer flow rates must be consistent to one another, using mass flow devices such as MC-Series or KC-Series help to better regulate the nebulizer and drying gases, improving the accuracy of mass spectrometry and allowing for more precise measurements.