Gas Chromatography Mass Spectrometry (GCMS)

You might have studied about gas chromatography and mass spectrometry techniques. The former is for the separation, whereas the latter works for the detection and identification of the components of the desired sample.

Gas chromatography-mass spectrometry is a combination of the two techniques. It facilitates the separation as well as identification of the constituents of any compound simultaneously.

The post discusses about the definition of GCMS, along with its principle, instrumentation, working, analysis, applications, diagrams and limitations.

What is GCMS?

In GCMS, the features of GC is paired with MS for the analysis of small, volatile compounds or multi-component quantification. Also, it is useful for determining the purity and ability of unknown compounds. Research professionals consider GCMS as a gold standard analytical technique for clinical testing, toxicological and pharmaceutical analysis and so forth.

For accomplishing different analytical tasks, different detection abilities are needed. In different GCMS, the gas chromatography system remains the same while the types of mass spectrometers differ based on the desired level of sensitivity and selectivity.

Content: Gas Chromatography-Mass Spectrometry

1. Principle of GCMS
2. How does GCMS Work?
3. Types of GCMS
4. GCMS Instrumentation
5. GCMS Analysis
6. GCMS Application
7. Limitations of GCMS
8. Frequently Asked Questions (FAQs)

Principle of Gas Chromatography-Mass Spectrometry

You can use the GCMS technique to study all kinds of solid, liquid, and gaseous samples. The analytical process begins with GC, where the injected sample gets vaporised at a high temperature. The vaporised sample moves within the column and uses it as a stationary phase for getting separated into its components. An inert carrier gas like helium, nitrogen etc., propels the separated constituents forward as per their retention time.

Each constituent elutes out at a different time on the basis of its boiling point and polarity. As soon as the component comes out of the GC column, the mass spectrometer ionises and fragments with the help of electron or chemical ionisation sources.

Further, the ionised fragments or molecules pass through a mass analyser, which accelerates them and segregates ions on the basis of their different mass-to-charge ratio (m/z). Most of the time, the mass analyser is a quadrupole or an iron trap.

When you analyse a complex sample with GCMS, you will obtain numerous peaks in the chromatogram, having each peak with a unique mass spectrum. This mass spectrum aids in the compound identification process by using commercially available mass spectra libraries.

This way, you can separate, identify and quantify targeted analytes or an unknown compound.

How does GCMS Work?

The science behind the GCMS is quite complicated as it combines two processes.

1. The process begins when a trained chemist places the s previously prepared sample vials in the autosampler.
2. When the process starts, the autosampler positions the sample vials as programmed into the autoinjector. A syringe draws a microlitre of the selected sample and injects it into the sample inlet of the GC oven.
3. Similar to the conventional GC, the high temperature of the sample port vaporises the sample, and it starts moving along with the carrier gas.
4. The sample uses the column surface to interact and slow down so that its constituents can get separated.
5. Now, as pre-programed, the oven raises the temperature, causing the movement of the volatile particles according to their retention time.
6. After the separation, the elute goes to the transfer line and then into the ion source of MS.
7. Here, the molecules of the eluted compound are ionised by an electric current.
8. Later the ionised particles pass via a mass filter (electromagnetic field) that segregates them based on their mass to charge ratio.
9. The MS detector multiplies and counts every ion linked with that specific mass of fragment.
10. This information is converted into electromagnetic signals and is sent to the computer, which generates a mass spectrum.
11. The mass spectrum provides a complete blueprint of the constituents and the chemical identity of the sample.

Types of GCMS

Some common examples of GCMS systems are:

Single Quadrupole GCMS

When the GC – MS combination involves only one quadrupole, then it is a Single quadrupole GCMS. This system is well suited for daily analysis of samples where targeted or untargeted detection is necessary.

Triple Quadrupole GCMS

When GC combines with Triple quadrupole MS, then we refer to it as GC-MS/MS. It delivers comparatively better sensitivity and selectivity levels, thus used for specified samples only.

HRAM GC-MS/MS

HRAM stands for high-resolution accurate mass. The HRAM spectrometer facilities the characterisation, discovery, identification and quantitation of the sample in a single analysis.
So, when GC combines with HRAM MS, it makes HRAM GC-MS/MS.

GCMS Instrumentation

The instrumentation of GC in GCMS is roughly similar with slight modifications. These modifications aid in effective connection and interfacing with the MS system.

• Gas cylinder: The first component is the same, i.e., a gas cylinder with inert carrier gas that works as a mobile phase.
• Autosampler: Then comes the autosampler that allows the introduction of the sample inside the GC. The autosampler consists of a tray with positions numbered inside out.
  This numbering helps the autosampler to instruct the system on when and what sample to inject as programmed. The automated needle withdraws the sample from the selected vial and delivers it into the sample port.
  Note: The automated needle is pre-programmed with washing stages before every consecutive injection.
• GCMS Column: Similar to regular GC, capillary columns are used in GCMS. The column’s low pressure and low flow rate aid proper interaction of the sample molecules with the stationary phase.
  Columns can have different affinities based on the polarity and type of compound.
• GC-MS Interphase: The interphase functions as a bridge between GC and MS. Pumps maintain gas-tight with low but constant pressure here. The temperature of the interphase is quite high, i.e. 50-350o C, so as to maintain the GC eluted compound in its volatile gaseous form.
  Technically you can say that the interphase works along with the vacuumed pumping system to lower the pressure for meeting the high requirement of MS.
• Mass Spectrometer: MS consist of several components that treat the GC elute. The ionisation source ionises the molecules, which are later detected by the mass analyser.
  The mass analyser amplifies, filters and counts the ions based on their mass to charge ratio.
  Different mass analysers are available depending on the process preferred for ion separation.

GCMS Analysis

You can utilise the data obtained from MS in either full scan or selective ion monitoring. GCMS can perform both the function simultaneously or parallelly, based on the pre-programmed setup

The primary motive of the GCMS system is to quantify the amount of any particular substance in a sample. You can do this by comparing relative concentrations between the atomic masses of the obtained spectrum.

For this, there are two types of analysis, original and comparative.

The former measures the peaks with respect to one another. Here, the computer assigns the tallest peak with a 100% value, and in comparison to that, the proportionate values of the other peak are found. The total mass of the unknown sample indicates the parent peak. The value of the parent peak helps to fit in the chemical formula of many elements that might be present in the sample.

While in the comparative analysis, the obtained spectrum is compared with the previously prepared spectrum so as to see any similarities between their characteristics.

GCMS Application

Food and Beverage Analysis

FMCG industries highly prefer GCMS to ensure the safe composition of our food and drinks. GCMS can determine any kind of contamination, such as residues of pesticides etc.,. This provides authenticity to the manufacturers and consumers about the product’s safety.

Environmental Analysis

GCMS is an effective tool for keeping the pollutant of air, water or soil in check. It helps in the quantification of harmful volatile or semi-volatile compounds (VOCs and VOCs), polycyclic aromatic hydrocarbons (PAH), and Polychlorinated biphenyls (PCBs) etc.

Clinical Toxicology

Helps to determine the presence of any unwanted toxic contaminant in a controlled substance.

Criminal Forensics

Forensic science also seeks the help of GCMS to analyse the casework samples. For instance, GCMS is useful in testing fire debris to know about causative explosives.

Anti-doping Analysis

Sports players have to pass the doping test for steroids etc. performed by using GCMS drugs before the game.

Pharmaceutical and Drug Analysis

Pharma industries use it to determine the standard composition of medicines. Also, it helps in finding out any contaminant in the respective drug.

Limitations

• The non-volatile samples require extra preparations such as outgassing, extraction etc.
• MS finds it difficult to distinguish between isomeric compounds.
• It is often difficult to determine the positional substitutions on the aromatic ring.

Frequently Asked Questions (FAQs)

Is GC-MS quantitative?

Yes, it is an analytical technique that detects as well as quantifies the component of the sample.

Can I use a solid sample in GCMS?

Yes, you can use all three kinds of matter -solid, liquid or gaseous as a sample in GCMS. Where liquids and gaseous samples are directly used, the solid samples need proper treatment before you can use them as a sample.