Gas ChromatographyMass Spectrometry

How Do I Get the Best Results?

Diane C Turner, Mathias Schäfer, Steven Lancaster, Imran Janmohamed, Anthony Gachanja, Jason Creasey

• Publisher: Royal Society of Chemistry
• Year: 2019
• Print ISBN: 9781782629283
• eBook ISBN: 9781839160042
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Gas chromatography–mass spectrometry (GC-MS) is a powerful way to analyse a range of substances. It is used in everything from food safety to medicine. It has even been used to protect endangered vultures through analysis of poisonous pesticide molecules in their environment!

I want to apply this technique, where do I begin? Is GC-MS is the right technique to use? How do I prepare my samples and calibrate the instruments? This textbook has the answers to all these questions and more.

Throughout the book, case studies illustrate the practical process, the techniques used and any common challenges. Newcomers can easily search for answers to their question and find clear advice with coloured images on how to get started and all subsequent steps involved in using GC-MS as part of a research process. Readers will find information on collecting and preparing samples, designing and validating methods, analysing results, and troubleshooting. Examples of pollutant, food, oil and fragrance analysis bring the theory to life.

The authors use their extensive experience teaching GC-MS theory and practice and draw on their combined backgrounds applying the technique in academic and industry settings to bring this practical reference together. The authors also design and teach the Royal Society of Chemistry's Pan Africa Chemistry Network GC-MS course, which is supported by GSK.

1

Sample Collection and Preparation: How Do I Get My Sample Ready for GC-MS Analysis?

For successful GC-MS analysis, there are some fundamental requirements. These ensure optimum conditions to ensure that the analytes are in the gas phase, and that a sufficient amount of the analyte reaches the detector. Samples will be in one of three states: solid, liquid or gas and the techniques used for successful introduction into the GC-MS will vary based on the sample state. This chapter is therefore sub-divided into these three states.

In order to conduct a successful GC-MS analysis there are some fundamental requirements. Gas chromatography relies on the analytes being in the gas phase; therefore, one important element is to ensure the optimal conditions are in place for this to occur. The second fundamental part is ensuring that a sufficient amount of the analyte reaches the detector. Although a mass spectrometer is a very sensitive detector, like every detector, it has limits. Therefore, any successful sample preparation must consider these limitations.

There are many different sample preparation techniques which can be used or automated for GC-MS. The best sampling or sample preparation technique to use is determined by:

1. Sample phase: gas/liquid/solid or something in-between?
2. Where is the sample? Can a portion be moved into the lab or must it be sampled in situ (can the instrument be taken to it)?
3. Analytes: volatile/semi-volatile/involatile?
4. Is it possible, if necessary, to automate the sampling/preparation technique?

A sample will be in one of three states, solid, liquid or gas. The technique used for its successful introduction into the GC-MS will vary based on the sample state. This chapter is therefore sub-divided into these three states.

1.1 How Do I Collect and Sample a Gas for GC or GC-MS Analysis?

Gas-phase samples are already in the state in which GC separations occur, therefore, there is no need for further transformation. Some gas-phase samples must be sampled in situ, for example air, breath from a patient or air from a processing plant. For others, a large sample can be taken and then a portion of this analysed, for example a cylinder of industrial gas, or a canister filled with the gas-phase sample.

The analytes in gas-phase samples are usually already gaseous, therefore the sampling of gases for analysis should be quite straight forward, as long as the sample is kept under leak-free conditions. However, even samples of this type have some challenges, these include:

1. Enrichment of the sample to ensure the concentration is high enough to allow successful detection.
2. Accurate sampling to ensure the sampled fraction is representative of the bulk.
3. Transfer to the GC-MS system which does not change the sample through reaction or absorption onto surfaces and ensures the sample volume entering the GC is optimal for the gas chromatographic process, for example, delivered in a narrow band onto the head of the column.
4. Storage of the sample before analysis to maintain its integrity both qualitatively and quantitatively.
5. The preparation of suitable standards for quantitative analysis.

Sampling an accurate representation of the bulk is a key step. The gaseous environment can be sampled in a variety of ways and these are described in a number of standards, for example ASTM (D3588, D5466), EPA (TO-14A & TO-15, EPA Method 18) and ISO (3171).

As these standards indicate, there are a wide variety of methods used to sample a gas and it usually takes place in three different ways: spot, continuous or representative.

1.1.1 What Is Spot Analysis?

Spot samples are taken at one time and at one point, usually via a pitot tube. The pitot tube is a pressure measurement instrument and its primary function is to measure the fluid flow velocity of liquid, air and gas flows. It is inserted into the process (e.g. stack monitoring) or pipeline (e.g. gas outlet), or via a valve. Analysis occurs immediately without the need for storage.

1.1.2 How Do I Sub-sample with a Canister or Sampling Bag?

There are various methods that can be employed to fill and use canisters and sampling bags, a full description of which is beyond the scope of this chapter. They all share the aim of creating a representative sample from the bulk being sampled. They all involve adding a flow controller to a sample over extended periods of time rather than immediate sampling.

For example, GPA Standard 2166 describes eight different sampling methods which are listed below:

1. Evacuated container method: gas is introduced into an evacuated sample container with a pressure of less than 1 mm Hg.
2. Reduced pressure method: similar to the evacuated contained method, but for higher inlet pressures.
3. Helium pop method: beginning with an evacuated sample container, this is filled with helium (to around 5 psi), and then filled with the gas sample.
4. Floating piston cylinder method: this method has a pre-charge chamber and sample chamber created by a piston. The pre-charge chamber is filled with an inert gas (slightly above line pressure). The outlet valve is opened and the sample displaces the piston and fills the cylinder.
5. Water displacement method: the sample cylinder is filled with clean water and a vessel to measure the displaced water is attached. The gas sample is slowly introduced and the outlet valve slowly opened. The gas is sampled until all the water is displaced (detected using the sound or by observation).
6. Glycol displacement method: the same as the water displacement method but using glycol rather than water.
7. Purging – fill and empty method: the sample is used to purge the container, it is then emptied by releasing the output valve. This process is repeated several times to obtain a representative sample.
8. Purging – controlled rate method: the rate of entry is controlled by flow controllers on the inlet and outlet.

1.1.2.1 How Do I Select and Use a Canister?

Deactivated SUMMA™ or SilcoSteel™ canisters have been internally treated to ensure the collected analytes do not react with the stainless steel surface. SUMMA™ canisters use a passivation process to apply a nickel-chromium oxide layer, whereas SilcoSteel™ canisters have an internal silica layer. However, the term SUMMA™ canister is sometimes used to describe both types of coated canister (ASTM D5466 – 15).

When selecting a canister, the volume and nature of the sample, and the location that the sample is being taken needs to be taken into consideration.

1.1.2.2 How Do I Select and Use a Gas Sampling Bag?

Before sampling, unused bags should be stored in a clean environment and sealed in an outer bag to prevent adsorption of contaminants. Bags should be pre-cleaned before use by flushing with high-purity nitrogen. For validation, compounds must be stable in the bag or canister over the period in which the validation is conducted. Overall, the leak rate from the bag must be low.

During sampling it should be ensured that any tubing used for the bag connections is clean. A known and predictable flow rate should be used. Bags should not be overfilled, no more than 80% of the stated maximum bag volume should be filled.

Bags are intended for a single use, owing to potential sample adsorption onto the bag film. Therefore, after sampling and analysis, best practice is to not re-use bags. Hold times in bags before analysis are typically recommended to be 48 h or less owing to concerns around adsorption onto bag surfaces, unless the validation study demonstrates a longer stability. Bags containing samples should be protected from direct sunlight and stored above 0 °C to prevent condensation. Bags should be transported in rigid containers to prevent bag puncture and not shipped by air unless samples will be kept in a pressurised area.

1.1.3 What Is Active Sampling?

Gas-phase samples can be actively sampled in situ by drawing the sample through a conditioned sorbent and packed into a thermal desorption (TD) tube using a constant pressure or constant flow pump. The tube is then sealed, to prevent the loss of analytes and the ingress of contaminants, then returned to the lab for analysis. Tubes can be stable for several weeks. For example, parts-per-trillion (ppt) levels of polyaromatic hydrocarbons (PAHs) can be detected using air analysis, by drawing 100 L of air through a packed TD tube. See Section 1.3.1 for background information on thermal desorption.

The solid sorbent can also be held within a device such as an ORBO™ tube or filter. An impinger enables collection with a liquid sorbent.

1.1.3.1 What Is Thermal Desorption?

Thermal desorption (TD) is a physical separation process, in which heat is applied to a sample to transfer analytes that are adsorbed or absorbed within the sample tube, into the gas-phase so that they can be analysed using gas chromatography. TD only uses temperatures up to 350 °C and therefore no chemical bonds are broken in the process, only interactions. TD can be used to analyse a range of species, from those as volatile as acetylene (with two carbon atoms) up to molecules with forty carbons, such as PAHs and phthalates.

Small solid or viscous liquid samples can be directly thermally desorbed by placing them in a conditioned TD tube (see Section 1.3.1). As mentioned previously, gas-phase analytes can be concentrated by drawing the gas-phase sample through a conditioned TD tube packed with a sorbent.

1.1.3.2 How Do I Select My TD Tube and Sorbent to Trap My Gas-phase Analytes?

The TD tube itself can be made from: glass, which is beneficial for observation of the position of solid samples placed directly into the tube (see Section 1.3.1); stainless steel, which makes it very robust, especially for those tubes sampled away from the lab; or coated (silco) steel, which makes the tube very inert and is much better for active analytes such as those molecules containing sulphur.

The tubes vary in size depending on the manufacturer, but the industry standard TD methods, such as ISO, CEN, ASTM and EPA, use a 3.5 × ¼ in. outside diameter (o.d.) tubes. Tubes should have a unique identifier, which enables the sample to be matched to the tube and the sampling direction must be known so that the tube is desorbed in the reverse direction to ensure that all the sampled analytes are recovered.

Similar to solid phase micro-extraction (SPME), the sorbents placed into the TD tube and the cold trap can either interact with the analytes through absorption or adsorption. The sorbent(s) selected is dependent on the target analytes. It must trap the target analytes at the ambient temperature of the sampling location and easily release them again when rapidly heated. This temperature must not be higher than the maximum temperature of the sorbent, with no irreversible ad/absorption or catalytic breakdown.

Common sorbents are polymers such as Tenax ®, Porapak, Hayesep or Chromosorb, a styrene divinylbenzene (DVB) polymer; carbon molecular sieves such as Sulficarb, Carbosieve or Carboxen; zeolite molecular sieves; or graphitised carbon black such as Carbopack, Carbotrap or Carbograph. Tenax® and graphitised carbon blacks are hydrophobic and are therefore beneficial for ‘wet’ samples. Carbon molecular sieves are mostly hydrophilic with Carboxen being the most hydrophobic. Zeolite molecular sieves are very hydrophilic and can collect water up to the mg level, in a typically sized TD tube.

Different types of sorbents are good for different volatilities and polarities of analytes and have different reten...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Preface
• Introduction
• Contents
• 1 Sample Collection and Preparation: How Do I Get My Sample Ready for GC-MS Analysis?
• 2 How Do I Introduce My Samples into the GC Column?
• 3 Chromatographic Separation
• 4 How Do I Detect My Analytes?
• 5 Mass Analysis
• 6 What Is Qualitative Analysis and How Do I Perform It?
• 7 Basic Aspects of Mass Spectra Interpretation
• 8 What Is Quantitative Analysis and How Do I Perform It?
• 9 How Do I Maintain My GC-MS?
• 10 How Do I Troubleshoot a Problem on My GC-MS?
• 11 Conclusions
• Subject Index