This can range from inaccurate results to instruments with significant downtime and high maintenance costs. The guide covers gas analysis from trace level analytes (ppb) that may require preconcentration devices (remote or instrument configured) or specialised detectors all the way up to bulk gas (100%).
<h3>Before Commencement</h3>
<ul>
<li>Ensure you understand the requirements of the analysis and sampling</li>
<li>If the instrument is not configured or cannot conform to your analysis requirements it may require modification or renewal (please refer to <a href="https://gcms.labrulez.com/library?search=%20Analytical%20Instrument%20Purchasing%20Quick%20Reference%20Guide">Analytical Instrument Purchasing Quick Reference Guide</a>)</li>
<li>The instrument has been installed to manufactures specification and all maintenance logs/service history are current and up to date.</li>
</ul>
<h3>Sample State</h3>
The sample state is the next item to consider. How can you introduce this into the gas chromatographic process in a safe and representative way to produce accurate and reliable results? 
The sample maybe from a reaction process and may not be at ambient temperature and pressure. 
There also maybe other compounds or the matrix that will interfere and either at best compromise results or actually react with the materials in the sample path. These are additional questions you should ask.
<ul>
<li>Vessel (Type, size, material, connections, seals/caps) </li>
<li>Sample pressure &amp; temperature (range)</li>
<li>Sample stability &amp; storage</li>
</ul>
<h3>Instrument Configuration</h3>
The configuration of a gas analysis GC could be simple a one injector , one column and one detector, however most tend to be a little more complex that are provided with plumbing diagram in order to understand what is happening and also to troubleshoot any problems that may occur. 
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Schema_zapojeni_a_ventilovy_diagram_plynoveho_chromatografu_pro_analyzu_plynu_26f8b32b4a/Schema-zapojeni-a-ventilovy-diagram-plynoveho-chromatografu-pro-analyzu-plynu.png" alt="ChromSolutions: Plumbing diagram of the valve GC system for Gas Analysis"/>
There maybe additional hardware such as methanizers/Jetanizer which can convert an inert gas to methane for a greater response on an FID. 
Valves maybe plumbed upstream of a injector to enabled variations in sample loading. 
Flows also maybe changed/diverted, analytes trapped and this may involve multi port valves and/or pressure switching.
<h3>Gas Sample Path/Introduction</h3>
Firstly if your sample is from an above temperature and pressure (e.g. reactor) you want this to be introduced directly and should use a valve. In this case you may have to consider the use of trace heating lines to the instrument and temperature/pressure ratings of the valves. 
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Ventil_pro_plynovy_chromatograf_pro_analyzu_plynu_e9d6d0e232/Ventil-pro-plynovy-chromatograf-pro-analyzu-plynu.png" alt="ChromSolutions: A Valve for Gas Analysis"/>
The other ways to introduce your sample is by a syringe. 
Both can be manual or automated. Automated is always a better option with regard to repeatability and valves are a preferred choice as the ingress of air can be minimised, through purging the loop and if necessary purging the valve housing. Rotary and diaphragm are the choices.
Typical sample volumes are between 0.25 - 1.0ml.
When sampling several different sample points a stream selection valve maybe incorporated into the GC configuration. 
Manual sampling/injection using a syringe (gas tight or glass to glass seal) can have safety and analysis implications from pressurised vessels and directly introduced into GC injection ports.
The compatibility of material that comes in contact with the sample always needs to be considered particularly on analytes and matrices that absorb or react on these surfaces.
<h3>Columns</h3>
Pick column dimensions and material, and phase which depends on the application.
<ul>
<li><a href="https://gcms.labrulez.com/library?search=Packed">Packed</a> (1/4”, 1/8”, 1/16”) or capillary (0.25mm - 0.53mm)</li>
<li><a href="https://gcms.labrulez.com/library?search=porous">Porous polymers</a>, <a href="https://gcms.labrulez.com/library?search=molecular%20sieve">molecular sieves</a>, thick film liquid phase columns</li>
</ul>
Porous polymers come in a range of polarities and molecular sieve columns are specifically to separate O₂ and N₂ and can separate H₂/He, CO &amp; CH₄. CO₂ and &gt;C₂ will not elute from molecular sieve columns. 
Conditioning and upper temperature limits may vary dramatically so care maybe needed when they are located in the same oven.
<h3>Detectors</h3>
There are some specifically used for gas analysis such as TCD, PDHID, IMS and PED, however PFD/PFPD/SCD, FID and MS are used in certain applications.
<h3>Conclusion</h3>
If these guidelines are considered you will encounter fewer issues with your gas chromatograph and gas analysis results.
<h3><a href="http://www.chromsolutions.co.uk/">ChromSolutions Ltd</a></h3>
What we offer at ChromSolutions is our wealth of experience in analytical instrument sales and support (over 110 years distributed through the members of our company). We can help you from defining your requirements to the implementation of a robust gas analytical method fit for purpose.

GC Gas Analysis Quick Reference Guide

This can range from inaccurate results to instruments with significant downtime and high maintenance costs. The objective being to develop automated reliable identification and quantitative results that are fit for purpose.
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/C3_C15_Hydrocarbons_Method_Translation_Example_c49d313631/C3-C15-Hydrocarbons-Method-Translation-Example.jpg" alt="C3 -C15 Hydrocarbons Method Translation Example.jpg"/>
<h3>Before Commencement</h3>
<ul>
<li>Ensure the new or existing instrument is configured and installed with the correct supplies, gases, column(s) and injector(s), valve(s), liner(s), detector, interfaces and software options for detection of the target analytes.
</li>
<li>The instrument has been installed to manufactures specification and all maintenance logs/service history are current up to date.
</li>
</ul>
<h3>Initial Testing</h3>
<ul>
<li>Ensure data capture method has sufficient speed to comply with accuracy requirement (8 points min across a peak for 5-10% repeatability).
</li>
<li>Adjust column flows and oven temperature profile to achieve the required sample throughput and separation for a mid range standard.
</li>
<li>Optimise detector conditions for best response and minimal interference.
</li>
<li>Optimise injector parameters for best response, chromatographic resolution and minimal peak distortion.
</li>
<li>Apply data processing method to obtain the correct integration parameters and identification windows for the chromatography.
</li>
<li>Adjust auto sampler settings and to obtain optimal repeatability (min 6 injections) on a mid range standard.
</li>
<li>If the method criteria cannot be reached for the mid range standard consider increasing data capture speed and/or the addition of internal standard(s).
</li>
</ul>
<h3>SQC &amp; Sample Testing</h3>
<ul>
<li>Inject a matrix matched standard or a sample (min 6 injections) to check identification, integration and repeatability.
</li>
<li>If the chromatograms/results differ significantly from the initial testing or than expected/acceptable, consider a more appropriate matrix elimination strategy.
</li>
</ul>
<h3>Carry Over Testing</h3>
<ul>
<li>Inject high std +20% followed by a blank and measure carry over and carry over re-peatability. Also ensuring overloaded peak shapes is kept to a minimum.
</li>
<li>Modify autosampler parameters and wash solvents/purge gas to minimise carry over.
</li>
<li>Modify data processing method to ensure correct integration of high level standard and blanks.
</li>
<li>Run a sample or independent QC standard six times with new auto sampler conditions to compare chromatography and repeatability data to the initial SQC and sample testing. If not comparable re-adjust modifications made in carry over tests.
</li>
</ul>
<h3>Calibration/QC/Validation Considerations</h3>
<ul>
<li>Initial Calibration for external/internal standard and mole %/normalised %.
</li>
<li>Run calibration range from low std 3*limit of detection and top std max level +20% (ideally 7 point). Run 3 points at each level, 6 points at LOQ, 6 points at mid point and 6 points at highest level. Plot calibration curve and assess linearity criteria and calculate % rsd at levels containing 6 points.
</li>
<li>If any adjustments are made to the method the sample or independent QC/sample spikes need to be recalculated or re-injected.
</li>
<li>Devise a QC/validation protocol with independent QC and calibrations to ensure method robustness and the control of the frequency of the maintenance procedures. Typical protocols include injection of low and high level standards/SQC’s/sample spikes measuring peak areas, retention times and peak shapes. In addition baseline noise and offsets from chromatogram start and end are typically measured.
</li>
</ul>
<h3>Conclusion</h3>
If these guidelines are adhered to you will avoid undesirable outcomes and have a fully utilized robust method providing accurate and reliable results fit for purpose.
<h3><a href="http://www.chromsolutions.co.uk/">ChromSolutions Ltd</a></h3>
What we offer at ChromSolutions is our wealth of experience in analytical instrument sales and support (over 110 years distributed through the members of our company). We can help you from defining your requirements to the implementation of a robust gas analytical method fit for purpose.

GC & GC/MS Method Development Quick Reference Guide

Also required is a knowledge of how GC/MS libraries are compiled and the options for the search algorithms. If this is not fully understood the result could be an unmatched spectra or of more concern, false identification of unknown compounds.
<h3>Library Considerations</h3>
<ul>
<li>1st library match may not be correct.</li>
<li>The compound may not be in the library(s) being searched.</li>
<li>The acquisition mass range may not match library mass range.</li>
<li>Different library search algorithms may provide different compound matches.</li>
<li>Smaller peaks may require background subtraction before library search.</li>
<li>Create your own libraries from data acquired on your instrumentation.</li>
</ul>
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Chrom_Solutions_Cl_Br_Isotopic_Abundances_11b3e38c1a/ChromSolutions-ClBr-Isotopic-Abundances.png" alt="ChromSolutions: Cl/Br Isotopic Abundances"/>
<h3>EI Interpretation Strategy</h3>
<ul>
<li>Note general appearance of spectrum to see if it is interpretable.</li>
<li>Select a candidate peak for the molecular ion (M⁺).</li>
<li>Examine spectrum for peak clusters of characteristic isotopic patterns.</li>
<li>Test M⁺ candidate by searching for peaks corresponding to reasonable loss.</li>
<li>Look for characteristic low-mass fragment ions. </li>
<li>Compare spectrum to reference spectra: <ul>
<li>using compilations in published references</li>
<li>using computerized search of reference spectra</li>
</ul>
</li>
<li>Spectral interpretation must be reconciled with:<ul>
<li>retention times of nearby peaks in relation to compound indices</li>
<li>all other information about the sample.</li>
</ul>
</li>
<li>Check interpretation by obtaining a spectrum of pure compound under the same instrumental conditions.</li>
</ul>
<h3>Molecular Ion (M⁺) Considerations</h3>
<ul>
<li>If compound is known, molecular ion will have an m/z value equal to the sum of the atomic masses of the most abundant isotope of each element that comprises the molecule. </li>
<li>The M⁺ peak must be the highest m/z value peak of significance (non-background, non isotope). </li>
<li>The nominal molecular weight or m/z value for the molecular ion will be an even number for any compound containing only C, H, O, S, Si, P and halogens. </li>
<li>Fragment ions derived via homolytic fission from these ions will have an odd m/z value. </li>
<li>Fragment ions derived from these molecular ions (even mass) via expulsion of neutral components (e.g., H₂O, CO, ethylene, etc.) will themselves have an even value of m/z. </li>
<li>Nitrogen Rule: A compound containing an odd number of nitrogen atoms in addition to C, H, O, S, Si, P and halogens will have an odd molecular weight. </li>
<li>No fragment ion may contain a larger number of atoms of any particular ele-ment than the M⁺.</li>
</ul>
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Chrom_Solutions_Molecular_Ion_M_Considerations_306bc73b75/ChromSolutions-Molecular-Ion-(M+)-Considerations.png" alt="ChromSolutions: Molecular Ion (M⁺) Considerations.png"/>
<h3>Conclusion</h3>
If these guidelines are adhered to you are more likely to avoid misidentification of unknown compounds and produce more accurate data.
<h3><a href="http://www.chromsolutions.co.uk/">ChromSolutions Ltd</a></h3>
What we offer at ChromSolutions is our wealth of experience in analytical instrument sales and support (over 110 years distributed through the members of our company). We use this experience and extensive libraries to provide concise guidance to GC/MS interpretation. This can be done by providing remote learning, or access to your GC/MS data to ensure the best possible identification of unknown compounds.

GC/MS (EI) Interpretation Quick Reference Guide

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ChromSolutions Ltd was formed in 2004 and has a wealth of experience in sample preparation for chromatography and spectroscopy techniques. With over 120 years of collective experience ChromSolutions offer an independent source of advice and solutions for modern analytical challenges.

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