Due to their nature, bioanalytical samples often require a pretreatment step prior to further cleanup by solid phase extraction (SPE). Each sample matrix poses its own unique challenges such as the removal of proteins from plasma and serum, the disruption of red blood cells in whole blood, hydrolysis of glucuronidated analytes in urine, and homogenization of tissue samples. This technical note outlines common sample pre-treatment procedures for bioanalytical samples.<h4>Plasma/Serum</h4>Plasma and serum pre-treatments are analyte dependent. If the analyte of interest is an acid, 2 % phosphoric acid can be used (20 µL 85 % H3PO4 to 1 mL of plasma or serum) to disrupt the drug-protein interaction. If the analyte of interest is basic, 0.1 M sodium hydroxide can be used to disrupt the drug-protein interaction. After addition of acid or base, the sample should be vortexed for 20-30 seconds followed by centrifugation. The supernatant is now ready for further analysis.<h4>Whole Blood</h4>There are several pre-treatment strategies that can be followed for whole blood. If the target analyte is present in red blood cells, a hemolysis step is necessary<ul><li>Hemolysis: To 0.2 mL whole blood (spiked with analytes and internal standard) in a 1.2 mL centrifuge tube, add 400 µL of 2 % zinc sulfate/80 % methanol. Vortex for 10-20 seconds followed by centrifugation at 14,000 rpm for 10 minutes. Collect the supernatant for further analysis. Preparation of zinc sulfate/methanol: Into a 100 mL volumetric flask add 20 mL water and 3.6 g ZnSO4, 7H2O. After the solution is clear and the salt crystals have dissolved, add 100 % methanol. Refrigerate the solution at 2-8 °C for 7 days.</li><li>Osmotic breakdown: To 1 mL of whole blood add internal standard and 4 mL of distilled water. Mix/vortex and let stand for 5 minutes. Centrifuge at 670 g for 10 minutes and discard the pellet. Adjust the pH of the supernatant accordingly with the addition of a buffer solution.</li><li>Sonication: Sonicate 1 mL whole blood for 15 minutes at room temperature. Add 3-6 mL of an appropriate pH buffer (such as potassium phosphate buffer). Mix/vortex. Let stand for 5 minutes. Centrifuge at 670 g for 15 minutes. Analyze supernatant.</li></ul>Note: A comparison of the above pre-treatment techniques for whole blood was performed for acidic, basic, and neutral drugs. Recoveries were generally the highest when the whole blood sample was diluted with buffer and subjected to physical denaturing (sonication) rather than chemical means. In fact, the sonication process disrupts the cell membranes to the extent that no clogging was observed when the procedure listed above was followed.1<h4>Saliva</h4>No hydrolysis is required for oral fluids and the generic protocol used for plasma/serum pre-treatment may be followed.<h4>Urine</h4>Enzymatic hydrolysis is necessary in case of conjugated forms (sulfated or glucuronide form) of the analyte present. Enzymatic hydrolysis requires specific pH (pH 4-5) and temperature ranges. An acid or base hydrolysis can be performed as well, depending on the stability of the compound.<ol><li>Enzymatic hydrolysis: To 500 µL sample (spiked with analyte and internal standard) add 100 µL acidic buffer (see below) and 20 µL beta-glucuronidase. Vortex for 5-6 seconds. Incubate in a water bath at 63 °C for 30 minutes. Transfer sample to a 96-well collection plate or autosampler vial. Seal and centrifuge for 10 minutes at 2,000 rpm. Preparation of acidic buffer (1.0 M acetate buffer, pH 4.0): Dissolve 3.0 g of glacial acetic acid and 4.1g of sodium acetate in a 1 L volumetric flask.</li><li>Base hydrolysis: To 1 mL urine (spiked with analyte and internal standard) add 100 µL 10 N KOH. Mix, vortex, and hydrolyze for 20 minutes at 60 ˚C. Cool and adjust pH to 3.5- 4.0 (by adding 200 µL glacial acetic acid).</li><li>Acid hydrolysis: To 1 mL urine add 0.25 mL HCl in a screw capped test tube. Screw the tube top on loosely and heat in a boiling water bath for 60 minutes. Adjust to pH 7 (or as needed) with 1.0 N NaOH.</li></ol><h4>Tissue</h4>Homogenize with organic or aqueous solvent depending upon analyte solubility. Settle, decant, centrifuge or filter supernatant. Perform direct Matrix Solid Phase Dispersion (MSPD) extraction on tissue.

Sample Pre-treatment Procedures for Bioanalytical Samples

More often than not, GC column problems are traceable to something improperly done during installation. The table below may assist you in identifying and correcting common installation problems highlighted by your Column Performance Test Mixture.&nbsp;<h3>Symptom: Tailing Peaks</h3><h4>Possible Cause &nbsp;&nbsp;/ &nbsp;&nbsp;Suggested Remedy</h4><ul><li>Contaminated or active injector liner or column:&nbsp;<ul><li>Clean or replace injector liner.</li><li>Solvent rinse or replace the column.</li></ul></li><li>Dead volume due to poorly installed liner or column: Reinstall liner and column as necessary.</li><li>Ragged column end: Score the tubing lightly with a sapphire scribe or a ceramic scorer wafer before breaking it. Examine the end (a 20-power magnifier glass is recommended). If the break is not clean and the end square, cut the column again. Point the end down while breaking it, and while installing a nut and ferrule, to prevent fragments from entering the column. Reinstall the column.</li><li>A bad match between the polarities of the stationary phase and the solvent: Change the solvent.</li><li>A cold region in the sample flow path: Remove any cold zones in the flow path. Debris in the liner or column:&nbsp;<ul><li>Clean or replace the liner.</li><li>Cut a centimeter or two off the ends of the column and reinstall.</li></ul></li><li>Injection takes too long: Improve injection technique.</li><li>Split ratio is too low: Increase split ratio.</li><li>Overloading the inlet: Decrease the sample volume.</li><li>Some types of compounds such as alcoholic amines, primary and secondary amines, and carboxylic acids tend to tail:&nbsp;<ul><li>Try a different column.</li><li>Make a derivative of the sample.</li></ul></li></ul>

Peak Shape Problems: Tailing Peaks

In this technical tip, we will address a common issue encountered in Gas Chromatography (GC) analysis: Broad solvent peaks/fronts. These distorted solvent’s peak shapes can adversely affect the accuracy and reliability of your results making determination of early eluting compounds very difficult. Let's explore possible causes and suggest remedies:<h3>Symptom: Broad solvent peaks / fronts</h3><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Peak_Shape_Problems_Broad_Solvent_Peaks_Fronts_b4c36131a7.jpg" alt="Phenomenex: Peak Shape Problems: Broad Solvent Peaks/Fronts" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Peak_Shape_Problems_Broad_Solvent_Peaks_Fronts_b4c36131a7.jpg 245w," sizes="100vw" width="627" height="352"><h4>Possible Cause &nbsp;/ &nbsp;Suggested Remedy</h4><ul><li>Bad column installation: Reinstall column.</li><li>Injector leak: Find and fix leak.</li><li>Injection volume too large: Decrease sample size.</li><li>Injection temperature too low: Increase injection temperature so the entire sample is vaporized “instantly”. An injection temperature higher than the temperature limit of the column will not damage the column.</li><li>Split ratio is too low: Increase split ratio.</li><li>Column temperature too low: Increase column temperature. Use a lower boiling solvent.</li><li>Initial column temperature too high for splitless injection: Decrease the initial column temperature. Use a less volatile solvent so the initial column temperature is at least 10°C below the boiling point of the solvent.</li><li>Purge time too long (splitless injection): Use a shorter purge activation time.</li><li>Septum purge plugged or turned off: Remove plug or turn septum purge on.</li></ul>By addressing these potential causes and implementing the suggested remedies, you can easily overcome the issue of broad solvent peaks/fronts, improving the peak shape and the overall accuracy of your GC analysis.

Peak Shape Problems: Broad Solvent Peaks/Fronts

<h4>No Peaks - Some or all peaks are missing from the run</h4>How many times have you found that some or all peaks are suddenly missing from the run?In this month’s GC technical tip, we explore some of the possible causes and solutions to overcome peak disappearance, by evaluating three possible scenarios that should cover all possible real cases.<h3>Symptom: All Peaks Missing</h3><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Peak_Shape_Problems_No_Peaks_b0d5e8c344.jpg" alt="Phenomenex: Peak Shape Problems: No Peaks" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Peak_Shape_Problems_No_Peaks_b0d5e8c344.jpg 245w," sizes="100vw" width="1516" height="943"><h4>Possible Cause &nbsp;/ &nbsp;Suggested Remedy</h4><ul><li>Defective or clogged syringe.: Try a new or proven syringe.</li><li>“Blown” septum or massive leaks at the inlet.: Find and fix leaks.</li><li>Problems with carrier gas flow.: Adjust gas flow.</li><li>Column may be broken or installed in the wrong detector or inlet.: If breakage is close to the beginning or end, cut off the short piece. Breakage in the middle can be repaired with a press-fit connector. For multiple breakages, replace or reinstall the column.<ul><li><a target="_blank" rel="noopener noreferrer" href="https://gcms.labrulez.com/labrulez-bucket-strapi-h3hsga3/GC_Column_Instalation_Guide_73f12586a8.pdf">See column installation</a>.</li></ul></li><li>The detector is not functioning or is not connected to the recorder or integrator.: Ensure detector is working properly. (e.g. is the flame in a FID lit?) Check connection to the output device.</li><li>Incorrect injector temperature:<ul><li>Injector too cold: sample is not vaporized.: Cold injector: check injector and oven temperature with an accurate thermometer. If accurate, increase temperature as necessary, but do not exceed the maximum temperature limit of the column. Inject the sample directly onto the column.</li><li>Injector too hot: thermally labile sample is decomposing.: Hot injector: check injector and oven temperature with an accurate thermometer. If accurate, reduce temperature as necessary, ensuring compatibility with sample and column minimum limit.</li></ul></li></ul><h3>Symptom: No Peaks After Solvent Peak</h3><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Peak_Shape_Problems_No_Peaks_2_5f1459f9d3.jpg" alt="Phenomenex: Peak Shape Problems: No Peaks" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Peak_Shape_Problems_No_Peaks_2_5f1459f9d3.jpg 245w," sizes="100vw" width="1481" height="943"><h4>Possible Cause &nbsp;/ &nbsp;Suggested Remedy</h4><ul><li>Sample volume is too high.: Inject less sample or use a higher split ratio.</li><li>For FID detectors, the flame is blown out by the solvent peak.: Check the detector temperature.</li><li>Carrier gas flow is too high.: Reduce the flow rate.</li><li>Incorrect column temperature; column is too hot and sample is eluting in solvent peak.: Check oven temperature with an accurate thermometer. If accurate, reduce temperature as necessary, ensuring compatibility with sample and column minimum limit.</li><li>Column cannot separate components from solvent.: Change solvent or column.</li></ul><h3>Symptom: Some Peaks Missing</h3><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Peak_Shape_Problems_No_Peaks_3_be7fd462a8.jpg" alt="Phenomenex: Peak Shape Problems: No Peaks" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Peak_Shape_Problems_No_Peaks_3_be7fd462a8.jpg 245w," sizes="100vw" width="1453" height="886"><h4>Possible Cause &nbsp;/ &nbsp;Suggested Remedy</h4><ul><li>Activity in the inlet liner or column if the missing peak is an active compound.: Clean or replace the inlet liner. Ensure an inert column is used. If necessary, replace the column. See column installation.</li></ul>When some or all peaks are missing in a GC analysis most causes are related to the detector, which plays a crucial role, but do not dismiss other possible causes, such as a defective syringe, an improper injection or an activated column or liner.See you on the next technical tip, feel free to contact us if you need support in troubleshooting or for your method development.

Peak Shape Problems: No Peaks

For most reversed-phase SPE applications using a silica-based SPE media (e.g. those using <a target="_blank" rel="noopener noreferrer" href="https://lcms.labrulez.com/products/1507">Strata™ C18-E</a>), cartridges (or wells in the case of 96-well plates) are initially conditioned using an organic solvent such as methanol, followed by rinsing with water or a buffer. If silica-based SPE media is allowed to dry out for extended periods of time during these conditioning steps, they may exhibit poor retention for target analytes, especially if the target compounds are polar in nature. It is therefore essential that SPE users pay close attention to their methods during this crucial conditioning step in order to obtain accurate and reproducible results.&nbsp;The figure below shows two <a target="_blank" rel="noopener noreferrer" href="https://lcms.labrulez.com/products/1546">Strata C18-T </a>cartridges (1 gm/6 mL) that were loaded with 4 mL of a blue dye (CuCl2). The cartridge on the left (A) was first conditioned using 12 mL of methanol, then was dried for ten minutes under high vacuum (10” of Hg), followed by 12 mL of water. As you can see from the figure, the blue dye is found uniformly distributed throughout the sorbent bed. In contrast, the cartridge on the right hand side (B) was conditioned with methanol, followed immediately by water without any drying. Under these conditions, the blue dye is visible as a tight, focused band at the head of the sorbent bed. After elution, recovery from cartridge B was far greater than was the recovery from cartridge A (the dried cartridge). This difference in recovery can be attributed primarily to the poor retention of the dye compound due to drying of the sorbent bed.&nbsp;<h3>Tips to Prevent Accidental over-drying to prevent loss of Analyte</h3><ul><li>Don’t allow your bed to dry out at any step (unless it is specifically indicated in the method).</li><li>Use gravity flow as an alternative to vacuum to prevent over-drying.</li><li>Centrifuge at low velocities (e.g. 500 RPM) or use positive pressure.</li><li>Switch to an equivalent polymeric sorbent, such as Strata-X, which do not display shifts in retention and recovery due to periodic drying</li></ul><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Over_Drying_of_Silica_Based_SPE_Cartridges_Can_Lead_to_Poor_Sample_Recovery_0839e0ef05.jpg" alt="Phenomenex: Over-Drying of Silica-Based SPE Cartridges Can Lead to Poor Sample Recovery: A. Silica C18 sorbent dried after conditioning with methanol; B. Silica C18 sorbent conditioned without drying." srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Over_Drying_of_Silica_Based_SPE_Cartridges_Can_Lead_to_Poor_Sample_Recovery_0839e0ef05.jpg 245w," sizes="100vw" width="548" height="312">

Over-Drying of Silica-Based SPE Cartridges Can Lead to Poor Sample Recovery

If you are working with an established method, we would recommend for you to:<ul><li>Check all solutions used for the SPE method to ensure that nothing is expired or has been mislabelled.</li><li>Check the process. Was flow potentially too fast during loading or elution? Could the cartridges have been allowed to dry between equilibration and sample loading?</li></ul>It is generally quickest to simply repeat the extraction, taking care of these points.If, however, you are developing a new method, it might be necessary to “track down” your analyte. The first thing we would recommend is to make up a solution of the analyte without sample matrix (to use as an ideal sample). Then work through the sample method capturing the eluent from each step potentially containing the analyte:<ol><li>Condition the cartridge.</li><li>Equilibrate the cartridge.</li><li>Load the sample and collect the flow-through in a vial.</li><li>Analyze this for analyte content. If analyte is found in this fraction it would suggest:<ul><li>Sample solvent is too strong, or the pH is incorrect.</li><li>Loading flow rate is too high.</li><li>You have breakthrough because either sample load is too high, or sorbent mass is too low.</li></ul></li><li>Wash the cartridge, collecting each wash fraction in a separate vial.</li><li>Again, analyze for analyte content. If analyte is found in any wash fraction, this suggests:<ul><li>Wash solvent is too strong.</li><li>Wash solvent is not at the correct pH.</li></ul></li><li>If the analyte has not been found in the load or wash fractions, then the analyte must be retained on the cartridge.<ul><li>Try using a stronger elution solvent.</li><li>Ensure pH of the elution solvent is correct.</li><li>If the first 2 suggestions don’t help, try using a less retentive sorbent (in reversed phase, try switching a <a target="_blank" rel="noopener noreferrer" href="https://lcms.labrulez.com/products?category=136&amp;manufacturerItems=50&amp;search=Strata">C18 cartridge for a C8</a> if the analytes are very hydrophobic).</li></ul></li></ol>

I am getting low recovery in my SPE method, how do I fix the problem?

When sensitivity loss occurs, it can significantly impact the accuracy and reliability of analytical results. Several factors can contribute to sensitivity loss, such as column contamination, sample matrix effects, injector and detector issues, or improper sample preparation. This technical note aims to guide analysts in identifying the common causes behind sensitivity loss and implementing effective strategies to address the issue.<h3>Symptom: Decreased Response</h3>Some or all peaks are displaying decreased response.<h4>Possible Cause &nbsp;/ &nbsp;Suggested Remedy</h4><ul><li>Contamination of column and/ or liner can lead to loss of sensitivity for active compounds.:&nbsp;<ul><li>Clean liner.</li><li>Bake out the column. Solvent rinse or replace the column.</li></ul></li><li>Injector leaks reduce the peak height of the most volatile components of a sample.:&nbsp;<ul><li>Find and fix any leaks</li></ul></li><li>If initial column temperature is too high for splitless injection, this prevents refocusing of sample. This affects the more volatile components most.:&nbsp;<ul><li>Lower temperature below the boiling point of the solvent. Decrease the initial column temperature, or use a less volatile solvent</li></ul></li><li>Inlet discrimination: injector temperature is too low. Later eluting and less volatile compounds have low response.:&nbsp;<ul><li>Increase the injection temperature or use on-column injection with direct connect liner</li></ul></li><li>Issues with the sample:&nbsp;<ul><li>Check the sample concentration, any sample preparation procedures, and shelf life. Prepare a fresh sample ensuring the proper concentration</li></ul></li></ul>In conclusion, sensitivity loss in GC is characterized by decreased response in one or more peaks, which can hinder accurate and reliable analysis. It is crucial to maintain a clean column and liner, address any injector leaks, optimize column conditions, and ensure proper sample preparation. Remember to regularly perform maintenance tasks and quality checks to ensure the ongoing performance of the GC system.By implementing these countermeasures, you can overcome sensitivity loss and achieve more accurate and reliable analytical results.

Peak Shape Problems: Sensitivity Loss

<h3>Negative Peaks - Some or All Peaks Dip Below the Baseline</h3>Negative peaks, where some or all peaks dip below the baseline, can occur in Gas Chromatography (GC) analysis and might impact the accuracy and reliability of your results. In this month's technical tip, we will explore the possible causes and suggest remedies to troubleshoot the presence of negative peaks.<h4>Symptom - Some Peaks Dip Below the Baseline</h4><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Peak_Shape_Problems_1_3aa9313384.jpg" alt="Phenomenex: Peak Shape Problems: Symptom - Some Peaks Dip Below the Baseline" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Peak_Shape_Problems_1_3aa9313384.jpg 245w," sizes="100vw" width="905" height="103"><h5>Possible Cause | Suggested Remedy</h5><ul><li>Detector overload in element specific detectors such as ECD, NPD, FPD, etc. can produce both positive and negative peaks: Have the compounds of interest arrive at the detector at a different time from the solvent or other compounds in high concentration. H₂ produces negative peaks with a TCD and helium carrier gas.</li><li>Dirty ECD detector can give a negative peak after a positive one: Clean or replace the ECD detector.</li><li>Sample contaminants (hydrocarbons or other non-responders) are present when using ECD, PID, or NPD (thermoionic specific) detectors: Improve sample preparation and cleanup methods prior to injection.</li></ul><h4>Symptom - All Peaks Dip Below the Baseline</h4><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Peak_Shape_Problems_2_67ef91aa12.jpg" alt="Phenomenex: Peak Shape Problems: Symptom - All Peaks Dip Below the Baseline" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Peak_Shape_Problems_2_67ef91aa12.jpg 245w," sizes="100vw" width="905" height="103"><h5>Possible Cause | Suggested Remedy</h5><ul><li>Incorrect polarity of the recorder connections results in nearly all peaks being negative: Reverse polarity of recorder connections.</li><li>Recorder-integrator wires reversed: Correct connections.</li><li>Sample injected onto the wrong column for dual-column setups: Reinject the sample onto the correct column.</li></ul><h4>Symptom - Dip after Solvent Peak</h4><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Peak_Shape_Problems_3_374ae71825.jpg" alt="Phenomenex: Peak Shape Problems: Symptom - Dip after Solvent Peak" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Peak_Shape_Problems_3_374ae71825.jpg 245w," sizes="100vw" width="905" height="103"><h5>Possible Cause | Suggested Remedy</h5><ul><li>Detector contamination: Clean or bake out the detector.</li><li>Sample contamination: For PID detectors, check that the sample has not been contaminated with methanol or water. If necessary, prepare a fresh sample.</li><li>Often normal for NPD (thermoionic specific) detectors: No correction necessary.</li></ul>Negative peaks in GC analysis can be addressed by understanding the possible causes and implementing the appropriate remedies. Most of the causes that has been observed are related to the detector that play a crucial role in negative peaks troubleshooting.

Peak Shape Problems in Gas Chromatography

In this technical tip we look at an issue that may occur before you start your gas chromatography run - how to solve offset baseline issues, where sudden unexplained changes in baseline position are seen.<h4>Troubleshooting Baseline Offset in Gas Chromatography</h4>Baseline offset in a chromatogram can result from several factors, including electrical issues, contamination, or improper column installation. The following guide describes common causes and practical steps to resolve the problem.<h5>1. Line Voltage Changes</h5>Fluctuations in the electrical line voltage can cause baseline drift or offset.<ul><li>Remedy: Monitor the line voltage and check for correlation with the observed offset. If the offset coincides with voltage variations, install a voltage regulator to stabilize the power supply.</li></ul><h5>2. Poor Electrical Connections</h5>Loose or corroded electrical connections may introduce noise or baseline instability.<ul><li>Remedy: Inspect all electrical connections in the GC system. Tighten any loose fittings and clean dirty or corroded contacts to ensure consistent conductivity.</li></ul><h5>3. Contaminated Injector</h5>A contaminated or dirty injector can contribute to baseline offset and irregular peaks.<ul><li>Remedy: Perform full injector maintenance — clean the injector body, replace the inlet liner and glass wool, and ensure the system is free of residue or buildup.</li></ul><h5>4. Contaminated Column</h5>Contaminants on the analytical column can cause baseline disturbances.<ul><li>Remedy: Trim approximately 10 cm (4 inches) from the front of the column and bake it out at high temperature according to manufacturer recommendations. If the problem persists, replace the column with a new one.</li></ul><h5>5. Column Inserted Too Far into the Detector</h5>Incorrect column installation can lead to signal instability, especially with FID, NPD, or FPD detectors.<ul><li>Remedy: Reinstall the column according to the instrument manual. Ensure the column tip is inserted exactly the correct distance into the detector to maintain optimal performance.</li></ul><h5>6. Contaminated Detector</h5>Residue or buildup inside the detector can distort the baseline or create false signals.<ul><li>Remedy: Follow the instrument’s detector maintenance procedure to clean or replace contaminated components.</li></ul><h5>7. Gas Generator Issues</h5>Fluctuations in carrier or detector gas pressure may arise from irregular performance of the gas generator.<ul><li>Remedy: Check the supply pressure regulator on the gas generator. For systems with long supply tubing, install a step-down regulator near the GC to stabilize pressure. Adding an additional regulator can also smooth out fluctuations caused by the gas source.</li></ul><h4>Summary</h4>Addressing baseline offset requires a systematic approach — verify electrical stability, cleanliness, and correct installation throughout the GC system. Routine maintenance of injectors, detectors, and gas supplies helps ensure stable baselines and reliable chromatographic performance.<blockquote><a target="_blank" rel="noopener noreferrer" href="https://gcms.labrulez.com/labrulez-bucket-strapi-h3hsga3/application::paper.paper/183bdef8-200f-4854-9e39-e61a49cdeded.pdf">Download GC Troubleshooting Guide</a>The complete guide to GC troubleshooting with hints, tips, and tricks for chromatographic remedies.</blockquote>

Phenomenex technical tip for Baseline problems - offset

When it comes to selecting the best GC column for your analysis, understanding the concepts of polarity and selectivity is crucial. While often used interchangeably, they have distinct meanings in the context of gas chromatography. Let's explore how you can make informed decisions for column selection.<h4>Overview</h4>Polarity and selectivity are often regarded as interchangeable, however in practice they are quite different when looking at gas chromatography. Selectivity refers to the difference in retention factors between two peaks whilst polarity is an assigned value specific to a GC column, based on an assignment using solute-solvent properties. With a plethora of GC phases available to an analyst, how can you ensure you are selecting the best stationary phase for your method?<h4>Tip</h4>Resolution between compounds is primarily determined by the selectivity of the stationary phase, and your choice will have the largest effect on this. Column selectivity is unique to column polarity, but it is important to also ensure the polarity of the phase you select is appropriate to the compound types you are analyzing. For example, if you have a selection of mid-polarity compounds generally these will resolve best on a mid-polarity column. What is important when selecting a stationary phase is to consider that two columns with almost equivalent polarity can offer very different selectivities. If we consider the <a target="_blank" rel="noopener noreferrer" href="https://gcms.labrulez.com/products/1576">Zebron ZB-35</a> and <a target="_blank" rel="noopener noreferrer" href="https://gcms.labrulez.com/products/1573">ZB-1701</a>, which have polarity indexes of 18 and 19 respectively, due to their very different stationary phases they work in different ways depending on the class of compound in question.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Zebron_ZB_35_and_ZB_1701_0793537614.jpg" alt="Phenomenex: GC columns Zebron ZB-35 and ZB-1701" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Zebron_ZB_35_and_ZB_1701_0793537614.jpg 245w," sizes="100vw" width="1057" height="238">Using PAHs as an example we can see the different selectivity profiles for these hugely aromatic compounds, with the <a target="_blank" rel="noopener noreferrer" href="https://gcms.labrulez.com/products/1573">ZB-1701</a> and its higher level of aromaticity giving better resolution, specifically of the critical pair of dibenzeno[a,h]acridine and dibenzeno[a,j]acridine (Fig. 1) when run using the same column dimensions and temperature profile.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_Zebron_ZB_35_and_ZB_1701_2_eac90b78f7.jpg" alt="Phenomenex: Comparison of GC columns Zebron ZB-35 and ZB-1701" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_Zebron_ZB_35_and_ZB_1701_2_eac90b78f7.jpg 245w," sizes="100vw" width="1279" height="727">In GC, 3 types of interactions are important for selectivity:<ol><li>Van der Waals Interactions, which occur between non- polar groups on the phase and non- polar compounds.</li><li>Dipole-dipole interactions, which can assist with separating compounds with similar boiling points where their chemical structures are different.</li><li>Hydrogen bonding, which can be a powerful tool for polar compounds. It can also lead to poor peak shape if not controlled appropriately.</li></ol>When selecting the most appropriate GC column for your application, polarity should be considered first, and using a column which matches your compound polarity is generally advised. Once this is selected, elution order of your compounds is primarily determined by boiling point and in general compounds with lower boiling points will elute earlier than those with higher boiling points. The caveat to this is when the lower boiling compound has more interaction with the stationary phase than the higher boiling compound. For a purely boiling point based separation you would be looking to use a stationary phase such as the <a target="_blank" rel="noopener noreferrer" href="https://gcms.labrulez.com/products/1572">ZB-1 </a>which is 100% dimethylpolysiloxane – where Van De Waals forces form the main interaction between the analytes and the stationary phase.<h4>View On Demand Webinar</h4>🎥 <a target="_blank" rel="noopener noreferrer" href="https://www.phenomenex.com/webinars/exploring-fast-gc-solutions-for-priority-pollutants-like-pah-and-pcb-in-food-and-environmental-sampl">Exploring Fast GC Solutions for Priority Pollutants like PAH and PCB in Food and Environmental Samples.</a>

Good Column Selection: Polarity vs Selectivity

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18th Mass Spectrometry School in Biotechnology and Medicine

10th EuChemS Chemistry Congress (ECC10)

LECO Training Course Pegasus BT/BT4D BTX/BTX4D

Microplastics Analysis You Can Defend: From Method to Results

Helium, PFAS & Recycled Plastics: Addressing Key Challenges in Industrial Laboratories

Expert Answers: GC Troubleshooting and Method Development