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Improving Sensitivity of Environmental GC-MS Analysis

As with many sectors, detection limits for analytes in environmental samples are becoming ever lower. This challenges every step of the analytical process from sampling, to sample preparation, and the use and implementation of instrumentation which can provide the best sensitivity. Every step must be optimized to meet these sensitivity criteria and in this article we will look at some areas of the GC-MS method which can be optimized to provide the optimum sensitivity.

Environmental samples often come in large volumes or large volumes are created during sample preparation. Injection of volumes, which are often hundreds of times greater than those normally found in GC, can be problematic; however, it is also advantageous as the minimum detectable limits will decrease. Conventional analyses would use techniques such as pre-concentration to reduce these large sample volumes, although this runs the risk of losing volatile components, causing analyte decomposition, and can be time consuming.

Programmed thermal vaporizing (PTV) inlets can be used to inject large volume samples without the need for pre-concentration (Figure 1). The sample is injected into a cool liner using either a continuous controlled flow or in multiple aliquots, a high carrier gas flow passes through the liner and out of the split vent which evaporates the sample solvent, once all of the sample has been introduced into the liner it is quickly heated to transfer the analytes on to the column.

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Figure 1: Programmed thermal vaporizing (PTV) inlet.

PTV offers an enhancement in sensitivity of 1-2 orders of magnitude vs. splitless injection.  For example, concentration ranges of 2.5-500 ng/mL are achieved using PTV vs. 100-2500 ng/mL using splitless injection.  This improved sensitivity meets trace level detection requirements, and furthermore, this method of injection improves the peak shape of early eluting compounds due to the removal of solvent.  PTV inlets do require some optimization with the sample flow rate, liner, gas flows and temperatures being important parameters.

Sample flow rate

  • Too short - loss of components due to inlet flooding
  • Too long - excessive loss of volatiles
  • Optimum time is 2-20 seconds

Liner

  • For injection volumes below 10 µL use an unpacked baffled liner
  • For larger volumes use glass wool or beads help both solvent evaporation and analyte trapping
  • Liners with selective adsorption materials (Tenax etc.) increase the range of components which can be trapped (i.e. more volatiles)

Flow and temperature

  • Lower temperatures with higher flow rates are more desirable
  • Boiling point difference between the solvent and the analytes should be at least 150 °C.  A smaller difference will result in some loss of analytes but, due to the design of the system, this loss will be reproducible and will not affect quantitation.  Lower boiling solvent swork best (b.p. < 120 °C)
  • An initial inlet temperature set at 30 °C below the solvent boiling point is a good starting point
  • Flow is between 200-250 mL/min.

The column can play an important role in improving sensitivity.  Fast GC columns are shorter (normally 10 m) with a smaller diameter (0.1 mm, although 0.15 or 0.18 mm are more practical diameters which will give you the same results without the need for any changes to the system), and a thinner film (0.1 μm).  Shorter columns with small diameters allow the use of increased carrier gas linear velocity and heating rate of the column oven; analysis time is reduced without reducing the peak resolution and sensitivity is increased due to the narrow peaks produced in addition to the increase in signal/noise (S/N) ratio.

Furthermore, columns with thinner films bleed less which reduces background noise, increases S/N ratio, ultimately improving sensitivity.  One potential drawback of fast GC and GC-MS methods is the need for detectors which can rapidly scan the narrow peaks which are generated in order to collect sufficient data for reliable quantitation.  Fast GC-MS(MS) analyses result in peaks that are very sharp and elute very quickly (FHMW is below 1 sec).

Mass spectrometers must be optimized in order to gain the best sensitivity for an application.  In relation to data collection, parameters such as mass range (must cover all analyte ions), scan speed (high scan speeds are required to maintain relative intensities across a narrow peak), and sampling frequency (number of scans per second) should be considered.  The type of GC-MS experiment being carried out will have a significant impact on sensitivity.  For example, greater sensitivity can be achieved through the increased specificity of a GC-MS/MS experiment in comparison to GC-MS measurements (Figure 2).

GC-MS/MS offers additional advantages, such as, additional spectral separation dimension for complex samples, acquisition of structural information (MS/MS or MSn), spectral deconvolution of isobaric interferences (HRMS), lower detection limits when tuned to specific ions (SRM/MRM), and analysis/characterization of very complex samples - 10s to 1000s of components.

Figure 2: Total ion chromatogram of 170 pesticides under GC-MS/MS conditions (top) and SRM of Monocrotophos (bottom).

Multiple reaction monitoring (MRM) eliminates matrix noise and allows only the selected product ion through the second quadrupole, although sensitivity is actually less, the noise is reduced which increases the signal-to-noise ratio (Figure 3).  MRM detection limits are usually 5-10 times lower than those achieved by SIM.  MRM experiments can be optimized to provide optimum sensitivity (Figure 4). 

An optimized MRM transition should include a precursor ion, product ion, and optimized collision energy.  Identify unique (high mass) and/or abundant pre-cursor ions and optimize instrument parameters to gain maximum sensitivity.  The product ion(s) (fragments) should give the highest signal abundance - this may be achieved using a scan function on Q3.  Attention should be paid to the uniqueness of the fragment ions; a larger mass difference between parent and precursor ions tends to be more specific. 

Figure 3: Example of a SIM and MRM chromatogram showing improvement of MRM S/N ratio.

 

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Figure 4: Single/multiple reaction monitoring (SRM/MRM).
 

Dwell time affects the abundance of each ion/transition, the use of smart MRM technology provided by vendors allows for high-speed scanning control and simultaneous Scan/MRM analysis, which produces high-quality library searchable fragmentation and accurate low-level quantitative data in a single analysis.  The software automatically adjusts the analytical dwell time for each transition, only acquiring data during peak elution, to fully optimize sensitivity.

Watch this webcast for more information Optimizing GC-MS and GC-MS/MS Analysis for Environmental Applications »

 

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Dr. Dawn Watson
 

This article was written by Dr. Dawn Watson.

Dawn received her PhD in synthetic inorganic chemistry from the University of Strathclyde, Glasgow. The focus of her PhD thesis was the synthesis and application of soft scorpionate ligands. As well as synthetic skills, this work relied on the use of a wide variety of analytical techniques, such as, NMR, mass spectrometry (MS), Raman spectroscopy, infrared spectroscopy (IR), UV-visible spectroscopy, electrochemistry, and thermogravimetric analysis.

Following her PhD she spent two years as a postdoctoral research fellow at Princeton University studying the reaction kinetics of small molecule oxidation by catalysts based on Cytochrome P450. In order to monitor these reactions stopped-flow kinetics, NMR, HPLC, GC-MS, and LC-MS techniques were utilized.

Prior to joining the Crawford Scientific and CHROMacademy technical team she worked for Gilson providing sales and support for the entire product range including, HPLC (both analytical and preparative), solid phase extraction, automated liquid handling, mass spec, pipettes, and laboratory consumables.

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