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Frequently Asked Questions: Low-Pressure GC-MS (LPGC-MS)

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  1. What is low-pressure GC-MS (LPGC-MS)?
  2. What is the biggest benefit I will see if I implement LPGC-MS in my lab?
  3. My retention times shifted when I installed a new LPGC column kit. Is that normal?
  4. What changes do I need to make to my GC-MS instrument or system settings to implement LPGC-MS in my lab?
  5. Can I use LPGC-MS with both split and splitless injection techniques?
  6. What is the best way to check for leaks at the column connector or the union between the LPGC column kit and the mass spectrometer?
  7. Does the LPGC column kit require specific conditioning?
  8. If I want to use a different column ID or film thickness, are there other formats available for LPGC-MS?
  9. Are analyte protectants required for pesticides in order to make this technique work well in my instrument?
  10. Do I need to trim my restrictor column and, if so, how often?
  11. Are the components of the LPGC column kit available for sale separately, and is it cost effective to connect them myself?
  12. Can I use hydrogen as a carrier gas for my LPGC-MS application?
  13. Will I lose resolution when switching to LPGC-MS?
  14. What do I do if I develop a leak at the connector between the restrictor column and the analytical column?
  15. What MS data acquisition rate is necessary for LPGC-MS?
 

1. What is low-pressure GC-MS (LPGC-MS)?

LPGC-MS is a technique that allows GC-MS users to leverage the vacuum system of their MS, along with a specially designed column kit, to significantly speed up their analysis. By using a 0.53 mm analytical column that is inserted directly into the MS and a flow restrictor on the GC inlet side, one can benefit from low pressure created inside the 0.53 mm column. Using this LPGC-MS technique, some efficiency is traded for speed. But, because a mass spectrometer is used, most coeluting components can be deconvoluted by the MS. You can learn more about how LPGC-MS works and whether it is a good fit for your lab in this technical article or by contacting your Restek representative.

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2. What is the biggest benefit I will see if I implement LPGC-MS in my lab?

The primary benefit of LPGC-MS is the ability to significantly shorten your GC-MS analysis times, in some cases speeding up analyses by three times or more compared to your previous analysis time. There may also be other benefits, such as increased sensitivity and less instrument maintenance, but these will vary based on your use level and sample types.

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3. My retention times shifted when I installed a new LPGC column kit. Is that normal?

Yes. Some degree of retention time shifting is usually observed between LPGC column kits. Depending on the approach used or the width of the monitoring retention time windows, some adjustment may be necessary upon installation of a new LPGC column kit. However, the retention times should stay within the expected stability range throughout the lifetime of the column kit.

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4. What changes do I need to make to my GC-MS instrument or system settings to implement LPGC-MS in my lab?

The principal changes needed when adopting LPGC-MS are presented below. They are also discussed in more detail in this technical article.

  1. When installing the LPGC column kit, make sure you are using a 0.8 mm ID Vespel/graphite ferrule (the correct size for 0.53 mm ID columns) for the connection between the LPGC column kit and the MS. We recommend using a 60:40 Vespel/graphite ferrule, if available. Note that the same ferrule used for a conventional 0.25 mm ID column is used on the restrictor end (inlet) of the LPGC column kit; only the MS end requires a 0.53 mm ferrule.
  2. When configuring the LPGC column kit in your instrument’s software, you will only use the dimensions of the restrictor column portion of the kit because that narrow length of column predominately defines the kit’s overall restriction to carrier gas flow. When it comes to providing a stationary phase film thickness, you can use the thickness of the analytical column stationary phase (1 µm), although this will have little impact on your instrument's ability to correctly determine flow rates.
  3. You will need to update your method to increase the oven ramp rate and, likely, the carrier gas flow rate to achieve the greatest increases in analysis speed. Do not to exceed the maximum oven ramp rates for your GC oven or the maximum carrier gas flow rate for your mass spectrometer.
  4. We recommend that you tune your mass spectrometer under the same flow conditions that you will use for your LPGC method.
  5. Note: It is also likely that you will have to confirm/adjust retention time windows when a new LPGC column kit is installed. Even though elution orders will not change from kit to kit, absolute retention times may vary by tens of seconds (e.g., 10-20 sec).
 

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5. Can I use LPGC-MS with both split and splitless injection techniques?

Yes, LPGC-MS is compatible with both split and splitless injection.

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6. What is the best way to check for leaks at the column connector or the union between the LPGC column kit and the mass spectrometer?

Initially, to identify large leaks, use an electronic leak detector at pressurized points (e.g., the inlet). After eliminating large leaks, which might destroy a filament, use your mass spectrometer to detect the presence of a smaller leak. Although using your mass spectrometer is an effective way to identify a smaller leak, it is not able to locate where the leak is. At points where the vacuum’s influence is strong, like the column connector between the restrictor column and the analytical column or the fitting between the column and the mass spectrometer, we recommend using some form of “canned air” (commonly used for dusting keyboards). Canned air commonly contains a halogenated compound that the mass spectrometer can monitor in real time. Short sprays of the canned air near leaking connections will result in an elevation of the relevant m/z ion from the halogenated compound, indicating the presence and location of the leak.

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7. Does the LPGC column kit require specific conditioning?

No. Conditioning procedures for the LPGC column kit would be the same as for any 5-type GC column. Check our GC Column Conditioning video to find out more about column conditioning.

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8. If I want to use a different column ID or film thickness, are there other formats available for LPGC-MS?

Yes, we may offer custom column formats for special applications; however, we recommend you try this one first, then contact us to discuss what changes you will need to optimize your system to your specific needs.

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9. Are analyte protectants required for pesticides in order to make this technique work well in my instrument? 

The use of analyte protectants is not necessary for LPGC-MS pesticides analysis, but it can improve your results (peak shape, sensitivity, lifetime). This blog provides additional information, and you can contact us if you have further questions.

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10. Do I need to trim my restrictor column and, if so, how often?

If you see a loss of peak symmetry and sensitivity, first try changing the inlet consumables. If that does not help, try trimming the restrictor column. Trim as much as your maintenance protocols call for, but typically 10-30 cm is sufficient. Note that the length of the restrictor determines the flow and retention times. If the restrictor tubing becomes too short, you can either (a) adjust the restrictor length in your setup and set the same flow, or (b) reduce the flow manually to match the original retention. 

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11. Are the components of the LPGC column kit available for sale separately, and is it cost effective to connect them myself? 

The LPGC analytical column is not available separately, and standard columns may not give the same performance. We strongly recommend purchasing the kit to gain the advantage of a strong, leak-free, manufacturer-made connection designed for LPGC-MS. The factory-coupled kit is the most robust and cost-effective way to implement LPGC-MS in your lab.

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12. Can I use hydrogen as a carrier gas for my LPGC-MS application?

There are a number of reasons why hydrogen can be challenging for GC-MS work, so be sure to consult your instrument manufacturer to determine if it is safe to do so and what effects to expect. However, even if it is safe, we do not recommend using hydrogen because the relatively high optimal flow rates for hydrogen may reduce MS sensitivity, and a vacuum pump suitable for higher flows will be needed.

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13. Will I lose resolution when switching to LPGC-MS?

There is an overall loss of resolving power when comparing a 30 m, 0.25 mm conventional GC-MS column dimension with the LPGC column kit, but that loss of chromatographic resolution is typically compensated by the resolving power of the mass spectrometer. However, careful monitoring of any isobaric compounds to ensure chromatographic resolution is maintained is important when implementing LPGC-MS.

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14. What do I do if I develop a leak at the connector between the restrictor column and the analytical column?

LPGC column kits are sold pre-connected and are tested to ensure a leak-free connection before being put in stock. If, after extended use, a leak does develop, we recommend buying a new kit because making robust connections for LPGC-MS conditions is very difficult. If your column forms a leak very early in its lifetime, please contact Restek.

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15. What MS data acquisition rate is necessary for LPGC-MS?

  1. Make sure you have a scan rate that is adequate to gather 7-8 data points across a 1-2 second wide peak at minimum. Otherwise, you may observe inconsistent peak definition and poor accuracy and precision.
  2. For MS/MS work, you may need to reduce the number of transitions monitored for a given compound to ensure adequate sensitivity.
 

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