In order to comprehend the development of LPGC-MS we must first understand the more traditional GC-MS.
Gas Chromatography (GC) is the technique of choice in separating smaller volatile and semi-volatile organic molecules such as hydrocarbons, alcohols and aromatics, as well as pesticides, steroids, fatty acids and hormones making this analytical technique common in many areas such as food safety and environmental testing.
When combined with mass spectrometry (MS), GC-MS can be used to isolate complex mixtures, quantify analytes, identify unknown peaks and determine trace levels of contamination.
Low-pressure GC-MS is a faster alternative to the more traditional GC-MS. LPGC-MS is a technique that uses the MS vacuum system, along with a specially designed column setup, to lower pressure inside the entire column, notably speeding up analysis. When using LPGC-MS, some efficiency is traded for speed, but because a mass spectrometer is used, most coeluting components can be filtered by the MS.
While the concept of LPGC has been around the 1960’s a practical solution for its use evaded analytical chemists until 2000, when de Jaap de Zeeuw constructed a simple guard column restrictor concept to maintain positive inlet pressure for a wide-bore analytical column under vacuum.
Originally MS commonly used long 0.25 mm columns in order to have adequate restriction when positive pressure is required in the injection port and the MS is running under vacuum.
Zeeuw’s idea was to see if it was possible to use a wide-bore column with MS rather than the customary narrow-bore.
Using the Van Deemeter equation, he realised that theoretically at a lower pressure a higher optimal linear velocity could be acquired. When investigating the literature for vaccum GC he saw that the trials being setup often presented challenges as the vacuum had to be created in the injection system.
Zeeuw proposed that pliers be used to restrict the flow on the inlet side of a metal 0.53 mm I.D. capillary, which led to a similar separation but had a run time which was approximately nine times faster.
What other areas of development led to this technique being what it is today?
In the first few years on the market Zeeuw began recognising some limitations in terms of the restriction lifetime/maintenance and the coupling so he developed a solution based on the coupling with PressFit and positioning it within the injector body, which meant that the coupling and restriction were constantly at a high temperature and an immobile atmosphere.
There have been many technological advancements within the past two decades which have constantly improved the performance and features of the LPGC-MS, notably the introduction of commercial triple quadrupole MS/MS instruments. It is with these introductions greater targeted analyte detectability and faster data acquisition speeds were produced.
High resolution MS tools which are compatible with the LPGC have also been established. Improvements on the QuEChERS (a solid phase extraction method for detection of pesticide residues in food) and analyte protectants streamlined sample preparation and improved peak shapes in GC. The production of a light and reliable capillary column union also assisted in making LPGC more practical for shipping and installation.
So, who within the industry should be considering LPGC-MS?
Anyone using GC in terms of analysis should be considering the LPGC-MS as the megabore columns are advantageous in routine monitoring using GC because of their substantial sample load ability and sturdiness. As a product LPGC can be used in extensive applications but as a technique it possesses the framework for investigation and innovation more so than standard GC-MS.
LPGC is not unique and is one of around ten possible routes towards a faster GC. However, its benefits lie in its rather simple technique. In common terms, it is the preferential method for straightforward separations which do not require a high peak capacity and use MS detection while sample preparation is not too much of an issue. Within these situations LPGC can increase your sample performance significantly.
Unfortunately, many laboratories will continue to use the same column and configuration for their applications which were installed with the instrument, without knowing the purpose of each step or better alternatives.
Since 1960 LPGC has been known for its advantages and for the past 20 years it has been shown that LPGC can be installed into GC–MS instruments without modifications. With exception to the analysis of volatiles that are already separated quickly, LPGC–MS are a faster and recommended alternative to a standard GC–MS.
Perhaps it is time to reconsider workflows, techniques, and technologies to improve analytical quality and efficiency.
Apex
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