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General Technical Overview of Vacuum Ultraviolet Detector for Gas Chromatography – VUV Detector

General Technical Overview of Vacuum Ultraviolet Detector for Gas Chromatography – VUV Detector

October 3, 2020 by Datespeck

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What is Vacuum Ultraviolet (Vacuum UV) spectrum?

The Vacuum Ultraviolet (VUV) spectrum is actually the range below ≈ 200 nm in electromagnetic spectrum that many of Analytical chemists/chromatographers are not familiar with. They are very familiar with UV-A, UV-B, UV-C as chromatographers used to working in UV-Vis range above 190 nm. Chromatographers are not familiar with workings below the 190 nm. Vast majority of the electromagnetic spectrum has already been commercialized so far to this point but not the vacuum ultraviolet (VUV) range.

Why didn’t Vacuum Ultraviolet (VUV) range was commercialized before?

Analytical chemists/Chromatographers have been told that the energies associated with electrons forming most single bonds are sufficiently high that absorptions by them is restricted to the vacuum ultraviolet (VUV) region (ʎ< 185nm) of electromagnetic spectrum. Also they were taught that experimental difficulties associated with vacuum ultraviolet (VUV) range are significant. As a result there was no discussion of absorption in the vacuum UV range. Therefore analytical chemists thought that vacuum ultraviolet (VUV) chromatography is not really possible.

History of Vacuum UV Gas Chromatography detector (VUV Detector)-

The scientists who first worked on vacuum ultraviolet detection principle are not analytical chemist rather they were physicist working in semiconductor space. They were utilizing vacuum ultraviolet (VUV) spectrum for measuring dielectric of semiconductors. Further research on this technology eventually led to the design and development of a great gas chromatography detector called Vacuum UV detector.

What is the application of VUV spectroscopy?

Virtually everything absorbs strongly in the Vacuum UV range. It enables us to see small molecules. Even it shows the vibrational structure in smaller molecules. It is not only that everything absorbs in the Vacuum UV range but absorbs strongly and gives very unique spectra. These vacuum UV spectra are little bit different from liquid chromatography above 190 nm range that we are used to see where spectra is not particularly distinct and from the issues based on mobile phase effect.

Challenges to the use of Vacuum UV Spectroscopy?

Because everything absorbs strongly in the VUV, we see things like water and oxygen which makes detection challenges of this technology fairly difficult. This is one of the reasons that this technology has not been commercialized up to this point.

VUV Spectrum

Main features of Vacuum Ultraviolet (VUV) detection technology-

Vacuum Ultraviolet (VUV) absorption provides strong detection capabilities and a new unique orthogonal separation methodology. Some of the analytical benefits of Vacuum Ultraviolet (Vacuum UV) detection technology are furnished below-

  •  Unparalled selectivity-
  •  Clear and unambiguous compound identification
  •  Because of the clear compound identification and that it is an absorbance spectrometer VUV detector gives the ability to de-convolve co-eluting analytes very easily
  •  The use of spectral filters greatly reduces matrix contribution
  •  Excellent Sensitivity-
  •  VUV spectroscopy detection provides low pictograms range sensitivity for most compounds
  •  Broad dynamic range
  •  Rapid multi-W/L acquisition in 120-240 nm spectral range
  •  Clear and Easy Isomer Differentiation-
  •  One of the biggest benefits of this technology is the ability to provide clear and easy isomer differentiation
  •  Shows differences between isomers with good sensitivity
  •  Explicit manifestation of spectral differences between different Trans isomers, positional isomers
  •  Non-destructive Analysis-
  •  VUV detection is a non-destructive technology; it does not do any kind of ionization so it is not limited by compounds that are easily ionized or compounds that may be fall apart in the source. Vacuum UV detector allows performing any number of potential hyphenated applications
  •  Make-up gas controlled flow cell
  •  Excellent Temporal Resolution-
  •  Vacuum UV detector is a very fast GC detector. Upto 100Hz sampling can be done which allows GCxGC operation
  •  Do not have issues with flow rate. So this detector is good technology for flow modulated GCxGC operation
  •  Dynamic control of Make-up gas
  •  Predictable Linear Response-
  •  Vacuum UV detection is an absorbance detection technique so it is predictable and provides linear response like many other absorbance detection technique.
  •  Involves 1st principle detection and makes calibration burden much lower
  •  Eliminates the need for continuous calibration so less calibration hassles
  •  Reliable & Easy to Use-
  •  It is very reliable and easy to use
  •  Easily connects to any GC
  •  This detector does not use vacuum pump
  •  Helium, Hydrogen and Nitrogen is used as carrier gas. Though using Hydrogen as carrier gas in mass spectroscopy is a challenge
  •  Although many compounds do absorb in the vacuum ultraviolet (VUV) region, it is a favor of nature that Helium, Hydrogen and Nitrogen do not absorb well or not at all in the vacuum ultraviolet (VUV) range
  •  The make-up gas can be any of Hydrogen, Helium, Argon and Nitrogen
  •  The deuterium lamp has >2000 hour life like any other UV detector
  •  Integrated absorption data-
  •  Total absorbance chromatogram
  •  Extracted absorbance chromatograms (spectral filters)

Read More: An Informative Overview of Thermal Desorption Gas Chromatography: TD-GC

Vacuum UV detector (GC-VUV) is easy to use and understand-

A VUV detector consists of a source module having a standard deuterium lamp and a detector module. Both of this module are pumped down at factory and air replaced with dry nitrogen and modules are sealed off. The atmosphere is maintained free of oxygen and water. A Flow cell is placed between the source module and detector module. Samples from gas chromatograph (GC) passes through the flow cell then exits through outlet port.

VUV detector can be connected to any GC. It has adjustable feet to move up and down. Vacuum UV detector connection to gas chromatograph (GC) is simple. It connects to GC through mass spectrometer port. This GC detector does not do any kind of sample ionization but use make-up gas which is required in the flow cell. 

Vacuum UV GC detector is pumped down at factory so no requirement of vacuum pump. As the name indicates VUV detector absorbs in the 120-240 nm spectral range with upto 100Hz data acquisition speed.

Vacuum UV GC detector itself is at ambient pressure. It shows everything that comes through gas chromatography (GC) into the flow cell. The cell itself is at ambient pressure and it shows everything that comes out of GC. If there is extremely high amount of water in the GC the VUV detector will show that. If the analyte is oxygen the detector will show that. But the detector itself is not under vacuum.

Vacuum UV detector can detect Nitrogen (N2) but at very high concentration because Nitrogen (N2) has very poor absorbance cross section. It can also detect Carbon di oxide (CO2) also at high concentration.

Evolution of VUV detector for GC

User-Friendly software interface of GC-VUV System-

Software user interface is navigation friendly with a number of sections as given below-

  •  Method Selection/Edit-
  •  Library based user-defined methods
  •  Spectral filter settings
  •  Start/Stop trigger settings. The detector picks start/stop signal from the gas chromatograph (GC)
  •  Acquisition settings
  •  Make-up gas flow/Timing sequence
  •  Spectra Fitting Engine-
  •  Spectral fitting and match routines
  •  Goodness of fit matric determinations
  •  Materials Library-
  •  Stored absorption cross-sections
  •  Absorption spectra
  •  General information such as (1) Retention indices (2) Chemical & Physical property information 
  •  Runtime Screen-
  •  Chromatogram per spectral filter
  •  Measured absorption spectra
  •  Peak processing tool kit

Compound Identification using GC-VUV System-

Vacuum UV GC detector has compound specific absorption spectra. It has library search program as with mass spectra and photo diode array detector (PDA).

A typical and powerful attribute of this detector is that the chosen compound comes first in the list of library search result which is different from mass spectroscopy. Another important aspect is that the compounds that come after the first compound are very similar in class. Whereas in total ion chromatogram of mass spectrometer the compound of interests comes second, third, fourth or fifth in library search list.

Identification of Similar but very distinct Compounds-

Vacuum UV detector can easily distinguish between visually similar compounds in fitting routine. The detector is able to perform this task in almost all cases.

Compound Class Analysis (Introducing spectral filters)-

Vacuum UV detector has spectral filters that separate compounds based on range of wavelength.

Also Read: Mass Spectrometry Technology – A Beginner’s Guide

Unique class signatures-

Compounds within a class exhibits unique properties in the “same wavelength region”. Compounds within a class have similar spectra. VUV detector allows the user to see separate compounds based on class and to eliminate a portion of spectra to reduce matrix effect. “Spectral filters” applied in software help distinguish families of compounds.

Spectral filters – Class discrimination-

Spectral filters used in Vacuum UV detector allows to easily separate one compound class from another. For example Alkanes class responses in the 125nm-175nm spectral range whereas Olefins class responses in the 175nm-205nmspectral range. Spectral filter helps to focus on a particular compound class by removing response from other classes.

Spectral filters – Highlight compound response-

Highlighting the response of a compound can be used to identify trace contamination in a trail of saturating compound.

Spectral de-convolution of co-eluting compounds-

  •  Total absorption is proportional to the product of the concentration and the absorption cross-section
  •  The Co-elution is simply a sum of this products
  •  Linear regression allows for easy de-convolution of the compound concentration

Easy Compound fitting and de-convolution software facilitates de-convolution which enables to accurately determine co-eluting compound concentrations

Isomer identification using GC-VUV System-

  •  Isomer identification is the most powerful attributes of Vacuum UV detector
  •  The ability to look at different isomers is a clear advantage over mass spectrometer
  •  Mass spectrometers does not show spectral differences between positional isomers and Trans isomers as isomers co-elute
  •  Since many isomers have unique absorption spectra a vacuum UV detector not only shows common class compounds but provides on the low wavelength some insight into bond location

Performance Ratings of Vacuum Ultraviolet Detection (VUV)-

Vacuum Ultraviolet (VUV) detection has wide range application for quantitative analysis of compounds in many industries including Food/ Beverage/Flavor/Fragrance/Oils, Oil and gas, Life sciences, Forensic, Environmental and others. Typical Instrument Detection Limits (IDLs) of Vacuum UV detection for some compounds are as follows:

  •  Paraffin IDLs averaged 41 pg on column with an R2 of 0.9945
  •  Polycyclic Aromatic Hydrocarbons (PAH) IDLs averaged 28 pg on column with an R2 of 0.9936
  •  Terpene IDLs averaged 28 pg on column with an R2 of 0.9971
  •  Fatty acids and their corresponding methyl esters (FAMEs) IDLs averaged 34 pg on column with an R2 of 0.9940
  •  Fragrance allergen [A], [B], and [C] IDLs averaged 35, 44, and 36 pg on column with R2 of 0.9938, 0.9952, 0.9910, respectively

Concluding Remarks-

Vacuum UV detector is a completely new tool. Analytical chemists/Chromatographers are very familiar with FID, TCD and mass spectroscopy but this detector allows the ability to see things that we have not been able to see before. Vacuum UV detector is complementary to FID and mass spectroscopy in many ways. Because it gives information that we are unable to obtain using FID and mass spectrometer. 

VUV detector is clearly superior to mass spectroscopy in terms of ease of use, in terms of spectral identification of isomers, in terms of sensitivity under low molecular weight compound. Low molecular weight compounds are no longer a problem to detect thanks to Vacuum UV detection technique.

Vacuum UV detection is clearly superior to mass spectroscopy, superior to FID. So Vacuum UV detector is a complementary and in some cases competitive technique to mass spectroscopy.


Related Reading

  • A Guide to Optical Spectrometry and Major Spectrometry Instruments
  • The Importance of Vacuum in Mass Spectroscopy
  • Technical Specifications of Gas Chromatograph with Flame Ionization Detector and Split/Splitless Inlet (GC with FID)

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