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.
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-
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.
User-Friendly software interface of GC-VUV System-
Software user interface is navigation friendly with a number of sections as given below-
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.
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-
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-
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:
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.