Aviv Amirav, Tel Aviv University and Aviv Analytical.
The United States Environmental Protection Agency method 8270 is one of the most widely used GC-MS analysis. It lists over 250 semivolatile compounds and usually GC-MS methods are developed around the analysis of available 8270 mixtures such as the Restek MegaMix with 76 compounds.
In this study we demonstrate and discuss this mixture analysis by GC-MS with Cold EI. We show that GC-MS with Cold EI can easily perform this analysis and with several important benefits of faster analysis of 10 minutes, complete elimination of ion source peak tailing even for compounds such as pentachlorophenol, benzidine and large PAHs and the provision of enhanced molecular ions while retaining NIST library identification. Thus, GC-MS with Cold EI is the best tool for environmental analysis that generates the greatest amount of sample and matrix information.
GC-MS with Cold EI is based on the interface of the GC and MS with a supersonic molecular beam (SMB) and on sample compounds ionization with a fly-through ion source for their electron ionization as vibrationally cold sample compounds in the SMB (hence the name Cold EI). Cold EI mass spectra are characterized by enhanced molecular ions yet they are fully compatible with NIST library identification. The magnitude of Cold EI enhancement of the molecular ion depends on the number of atoms in the compounds thus it can be very little for polycyclic aromatic hydrocarbons and small molecules while for large hydrocarbons it can be higher than a factor of 1000. With Cold EI one can use shorter columns and/or higher column flow rates thereby significantly reduce the analysis time and lower the GC elution temperatures which combined with contact-free fly-through ion source leads to significantly extended range of compounds amenable for analysis. Furthermore, in cold EI there is no ion source related peak tailing and/or degradation in view of its use of a contact-free fly-through ion source and thus it provides peak shapes as eluted from the column. Cold EI was initially developed by Amirav and his group in 1990 [1, 2], it is reviewed in [3, 4] and a book on GC-MS with Cold EI was recently published .
We wish to emphasize that unlike other soft ionization methods such as chemical ionization, Cold EI is not a supplementary ion source to standard EI but rather it is a highly superior replacement ion source that can perform all the analysis methods of standard EI plus significantly extend the range of compounds and applications that are amenable for analysis by GC-MS .
In this application note we wish to demonstrate and explain that GC-MS with Cold EI can easily perform simple analysis methods such as the EPA 8270 mixture analysis  and with several important benefits of faster analysis, elimination of ion source peak tailing and the provision of enhanced molecular ions while retaining NIST library identification capability.
We used the Aviv Analytical GC-MS with Cold EI (model 5975-SMB, Aviv Analytical Ltd, Hod Hasharon, Israel). This instrument consists of an Agilent 7890 GC and 5975C MSD from 2011 (Agilent Technologies, Santa Clara, CA, USA) combined with the Aviv Analytical supersonic molecular beam (SMB) interface and its fly-through EI ion source for the 70 eV electron ionization of internally vibrationally cold molecules in the SMB (hence the name Cold EI). The Agilent 5975C MSD was upgraded to Cold EI in 2017 and the Cold EI ion source was never serviced in its 4 years of use. The standard EI experiments were performed with an Agilent 7890 GC and 5977 MSD from 2017 using 70 eV electron energy and GC conditions as described below for both experiments.
Column: 30 m 0.25 mm I.D. with 0.25µ DB-5MS-UI film in both systems.
Sample: Restek 8270 MegaMix part number 31687 with 76 components that was diluted to 50 µg/ml with dichloromethane and injected with split 10 for having 5 ng on-column each compound. We also used the Restek GC-MS 8270 tuning mixture part number 31615 with four compounds that was diluted to 100 and 10 ppm in dichloromethane (with split ratio of 20).
Ion Sources: For standard EI Agilent Inert ion source was used at 300ºC. For Cold EI an Aviv Analytical dual cage fly-through ion source was used whose temperature is irrelevant (~400ºC).
Transfer line: In standard EI it was at 320ºC while in Cold EI it was at 250ºC with temperature program to 300ºC.
GC Oven Program. 40ºC hold for 0.5 min, 10ºC/min to 100ºC, then 25ºC/min to 260ºC, then 5ºC to 280ºC followed by 15ºC/min to 320ºC with hold for 2 min. This method was taken from Agilent 8270 application note . For the fast GC-MS with Cold EI it was 40ºC hold for 0.5 min followed by 35ºC/min to 320ºC with hold for 1.5 min for the total of 10 min.
Injector: Agilent standard split splitless at 250ºC with split 10 (9:1) or split 20 for the Tune mixture.
Column Flow rate: 1.2 ml/min Helium. For the fast GC-MS with Cold EI it was 3 ml/min with flow program to 5 ml/min after 8 min.
Mass Spectrometer: Filament emission current of 36 µA in standard EI as obtained in the tune method and the triple axis ion detector was at the tune voltage. In Cold EI the emission current was 6 mA. The mass spectral range was 50-500 u and scan speed was 3.1 Hz while in the fast GC-MS with Cold EI it was 6.2 Hz.
Results and Discussion
In Figure 1 we show a comparison of the 8270 Restek MegaMix mixture analysis by standard EI using an Agilent 7890 GC + 5977 MSD with the Agilent method of analysis as described in its 8270 application note  (bottom mass chromatogram) and by GC-MS with Cold EI (upper mass chromatogram) using the same GC column and method. As shown, the mass chromatograms are basically similar which means that Cold EI, unlike other soft ionization methods, generates EI mass chromatograms that are similar to standard EI since it similarly uses electrons with 70 eV electron energy for ionization. A closer look reveals that the early eluting 2 peaks are without tailing in Cold EI while they tail in standard EI (possibly due to lower injector pressure in standard EI via its vacuum outlet). The overall separation is good and similar to what is published by Agilent in their application note for 8270 analysis  and even the last to elute 6 rings PAHs are well separated without tailing. The elution time of the last to elute compound is about half a minute later in Cold EI than in standard EI. Since the same GC method (and column) was used this difference is attributed to the fact that in Cold EI the elution is into 730 mBar supersonic nozzle pressure while in standard EI it is into vacuum thus having faster He velocity for the same flow rate. However, both chromatograms end according to the method in 21.6 min. We injected 5 ng each compound on-column (50 ppm split 10) and the sensitivity is good in both mass chromatograms. In standard EI we obtain TIC signal to early elution time noise ratio of about 4000 while in Cold EI it is up to 10,000 but there are no major sensitivity challenges in the 8270 method and thus split injection can be used for the requested LOD of 0.2 ppm.
|Figure 1. A comparison of 8270 Restek MegaMix mixture analysis by Agilent 7890 GC + 5977 MSD with the Agilent method of analysis (bottom mass chromatogram) and by GC-MS with Cold EI (upper mass chromatogram) using the same GC column and method.|
However, while in the overall the separation is good and similar in both standard EI and Cold EI mass chromatograms and the column used DB-5MS UI is the same in both GC-MS systems the separation is not exactly the same. In Figure 2 we show a comparison of the 8270 Restek MegaMix mixture analysis of Figure 1 as obtained by GC-MS with standard EI (bottom mass chromatogram) and GC-MS with Cold EI (upper mass chromatogram) while zooming the mass chromatograms around the elution times of early eluters according to their appearance in the Restek MegaMix (compounds number 3-16 in ). Despite the fact that we used the same GC column and oven temperature program the obtained mass chromatograms are not exactly the same. The main reason for this difference is since the column in GC-MS with standard EI ends in vacuum while in Cold EI it ends behind the supersonic nozzle at 730 mBar thus the carrier gas linear velocity is a little different.
Thus, the standard EI mass chromatogram begins earlier than that of Cold EI and as shown the mass chromatograms are a little different. We marked with arrows peaks with partial overlap and in standard EI three such overlaps were found while in Cold EI one such overlap was found but in general the GC separation and peak widths are similar. We note that the EPA 8270 method  includes over 250 compounds and thus some compounds coelution or partial coelution is unavoidable and thus another requirement is that the retention time in combination with the molecular ion and/or major mass spectral fragment ions will enable unique compound identification.
In Figure 3 we show a comparison of Cold EI mass spectra (upper MS) with standard EI mass spectra (bottom MS) of three representative compounds in the 8270 mixture of benzoperylene, azobenzene and diisooctyladipate. The names and NIST library identification probabilities are included in the inserts. Cold EI is known for its ability to enhance molecular ions [1-5, 9, 10] and in Figure 3 we demonstrate this feature for compounds in the Restek MegaMix of EPA 8270 compounds. In Cold EI, compounds with dominant molecular ions and those with limited number of atoms (below 20 atoms) exhibit little or no enhancement of their molecular ions since for relatively small molecules the vibrational heat capacity is small thus the effect of the cooling in the SMB is limited. Accordingly, the Cold EI and standard EI mass spectra of benzoperylene are practically identical as shown at the left side of Figure 3. For an unknown reason the NIST library identification probability for Cold EI of 41.1% is a little higher than the 38.6% for standard EI MS. The main reason for these low probabilities is since there are a few other isomers such as indenopyrene that exhibit practically identical mass spectra and thus for isomer level identification with both Cold EI and standard EI unique retention time is needed.
While similar Cold EI and standard EI mass spectra are exhibited for PAHs, for most compounds Cold EI is characterized by visibly enhanced molecular ions as demonstrated in Figure 3 center portion for azobenzene. As shown, while the molecular ion abundance in standard EI is 24% it is dominant in Cold EI and higher by a factor of 1.7 from the most abundant m/z = 71 fragment ion. In addition, the NIST library identification probability of azobenzene in its Cold EI mass spectrum is 93.2% which is noticeably higher than the 87.2% of its standard EI mass spectrum. Thus, Cold EI is unique in combining enhanced molecular ions with improved NIST library identification probabilities . One reason for this improved NIST library identification probability versus standard EI is that the standard EI ion source temperature was 300ºC which is higher than what was typically used for the mass spectra given to the NIST library. Thus, while the Cold EI mass spectrum exhibits enhanced molecular ion versus the library, the standard EI mass spectrum exhibits somewhat depleted molecular ion abundance. We note also that the Cold EI mass spectrum exhibits all the fragment ions of azobenzene and while the molecular ion and the high mass fragment ion m/z = 152 are enhanced, the lower mass fragment ions are depleted yet they are all observed and thus resulted in very high NIST library identification probability.
While the molecular ion of azobenzene is enhanced in Cold EI it is also observed in standard EI. However, typically 30% of the NIST library compounds have weak or no molecular ions and in the Restek MegaMix we found that 13 compounds have weak or no molecular ions (such as the phthalate esters) and thus for this group of compounds the availability of molecular ions is very important for their proper identification.
In Figure 3 right side we show the Cold EI (upper) and standard EI (bottom) mass spectra of Hexanedioic acid, bis(2-ethylhexyl) ester (in short, diisooctyladipate). As demonstrated, the standard EI mass spectrum is without any molecular ion plus its m/z = 259.2 and m/z = 241.2 fragment ions are weak. In contrast, the Cold EI mass spectrum exhibits both molecular ions and abundant high mass compound characteristic fragment ions while the low mass fragment ion abundances are reduced. Despite the effect of the cooling on enhanced molecular ions the Cold EI MS NIST identification probability of 68% is significantly higher than 47% of standard EI, in part since the standard EI ion source temperature is high at 300ºC.
Another important aspect of the 8270 analysis is peak tailing for polar and late eluting compounds such as pentachlorophenol, benzidine and 5 and 6 rings PAHs. The EPA 8270 describes the need to measure and characterize such peak tailing for pentachlorophenol and benzidine as example compounds. In Figure 4 we show a comparison of the analysis of pentachlorophenol in its 8270 Restek MegaMix 76 compounds mixture by GC-MS with Cold EI (upper mass chromatogram) and by GC-MS with standard EI (bottom mass chromatogram). Note the small yet visible tailing in the standard EI mass chromatogram and its elimination in Cold EI that as a result reveals another small peak that is indicated by the arrow. It is important to note that the relative magnitude of peak tailing strongly depends on the on-column amount and the analysis of 5 ng on-column as in our case under-estimates the amount of peak tailing and its implications. We studied this ion source related peak tailing with cholesterol and n-C32H66 as described in . In that study we found that ion source peak tailing and its related non-linear response are magnified at sub ng on-column amounts. On the other hand and as demonstrated in Figure 4 the Cold EI fly-through ion source is inherently immune against peak tailing at any on-column amount and thus it also exhibits uniform compound independent response.
In Figure 5 we demonstrate the analysis of Restek 8270 Tune mixture with pentachlorophenol, Bis(pentafluorophenyl)phenyl phosphine (DFTPP), Benzidine and 4,4'-DDT at 5 ng on-column amount (100 µg/ml split 20) and with the same GC oven analysis conditions as in Figure 1 of the MegaMix. The upper mass chromatogram was obtained with Cold EI and the bottom mass chromatogram was obtained by standard EI. The Cold EI and standard EI mass spectra of DFTPP are at the right side. As shown, the Cold EI mass spectrum of DFTPP exhibits enhanced molecular ion together with depleted low mass fragment ions below m/z = 198. However, the Cold EI mass spectrum conforms with all the US EPA 8270E DFTPP ion abundance criteria as described in  (page 4 Table 4) by Agilent which is the manufacturer of the GC and quadrupole MS of the GC-MS with Cold EI. The NIST identification probability of the Cold EI mass spectrum of DFTPP is high with 96.9% due to the high uniqueness of this compound mass spectrum while that of standard EI is 93.2%. The mass chromatograms shown in Figure 5 are also interesting in that the peak height of pentachlorophenol is similar to that of DFTPP in Cold EI (97% of DFTPP) while it is only 24% of the DFTPP peak height in standard EI. The reason for this large effect of the peak tailing of pentachlorophenol on its intensity is that like an iceberg peak tailing hides more intensity than commonly perceived via the small exhibited chromatographic peak tail since the peak tail also have a very long time component as can be seen for pentachlorophenol via RSIM on its molecular ion plus the peak height reduction can also result from intra-ion-source degradation. Furthermore, we also found that this peak tailing results in high RSD and its magnitude is much higher at low on-column amounts. In the Agilent 8270 application note  one can see in its figure 1 that the pentachlorophenol peak height is half of that of the DFTPP thus manifesting sizable peak tailing. The main reasons why the Agilent relative peak height of pentachlorophenol is higher than in our Figure 5 are that; a) Agilent probably used a new ion source with electropolished internal surface unlike our used ion source that was serviced with abrasive material; b) Also, as written  Agilent used an ion extraction lens with 9 mm hole diameter which is 9 times bigger (in its area) than the standard ion extraction lens with 3 mm hole diameter. However, the 9 mm lens reduces the signal by an estimated factor of four (much less intra ion source thermal scattering due to higher gas conductivity) and thus most users are reluctant to use it and spend the time to replace it for other analysis types. In addition, benzidine also exhibits in Figure 5 some peak tailing in standard EI and thus its peak height in standard EI is lower than that of DFTPP while in Cold EI it is the most abundant peak in the mass chromatogram. We did not find any DDT degradation in both Cold EI and standard EI. Thus, both pentachlorophenol and benzidine exhibit some ion source related peak tailing in standard EI, even at 300ºC ion source temperature and 5 ng on-column amount, which is inherently eliminated in Cold EI in its contact-free fly-through ions source.
In sharp contrast, the relative peak height of pentachlorophenol declined in its standard EI mass chromatogram from 24% of the DFTPP peak height at 5 ng on-column amounts to only 4% of the DFTPP peak height at 0.5 ng on-column amount. Thus, ten times reduction in the on-column amount of pentachlorophenol resulted in its non-linear signal reduction by a factor of 60. Furthermore, while benzidine exhibited in standard EI at 5 ng only slight peak tailing and relative total ion count signal reduction versus that of DFTPP and DDT, at 0.5 ng its peak exhibits major peak tailing and its intensity is now only 11.5% of that of the DFTPP. Thus, Figure 6 clearly demonstrates the non-linear dependence of standard EI signal of compounds with OH, COOH and NH (or those with low volatility) as further discussed in . In fact, this standard EI ion source activity of compounds with OH, COOH and NH is well known and thus these compounds require derivatization for their low levels analysis. Accordingly, we also believe that the major vendors selected 5 ng on-column amounts also for the reason to reduce the appearance of this peak tailing and non-linear behavior and they show similar data in this regard [8, 13].
Another important aspect of environmental 8270 analysis is the chromatography separation and analysis time. In Cold EI we can increase the column flow rate to 3 and even 5 ml/min and thus we can use faster GC oven temperature program rate to obtain 10 minutes 8270 mixture separation time, as shown in Figure 7. As shown, Cold EI facilitates over twice faster GC-MS separation time of under 10 minutes. We started as usual at 40ºC to be able to observe the early eluting couple of compounds and used 35ºC/min temperature program rate for 8 min to 320ºC and hold time of 1.5 min for total of 10 minutes.
|Figure 7. Fast 10 minutes analysis of 8270 Restek MegaMix mixture by GC-MS Cold EI. 5 ng on-column each compound was injected as in Figure 1.|
We found that using this higher temperature program rate resulted in narrow GC peaks of 0.6-0.7 s which are 1.6 times narrower than with the standard method but are with about a factor of up to 1.4 poorer separation in some portions of the mass chromatogram such as around the elution time of the late eluting 6 rings PAHs. However, the overall separation is good and we need to remember that some peaks coelutions are unavoidable with any method considering the EPA list of over 250 compounds. Accordingly, in any case we need to also use the separation power of the mass spectrometer which is greater with Cold EI in view of its enhancement of the molecular ions and high mass fragment ions that are the most selective ions. With Cold EI we can also use shorter columns such as 15 m with 0.32 mm I.D. and with it higher column flow rates up to 32 ml/min. With such a 15 m column and 4 ml/min column flow rate we obtained the 8270 mixture chromatography in less than 6 minutes but with some further loss of separation. However, since the EPA 8270 method specifies the use of 30 m column length we do not show these results.
We demonstrated in this study that GC-MS with Cold EI can effectively serve for optimized 8270 mixture analysis and with several important demonstrated benefits of: a) Enhance molecular and high mass fragment ions; b) Twice faster analysis time of 10 minutes; c) Elimination of ion source related peak tailing for polar and low volatility compounds. We demonstrated the reduction of the separation time from 21.6 min to 10 min. In addition, in standard EI one needs to add about 5 minutes for backflush or column cleaning from heavy matrix residue while in Cold EI only 1 minute is required in view of using 5-6 ml/min final column flow rate with the same column. Thus, considering 6 minutes cooling down time from 320ºC to 40ºC and one minute autosampler preparations time Cold EI results in full analysis cycle time reduction from 34 minutes to 18 minutes which represents about (almost) twice higher productivity. We note that faster 8270 mixture analysis can also be obtained in standard EI with smaller columns such as with 0.15 mm I.D. However, such a small column is expected to have much lower robustness and shorter lifetime (estimated factor of 5), lower sample capacity, earlier onset of peak fronting and smaller linear dynamic range and thus is not recommended for use (also narrower GC peaks are further subjected to ion source related peak tailing). The availability of molecular ions is more important for this analysis type than commonly perceived since real analyses are involved with matrix interference and we found that the higher the mass the lower are the matrix interference by about a factor of 20 per 100 u as described in . Furthermore, the availability of molecular ions also helps to better characterize the matrix and thus the source of 8270 impurities. We demonstrated that Cold EI mass spectra are fully compatible with NIST library identification which Cold EI even improves versus standard EI as simulated and fully explained in . We also note that Cold EI in combination with quadrupole mass analyzers with unit mass resolution is able to provide elemental formula with our TAMI software that inverts isotope abundances into elemental formulae [15, 16] in cases of compounds that are not in the NIST library and further confirms or rejects NIST library identifications for best identification quality. We showed in Figures 4-6 that Cold EI is immune against ion source related peak tailing. On the other hand, standard EI is characterized by ion source related peak tailing, particularly for compounds with OH, NH and COOH groups or that are low volatility. The fact that Cold EI unlike standard EI exhibits with the same column type (Figures 5 and 6) uniform response proves that the observed standard EI peak tailing is due to its ion source metallic surface. Another independent proof is that we also found that the reduction of the standard EI ion source temperature from 300ºC to 250ºC increases this tailing and reduces the relative peak height of pentachlorophenol by almost a factor of 2. This peak tailing is not just a matter of reduced chromatography separation but it also significantly reduces the signal (as demonstrated in Figure 6) and increases the limits of detection possibly by over two orders of magnitude [4, 5]. Thus, we consider the evaluation of pentachlorophenol and benzidine peak heights to that of DFTPP as a much better measure than their peak asymmetry. Furthermore, such ion source related peak tailing induces non-linear response that requires careful multi-points calibration for quantitation and it also results in high irreproducibility and RSD. Accordingly, the Cold EI merit of elimination of ion source related peak tailing is very important. In order to reduce such peak tailing especially at low on-column amounts Agilent recommends the use of 9 mm I.D. ion source extractor lens  which is 9 times bigger than the usual lens. However, such a lens also reduces the signal by an estimated factor of 4 (reduced number of ion source internal scattering and ionization time) and thus such a change is unacceptable by most GC-MS users that need their system for both 8270 and other analyses. In addition, while not fully demonstrated here (partially demonstrated in Figure 5 and 6) Cold EI also exhibits approximately uniform compound independent response  within a factor of 3. Thus, if one finds an 8270 compound at low levels one can estimate its concentration with a one calibration compound at one concentration without the need to have over 250 EPA listed 8270 compounds. While in this report we analyzed the Restek 8270 MegaMix with 76 compounds, the EPA 8270 lists over 250 compounds including those that are difficult to analyze by GC-MS such as Captan and Captafol. Such compounds can specifically benefit from the use of Cold EI since the harder the compound analysis the greater is the gain from the use of Cold EI [4, 5]. Finally, if any user wishes to have classical EI mass spectra the Cold EI system with its fly-through ion source enables easy switching into classical EI mode of operation via the reduction of helium make up gas flow rate from 50 ml/min to 2-4 ml/min  while retaining the benefits of no ion source tailing and 10 minutes fast chromatography analysis time. Accordingly, GC-MS with Cold EI can serve well for 8270 environmental analysis and with several important benefits.
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