- Email: [email protected]
A low thermal mass fast gas chromatograph and its implementation in fast gas chromatography mass spectrometry with supersonic molecular beams
Journal of Chromatography A, 1218 (2011) 9375–9383
Contents lists available at SciVerse ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
A low thermal mass fast gas chromatograph and its implementation in fast gas chromatography mass spectrometry with supersonic molecular beams Alexander B. Fialkov, Mati Morag, Aviv Amirav ∗ School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 18 October 2011 Accepted 20 October 2011 Available online 11 November 2011 Keywords: Fast GC Fast GC–MS Supersonic molecular beams Low thermal mass High throughput analysis
a b s t r a c t A new type of low thermal mass (LTM) fast gas chromatograph (GC) was designed and operated in combination with gas chromatography mass spectrometry (GC–MS) with supersonic molecular beams (SMB), including GC–MS–MS with SMB, thereby providing a novel combination with unique capabilities. The LTM fast GC is based on a short capillary column inserted inside a stainless steel tube that is resistively heated. It is located and mounted outside the standard GC oven on its available top detector port, while the capillary column is connected as usual to the standard GC injector and supersonic molecular beam interface transfer line. This new type of fast GC–MS with SMB enables less than 1 min full range temperature programming and cooling down analysis cycle time. The operation of the fast GC–MS with SMB was explored and 1 min full analysis cycle time of a mixture of 16 hydrocarbons in the C10 H22 up to C44 H90 range was achieved. The use of 35 mL/min high column flow rate enabled the elution of C44 H90 in less than 45 s while the SMB interface enabled splitless acceptance of this high flow rate and the provision of dominant molecular ions. A novel compound 9-benzylazidanthracene was analyzed for its purity and a synthetic chemistry process was monitored for the optimization of the chemical reaction yield. Biodiesel was analyzed in jet fuel (by both GC–MS and GC–MS–MS) in under 1 min as 5 ppm fatty acid methyl esters. Authentic iprodion and cypermethrin pesticides were analyzed in grapes extract in both full scan mode and fast GC–MS–MS mode in under 1 min cycle time and explosive mixture including TATP, TNT and RDX was analyzed in under 1 min combined with exhibiting dominant molecular ion for TATP. Fast GC–MS with SMB is based on trading GC separation for speed of analysis while enhancing the separation power of the MS via the enhancement of the molecular ion in the electron ionization of cold molecules in the SMB. This paper further discusses several features of fast GC and fast GC–MS and the various trade-offs involved in having powerful and practical fast GC–MS. © 2011 Elsevier B.V. All rights reserved.
1. Background Gas chromatography is a central analytical technology which is applied in a large variety of applications in a broad range of fields, especially when used in association with mass spectrometry. However, GC analysis requires long time, typically in the order of 20–60 min while many analyses require high throughput. In view of the long time associated with standard GC analysis, several fast GC systems have been developed that incorporate low thermal mass devices which provide fast heating temperature program and cooling rates for the GC separation columns. For example, Rounbehler et al. describe in US Patent No. 5,808,178 [1] a fast GC module named “Flash GC” [2], which is based on a capillary GC column inside a resistively heated metal tube which can be quickly heated and cooled due to its low thermal mass to achieve rapid separation
∗ Corresponding author. Tel.: +972 36408253; fax: +972 36424048. E-mail address: [email protected] (A. Amirav). 0021-9673/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2011.10.053
of analytes. This concept was also used in a portable GC with PFPD [3] and a portable electrolyzer operated fast GC-FID [4]. The concept of using resistive heating for fast GC is traced back to Hopkins and Pretorius [5] and later to Ewels and Sacks [6]. However, in these early publications the use of resistive heating was limited to the inlet systems. Hail and Yost used resistively heated aluminum clad capillary columns [7] and later on the use of various forms of resistive heating became more abundant [8–11] in view of its ability to reduce the thermal mass of the GC oven hence to reduce both its heating and cooling rates. However, when it comes to fast GC–MS, the mass spectrometer introduces additional important considerations which change several central aspects of the fast GC design. The area of fast GC–MS is reviewed and discussed in details by a few authors [12–15]. As is known, fast GC is the art of compromises, and speeding up the GC temperature program rate alone can result in the reduction of GC separation efficiency, column lifetime, range of compounds amenable for analysis, sample capacity, linear dynamic range and sensitivity, combined with increased cost of columns which can be
9376
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
coupled with the cost of the whole fast GC module. Furthermore, fast GC and particularly fast GC–MS, require much more than just fast temperature program rate of the GC oven. For example, standard splitless sample injection takes a few minutes, since it requires 1 min just for sample thermal-focusing at a low GC oven temperature plus additional time for heating to the analytically useful column temperature and cooling back. While split injection may reduce the time required for sample introduction into the column, it leads to unavoidable and often unacceptable loss in limit of detection and sensitivity. In addition, fast GC may reduce the GC peak widths, which in the case of mass spectrometry it requires the combination of fast scan speed and fast ion source response time. It is important to note that there is a major difference between fast GC and fast GC–MS in that the mass spectrometer adds an additional dimension of sample separation and selectivity, which can be further enhanced with tandem mass spectrometry (MS–MS) such as in triple quadrupole mass spectrometry systems. The basic idea is that, in fast GC–MS, the GC separation can be traded and some chromatographic peaks co-elution can be allowed in view of the additional separation of the MS, while, in fast GC, when a universal detector such as FID or TCD are used, the GC separation is its prime feature which often cannot be sacrificed. Thus, despite the obvious merit of having fast GC and/or fast GC–MS and its availability in the market, the vast majority of GC and GC–MS analysis still takes more than 20 min. In the last 18 years our group research has been focused on the development of GC–MS with supersonic molecular beams (SMB) (also named Supersonic GC–MS) [16–23]. Supersonic GC–MS is based on a GC and MS interface with SMB and on the electron ionization (EI) of vibrationally cold analytes in the SMB (cold EI) in a fly-through ion source. This ion source is inherently inert and further characterized by ultra-fast response time and vacuum background filtration capability [17,24]. The same ion source also offers a mode of classical EI [25]. Cold EI, as a main mode, provides enhanced molecular ion combined with effective library sample identification which is supplemented and complemented by a powerful isotope abundance analysis method and software [26]. We note that the feature of enhanced molecular ion also implies enhanced separation power of the mass spectrometer since, as is known, matrix interference is exponentially reduced with mass [20]. In addition, the range of low volatility and thermally labile compounds amenable for analysis is significantly increased with the Supersonic GC–MS due to the use of contact-free fly-through ion source and the ability to lower sample elution temperatures through the use of high GC column carrier gas flow rates [27]. Another important feature of the Supersonic GC–MS is its compatibility with high column flow rates without any adverse effect on its sensitivity, due to the availability of a differential vacuum chamber for the supersonic nozzle. As was demonstrated [27] and as will be shown below, this feature is very important for the combination of our fast GC method and device with the Supersonic GC–MS.
2. Experimental: low thermal mass fast GC and fast GC–MS Fig. 1 schematically describes our low thermal mass (LTM) fast gas chromatograph device which is mounted on the top of the standard GC oven [28]. Samples are injected as usual, manually or preferably with an autosampler into a GC injector, typically split splitless (SSL), which is connected to a capillary column. The capillary column itself is the standard polyimide coated fused silica capillary column albeit usually shorter. Thus, the column is immersed in a standard GC oven which as a result must be heated to a temperature that corresponds to the temperatures of the injector and detector to eliminate cold spots. The capillary column is connected to our LTM fast GC in a flexible way as is normally done with
Fig. 1. A schematic diagram of our low thermal mass fast GC module, installed on an available detector port on the top plate of a standard GC oven of a GC–MS.
standard GCs and in contrast to some other fast GCs. The capillary column is introduced into a LTM metal tube that can be resistively heated and that possesses an inlet and an outlet, both of which are connected to a current programmed power supply via electrical connections. The current program of the power supply provides a temperature program (with time). The temperature can be measured with a thin thermocouple, or via the resistance of the metal tube which increases with its temperature in a known manner. This way, the current value can be used alone to represent the temperature via temperature calibration tables. The capillary column in its resistively heated metal tube is mostly located in an air-cooled enclosure which is mounted on the detector place on the top plate of the standard GC oven (Varian 3800 in our experiments). The inlet and the outlet assembly of the heated metal tube is located at the inner edge of the standard GC oven housing so that its portion that is not resistively heated (about 1 cm) is located inside the GC oven hence heated to the standard GC oven temperature. The portion that is resistively heated is located mostly outside the GC oven to avoid or significantly reduce its double heating by both the GC oven and the resistively heated tube, which can lead to an over heated column section (hot spot), as well as to avoid unheated sections of the GC column (cold spot) between the resistively heated tube and the GC oven. Thus, the proper location of the inlet and outlet unions and their design is aimed at minimizing both hot and cold spots. The capillary column output is flexibly connected to the separately heated detector transfer line. In our experiments we used mostly 1.7 m, 0.53 mm ID, 0.75 mm OD metal tube as the heating tube (MXT Guard Column, Restek, Bellefonte, PA, USA) and 2.5 m standard GC column with 0.25 mm ID and 0.25 Rxi-5SIL MS film. Our LTM fast GC was capable of temperature program rate in excess of 2000 ◦ C/min but our standard temperature program
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
rate was limited to 600 ◦ C/min as explained in the discussion. The temperature program was achieved by current program of an external power supply in the 0–3.4 A range and the temperatures were in the 50–340 ◦ C range as calibrated indirectly via thermocouple which served to calibrate the heating tube temperature via its resistance. The helium column flow rate was in the 8–40 mL/min initial flow rate range (depending on the experiment) and was somewhat reduced as the LTM fast GC column temperature was increased. We note that our LTM fast GC module differ from Rounbehler et al. “Flash GC” [1,2] and other LTM fast GC designs in that our LTM fast GC module is placed outside the GC oven on the top of the standard GC oven while the column is flexibly connected to both injector and detector, hence during the fast GC analysis the standard GC oven must be heated and serve as a portion of the transfer line. It is also unique in comparison with other LTM fast GC designs in that the separation column is a standard fused silica column that is cut into a short length and that is inserted into a thin walled metal tube for its resistive heating hence it possesses very low thermal mass. We found that we are unable to use available metal columns since due to differences in thermal expansion between the PDMS film and the metal the column lifetime was significantly reduced. Thus, our solution was the use of standard columns that are available at low cost in large number of coating film types and thickness. In our experiments the detector is MS with SMB based on the conversion of a Varian 1200 triple quad GC–MS(–MS) into a Supersonic GC–MS as described in reference [22]. GC-SMB–MS is based on the use of an SMB for interfacing the GC to the MS and as a medium for electron ionization of sample compounds [17]. SMBs are characterized by intra-molecular vibrational supercooling, unidirectional molecular motion with controlled hyperthermal kinetic energy (1–20 eV), mass focusing similar to that in a jet separator, and capability to handle very broad range of column flow rates from standard 1 mL/min (or lower) up to 90 mL/min. The system used in this research is based on the combination of the SMB interface and its related fly-through electron ionization (EI) ion source, with the Varian 1200 L GC–MS and MS–MS (Varian Inc. Walnut Creek, CA). This system, named 1200-SMB, is described in detail in reference [22], and GC–MS with SMB is reviewed in reference [17], thus, it will be discussed here only briefly. In the 1200-SMB the column output flow is mixed with helium make-up gas (∼90 mL/min total), and flows to the supersonic nozzle through a heated and temperature controlled transfer line. The helium flow can be mixed (via the opening of a one valve) with perfluorotributylamine (PFTBA) for system tuning and calibration. The sample compounds seeded in the helium gas expand from a 90 m diameter supersonic nozzle into a nozzle vacuum chamber that is differentially pumped by a Varian Navigator 301 turbomolecular pump (Varian Inc., Torino, Italy) with 250 L/s pumping speed. The helium pressure at this vacuum chamber is about 6 × 10−3 mbar. The supersonic expansion vibrationally cools the sample compounds and the expanded supersonic free jet is skimmed by a 0.8 mm skimmer and collimated in a second differentially pumped vacuum chamber, where an SMB is formed. The second vacuum chamber is pumped by a Varian 300/400 split turbomolecular pump that pumps both the second vacuum chamber (400 L/s) and main MS vacuum chamber (300 L/s). The SMB, seeded with vibrationally cold sample compounds flies through a dual cage EI ion source [24] where these beam species are ionized by 70 eV electrons with 10–15 mA emission current. The ions are focused by an ion lens system, deflected 90◦ by an ion mirror and enter a radiofrequency (RF)-only hexapole ion transfer optics (Q0 of the original 1200 system). The 90◦ ion mirror is separately heated and serves to keep the mass analyzers clean from sample induced contaminations. The ions are further transferred through an ion lens into the MS vacuum chamber and are analyzed by a quadrupole MS–MS mass analyzer system. It consists of two quadrupole mass
9377
Fig. 2. Fast GC–MS with SMB analyses of aliphatic hydrocarbons n-C10 H22 –n-C44 H90 mixture. Full scan TIC chromatogram is shown in the upper trace while cold EI mass spectrum of C40 H82 is shown in the bottom panel. The sample was injected into a Varian 1177 injector with split 10 at 350 ◦ C; and standard 3800 GC oven temperature of 330 ◦ C. The short fast GC column flow rate was 35 mL/min.
analyzers (Q1 and Q3) and a collision cell (Q2). As any quadrupole MS–MS system, it can operate in SIM or full scan mode, as well as in all common MS–MS scan modes. Since Q2 is a 180 degrees curved RF-only quadrupole ion transfer system in the 1200 L, a head-on ion detector is positioned directly in the path of ions exiting Q3. Its entrance is biased at 5 kV, serving as an efficient ion to electron converter. Due to the combination of the 90◦ ion mirror and the 180◦ bend of Q2, the mass-independent neutral noise (produced by the ion source) was lower than one count per 10 s in the 1200-SMB system. 3. Results In Fig. 2 typical and representative results of the analysis of a broad volatility range of linear chain hydrocarbons are shown, using the fast GC in combination with a mass spectrometer with supersonic molecular beams (1200-SMB). The hydrocarbons mixture included 16 compounds in the range from C10 H22 up to C44 H90 , each at 100 ng/L concentration in hexane. It was injected into a Varian model 1177 injector with a split ratio of 9:1 so that 10 ng of each sample compound was injected into the column. The column was a 2.5 m long 0.25 mm ID fused silica capillary column with 0.25 DB5MS UI film (Agilent Technologies, Folsom, CA) that was inserted into 1.7 m metal tube heater with 0.53 mm ID and 0.75 mm OD (MXT Guard Column, Restek, Bellefonte, PA). The metal tube was coiled three times in 12–13 cm diameter loops and contacted only Kapton foil (0.125 mm thick) on its way from the top of the Varian 3800 GC into its oven where inlet and outlet connector unions penetrated 1 cm inside the oven. Helium served as the carrier gas at a flow rate of 35 mL/min. Home-made fast GC software controlled
9378
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
Fig. 3. Fast GC–MS with SMB analysis of a novel synthetic organic compound for products identification and for synthetic reaction yield (purity) evaluation. Full scan TIC is shown on upper trace. The mass spectra of one impurity (molecular ion 205 amu) and the desired product (9-benzylazidanthracene, molecular ion 233 amu) are depicted in the lower left and right panels, respectively. The sample was injected into a Varian 1177 injector with split 10 at 180 ◦ C; and standard 3800 GC oven temperature at 220 ◦ C. The short fast GC column flow rate was 24 mL/min.
the fast GC heating rate via a current programmable power supply that was programmed in the 0–3.4 A range. A cooling fan was also used to speed up the fast GC cooling and its operation was timeprogrammed by the control software. As demonstrated in Fig. 2, all the 16 hydrocarbons were fully separated in 45 s despite the large difference in their boiling points which was 142 ◦ C for C10 H22 and 693 ◦ C for C44 H90 . In addition, only 10 s were required for cooling down the column and being ready for the next analysis cycle, thus providing ultra fast GC–MS analysis cycle time. In addition, it is important to note the observation of molecular ion even for C40 H82 [23], which is unique to the Supersonic GC–MS and which demonstrates the increased selectivity and information content provided by GC–MS with SMB. In Fig. 3 the application of fast GC–MS with SMB for the analysis of a new thermally labile organic compound is shown. Some of the unique features and benefits of GC–MS with SMB for the analysis of synthetic organic compounds were previously discussed in reference [21]. This is a typical example of service mass spectrometry in which synthetic organic chemists wish to verify the success of their synthesis. The main goal is to provide a GC–MS peak of the new compounds, and as shown in Fig. 3, 9-benzylazidanthracene was synthesized and mostly properly cleaned. Benzylazidanthracene was synthesized by the group of Prof. M. Gozin, School of Chemistry, Tel Aviv University. The full analysis time was less than 1 min and the obtained mass spectrum shows abundant molecular ion. However, two other peaks are also observed; the first is
of an impurity from the degradation and loss of the azide N2 (MS is given at the bottom left trace) while the second is an isomer of the benzylazidanthracene. We know that the nitrogen loss is not a matter of injector degradation since lowering the injector temperature from 180 ◦ C to 130 ◦ C did not affect the results. The availability of the results in real time is useful and appealing for the synthetic organic chemists which get real time feedback. The availability of molecular ions was complemented by isotope abundance analysis [26] that provided elemental formula and superior verification of the compound identity. We note that unlike standard GC–MS, fast Supersonic GC–MS can be used for the analysis of such thermally labile compounds. Furthermore, such analysis cannot be properly performed by ESI LC–MS since the ESI ionization yields are highly compound dependent (inapplicable for this compound), hence without compound specific calibration (which is not practical) all the information on sample cleanliness and organic chemistry yield is lost. In addition, unlike GC–MS which has a one universal method, LC–MS method development is very long and not practical in service MS, thus typical ESI-mass spectrometry is performed with flow injection in which quantitation is lost upfront. As shown in Fig. 3 the fast GC–MS with SMB enables real time analysis of synthetic organic compounds. This can be explored in on-line monitoring of the progress of few synthetic organic chemical reactions for its optimization. Results from this research which seems very important for synthetic chemistry, will be separately reported. Another application that was explored with the fast GC–MS with SMB is the analysis of biodiesel fatty acid methyl esters (FAMEs) in jet fuel. FAMEs analysis was explored by fast GC [29,30] and the emerging need of FAMEs in jet fuel analysis was explored by GC × GC [31,32]. Obviously, reduced analysis time to 1 min is highly beneficial and can enable improved jet fuel quality monitoring. The LOD requirement for biodiesel analysis as FAMEs in jet fuel is modest at 5 ppm, but the problem is selectivity and matrix interference. This problem is further exacerbated with fast GC in view of the unavoidable reduction of GC separation. Thus, the added selectivity of enhanced molecular ions with cold EI and the use of further selectivity of MS–MS are highly desirable as shown in Fig. 4. Fig. 4 portrays four sequential analyses of FAMEs in jet fuel, done within 4 min, one after the other, and each at 1 min full analysis cycle time. The first run is of 20 ppm of each methylpalmitate, methylheptadecanoate and methyllinolenate in methanol solution that contains also 1% jet fuel. Full scan TIC is shown and the FAMEs are revealed as small peaks eluting after the majority of the jet fuel compounds. The next analysis is full scan of 5 ppm of the above FAMEs in neat jet fuel. The TIC is void of structure and the FAMEs are not observed. The third analysis is with three ions SIM of each of the FAMEs molecular ions (5 ppm in neat jet fuel). The selectivity of the molecular ions is sufficient now to resolve the FAMEs but they are riding on a hump of un-separated jet fuel background. The fourth run is of MS-MS (5 ppm in neat jet fuel) while using the FAMEs molecular ions as the parent ions and the indicated daughter ions were transferred by Q3. Very good selectivity and sensitivity is observed and clearly fast GC–MS with SMB can be used for 1 min analysis of biodiesel in jet fuel. Note that the FAMEs are chromatographically separated even at such fast chromatography. We used a standard DB5MS column and the use of a more polar column such as wax can further improve the separation of the biodiesel related FAMEs from the jet fuel matrix but evidently it was not needed as the fast GC–MS–MS with SMB provided sufficient selectivity. An important fast GC–MS application is pesticide residue analysis in agricultural products [33–35]. So far all attempts to achieve fast GC–MS analysis were limited to 8–10 min full analysis cycle time. Such analysis is particularly challenging in view of the high complexity of the matrix which is further significantly (10–20×) increased with the reduced separation efficiency of the fast GC. In addition, the short cycle time naturally reduces the GC column life
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
9379
Fig. 4. Sequence of four fast GC–MS with SMB analyses of FAMEs in jet fuel. Each full analysis cycle time as shown was 60 s, including 5 s injection time, 40 s temperature programming time and 15 s cooling down time (see bottom middle panel). Sample injections and GC programs were repeated every 1 min, while the mass spectra were recorded continuously with four time segments 1 min each: full scan, full scan, SIM and MS–MS. First injection was of a solution of 20 ppm FAMEs (methyl palmetate, methyl heptadeconate, and methyl linolenate) in methanol, contaminated with about 1% jet fuel and the obtained mass spectrum of methyl linolenate is shown on the bottom left panel. Than, neat jet fuel with 5 ppm FAMEs was injected 3 times at 1, 2, and 3 min (see upper record). Full scan MS (0–1 min and 1–2 min in upper trace), SIM on FAMEs molecular ions (2–3 min) and MS–MS on FAMEs molecular ions-high mass fragments transitions (3–4 min), (see bottom right panel) were recorded, respectively. The sample was injected with split 20 at 280 ◦ C injector temperature and standard 3800 GC oven temperature at 290 ◦ C. The short fast GC column flow rate was 7 mL/min.
time. Furthermore, sample preparation currently seem to be the bottle neck in attempts to reduce the full time of pesticide analysis, but the recent advances in QuEChERS sample preparation [36,37] and the fact that sample preparation can be performed in batches and by few people (or automated in the future) brings back the need for faster GC–MS. In Fig. 5, fast pesticide analysis in 1 min full analysis cycle time is demonstrated. Results of two consecutive analyses are shown in Fig. 5. The first analysis is via full scan fast GC–MS with SMB and the second run is of fast GC–MS–MS with SMB while using the molecular ions m/z = 329 of iprodion as the parent ion and m/z = 314 as the daughter ion and m/z = 415 which is the molecular ion of cypermethrin as the parent ion and m/z = 163 as the daughter ion for that pesticide. Both iprodion and cypermethryn were independently found at 0.23 and 0.22 mg/kg in grape extract. (sample with authentic pesticides was kindly donated by Paulina Goldshlag from the Israel Plant Protection Center). The bottom trace shows the cold EI mass spectrum of iprodion which could be extracted from the congested TIC. Our suggested strategy for fast pesticide analysis is to have a 1 min full scan analysis, as demonstrated in Fig. 5, followed by any number of pesticides screening using 2 ions RSIM on the molecular ion and a one high mass fragment ion, as explored in details before [20]. While the enhancement of the molecular ion in cold EI is helpful the reduced separation of the GC short column implies increased rate of false positive. Thus, the second 1 min run can serve for fast GC–MS–MS with SMB confirmation. Currently there is no software that can perform the needed screening in a few seconds for launching an appropriate MS–MS method but the
MS–MS run can be performed any time and not necessarily right after the sample analysis in which the suspected pesticides were found. Note that even if some people may not accept the 1 min full scan method as a viable opportunity, one can use a more standard 8 min fast GC–MS with SMB [20], or even GC–MS with SMB with standard column and analysis time and operate the confirmation fast GC–MS–MS with SMB during the end of the run plateau, in which the standard column is being cleaned from matrix by the end of run high temperature or via backflush. In that case a second injector can be used for performing fast GC–MS–MS with SMB for confirmation of the pesticides found in the last run or previous runs. This way the fast MS-MS confirmation does not take any additional time since it is performed during the run of the other column, while the 2 columns can be simultaneously connected to the supersonic nozzle with its unlimited flow rate acceptance. We explored several combinations of pesticides and matrixes including also authentic phosmet and azoxystrobin in apricot while using a ChromatoProbe for sample introduction [38] which enables the use of minimal sample preparation [38,39]. Note that our fast GC–MS with SMB can also be used for the analysis of such thermally labile compound, as Iprodion, which cannot be properly analyzed with standard GC–MS due to its injector and column degradation. Another application that clearly benefits from fast analysis is explosives analysis, which was already explored with the Supersonic GC–MS [27]. In Fig. 6, fast, 1 min cycle time analysis of explosive mixture is demonstrated. Triacetonetriperoxide (TATP), trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX)
9380
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
Fig. 6. One minute analysis of explosives TATP, TNT and RDX. The upper trace demonstrates 50 s total ion chromatography within 60 s full analysis cycle time. The bottom traces show the obtained mass spectra with enhanced molecular ions. The sample was injected with split 9 at 170 ◦ C injector temperature and standard 3800 GC oven temperature at 190 ◦ C. The short fast GC column flow rate was 16 mL/min. Fig. 5. Pesticides analysis with fast GC–MS with SMB. Authentic Iprodion (0.23 mg/kg) and Cypermethrin (0.22 mg/kg) in grapes were analyzed in full scan (first minute) followed by fast GC–MS–MS analysis in the second minute. The broken line in the upper trace provides the temperature program used for consequent runs. The elution times of the pesticides in the first run are pointed. Iprodion was sufficiently separated from the matrix thus provided library searchable mass spectrum with enhanced molecular ion (bottom trace). The second run recorded from 1.0 min to 2.0 min was for MS–MS on molecular ions-high mass fragments transitions. The sample was injected splitless for 0.1 min at 220 ◦ C injector temperature and standard 3800 GC oven temperature of 300 ◦ C. The short fast GC column flow rate was ∼32 mL/min.
were analyzed and despite being thermally labile (TATP), both TATP conformers are shown in Fig. 6 and a dominant molecular ion is exhibited. Similarly, an abundant molecular ion is exhibited in cold EI for TNT while for RDX a small molecular ion plus an abundant high mass fragment are observed. While there is a clear need for fast explosive screening such analysis also includes sample collection and injection or introduction. Recently we developed a new device that enables fast sampling and sample introduction, named open probe [40], and work is underway to combine the open probe with our fast GC–MS with SMB in order to facilitate fast explosive analysis all the way for sample collection through data analysis. 4. Discussion Resistive heating as a concept for fast GC has been well established, but not widely practiced. This trend changed with the invention of Low Thermal Mass (LTM) fast GC by Mustacich et al. [41] with Agilent Technologies currently owns the intellectual properties associate with that innovation. Thus, LTM fast GC is now a widely used technology which is available by several vendor including Agilent, Thermo, Leco and VICI Valco to name a few. However, currently LTM fast GC is not yet widely used in combination
with mass spectrometry since as discussed in this manuscript fast GC–MS is very different from fast GC in view of availability of mass spectrometry separation and flow rate constraints. Thus, our LTM Fast GC–MS with SMB is aimed at advancing LTM fast GC–MS and in exploring a few of its important applications. While the essence of our fast GC is shown in Fig. 1 and described above, additional details can further help in the illumination of a few more aspects and advantages of our LTM fast GC: (A) Ultra fast GC and GC–MS: As demonstrated, our fast GC–MS approach provides a solution for all the requirements of fast analysis including fast high flow rate splitless injections, fast temperature programming rate and cooling down with our low thermal mass resistively heated metal tube, tailing-free ultra fast ion source response time and increased MS separation power through the provision of enhanced molecular ions with the Supersonic GC–MS. Thus, sub 1 min total analysis cycle time can be routinely obtained, and, when only a limited GC column temperature range is needed, analysis cycle time of only a few seconds can be obtained. (B) High carrier gas flow rate: The carrier gas flow rate as used in the examples given above was typically in the range of 20–40 mL/min. This approach represents a reduction in the fast GC separation power for achieving faster analysis, greater sample capacity and linear dynamic range (LDR), and column robustness as well as extended range of compounds amenable for analysis. The reduced fast GC separation is compensated by increased separation power of the mass spectrometer with supersonic molecular beams that serves as the detector. Similarly, the fast GC column is short, typically with only 1.7 m column length. Column length reduction is required to enable the desirable high column flow rate and facilitate ultra fast GC
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
(C)
(D)
(E)
(F)
(G)
(H)
separation. In addition, it enables easier column replacement into the heating metal tube which is practically impossible with standard 30 m long columns. Consequently, the combination of short column, high column flow rate and low thermal mass of the resistively heated metal tube enables a full analysis cycle time of less than 1 min. High sample capacity and linear dynamic range: The use of high column flow rate and standard 0.25 mm ID column increases the sample capacity by about two orders of magnitude in comparison with the use of microbore columns with 0.1 mm ID and 1 mL/min or less column flow rate. Sample capacity depends on the volume of the adsorption film of a separation plate, hence it usually increases with the third power of the column diameter and linearly with the column flow rate which are increased by about two orders of magnitude in our experiments. In addition, the linear dynamic range (LDR) similarly increases with the sample capacity since more sample can be injected without peak shape distortion with the same limit of detection. This improved LDR and sample capacity are important for broad range of practical applications. Extended range of compounds amenable for analysis: The use of a very short column and high column flow rate provides extended range of compounds amenable for analysis. For example, we analyzed a mixture of hexadecane, anthracene, methylstearate, cholesterol and a heavy compound with molecular weight of 774 (C54 H78 O3 ) in less then 20 s while the last compound and even cholesterol are not amenable for certain other types of fast GC. Flexible column connections and easy column insertion: One of the important features of our fast GC is that the capillary column is not rigidly connected to the metal tube and can freely slide (inserted) into it. Therefore, the column can be pushed into the heated metal tube or pulled out and as a result column replacement can be easily performed. Our method of fast GC requires that the standard GC oven will serve as a part of the transfer line from the injector and to the detector during the fast GC operation, hence it must be heated to a temperature around that of the detector transfer line such as in the range of 250–350 ◦ C. Since such high temperature can lead to column bleed, the analytical column of the fast GC can also be connected to the injector and detector via unions and deactivated fused silica capillary transfer lines. Mounting on the top of the GC: Our fast GC is based on the mounting of the fast GC module on top of the oven of the standard GC. Consequently, the fast GC is typically interfaced with the standard GC through available holes for a second injector or detector. As a result, the standard GC oven door can be fully and freely open as usual for service, including for column assembly and its connection and insertion into the injector and detector. This arrangement also implies that the fast GC module is relatively small and that its control electronics is separated from it and conveniently located at a side of the GC. Increased fast GC column lifetime: Since the separation plate height is increased linearly with the flow rate, the use of high column flow rate increases the column sample capacity, system linear dynamic range and column robustness and lifetime. Furthermore, since the column freely extends beyond the heating metal tube it can be cut as needed to remove its upstream portion near the injector which is periodically contaminated by low volatility matrix compounds and dirt. Lower cost of use: The price of columns for our fast GC is far lower than with most other fast GCs since our replacement column is just a piece of any standard capillary GC column of choice, the column lifetime is much longer as explained above, and a new short capillary column can be inserted many times into the fast GC oven tube for new use.
9381
(I) Supersonic GC–MS advantages: Our fast GC is uniquely compatible and combined with the Supersonic GC–MS as demonstrated in the results described above. As a result, it forms a powerful fast GC–MS in terms of speed of analysis, sensitivity, range of compounds amenable for analysis and improved sample identification capability. Thus, we named it Supersonic Fast GC–MS. (J) Combined fast GC–MS and standard GC–MS in a single system: Our fast GC when coupled with the Supersonic GC–MS is further uniquely characterized by having the ability to connect two columns to the same MS via a simple double hole ferrule at the GC end of the heated transfer line since the Supersonic GC–MS can handle any column flow rate. Thus, one can use the fast GC module in combination with a standard 30 m GC column that is connected from a second standard GC injector. As a result, our fast GC–MS can uniquely serve for fast screening, while in the case of detection of a suspected compound, the method can be changed by a click of the mouse and the screening can be followed by a confirmation run with increased separation power of a standard GC column.
Standard GC is typically operated with 30 m long capillary columns with 0.25 mm ID and 1 mL/min column flow rate. According to the theory of GC, the maximum allowed temperature programming rate in which the column separation is retained is 10 ◦ C per void time [42] which for the standard column and flow rate as above is ∼90 s. As a result, the maximum temperature programming rate which is used in combination with full standard GC separation power is 7 ◦ C/min. With such temperature programming rate, full temperature range analysis from 50 ◦ C to 350 ◦ C sums up from: 1 min for splitless injection at 50 ◦ C, 43 min temperature programming to 350 ◦ C, 2 min at 350 ◦ C and about 4 min cooling down and equilibration time with total analysis cycle time of 50 min. Obviously, most analysis can employ 10 ◦ C/min with only negligible/minor loss in separation resolution and with a typical range of temperature program of only 250 ◦ C such analysis can take only 30 min. If reduced GC separation can be tolerated, 5 m columns can be used with 60 ◦ C/min temperature programming rate and the total analysis time can be reduced even to 10 min with standard GC but with a loss of more than a factor of 6 in the number of separation plates and about a factor of 2.5 in peak capacity. The standard column with 0.25 mm can be replaced with a short microbore column, which can provide such fast 10 min analysis with smaller loss in the GC separation capability but with a major loss in sample capacity, column robustness, linear dynamic range and column lifetime. In fact, the use of such microbore columns defies, in our opinion, the purpose of fast GC since the combination of reduced column life time with reduced analysis time implies that the column replacement frequency will be significantly increased along with its associated cost and downtime. It is important to note that while standard GC is typically operated with a temperature programming rate in the range of 5–10 ◦ C/min the standard GC oven is capable of temperature programming rate of up to 120 ◦ C at low temperatures and 50–60 ◦ C around 300 ◦ C. However, if 60 ◦ C/min is used with standard columns the analysis will be hampered by a loss factor of about 8 in the number of separation plates, since most of the time the sample compounds will not be retained in the too hot column and only the initial portion of the column will be used for effective separation. Furthermore, the elution temperatures will be increased by over 60 ◦ C (20 ◦ C per each factor of two increased temperature programming rate above 7 ◦ C/min [27]) and as a result thermally labile compounds will degrade, low volatility compounds will elute at the high temperature plateau end of the run (or will not elute) with increased column bleed and ghost peaks chemical noise, and thus, the range of compounds amenable for analysis will be sacrificed and reduced.
9382
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
In 1996 Dagan and Amirav [18] classified fast GC–MS as fast, very fast and ultra fast GC–MS based on a speed enhancement factor (SEF) which is the void time reduction factor versus standard GC–MS operation, meaning the reduction of the column length, times column flow rate increase, versus standard 30 m 0.25 mm ID column with 1 mL/min column flow rate. In 1998 Blumberg and Klee introduced another measure of the speed of a GC analysis [43]. They took the peak width as a measure of analysis speed and defined “A fast capillary GC analysis as one with the average peak width of less than 1 s”. They also classified approaches involving an average peak width of around 100 ms as Superfast GC and those in which it is below 10 ms as Ultrafast GC. Magni et al. [44] went into more details and defined “Fast GC” as an analysis performed in less than 10 min with columns with i.d. between 0.25 mm and 0.1 mm, length from 5 m to 15 m, temperature programming rates of 20–60 ◦ C/min and peak widths in the range between 0.5 s and 2 s. They used the term Ultrafast GC for analyses of 1 min or less, entailing the use of short (2–10 m) narrow-bore columns (0.1–0.05 mm i.d.) and temperature programming rates above 1 ◦ C/s, leading to peak widths of 50–200 ms. Later on Bicchi et al. followed Magni et al. in their terminology. In our opinion fast GC–MS is different than fast GC in that due to the added separation power of the MS (plus enhanced molecular ion and MS–MS in this manuscript) the emphasis in the terminology should be given to the total analysis cycle time and not to GC peak width. The total analysis cycle time relate more closely to the Dagan and Amirav SEF factor which in our experiment is 600 hence closer to ultra fast GC–MS than to very fast GC–MS. According to our preferred method of operation of our fast GC we use 2.5 m, 0.25 mm ID capillary column with 20–30 mL/min column flow rate, from which only 1.7 m is inside the TLM fast GC heating tube hence active in the separation. Even bigger capillary columns with 0.53 mm ID can be used with the advantage of having larger column capacity and much lower flow impedance. Due to the combination of column length reduction by a factor of ∼20 and column carrier gas flow rate increase by a factor of 20 the number of separation plates is significantly reduced by about a factor of 400 which results in a loss of GC separation resolution and peak capacity by a factor of ∼20. Such loss cannot be tolerated by most standard GC analysis requirements. However, when fast GC–MS analysis is considered, particularly with the Supersonic GC–MS and/or GC–MS–MS, the merits of our method and device far outweighs its only limitation in reduced GC separation capability. The use of 1.5 m column with 20–30 mL/min column flow rate results in the reduction of the void time by a huge factor of 400. Consequently a temperature programming rate of 45 ◦ C/s (2700 ◦ C/min) can be used and full range temperature programming can take only 6–7 s. However, since even with the low thermal mass fast GC heating tube, cooling down may take 10 s, only limited gain in time is achieved by using such fast temperature programming rate. A better method of fast GC operation is based on using a temperature programming rate of 10 ◦ C/s which results in 40 ◦ C lower elution temperatures to significantly increase the range of compounds amenable for analysis. Consequently, full analysis cycle takes a few seconds for sample thermal-focusing, 40 s temperature programming, 10 s cooling down time and less than 5 s equilibration time for the total of less than 1 min per analysis, which is an order of magnitude faster than can be achieved with standard GC. It is important to realize that the use of high column flow rate is essential for having a useful fast GC–MS since flow rate reduces the void time, thereby enabling fast temperature programming rate combined with increased range of compounds amenable for analysis as opposed to the reduced range encountered with other fast GC methods and devices. An additional very important attribute of the use of high column flow rate is that standard sample injections are correspondingly faster, and with 30 mL/min standard splitless
injection may take only two seconds instead of the usual 1 min. Without such injection time reduction, splitless (or split with low split ratio) injection may either prohibit fast GC cycle time of less than 1 min or correspondingly reduce the sensitivity (with high split ratio split injections) which is unacceptable for many types of trace level analyses. Finally, we note that the ability to use very high column flow rates enables the use of effective flow programming with large high to low flow rates ratios, which provides a faster alternative to temperature programming rate and eliminates the time needed for cooling back. Thus, flow programming can provide an ultra fast alternative to fast temperature programming when only a limited temperature range of up to 100 ◦ C is needed. 5. Conclusions Fast GC–MS with SMB with full analysis cycle time of 1 min was demonstrated for several selected applications. Our fast GC is based on the use of standard short capillary column that is inserted in a resistively heated stainless steel tube with low thermal mass. Our fast GC method is uniquely based on having high column flow rate with a trade off of GC separation for speed while combining it with mass spectrometry with Supersonic molecular beams that provides enhanced molecular ions hence enhanced MS separation. Such mass spectrometry is unique in its compatibility with high column flow rate. Thus, the combination of our LTM fast GC and Supersonic GC–MS provides a powerful Supersonic Fast GC–MS as demonstrated in five challenging applications. Acknowledgements This research was supported by the Israel Science Foundation founded by the Israel Academy of Sciences and Humanities (grant No. 1172/07 and 393/11). This research was also supported by a Research Grant Award No. US-4273-09 from BARD, the United States – Israel Binational Agricultural Research and Development Fund, by the James Franck Center for Laser Matter Interaction Research and by the infrastructure research program of the ministry of science and technology (MOST) of the state of Israel. References [1] D. Rounbelher, E. Achter, D. Fine, G. Jarvis, S. Macdonald, D. Wheeler, C. Wood, US Patent No. 5,808,178, (1998). [2] S.J. Macdonald, D. Wheeler, Am. Lab. 30 (1998) 27. [3] G. Frishman, A. Amirav, Field. Anal. Chem. Technol. 4 (2000) 170. [4] G. Frishman, N. Tzanani, A. Amirav, Field. Anal. Chem. Technol. 5 (2001) 107. [5] B.J. Hopkins, V. Pretorius, J. Chromatogr. 158 (1978) 465. [6] B.A. Ewels, R.D. Sacks, Anal. Chem. 57 (1985) 2774. [7] M.E. Hail, R.A. Yost, Anal. Chem. 61 (1989) 2410. [8] E.U. Ehrmann, H.P. Dharmasena, K. Carney, E.B. Overton, J. Chromatogr. Sci. 34 (1996) 533. [9] K.M. Sloan, R.V. Mustacich, B.A. Eckenrode, Field Anal. Chem. Technol. 5 (2001) 288. [10] G. Sacks, T. Brenna, Am. Lab. 35 (2003) 22. [11] C. Bicchi, C. Brunelli, C. Cordero, P. Rubiolo, M. Galli, A. Sironi, J. Chromatogr. A 1024 (2004) 195. [12] C.A. Cramers, P.A. Leclecq, J. Chromatogr. A 842 (1999) 3. [13] K. Mastovska, S.J. Lehotay, J. Chromatogr. A 1000 (2003) 153. [14] M. Kirchner, E. Matisova, Chem. Listy 99 (2005) 789. [15] J. de Zeeuw, J. Peene, H.G. Jansen, X.W. Lou, HRC-J. High Res. Chromatogr. 23 (2000) 677. [16] S. Dagan, A. Amirav, Int. J. Mass. Spectrom. Ion Proc. 133 (1994) 187. [17] A. Amirav, A. Gordin, M. Poliak, A.B. Fialkov, J. Mass Spectrom. 43 (2008) 141. [18] S. Dagan, A. Amirav, J. Am. Soc. Mass Spectrom. 7 (1996) 737. [19] A. Amirav, A. Gordin, N. Tzanani, Rapid. Commun. Mass Spectrom. 15 (2001) 811. [20] M. Kochman, A. Gordin, P. Goldshlag, S.J. Lehotay, A. Amirav, J. Chromatogr. A 974 (2002) 185. [21] A.B. Fialkov, A. Amirav, J. Chromatogr. A 1058 (2004) 233. [22] A.B. Fialkov, U. Steiner, L. Jones, A. Amirav, Int. J. Mass Spectrom. 251 (2006) 47. [23] A.B. Fialkov, A. Gordin, A. Amirav, J. Chromatogr. A 1195 (2008) 127. [24] A. Amirav, A.B. Fialkov, A. Gordin, Rev. Sci. Instrum. 73 (2002) 2872.
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383 [25] A. Gordin, A. Amirav, A.B. Fialkov, Rapid. Commun. Mass Spectrom. 22 (2008) 2660. [26] T. Alon, A. Amirav, Rapid Commun. Mass Spectrom. 20 (2006) 2579. [27] A.B. Fialkov, A. Gordin, A. Amirav, J. Chromatogr. A 991 (2003) 217. [28] A. Amirav, A.B. Fialkov, Fast gas chromatograph method and device for analyzing a sample in a supersonic molecular beam US Patent application number 12/899,288, (2010). [29] C. Cruz-Hernandez, J. Liq. Chromatogr. Related Technol. 32 (2009) 1672. [30] L. Mondello, P.Q. Tranchida, R. Costa, A. Casilli, P. Dugo, A. Cotroneo, G. Dugo, J. Sep. Sci. 26 (2003) 1467. [31] J.V. Seeley, S.K. Seeley, E.K. Libby, J.D. McCurry, J. Chromatogr. Sci. 45 (2007) 650. [32] F. Adam, F. Bertoncini, V. Coupard, N. Charon, D. Thiebaut, D. Esoinat, M.C. Hennion, J. Chromatogr. A 1186 (2008) 236. [33] R.S. Sheridan, J.R. Meola, J. AOAC Int. 82 (1999) 982. [34] K. Mastovska, J. Hajslova, S.J. Lehotay, J. Chromatogr. A 1054 (2004) 335. [35] T. Cajka, J. Hajlova, O. Lacina, K. Mastovska, S.J. Lehotay, J. Chromatogr. A 1186 (2008) 281.
9383
[36] M. Anastassiades, S.J. Lehotay, D. Stainbaher, F.J. Schenck, J. AOAC Int. 86 (2003) 412. [37] S.J. Lehotay, K.A. Son, H. Kwon, U. Koesukwiwat, W.S. Fu, K. Mastovska, E. Hoh, N. Leepipatpiboon, J. Chromatogr. A 16 (2010) 2548. [38] A. Amirav, S. Dagan, Eur. Mass Spectrom. 3 (1997) 105. [39] H. Jing, A. Amirav, Anal. Chem. 69 (1997) 1426. [40] M. Poliak, A. Gordin, A. Amirav, Anal. Chem. 82 (2010) 5777. [41] R.V. Mustacich, US patents 5782964 (1998), 6209386 (2001) and 6217829 (2001). [42] L.M. Blumberg, M.S. Klee, J. Microcolumn Sep. 12 (2000) 508. [43] L.M. Blumberg, M.S. Klee, in: P. Sandra (Ed.), Proceedings of 20th International Symposium of Capillary Chromatography, Riva del Garda, Italy, 26–29 May 1998, 1998, p. PL9. [44] P. Magni, R. Facchetti, D. Cavagnino, S. Trestianu, in: P. Sandra (Ed.), Proceedings of 25th International Symposium of Capillary Chromatography, Riva del Garda, Italy, 13–17 May 2002, 2002, p. KLN05, and references cited therein.
Contents lists available at SciVerse ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
A low thermal mass fast gas chromatograph and its implementation in fast gas chromatography mass spectrometry with supersonic molecular beams Alexander B. Fialkov, Mati Morag, Aviv Amirav ∗ School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 18 October 2011 Accepted 20 October 2011 Available online 11 November 2011 Keywords: Fast GC Fast GC–MS Supersonic molecular beams Low thermal mass High throughput analysis
a b s t r a c t A new type of low thermal mass (LTM) fast gas chromatograph (GC) was designed and operated in combination with gas chromatography mass spectrometry (GC–MS) with supersonic molecular beams (SMB), including GC–MS–MS with SMB, thereby providing a novel combination with unique capabilities. The LTM fast GC is based on a short capillary column inserted inside a stainless steel tube that is resistively heated. It is located and mounted outside the standard GC oven on its available top detector port, while the capillary column is connected as usual to the standard GC injector and supersonic molecular beam interface transfer line. This new type of fast GC–MS with SMB enables less than 1 min full range temperature programming and cooling down analysis cycle time. The operation of the fast GC–MS with SMB was explored and 1 min full analysis cycle time of a mixture of 16 hydrocarbons in the C10 H22 up to C44 H90 range was achieved. The use of 35 mL/min high column flow rate enabled the elution of C44 H90 in less than 45 s while the SMB interface enabled splitless acceptance of this high flow rate and the provision of dominant molecular ions. A novel compound 9-benzylazidanthracene was analyzed for its purity and a synthetic chemistry process was monitored for the optimization of the chemical reaction yield. Biodiesel was analyzed in jet fuel (by both GC–MS and GC–MS–MS) in under 1 min as 5 ppm fatty acid methyl esters. Authentic iprodion and cypermethrin pesticides were analyzed in grapes extract in both full scan mode and fast GC–MS–MS mode in under 1 min cycle time and explosive mixture including TATP, TNT and RDX was analyzed in under 1 min combined with exhibiting dominant molecular ion for TATP. Fast GC–MS with SMB is based on trading GC separation for speed of analysis while enhancing the separation power of the MS via the enhancement of the molecular ion in the electron ionization of cold molecules in the SMB. This paper further discusses several features of fast GC and fast GC–MS and the various trade-offs involved in having powerful and practical fast GC–MS. © 2011 Elsevier B.V. All rights reserved.
1. Background Gas chromatography is a central analytical technology which is applied in a large variety of applications in a broad range of fields, especially when used in association with mass spectrometry. However, GC analysis requires long time, typically in the order of 20–60 min while many analyses require high throughput. In view of the long time associated with standard GC analysis, several fast GC systems have been developed that incorporate low thermal mass devices which provide fast heating temperature program and cooling rates for the GC separation columns. For example, Rounbehler et al. describe in US Patent No. 5,808,178 [1] a fast GC module named “Flash GC” [2], which is based on a capillary GC column inside a resistively heated metal tube which can be quickly heated and cooled due to its low thermal mass to achieve rapid separation
∗ Corresponding author. Tel.: +972 36408253; fax: +972 36424048. E-mail address: [email protected] (A. Amirav). 0021-9673/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2011.10.053
of analytes. This concept was also used in a portable GC with PFPD [3] and a portable electrolyzer operated fast GC-FID [4]. The concept of using resistive heating for fast GC is traced back to Hopkins and Pretorius [5] and later to Ewels and Sacks [6]. However, in these early publications the use of resistive heating was limited to the inlet systems. Hail and Yost used resistively heated aluminum clad capillary columns [7] and later on the use of various forms of resistive heating became more abundant [8–11] in view of its ability to reduce the thermal mass of the GC oven hence to reduce both its heating and cooling rates. However, when it comes to fast GC–MS, the mass spectrometer introduces additional important considerations which change several central aspects of the fast GC design. The area of fast GC–MS is reviewed and discussed in details by a few authors [12–15]. As is known, fast GC is the art of compromises, and speeding up the GC temperature program rate alone can result in the reduction of GC separation efficiency, column lifetime, range of compounds amenable for analysis, sample capacity, linear dynamic range and sensitivity, combined with increased cost of columns which can be
9376
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
coupled with the cost of the whole fast GC module. Furthermore, fast GC and particularly fast GC–MS, require much more than just fast temperature program rate of the GC oven. For example, standard splitless sample injection takes a few minutes, since it requires 1 min just for sample thermal-focusing at a low GC oven temperature plus additional time for heating to the analytically useful column temperature and cooling back. While split injection may reduce the time required for sample introduction into the column, it leads to unavoidable and often unacceptable loss in limit of detection and sensitivity. In addition, fast GC may reduce the GC peak widths, which in the case of mass spectrometry it requires the combination of fast scan speed and fast ion source response time. It is important to note that there is a major difference between fast GC and fast GC–MS in that the mass spectrometer adds an additional dimension of sample separation and selectivity, which can be further enhanced with tandem mass spectrometry (MS–MS) such as in triple quadrupole mass spectrometry systems. The basic idea is that, in fast GC–MS, the GC separation can be traded and some chromatographic peaks co-elution can be allowed in view of the additional separation of the MS, while, in fast GC, when a universal detector such as FID or TCD are used, the GC separation is its prime feature which often cannot be sacrificed. Thus, despite the obvious merit of having fast GC and/or fast GC–MS and its availability in the market, the vast majority of GC and GC–MS analysis still takes more than 20 min. In the last 18 years our group research has been focused on the development of GC–MS with supersonic molecular beams (SMB) (also named Supersonic GC–MS) [16–23]. Supersonic GC–MS is based on a GC and MS interface with SMB and on the electron ionization (EI) of vibrationally cold analytes in the SMB (cold EI) in a fly-through ion source. This ion source is inherently inert and further characterized by ultra-fast response time and vacuum background filtration capability [17,24]. The same ion source also offers a mode of classical EI [25]. Cold EI, as a main mode, provides enhanced molecular ion combined with effective library sample identification which is supplemented and complemented by a powerful isotope abundance analysis method and software [26]. We note that the feature of enhanced molecular ion also implies enhanced separation power of the mass spectrometer since, as is known, matrix interference is exponentially reduced with mass [20]. In addition, the range of low volatility and thermally labile compounds amenable for analysis is significantly increased with the Supersonic GC–MS due to the use of contact-free fly-through ion source and the ability to lower sample elution temperatures through the use of high GC column carrier gas flow rates [27]. Another important feature of the Supersonic GC–MS is its compatibility with high column flow rates without any adverse effect on its sensitivity, due to the availability of a differential vacuum chamber for the supersonic nozzle. As was demonstrated [27] and as will be shown below, this feature is very important for the combination of our fast GC method and device with the Supersonic GC–MS.
2. Experimental: low thermal mass fast GC and fast GC–MS Fig. 1 schematically describes our low thermal mass (LTM) fast gas chromatograph device which is mounted on the top of the standard GC oven [28]. Samples are injected as usual, manually or preferably with an autosampler into a GC injector, typically split splitless (SSL), which is connected to a capillary column. The capillary column itself is the standard polyimide coated fused silica capillary column albeit usually shorter. Thus, the column is immersed in a standard GC oven which as a result must be heated to a temperature that corresponds to the temperatures of the injector and detector to eliminate cold spots. The capillary column is connected to our LTM fast GC in a flexible way as is normally done with
Fig. 1. A schematic diagram of our low thermal mass fast GC module, installed on an available detector port on the top plate of a standard GC oven of a GC–MS.
standard GCs and in contrast to some other fast GCs. The capillary column is introduced into a LTM metal tube that can be resistively heated and that possesses an inlet and an outlet, both of which are connected to a current programmed power supply via electrical connections. The current program of the power supply provides a temperature program (with time). The temperature can be measured with a thin thermocouple, or via the resistance of the metal tube which increases with its temperature in a known manner. This way, the current value can be used alone to represent the temperature via temperature calibration tables. The capillary column in its resistively heated metal tube is mostly located in an air-cooled enclosure which is mounted on the detector place on the top plate of the standard GC oven (Varian 3800 in our experiments). The inlet and the outlet assembly of the heated metal tube is located at the inner edge of the standard GC oven housing so that its portion that is not resistively heated (about 1 cm) is located inside the GC oven hence heated to the standard GC oven temperature. The portion that is resistively heated is located mostly outside the GC oven to avoid or significantly reduce its double heating by both the GC oven and the resistively heated tube, which can lead to an over heated column section (hot spot), as well as to avoid unheated sections of the GC column (cold spot) between the resistively heated tube and the GC oven. Thus, the proper location of the inlet and outlet unions and their design is aimed at minimizing both hot and cold spots. The capillary column output is flexibly connected to the separately heated detector transfer line. In our experiments we used mostly 1.7 m, 0.53 mm ID, 0.75 mm OD metal tube as the heating tube (MXT Guard Column, Restek, Bellefonte, PA, USA) and 2.5 m standard GC column with 0.25 mm ID and 0.25 Rxi-5SIL MS film. Our LTM fast GC was capable of temperature program rate in excess of 2000 ◦ C/min but our standard temperature program
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
rate was limited to 600 ◦ C/min as explained in the discussion. The temperature program was achieved by current program of an external power supply in the 0–3.4 A range and the temperatures were in the 50–340 ◦ C range as calibrated indirectly via thermocouple which served to calibrate the heating tube temperature via its resistance. The helium column flow rate was in the 8–40 mL/min initial flow rate range (depending on the experiment) and was somewhat reduced as the LTM fast GC column temperature was increased. We note that our LTM fast GC module differ from Rounbehler et al. “Flash GC” [1,2] and other LTM fast GC designs in that our LTM fast GC module is placed outside the GC oven on the top of the standard GC oven while the column is flexibly connected to both injector and detector, hence during the fast GC analysis the standard GC oven must be heated and serve as a portion of the transfer line. It is also unique in comparison with other LTM fast GC designs in that the separation column is a standard fused silica column that is cut into a short length and that is inserted into a thin walled metal tube for its resistive heating hence it possesses very low thermal mass. We found that we are unable to use available metal columns since due to differences in thermal expansion between the PDMS film and the metal the column lifetime was significantly reduced. Thus, our solution was the use of standard columns that are available at low cost in large number of coating film types and thickness. In our experiments the detector is MS with SMB based on the conversion of a Varian 1200 triple quad GC–MS(–MS) into a Supersonic GC–MS as described in reference [22]. GC-SMB–MS is based on the use of an SMB for interfacing the GC to the MS and as a medium for electron ionization of sample compounds [17]. SMBs are characterized by intra-molecular vibrational supercooling, unidirectional molecular motion with controlled hyperthermal kinetic energy (1–20 eV), mass focusing similar to that in a jet separator, and capability to handle very broad range of column flow rates from standard 1 mL/min (or lower) up to 90 mL/min. The system used in this research is based on the combination of the SMB interface and its related fly-through electron ionization (EI) ion source, with the Varian 1200 L GC–MS and MS–MS (Varian Inc. Walnut Creek, CA). This system, named 1200-SMB, is described in detail in reference [22], and GC–MS with SMB is reviewed in reference [17], thus, it will be discussed here only briefly. In the 1200-SMB the column output flow is mixed with helium make-up gas (∼90 mL/min total), and flows to the supersonic nozzle through a heated and temperature controlled transfer line. The helium flow can be mixed (via the opening of a one valve) with perfluorotributylamine (PFTBA) for system tuning and calibration. The sample compounds seeded in the helium gas expand from a 90 m diameter supersonic nozzle into a nozzle vacuum chamber that is differentially pumped by a Varian Navigator 301 turbomolecular pump (Varian Inc., Torino, Italy) with 250 L/s pumping speed. The helium pressure at this vacuum chamber is about 6 × 10−3 mbar. The supersonic expansion vibrationally cools the sample compounds and the expanded supersonic free jet is skimmed by a 0.8 mm skimmer and collimated in a second differentially pumped vacuum chamber, where an SMB is formed. The second vacuum chamber is pumped by a Varian 300/400 split turbomolecular pump that pumps both the second vacuum chamber (400 L/s) and main MS vacuum chamber (300 L/s). The SMB, seeded with vibrationally cold sample compounds flies through a dual cage EI ion source [24] where these beam species are ionized by 70 eV electrons with 10–15 mA emission current. The ions are focused by an ion lens system, deflected 90◦ by an ion mirror and enter a radiofrequency (RF)-only hexapole ion transfer optics (Q0 of the original 1200 system). The 90◦ ion mirror is separately heated and serves to keep the mass analyzers clean from sample induced contaminations. The ions are further transferred through an ion lens into the MS vacuum chamber and are analyzed by a quadrupole MS–MS mass analyzer system. It consists of two quadrupole mass
9377
Fig. 2. Fast GC–MS with SMB analyses of aliphatic hydrocarbons n-C10 H22 –n-C44 H90 mixture. Full scan TIC chromatogram is shown in the upper trace while cold EI mass spectrum of C40 H82 is shown in the bottom panel. The sample was injected into a Varian 1177 injector with split 10 at 350 ◦ C; and standard 3800 GC oven temperature of 330 ◦ C. The short fast GC column flow rate was 35 mL/min.
analyzers (Q1 and Q3) and a collision cell (Q2). As any quadrupole MS–MS system, it can operate in SIM or full scan mode, as well as in all common MS–MS scan modes. Since Q2 is a 180 degrees curved RF-only quadrupole ion transfer system in the 1200 L, a head-on ion detector is positioned directly in the path of ions exiting Q3. Its entrance is biased at 5 kV, serving as an efficient ion to electron converter. Due to the combination of the 90◦ ion mirror and the 180◦ bend of Q2, the mass-independent neutral noise (produced by the ion source) was lower than one count per 10 s in the 1200-SMB system. 3. Results In Fig. 2 typical and representative results of the analysis of a broad volatility range of linear chain hydrocarbons are shown, using the fast GC in combination with a mass spectrometer with supersonic molecular beams (1200-SMB). The hydrocarbons mixture included 16 compounds in the range from C10 H22 up to C44 H90 , each at 100 ng/L concentration in hexane. It was injected into a Varian model 1177 injector with a split ratio of 9:1 so that 10 ng of each sample compound was injected into the column. The column was a 2.5 m long 0.25 mm ID fused silica capillary column with 0.25 DB5MS UI film (Agilent Technologies, Folsom, CA) that was inserted into 1.7 m metal tube heater with 0.53 mm ID and 0.75 mm OD (MXT Guard Column, Restek, Bellefonte, PA). The metal tube was coiled three times in 12–13 cm diameter loops and contacted only Kapton foil (0.125 mm thick) on its way from the top of the Varian 3800 GC into its oven where inlet and outlet connector unions penetrated 1 cm inside the oven. Helium served as the carrier gas at a flow rate of 35 mL/min. Home-made fast GC software controlled
9378
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
Fig. 3. Fast GC–MS with SMB analysis of a novel synthetic organic compound for products identification and for synthetic reaction yield (purity) evaluation. Full scan TIC is shown on upper trace. The mass spectra of one impurity (molecular ion 205 amu) and the desired product (9-benzylazidanthracene, molecular ion 233 amu) are depicted in the lower left and right panels, respectively. The sample was injected into a Varian 1177 injector with split 10 at 180 ◦ C; and standard 3800 GC oven temperature at 220 ◦ C. The short fast GC column flow rate was 24 mL/min.
the fast GC heating rate via a current programmable power supply that was programmed in the 0–3.4 A range. A cooling fan was also used to speed up the fast GC cooling and its operation was timeprogrammed by the control software. As demonstrated in Fig. 2, all the 16 hydrocarbons were fully separated in 45 s despite the large difference in their boiling points which was 142 ◦ C for C10 H22 and 693 ◦ C for C44 H90 . In addition, only 10 s were required for cooling down the column and being ready for the next analysis cycle, thus providing ultra fast GC–MS analysis cycle time. In addition, it is important to note the observation of molecular ion even for C40 H82 [23], which is unique to the Supersonic GC–MS and which demonstrates the increased selectivity and information content provided by GC–MS with SMB. In Fig. 3 the application of fast GC–MS with SMB for the analysis of a new thermally labile organic compound is shown. Some of the unique features and benefits of GC–MS with SMB for the analysis of synthetic organic compounds were previously discussed in reference [21]. This is a typical example of service mass spectrometry in which synthetic organic chemists wish to verify the success of their synthesis. The main goal is to provide a GC–MS peak of the new compounds, and as shown in Fig. 3, 9-benzylazidanthracene was synthesized and mostly properly cleaned. Benzylazidanthracene was synthesized by the group of Prof. M. Gozin, School of Chemistry, Tel Aviv University. The full analysis time was less than 1 min and the obtained mass spectrum shows abundant molecular ion. However, two other peaks are also observed; the first is
of an impurity from the degradation and loss of the azide N2 (MS is given at the bottom left trace) while the second is an isomer of the benzylazidanthracene. We know that the nitrogen loss is not a matter of injector degradation since lowering the injector temperature from 180 ◦ C to 130 ◦ C did not affect the results. The availability of the results in real time is useful and appealing for the synthetic organic chemists which get real time feedback. The availability of molecular ions was complemented by isotope abundance analysis [26] that provided elemental formula and superior verification of the compound identity. We note that unlike standard GC–MS, fast Supersonic GC–MS can be used for the analysis of such thermally labile compounds. Furthermore, such analysis cannot be properly performed by ESI LC–MS since the ESI ionization yields are highly compound dependent (inapplicable for this compound), hence without compound specific calibration (which is not practical) all the information on sample cleanliness and organic chemistry yield is lost. In addition, unlike GC–MS which has a one universal method, LC–MS method development is very long and not practical in service MS, thus typical ESI-mass spectrometry is performed with flow injection in which quantitation is lost upfront. As shown in Fig. 3 the fast GC–MS with SMB enables real time analysis of synthetic organic compounds. This can be explored in on-line monitoring of the progress of few synthetic organic chemical reactions for its optimization. Results from this research which seems very important for synthetic chemistry, will be separately reported. Another application that was explored with the fast GC–MS with SMB is the analysis of biodiesel fatty acid methyl esters (FAMEs) in jet fuel. FAMEs analysis was explored by fast GC [29,30] and the emerging need of FAMEs in jet fuel analysis was explored by GC × GC [31,32]. Obviously, reduced analysis time to 1 min is highly beneficial and can enable improved jet fuel quality monitoring. The LOD requirement for biodiesel analysis as FAMEs in jet fuel is modest at 5 ppm, but the problem is selectivity and matrix interference. This problem is further exacerbated with fast GC in view of the unavoidable reduction of GC separation. Thus, the added selectivity of enhanced molecular ions with cold EI and the use of further selectivity of MS–MS are highly desirable as shown in Fig. 4. Fig. 4 portrays four sequential analyses of FAMEs in jet fuel, done within 4 min, one after the other, and each at 1 min full analysis cycle time. The first run is of 20 ppm of each methylpalmitate, methylheptadecanoate and methyllinolenate in methanol solution that contains also 1% jet fuel. Full scan TIC is shown and the FAMEs are revealed as small peaks eluting after the majority of the jet fuel compounds. The next analysis is full scan of 5 ppm of the above FAMEs in neat jet fuel. The TIC is void of structure and the FAMEs are not observed. The third analysis is with three ions SIM of each of the FAMEs molecular ions (5 ppm in neat jet fuel). The selectivity of the molecular ions is sufficient now to resolve the FAMEs but they are riding on a hump of un-separated jet fuel background. The fourth run is of MS-MS (5 ppm in neat jet fuel) while using the FAMEs molecular ions as the parent ions and the indicated daughter ions were transferred by Q3. Very good selectivity and sensitivity is observed and clearly fast GC–MS with SMB can be used for 1 min analysis of biodiesel in jet fuel. Note that the FAMEs are chromatographically separated even at such fast chromatography. We used a standard DB5MS column and the use of a more polar column such as wax can further improve the separation of the biodiesel related FAMEs from the jet fuel matrix but evidently it was not needed as the fast GC–MS–MS with SMB provided sufficient selectivity. An important fast GC–MS application is pesticide residue analysis in agricultural products [33–35]. So far all attempts to achieve fast GC–MS analysis were limited to 8–10 min full analysis cycle time. Such analysis is particularly challenging in view of the high complexity of the matrix which is further significantly (10–20×) increased with the reduced separation efficiency of the fast GC. In addition, the short cycle time naturally reduces the GC column life
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
9379
Fig. 4. Sequence of four fast GC–MS with SMB analyses of FAMEs in jet fuel. Each full analysis cycle time as shown was 60 s, including 5 s injection time, 40 s temperature programming time and 15 s cooling down time (see bottom middle panel). Sample injections and GC programs were repeated every 1 min, while the mass spectra were recorded continuously with four time segments 1 min each: full scan, full scan, SIM and MS–MS. First injection was of a solution of 20 ppm FAMEs (methyl palmetate, methyl heptadeconate, and methyl linolenate) in methanol, contaminated with about 1% jet fuel and the obtained mass spectrum of methyl linolenate is shown on the bottom left panel. Than, neat jet fuel with 5 ppm FAMEs was injected 3 times at 1, 2, and 3 min (see upper record). Full scan MS (0–1 min and 1–2 min in upper trace), SIM on FAMEs molecular ions (2–3 min) and MS–MS on FAMEs molecular ions-high mass fragments transitions (3–4 min), (see bottom right panel) were recorded, respectively. The sample was injected with split 20 at 280 ◦ C injector temperature and standard 3800 GC oven temperature at 290 ◦ C. The short fast GC column flow rate was 7 mL/min.
time. Furthermore, sample preparation currently seem to be the bottle neck in attempts to reduce the full time of pesticide analysis, but the recent advances in QuEChERS sample preparation [36,37] and the fact that sample preparation can be performed in batches and by few people (or automated in the future) brings back the need for faster GC–MS. In Fig. 5, fast pesticide analysis in 1 min full analysis cycle time is demonstrated. Results of two consecutive analyses are shown in Fig. 5. The first analysis is via full scan fast GC–MS with SMB and the second run is of fast GC–MS–MS with SMB while using the molecular ions m/z = 329 of iprodion as the parent ion and m/z = 314 as the daughter ion and m/z = 415 which is the molecular ion of cypermethrin as the parent ion and m/z = 163 as the daughter ion for that pesticide. Both iprodion and cypermethryn were independently found at 0.23 and 0.22 mg/kg in grape extract. (sample with authentic pesticides was kindly donated by Paulina Goldshlag from the Israel Plant Protection Center). The bottom trace shows the cold EI mass spectrum of iprodion which could be extracted from the congested TIC. Our suggested strategy for fast pesticide analysis is to have a 1 min full scan analysis, as demonstrated in Fig. 5, followed by any number of pesticides screening using 2 ions RSIM on the molecular ion and a one high mass fragment ion, as explored in details before [20]. While the enhancement of the molecular ion in cold EI is helpful the reduced separation of the GC short column implies increased rate of false positive. Thus, the second 1 min run can serve for fast GC–MS–MS with SMB confirmation. Currently there is no software that can perform the needed screening in a few seconds for launching an appropriate MS–MS method but the
MS–MS run can be performed any time and not necessarily right after the sample analysis in which the suspected pesticides were found. Note that even if some people may not accept the 1 min full scan method as a viable opportunity, one can use a more standard 8 min fast GC–MS with SMB [20], or even GC–MS with SMB with standard column and analysis time and operate the confirmation fast GC–MS–MS with SMB during the end of the run plateau, in which the standard column is being cleaned from matrix by the end of run high temperature or via backflush. In that case a second injector can be used for performing fast GC–MS–MS with SMB for confirmation of the pesticides found in the last run or previous runs. This way the fast MS-MS confirmation does not take any additional time since it is performed during the run of the other column, while the 2 columns can be simultaneously connected to the supersonic nozzle with its unlimited flow rate acceptance. We explored several combinations of pesticides and matrixes including also authentic phosmet and azoxystrobin in apricot while using a ChromatoProbe for sample introduction [38] which enables the use of minimal sample preparation [38,39]. Note that our fast GC–MS with SMB can also be used for the analysis of such thermally labile compound, as Iprodion, which cannot be properly analyzed with standard GC–MS due to its injector and column degradation. Another application that clearly benefits from fast analysis is explosives analysis, which was already explored with the Supersonic GC–MS [27]. In Fig. 6, fast, 1 min cycle time analysis of explosive mixture is demonstrated. Triacetonetriperoxide (TATP), trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX)
9380
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
Fig. 6. One minute analysis of explosives TATP, TNT and RDX. The upper trace demonstrates 50 s total ion chromatography within 60 s full analysis cycle time. The bottom traces show the obtained mass spectra with enhanced molecular ions. The sample was injected with split 9 at 170 ◦ C injector temperature and standard 3800 GC oven temperature at 190 ◦ C. The short fast GC column flow rate was 16 mL/min. Fig. 5. Pesticides analysis with fast GC–MS with SMB. Authentic Iprodion (0.23 mg/kg) and Cypermethrin (0.22 mg/kg) in grapes were analyzed in full scan (first minute) followed by fast GC–MS–MS analysis in the second minute. The broken line in the upper trace provides the temperature program used for consequent runs. The elution times of the pesticides in the first run are pointed. Iprodion was sufficiently separated from the matrix thus provided library searchable mass spectrum with enhanced molecular ion (bottom trace). The second run recorded from 1.0 min to 2.0 min was for MS–MS on molecular ions-high mass fragments transitions. The sample was injected splitless for 0.1 min at 220 ◦ C injector temperature and standard 3800 GC oven temperature of 300 ◦ C. The short fast GC column flow rate was ∼32 mL/min.
were analyzed and despite being thermally labile (TATP), both TATP conformers are shown in Fig. 6 and a dominant molecular ion is exhibited. Similarly, an abundant molecular ion is exhibited in cold EI for TNT while for RDX a small molecular ion plus an abundant high mass fragment are observed. While there is a clear need for fast explosive screening such analysis also includes sample collection and injection or introduction. Recently we developed a new device that enables fast sampling and sample introduction, named open probe [40], and work is underway to combine the open probe with our fast GC–MS with SMB in order to facilitate fast explosive analysis all the way for sample collection through data analysis. 4. Discussion Resistive heating as a concept for fast GC has been well established, but not widely practiced. This trend changed with the invention of Low Thermal Mass (LTM) fast GC by Mustacich et al. [41] with Agilent Technologies currently owns the intellectual properties associate with that innovation. Thus, LTM fast GC is now a widely used technology which is available by several vendor including Agilent, Thermo, Leco and VICI Valco to name a few. However, currently LTM fast GC is not yet widely used in combination
with mass spectrometry since as discussed in this manuscript fast GC–MS is very different from fast GC in view of availability of mass spectrometry separation and flow rate constraints. Thus, our LTM Fast GC–MS with SMB is aimed at advancing LTM fast GC–MS and in exploring a few of its important applications. While the essence of our fast GC is shown in Fig. 1 and described above, additional details can further help in the illumination of a few more aspects and advantages of our LTM fast GC: (A) Ultra fast GC and GC–MS: As demonstrated, our fast GC–MS approach provides a solution for all the requirements of fast analysis including fast high flow rate splitless injections, fast temperature programming rate and cooling down with our low thermal mass resistively heated metal tube, tailing-free ultra fast ion source response time and increased MS separation power through the provision of enhanced molecular ions with the Supersonic GC–MS. Thus, sub 1 min total analysis cycle time can be routinely obtained, and, when only a limited GC column temperature range is needed, analysis cycle time of only a few seconds can be obtained. (B) High carrier gas flow rate: The carrier gas flow rate as used in the examples given above was typically in the range of 20–40 mL/min. This approach represents a reduction in the fast GC separation power for achieving faster analysis, greater sample capacity and linear dynamic range (LDR), and column robustness as well as extended range of compounds amenable for analysis. The reduced fast GC separation is compensated by increased separation power of the mass spectrometer with supersonic molecular beams that serves as the detector. Similarly, the fast GC column is short, typically with only 1.7 m column length. Column length reduction is required to enable the desirable high column flow rate and facilitate ultra fast GC
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
(C)
(D)
(E)
(F)
(G)
(H)
separation. In addition, it enables easier column replacement into the heating metal tube which is practically impossible with standard 30 m long columns. Consequently, the combination of short column, high column flow rate and low thermal mass of the resistively heated metal tube enables a full analysis cycle time of less than 1 min. High sample capacity and linear dynamic range: The use of high column flow rate and standard 0.25 mm ID column increases the sample capacity by about two orders of magnitude in comparison with the use of microbore columns with 0.1 mm ID and 1 mL/min or less column flow rate. Sample capacity depends on the volume of the adsorption film of a separation plate, hence it usually increases with the third power of the column diameter and linearly with the column flow rate which are increased by about two orders of magnitude in our experiments. In addition, the linear dynamic range (LDR) similarly increases with the sample capacity since more sample can be injected without peak shape distortion with the same limit of detection. This improved LDR and sample capacity are important for broad range of practical applications. Extended range of compounds amenable for analysis: The use of a very short column and high column flow rate provides extended range of compounds amenable for analysis. For example, we analyzed a mixture of hexadecane, anthracene, methylstearate, cholesterol and a heavy compound with molecular weight of 774 (C54 H78 O3 ) in less then 20 s while the last compound and even cholesterol are not amenable for certain other types of fast GC. Flexible column connections and easy column insertion: One of the important features of our fast GC is that the capillary column is not rigidly connected to the metal tube and can freely slide (inserted) into it. Therefore, the column can be pushed into the heated metal tube or pulled out and as a result column replacement can be easily performed. Our method of fast GC requires that the standard GC oven will serve as a part of the transfer line from the injector and to the detector during the fast GC operation, hence it must be heated to a temperature around that of the detector transfer line such as in the range of 250–350 ◦ C. Since such high temperature can lead to column bleed, the analytical column of the fast GC can also be connected to the injector and detector via unions and deactivated fused silica capillary transfer lines. Mounting on the top of the GC: Our fast GC is based on the mounting of the fast GC module on top of the oven of the standard GC. Consequently, the fast GC is typically interfaced with the standard GC through available holes for a second injector or detector. As a result, the standard GC oven door can be fully and freely open as usual for service, including for column assembly and its connection and insertion into the injector and detector. This arrangement also implies that the fast GC module is relatively small and that its control electronics is separated from it and conveniently located at a side of the GC. Increased fast GC column lifetime: Since the separation plate height is increased linearly with the flow rate, the use of high column flow rate increases the column sample capacity, system linear dynamic range and column robustness and lifetime. Furthermore, since the column freely extends beyond the heating metal tube it can be cut as needed to remove its upstream portion near the injector which is periodically contaminated by low volatility matrix compounds and dirt. Lower cost of use: The price of columns for our fast GC is far lower than with most other fast GCs since our replacement column is just a piece of any standard capillary GC column of choice, the column lifetime is much longer as explained above, and a new short capillary column can be inserted many times into the fast GC oven tube for new use.
9381
(I) Supersonic GC–MS advantages: Our fast GC is uniquely compatible and combined with the Supersonic GC–MS as demonstrated in the results described above. As a result, it forms a powerful fast GC–MS in terms of speed of analysis, sensitivity, range of compounds amenable for analysis and improved sample identification capability. Thus, we named it Supersonic Fast GC–MS. (J) Combined fast GC–MS and standard GC–MS in a single system: Our fast GC when coupled with the Supersonic GC–MS is further uniquely characterized by having the ability to connect two columns to the same MS via a simple double hole ferrule at the GC end of the heated transfer line since the Supersonic GC–MS can handle any column flow rate. Thus, one can use the fast GC module in combination with a standard 30 m GC column that is connected from a second standard GC injector. As a result, our fast GC–MS can uniquely serve for fast screening, while in the case of detection of a suspected compound, the method can be changed by a click of the mouse and the screening can be followed by a confirmation run with increased separation power of a standard GC column.
Standard GC is typically operated with 30 m long capillary columns with 0.25 mm ID and 1 mL/min column flow rate. According to the theory of GC, the maximum allowed temperature programming rate in which the column separation is retained is 10 ◦ C per void time [42] which for the standard column and flow rate as above is ∼90 s. As a result, the maximum temperature programming rate which is used in combination with full standard GC separation power is 7 ◦ C/min. With such temperature programming rate, full temperature range analysis from 50 ◦ C to 350 ◦ C sums up from: 1 min for splitless injection at 50 ◦ C, 43 min temperature programming to 350 ◦ C, 2 min at 350 ◦ C and about 4 min cooling down and equilibration time with total analysis cycle time of 50 min. Obviously, most analysis can employ 10 ◦ C/min with only negligible/minor loss in separation resolution and with a typical range of temperature program of only 250 ◦ C such analysis can take only 30 min. If reduced GC separation can be tolerated, 5 m columns can be used with 60 ◦ C/min temperature programming rate and the total analysis time can be reduced even to 10 min with standard GC but with a loss of more than a factor of 6 in the number of separation plates and about a factor of 2.5 in peak capacity. The standard column with 0.25 mm can be replaced with a short microbore column, which can provide such fast 10 min analysis with smaller loss in the GC separation capability but with a major loss in sample capacity, column robustness, linear dynamic range and column lifetime. In fact, the use of such microbore columns defies, in our opinion, the purpose of fast GC since the combination of reduced column life time with reduced analysis time implies that the column replacement frequency will be significantly increased along with its associated cost and downtime. It is important to note that while standard GC is typically operated with a temperature programming rate in the range of 5–10 ◦ C/min the standard GC oven is capable of temperature programming rate of up to 120 ◦ C at low temperatures and 50–60 ◦ C around 300 ◦ C. However, if 60 ◦ C/min is used with standard columns the analysis will be hampered by a loss factor of about 8 in the number of separation plates, since most of the time the sample compounds will not be retained in the too hot column and only the initial portion of the column will be used for effective separation. Furthermore, the elution temperatures will be increased by over 60 ◦ C (20 ◦ C per each factor of two increased temperature programming rate above 7 ◦ C/min [27]) and as a result thermally labile compounds will degrade, low volatility compounds will elute at the high temperature plateau end of the run (or will not elute) with increased column bleed and ghost peaks chemical noise, and thus, the range of compounds amenable for analysis will be sacrificed and reduced.
9382
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383
In 1996 Dagan and Amirav [18] classified fast GC–MS as fast, very fast and ultra fast GC–MS based on a speed enhancement factor (SEF) which is the void time reduction factor versus standard GC–MS operation, meaning the reduction of the column length, times column flow rate increase, versus standard 30 m 0.25 mm ID column with 1 mL/min column flow rate. In 1998 Blumberg and Klee introduced another measure of the speed of a GC analysis [43]. They took the peak width as a measure of analysis speed and defined “A fast capillary GC analysis as one with the average peak width of less than 1 s”. They also classified approaches involving an average peak width of around 100 ms as Superfast GC and those in which it is below 10 ms as Ultrafast GC. Magni et al. [44] went into more details and defined “Fast GC” as an analysis performed in less than 10 min with columns with i.d. between 0.25 mm and 0.1 mm, length from 5 m to 15 m, temperature programming rates of 20–60 ◦ C/min and peak widths in the range between 0.5 s and 2 s. They used the term Ultrafast GC for analyses of 1 min or less, entailing the use of short (2–10 m) narrow-bore columns (0.1–0.05 mm i.d.) and temperature programming rates above 1 ◦ C/s, leading to peak widths of 50–200 ms. Later on Bicchi et al. followed Magni et al. in their terminology. In our opinion fast GC–MS is different than fast GC in that due to the added separation power of the MS (plus enhanced molecular ion and MS–MS in this manuscript) the emphasis in the terminology should be given to the total analysis cycle time and not to GC peak width. The total analysis cycle time relate more closely to the Dagan and Amirav SEF factor which in our experiment is 600 hence closer to ultra fast GC–MS than to very fast GC–MS. According to our preferred method of operation of our fast GC we use 2.5 m, 0.25 mm ID capillary column with 20–30 mL/min column flow rate, from which only 1.7 m is inside the TLM fast GC heating tube hence active in the separation. Even bigger capillary columns with 0.53 mm ID can be used with the advantage of having larger column capacity and much lower flow impedance. Due to the combination of column length reduction by a factor of ∼20 and column carrier gas flow rate increase by a factor of 20 the number of separation plates is significantly reduced by about a factor of 400 which results in a loss of GC separation resolution and peak capacity by a factor of ∼20. Such loss cannot be tolerated by most standard GC analysis requirements. However, when fast GC–MS analysis is considered, particularly with the Supersonic GC–MS and/or GC–MS–MS, the merits of our method and device far outweighs its only limitation in reduced GC separation capability. The use of 1.5 m column with 20–30 mL/min column flow rate results in the reduction of the void time by a huge factor of 400. Consequently a temperature programming rate of 45 ◦ C/s (2700 ◦ C/min) can be used and full range temperature programming can take only 6–7 s. However, since even with the low thermal mass fast GC heating tube, cooling down may take 10 s, only limited gain in time is achieved by using such fast temperature programming rate. A better method of fast GC operation is based on using a temperature programming rate of 10 ◦ C/s which results in 40 ◦ C lower elution temperatures to significantly increase the range of compounds amenable for analysis. Consequently, full analysis cycle takes a few seconds for sample thermal-focusing, 40 s temperature programming, 10 s cooling down time and less than 5 s equilibration time for the total of less than 1 min per analysis, which is an order of magnitude faster than can be achieved with standard GC. It is important to realize that the use of high column flow rate is essential for having a useful fast GC–MS since flow rate reduces the void time, thereby enabling fast temperature programming rate combined with increased range of compounds amenable for analysis as opposed to the reduced range encountered with other fast GC methods and devices. An additional very important attribute of the use of high column flow rate is that standard sample injections are correspondingly faster, and with 30 mL/min standard splitless
injection may take only two seconds instead of the usual 1 min. Without such injection time reduction, splitless (or split with low split ratio) injection may either prohibit fast GC cycle time of less than 1 min or correspondingly reduce the sensitivity (with high split ratio split injections) which is unacceptable for many types of trace level analyses. Finally, we note that the ability to use very high column flow rates enables the use of effective flow programming with large high to low flow rates ratios, which provides a faster alternative to temperature programming rate and eliminates the time needed for cooling back. Thus, flow programming can provide an ultra fast alternative to fast temperature programming when only a limited temperature range of up to 100 ◦ C is needed. 5. Conclusions Fast GC–MS with SMB with full analysis cycle time of 1 min was demonstrated for several selected applications. Our fast GC is based on the use of standard short capillary column that is inserted in a resistively heated stainless steel tube with low thermal mass. Our fast GC method is uniquely based on having high column flow rate with a trade off of GC separation for speed while combining it with mass spectrometry with Supersonic molecular beams that provides enhanced molecular ions hence enhanced MS separation. Such mass spectrometry is unique in its compatibility with high column flow rate. Thus, the combination of our LTM fast GC and Supersonic GC–MS provides a powerful Supersonic Fast GC–MS as demonstrated in five challenging applications. Acknowledgements This research was supported by the Israel Science Foundation founded by the Israel Academy of Sciences and Humanities (grant No. 1172/07 and 393/11). This research was also supported by a Research Grant Award No. US-4273-09 from BARD, the United States – Israel Binational Agricultural Research and Development Fund, by the James Franck Center for Laser Matter Interaction Research and by the infrastructure research program of the ministry of science and technology (MOST) of the state of Israel. References [1] D. Rounbelher, E. Achter, D. Fine, G. Jarvis, S. Macdonald, D. Wheeler, C. Wood, US Patent No. 5,808,178, (1998). [2] S.J. Macdonald, D. Wheeler, Am. Lab. 30 (1998) 27. [3] G. Frishman, A. Amirav, Field. Anal. Chem. Technol. 4 (2000) 170. [4] G. Frishman, N. Tzanani, A. Amirav, Field. Anal. Chem. Technol. 5 (2001) 107. [5] B.J. Hopkins, V. Pretorius, J. Chromatogr. 158 (1978) 465. [6] B.A. Ewels, R.D. Sacks, Anal. Chem. 57 (1985) 2774. [7] M.E. Hail, R.A. Yost, Anal. Chem. 61 (1989) 2410. [8] E.U. Ehrmann, H.P. Dharmasena, K. Carney, E.B. Overton, J. Chromatogr. Sci. 34 (1996) 533. [9] K.M. Sloan, R.V. Mustacich, B.A. Eckenrode, Field Anal. Chem. Technol. 5 (2001) 288. [10] G. Sacks, T. Brenna, Am. Lab. 35 (2003) 22. [11] C. Bicchi, C. Brunelli, C. Cordero, P. Rubiolo, M. Galli, A. Sironi, J. Chromatogr. A 1024 (2004) 195. [12] C.A. Cramers, P.A. Leclecq, J. Chromatogr. A 842 (1999) 3. [13] K. Mastovska, S.J. Lehotay, J. Chromatogr. A 1000 (2003) 153. [14] M. Kirchner, E. Matisova, Chem. Listy 99 (2005) 789. [15] J. de Zeeuw, J. Peene, H.G. Jansen, X.W. Lou, HRC-J. High Res. Chromatogr. 23 (2000) 677. [16] S. Dagan, A. Amirav, Int. J. Mass. Spectrom. Ion Proc. 133 (1994) 187. [17] A. Amirav, A. Gordin, M. Poliak, A.B. Fialkov, J. Mass Spectrom. 43 (2008) 141. [18] S. Dagan, A. Amirav, J. Am. Soc. Mass Spectrom. 7 (1996) 737. [19] A. Amirav, A. Gordin, N. Tzanani, Rapid. Commun. Mass Spectrom. 15 (2001) 811. [20] M. Kochman, A. Gordin, P. Goldshlag, S.J. Lehotay, A. Amirav, J. Chromatogr. A 974 (2002) 185. [21] A.B. Fialkov, A. Amirav, J. Chromatogr. A 1058 (2004) 233. [22] A.B. Fialkov, U. Steiner, L. Jones, A. Amirav, Int. J. Mass Spectrom. 251 (2006) 47. [23] A.B. Fialkov, A. Gordin, A. Amirav, J. Chromatogr. A 1195 (2008) 127. [24] A. Amirav, A.B. Fialkov, A. Gordin, Rev. Sci. Instrum. 73 (2002) 2872.
A.B. Fialkov et al. / J. Chromatogr. A 1218 (2011) 9375–9383 [25] A. Gordin, A. Amirav, A.B. Fialkov, Rapid. Commun. Mass Spectrom. 22 (2008) 2660. [26] T. Alon, A. Amirav, Rapid Commun. Mass Spectrom. 20 (2006) 2579. [27] A.B. Fialkov, A. Gordin, A. Amirav, J. Chromatogr. A 991 (2003) 217. [28] A. Amirav, A.B. Fialkov, Fast gas chromatograph method and device for analyzing a sample in a supersonic molecular beam US Patent application number 12/899,288, (2010). [29] C. Cruz-Hernandez, J. Liq. Chromatogr. Related Technol. 32 (2009) 1672. [30] L. Mondello, P.Q. Tranchida, R. Costa, A. Casilli, P. Dugo, A. Cotroneo, G. Dugo, J. Sep. Sci. 26 (2003) 1467. [31] J.V. Seeley, S.K. Seeley, E.K. Libby, J.D. McCurry, J. Chromatogr. Sci. 45 (2007) 650. [32] F. Adam, F. Bertoncini, V. Coupard, N. Charon, D. Thiebaut, D. Esoinat, M.C. Hennion, J. Chromatogr. A 1186 (2008) 236. [33] R.S. Sheridan, J.R. Meola, J. AOAC Int. 82 (1999) 982. [34] K. Mastovska, J. Hajslova, S.J. Lehotay, J. Chromatogr. A 1054 (2004) 335. [35] T. Cajka, J. Hajlova, O. Lacina, K. Mastovska, S.J. Lehotay, J. Chromatogr. A 1186 (2008) 281.
9383
[36] M. Anastassiades, S.J. Lehotay, D. Stainbaher, F.J. Schenck, J. AOAC Int. 86 (2003) 412. [37] S.J. Lehotay, K.A. Son, H. Kwon, U. Koesukwiwat, W.S. Fu, K. Mastovska, E. Hoh, N. Leepipatpiboon, J. Chromatogr. A 16 (2010) 2548. [38] A. Amirav, S. Dagan, Eur. Mass Spectrom. 3 (1997) 105. [39] H. Jing, A. Amirav, Anal. Chem. 69 (1997) 1426. [40] M. Poliak, A. Gordin, A. Amirav, Anal. Chem. 82 (2010) 5777. [41] R.V. Mustacich, US patents 5782964 (1998), 6209386 (2001) and 6217829 (2001). [42] L.M. Blumberg, M.S. Klee, J. Microcolumn Sep. 12 (2000) 508. [43] L.M. Blumberg, M.S. Klee, in: P. Sandra (Ed.), Proceedings of 20th International Symposium of Capillary Chromatography, Riva del Garda, Italy, 26–29 May 1998, 1998, p. PL9. [44] P. Magni, R. Facchetti, D. Cavagnino, S. Trestianu, in: P. Sandra (Ed.), Proceedings of 25th International Symposium of Capillary Chromatography, Riva del Garda, Italy, 13–17 May 2002, 2002, p. KLN05, and references cited therein.