Gas chromatography-mass spectrometry with supersonic molecular beams

Gas chromatography-mass spectrometry (GC-MS) with supersonic molecular beams (SMBs) (also named Supersonic GC-MS) is based on GC and MS interface with SMBs and on the electron ionization (EI) of vibrationally cold analytes in the SMBs (cold EI) in a fly-through ion source. This ion source is inherently inert and further characterized by fast response and vacuum background filtration capability. The same ion source offers three modes of ionization including cold EI, classical EI and cluster chemical ionization (CI). Cold EI, as a main mode, provides enhanced molecular ions combined with an effective library sample identification, which is supplemented and complemented by a powerful isotope abundance analysis method and software. The range of low-volatility and thermally labile compounds amenable for analysis is significantly increased owing to the use of the contact-free, fly-through ion source and the ability to lower sample elution temperatures through the use of high column carrier gas flow rates. Effective, fast GC-MS is enabled particularly owing to the possible use of high column flow rates and improved system selectivity in view of the enhancement of the molecular ion. This fast GC-MS with SMB can be further improved via the added selectivity of MS-MS, which by itself benefits from the enhancement of the molecular ion, the most suitable parent ion for MS-MS. Supersonic GC-MS is characterized by low limits of detection (LOD), and its sensitivity is superior to that of standard GC-MS, particularly for samples that are hard for analysis. The GC separation of the Supersonic GC-MS can be improved with pulsed flow modulation (PFM) GC x GC-MS. Electron ionization LC-MS with SMB can also be combined with the Supersonic GC-MS, with fast and easy switching between these two modes of operation.     

Identification of novel synthetic organic compounds with supersonic gas chromatography-mass spectrometry

Several novel synthetic organic compounds were successfully analyzed with a unique type of GC-MS titled Supersonic GC-MS following a failure in their analysis with standard GC-MS. Supersonic GC-MS is based on interfacing GC and MS with a supersonic molecular beam (SMB) and on electron ionization of sample compounds as vibrationally cold molecules while in the SMB, or by cluster chemical ionization. The analyses of novel synthetic organic compounds significantly benefited from the extended range of compounds amenable to analyses with the Supersonic GC-MS. The Supersonic GC-MS enabled the analysis of thermally labile compounds that usually degrade in the GC injector, column and/or ion source. Due to the high carrier gas flow rate at the injector liner and column these compounds eluted without degradation at significantly lower elution temperatures and the use of fly-through EI ion source eliminated any sample degradation at the ion source. The cold EI feature of providing trustworthy enhanced molecular ion (M+), complemented by its optional further confirmation with cluster CI was highly valued by the synthetic organic chemists that were served by the Supersonic GC-MS. Furthermore, the provision of extended mass spectral structural, isomer and isotope information combined with short (a few minutes) GC-MS analysis times also proved beneficial for the analysis of unknown synthetic organic compounds. As a result, the synthetic organic chemists were provided with both qualitative and quantitative data on the composition of their synthetic mixture, and could better follow the path of their synthetic chemistry. Ten cases of such analyses are demonstrated in figures and discussed.     

Fast,high-sensitivity,multipesticide analysis of complex mixtures with supersonic gas chromatography-mass spectrometry

We developed a new instrumental approach, termed Supersonic GC-MS, which achieves fast, sensitive, confirmatory and quantitative analysis of a broad range of pesticides in complex agricultural matrices. Our Supersonic GC-MS system is a modification of a bench-top Agilent 6890 GC+5972 MSD with a supersonic molecular beam (SMB) interface and fly-through EI ion source. One of the main advantages of Supersonic GC-MS is an enhanced molecular ion (M+) in the resulting mass spectra. For example, the M+ was observed in all 88 pesticides that we studied using the Supersonic GC-MS whereas only 36 of 63 (57%) pesticides that we investigated in standard GC-MS exhibited a M+. We also found that the degree of matrix interference is exponentially reduced with the fragment mass by about 20-fold per 100 amu increasing mass. The enhancement of the M+ combined with the reduction in matrix background noise permit rapid full scan analysis of a potentially unlimited number of pesticides, unlike selected ion monitoring or MS-MS in which specific conditions are required in segments for targeted pesticides. Furthermore, unlike the case with chemical ionization, EI-SMB-MS spectra still give accurate identification of compounds using common mass spectral libraries. In practice,we found thatlibraries favor mass spectra in which the M+ appears, thus Supersonic GC-MS produced better spectra for compound identification than standard GC-MS. To achieve even lower identification limits, the M+ plus a second major ion (still using full scan data) gives higher signal-to-chemical noise ratios than the traditional 3-ion approach. The replacement of two low-mass ions with the M+ (supersonic two-ions method) results in a significant reduction of matrix interference by a factor of up to 90. Another main advantage of Supersonic GC-MS is its exceptional suitability for fast GC-MS with high carrier gas flow-rate. Fast Supersonic GC-MS was able to analyze thermally labile pesticides, such as carbamates, that are difficult or impossible to analyze in standard GC-MS. Large volume injection using a ChromatoProbe was also demonstrated, in the 6 min analysis of pesticides at 20 ng/g in a spice matrix.     

A low thermal mass fast gas chromatograph and its implementation in fast gas chromatography mass spectrometry with supersonic molecular beams

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 C(10)H(22) up to C(44)H(90) range was achieved. The use of 35 mL/min high column flow rate enabled the elution of C(44)H(90) 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.     

GC–MS with photoionization of cold molecules in supersonic molecular beams—Approaching the softest ionization method

A new type of photoionization ion source was developed for the ionization of cold molecules in supersonic molecular beams (named Cold PI). The system was based on a GC–MS with supersonic molecular beams and its fly‐through EI of cold molecules ion source (Cold EI) plus quadrupole mass analyzer. A continuously operated deuterium VUV photoionization lamp was added and placed above and between the supersonic nozzle and skimmer whereas the Cold EI ion source served only as a portion of the ion transfer ion optics. The supersonic nozzle and skimmer were voltage biased and the VUV light crossed the supersonic expansion about 10 mm from the nozzle. We obtained over three orders of magnitude enhancement in the relative abundance of the molecular ion of squalane in Cold PI versus in photoionization of this compound as a thermal compound. Accordingly, we also proved that standard photoionization is not as soft ionization method as previously perceived for large compounds. We found that Cold PI is as soft as and possibly softer than field ionization; thus, it could be the softest known ionization method. The ionization yield was about 200–300 times weaker than with Cold EI yet our limit of detection was about 200 femtogram in SIM mode for cholesterol and pyrene which is reasonable. Practically, all hydrocarbons gave only molecular ions with rather uniform response whereas alcohols gave some molecular ions plus major fragment ions particularly with a loss of water (similarly to field ionization). We tested Cold PI in the GC–MS analysis of diesel fuels and analyzed the time averaged data for group type information. We also found that we can analyze the diesel fuels by fast under 20‐s flow injection analysis in which the generated averaged mass spectrum of molecular ions only could serve for the characterization of fuels.     

Comparing the weathering patterns of six oils using 3-way generalized Procrustes rotation and matrix-augmentation principal components

A case study is presented in which advanced chemical fingerprinting and data interpretation techniques are used to characterize and compare the weathering processes of six oil spillages made under controlled conditions (including the heavy oil released after the Prestige tanker sunk off the Galician coast-NW Spain on 2002). A tiered analytical approach using gas chromatography-flame ionization detection (GC-FID) and gas chromatography-mass spectrometry (GC-MS) was applied along with two different approaches for 3-way analyses; namely, generalized Procrustes rotation, and matrix-augmented principal components analysis. Results showed that the two 3-way chemometric techniques leaded to essentially the same conclusions when analyzing three sets of compounds fingerprinting the spilled hydrocarbons (aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and a set of diagnostic ratios). A steady evolution on the weathering of the oils was observed with both techniques, but for the diagnostic ratios. The variables involved on the weathering were the lightest aliphatic hydrocarbons and a general combination of the PAHs, which differentiated mostly among the light and the heavy products (fuel oils).     

Mass spectral determination of long-chain quaternary amines in mixtures

The analytical utility of several ionization techniques, which are known for their abilities to produce molecular ions of non-volatile/thermally labile compounds, is compared for the determination of the composition of quaternary ammonium salt mixtures derived from beef tallow. The ionization techniques include direct-exposure chemical ionization, field desorption, and fast atom bombardment. While all three techniques produce molecular cations from the salts, fast atom bombardment produces these in greatest relative abundance. Both fast atom bombardment and direct-exposure chemical ionization can be used for characterization of the composition of chain lengths in the mixture. Fast atom bombardment also produces different fragmentation/decomposition ions (imminium ions) than direct-exposure chemical ionization (protonated tertiary amines).     

Analysis of polycyclic aromatic hydrocarbons by ion trap tandem mass spectrometry

An ion-trap mass spectrometer with a wave board and tandem mass spectrometry software was used to analyze gas chromatographically separated polycyclic aromatic hydrocarbons (PAHs) by using collision-induced dissociation (CID). The nonresonant (multiple collision) mode was used to determine the conditions for CID ionization of 18 PAHs. Unlike in electron impact (EI) analysis, the relative abundances of progeny ions of isomers were statistically different (using Student’s t-test) in CID analysis, thus making isomer identification by CID possible. For comparison, CID and EI were applied to the analysis of used motor oil. CID analysis was shown to be more sensitive than EI analysis of the used motor oil. Precision at the 10-ppb level for EI and CID showed relative standard deviations of 5. 2 and 7. 7%, respectively.     

全二维气相色谱-飞行时间质谱联用技术分析重馏分油中芳烃组成

通过对升温速度、二维补偿温度、调制周期等关键实验参数的优化,建立了全二维气相色谱-飞行时间质谱(GC×GC-TOF MS)分析重馏分油中芳烃组分的方法,得到了重馏分油芳烃组分按环数分布的二维点阵图。根据谱库检索、标准化合物对照及文献报道,对重馏分油芳烃组分中菲、甲基菲及芘、苯并蒽等常见多环芳烃(PAH)进行了准确定性,并将该方法应用到重馏分油加氢处理工艺研究中,对菲、芘的加氢处理产物进行了定性分析。该研究为重馏分油芳烃组分的准确定性提供了新的技术手段,为加深对油品加氢规律的认识提供了技术支持。全二维气相色谱与普通一维色谱对比,在重馏分油的芳烃组分分析上体现了极大优势。     

Fast,very fast,and ultra-fast gas chromatography-mass spectrometry of thermally labile steroids,carbamates, and drugs in supersonic molecular beams

Gas chromatography-mass spectrometry (GC-MS) analyses of thermally labile compounds have been studied by using a short column fast gas chromatograph, coupled with fly-through electron ionization in supersonic molecular beams. Thirty-two compounds, which include steroids, carbamate pesticides, antibiotic drugs, and other pharmaceutical compounds, have been analyzed and the details of their GC-MS analysis are provided. The ability to analyze thermally labile compounds is discussed in relation to the speed of analysis. A new term, “speed enhancement factor” (SEF), is defined as the product of column length reduction and the carrier gas linear velocity increase, as compared with normal GC-MS conditions. Fast, very fast, and ultra-fast GC-MS are defined with a SEF in the ranges of 5–30, 30–400, and 400–4000, respectively. Trade-offs in the degree of dissociation, speed, gas chromatograph resolution, and sensitivity were studied and examined with thermally labile molecules. The experimental factors that affect the dissociation are described with emphasis on its reduction. We claim that the use of supersonic molecular beams for sampling and ionization provides the ultimate capability in the GC-MS of thermally labile compounds. The obtained 70-eV electron ionization mass spectra are shown, and an enhanced relative abundance of the molecular ion is demonstrated together with library search capability of these mass spectra, which is better than that reported with particle beam liquid chromatography-mass spectrometry. The performance of fast GC-MS in supersonic molecular beams is compared with other methods of fast GC-MS and with particle beam liquid chromatography-mass spectrometry.     

Extending the range of compounds amenable for gas chromatography-mass spectrometric analysis

Gas chromatography-mass spectrometry (GC-MS) suffers from a major limitation in that an expanding number of thermally labile or low volatility compounds of interest are not amenable for analysis. We found that the elution temperatures of compounds from GC can be significantly lowered by reducing the column length, increasing the carrier gas flow rate, reducing the capillary column film thickness and lowering the temperature programming rate. Pyrene is eluted at 287 degrees C in standard GC-MS with a 30 m x 0.25 mm I.D. column with 1-microm DB5ms film and 1-ml/min He column flow rate. In contrast, pyrene is eluted at 79 degrees C in our "Supersonic GC-MS" system using a 1 m x 0.25 mm I.D. column with 0.1-microm DB5ms film and 100-ml/min He column flow rate. A simple model has been invoked to explain the significantly (up to 208 degrees C) lower elution temperatures observed. According to this model, every halving of the temperature programming rate, or number of separation plates (either through increased flow rate or due to reduced column length), results in approximately 20 degrees C lower elution temperature. These considerably lower elution temperatures enable the analysis of an extended range of thermally labile and low volatility compounds, that otherwise could not be analyzed by standard GC-MS. We demonstrate the analysis of large polycyclic aromatic hydrocarbons (PAHs) such as decacyclene with ten fused rings, well above the current GC limit of PAHs with six rings. Even a metalloporphirin such as magnesiumoctaethylporphin was easily analyzed with elution temperatures below 300 degrees C. Furthermore, a range of thermally labile compounds were analyzed including carbamates such as methomyl, aldicarb, aldicarbsulfone and oxamyl, explosives such as pentaerythritol tetranitrate, Tetryl and HMX, and drugs such as reserpine (608 a.m.u.). Supersonic GC-MS was used, based on the coupling of a supersonic molecular beam (SMB) inlet and ion sources with a bench-top Agilent 6890 GC plus 5972 MSD. The Supersonic GC-MS provides enhanced molecular ion without any ion source related peak tailing. Thus, the lower GC separation power involved in the analysis of thermally labile and low volatility compounds is compensated by increased separation power of the MS gained from the enhanced molecular ion. Several implications of these findings are discussed, including our conclusion that slower chromatography leads to better analysis of thermally labile compounds.     

Compositional elucidation of heavy petroleum base oil by GC × GC‐EI/PI/CI/FI‐TOFMS

Comprehensive two‐dimensional gas chromatography (GC × GC) coupled to time‐of‐flight mass spectrometry is a powerful separation tool for complex petroleum product analysis. However, the most commonly used electron ionization (EI) technique often makes the identification of the majority of hydrocarbons impossible due to the exhaustive fragmentation and lack of molecular ion preservation, prompting the need of soft‐ionization energies. In this study, three different soft‐ionization techniques including photo ionization (PI), chemical ionization (CI), and field ionization (FI) were compared against EI to elucidate their relative capabilities to reveal different base oil hydrocarbon classes. Compared with EI (70 eV), PI (10.8 eV) retained significant molecular ion (M^+·) information for a large number of isomeric species including branched‐alkanes and saturated monocyclic hydrocarbons along with unique fragmentation patterns. However, for bicyclic/polycyclic naphthenic and aromatic compounds, EI played upper hand by retaining molecular as well as fragment ions to identify the species, whereas PI exhibited mainly molecular ion signals. On the other hand, CI revealed selectivity towards different base oil groups, particularly for steranes, sulfur‐containing thiophenes, and esters, yielding protonated molecular ions (M + H)⁺ for unsaturated and hydride abstracted ions (M‐H⁺) for saturated hydrocarbons. FI, as expected, generated intact molecular ions (M^+·) irrespective to the base oil chemical classes. It allowed elemental composition by TOFMS with a mass resolving power up to 8000 (FWHM) and a mass accuracy of 1 mDa, leading to the calculation of heteroatomic content, double bond equivalency, and carbon number of the compounds. The qualitative and quantitative results presented herein offer a unique perspective into the detailed comparison of different ionization techniques corresponding to several hydrocarbon classes.     

Characterization of complex hydrocarbons in cigarette smoke condensate by gas chromatography-mass spectrometry and comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry

Gas chromatography-mass spectrometry with electron ionization and positive-ion chemical ionization and comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC x GC-TOF-MS) were applied for the characterization of the chemical composition of complex hydrocarbons in the non-polar neutral fraction of cigarette smoke condensates. Automated data processing by TOF-MS software combined with structured chromatograms and manual review of library hits were used to assign the components from GC x GC-TOF-MS analysis. The distributions of aliphatic hydrocarbons and aromatics were also investigated. Over 100 isoprenoid hydrocarbons were detected, including carotene degradation products, phytadiene isomers and carbocyclic diterpenoids. A total of 1800 hydrocarbons were tentatively identified, including aliphatic hydrocarbons, aromatics, and isoprenoid hydrocarbons. The identified hydrocarbons by GC x GC-TOF-MS were far more than those by GC-MS.     

Screening the origin and weathering of oil slicks by attenuated total reflectance mid-IR spectrometry

The combination of attenuated total reflectance-fourier transform mid-infrared spectrometry (ATR-FTMIR) and multivariate pattern recognition is presented as a fast and convenient methodology to ascertain the source product an oil slick comes from and to evaluate the extent of its weathering. Different types of hydrocarbons (including crude oils, several heavy distillates and the Prestige's heavy fuel oil) were spilled on metallic containers designed ad hoc and their fate monitored by ATR-FTMIR. Not only environmental conditions were considered for weathering but artificial IR- and UV-irradiation. Pattern-recognition studies revealed that the different hydrocarbons clustered at different locations on the score plots and that the samples corresponding to each oil became ordered according to the extent of their weathering. Among them, fuel oil samples coming from the recent disaster of the Prestige tanker off the Galician shoreline showed a distinctive behaviour. Comparison of natural-, IR- and UV-weathering of a crude oil showed that IR solar radiation can be important in oil-weathering, in addition to broadly-reported UV degradation.     

Atmospheric pressure laser ionization (APLI) coupled with Fourier transform ion cyclotron resonance mass spectrometry applied to petroleum samples analysis: comparison with electrospray ionization and atmospheric pressure photoionization methods

The analysis of crude oil samples remains a tough challenge due to the complexity of the matrix and the broad range of physical and chemical properties of the various individual compounds present. In this work, atmospheric pressure laser ionization (APLI) is utilized as a complementary tool to other ionization techniques for crude oil analysis. Mass spectra obtained with electrospray ionization (ESI) and atmospheric pressure photoionization (APPI) are compared. APLI is primarily sensitive towards non‐polar aromatic hydrocarbons, which are generally present in high amounts especially in heavy crude oil samples. The ionization mechanisms of APLI vs. APPI are further investigated. The results indicate the advantages of APLI over established methods like ESI and APPI. The application of APLI in combination with Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) is thus demonstrated to be a powerful tool for the analysis of aromatic species in complex crude oil fractions. Copyright © 2011 John Wiley & Sons, Ltd.     

Differentiation and electron ionization-induced isomerization of 4,5-Bis(alkylthio)-1,3-dithiole-2-thiones and 1,2-dithiole-3-thiones

The electron impact ionization mass spectra of 4,5-bis(alkylthio)-1,3-dithiole-2-thiones and their 1,2-dithiole-3-thione isomers were studied by accurate mass measurements and linked scans. The relative abundance of ions formed following the extrusion of S₂, CS or CS₂ allows an unambiguous isomer differentiation. Isomerization of molecular ions was studied by means of metastable ion analysis and collision-induced dissociation. The order of reactivity was analogous to that observed in isomerization under photochemical conditions.     

两种离子化技术气相色谱-质谱法测定橄榄油中甾醇烯类物质的比较

分别采用电子轰击(EI)和正化学电离(PCI)两种离子源技术建立了气相色谱-质谱联用法(GC-MS)同时测定橄榄油中3,5-菜甾二烯、3,5,22-豆甾三烯和3,5-豆甾二烯3种甾醇烯含量的方法,并对这两种方法进行了比较。样品经石油醚溶解,硅胶柱净化后,分别采用GC-EI/MS和GC-PCI/MS分时段选择离子监测模式进行测定,以3,5-胆甾二烯为内标进行定量。结果表明,两种方法的线性、准确度、精密度、灵敏度均较好。3,5-菜甾二烯、3,5,22-豆甾三烯和3,5-豆甾二烯分别在0.024~0.48、0.02~0.50和0.03~0.75 mg/L浓度范围内线性关系良好(r>0.999)。在3个加标水平下,GC-EI/MS和GC-PCI/MS的平均回收率分别为88.7%~99.5%、87.1%~109.2%,两种检测方法的相对标准偏差(RSD,n=6)均不超过8.3%。定量限(S/N=10)分别为0.03 mg/kg(EI),0.03~0.10 mg/kg(PCI)。通过对两种方法的比较研究发现,EI能提供更多碎片离子和结构信息,而PCI中则主要为准分子离子及反应气加合离子。应用于样品中甾醇烯的测定时,PCI的选择性和抗干扰能力明显优于EI。两种方法可相互补充和替代应用于日常检测中。     

Sequence-specific fragmentation of benzyloxycarbonyl dipeptides by resonance-enhanced multiphoton ionization

The mechanism of fragment ion formation in resonance-enhanced multiphoton ionization (REMPI) of benzyloxycarbonyl (CBZ)-derivitized dipeptides is presented. At 266 nm, the entire multiphoton process can be thought of as a two-part scheme where ionization occurs by resonant two-photon ionization followed by photodissociation of the created ions. When the energy of two photons exceeds the molecular ionization energy by a significant amount, REMPI has the advantage of producing both parent ions and low appearance energy fragments in large amounts. For CBZ dipeptides, resonant two-photon ionization at 266 nm produces parent ions as well as A type sequence ions with high abundance. On the other hand, a three-photon process (resonant two-photon ionization followed by parent ion photodissociation) forms sequence-related ions which also involve complex fragmentations of the CBZ chromophore. These results are compared to mass spectra obtained by other ionization/dissociation methods and to REMPI mass spectra of related compounds. Factors related to molecular structure elucidation based upon REMPI mass spectra are discussed. Enhanced isomer distinction is demonstrated for CBZ-leu-ala-OCH₃ and CBZ-ile-ala-OCH₃ based upon REMPI fragmentation.     

稠油催化裂解前后咔唑类化合物的变化及其催化降黏机理的研究

在稠油族组成分离中,采用低毒试剂正己烷、二氯甲烷、无水乙醇及其混合溶剂,将反应前后已除去沥青质的稠油分离为饱和烃、芳烃和含氮化合物。优化并确立了各步分离条件,用GC-MS对各组分进行检测,了解水热催化裂解降黏前后各组分的变化,剖析了胶质中咔唑类化合物的变化。并结合¹HNMR、元素分析等分析手段,证明稠油经水热催化裂解反应后,重质组分中的含氮组分发生了变化,生成了部分咔唑类化合物, 同时,重质组分的组成和结构也发生了一定程度的改变,其超分子结构在一定程度上遭到破坏,生成二环、三环低环数芳烃和小分子量直链烃。这些变化使得稠油黏度降低。