Electron Impact Ion Source Research Articles

Focus on test, measurement, and analytical equipment

Focus on test, measurement, and analytical equipment

Numerical simulations of magnetic electron-impact ion source

An electron impact ion source equipped with a magnet has been designed for enhancing the ionization efficiency of such ion sources with potential applications in Knudsen effusion mass spectrometry or electron impact sources used in gas mass spectrometry. This modernized device utilizes the trapping of electrons in parallel electrical and magnetic fields with the goal of maximizing the probability of impact between an electron and an atom or molecule to be ionized. The influence of magnetic and electrical fields on ions and electron trajectories inside the ion source was studied with numerical simulations. Based on the electron and ion trajectories we have modified the classical Nier-type ion source and found the optimal geometrical and potential parameters to provide better electron ionization and higher ion extraction efficiency. This design has the potential to enhance ionization efficiency by a factor of ∼15–20 compared with existing designs.

Research and Application of Mass Spectrometry with Low Energy Electron Impact Ionization Source

Abstract The changes of concentrations of O2 and CO2 in tail gas of fermentation industry reflect some important physiological and metabolic parameters in the fermentation process. Monitoring of the tail gas of fermentation has important significance for guiding the fermentation process. However, traditional monitoring methods are time-consuming with low precision and poor versatility, and are not suitable to monitor fermentation tail gas accurately in real time. In the process mass spectrometry, the traditional 70 eV electron impact ionization source will produce a large number of fragment ions, which may overlap with different substances, thus increasing the difficulty of qualitative analysis and causing inaccurate monitoring results. In order to realize real-time, on-line accurate monitoring of fermentation tail gas, a quadrupole process mass spectrometry based on electron impact ionization was developed for on-line analysis in this study. Through investigation of the relationship between emission current of filament and the electron receiving pole at low ionization energy, the ionization efficiency of the electron impact ionization source under low ionization energy mode was improved, and CO2, O2 and N2 were tested. The results showed that under the conditions of electron energy of 22 eV, electron receiving electrode of −22 V and filament emission current of 20–25 mA, the relative errors of quantitative detection of CO2 with volume fraction of higher than 0.023% and O2 with volume fraction of higher than 0.021% were all less than 1%. The instrument and the developed method had ideal application prospect in the rapid real-time, on-line monitoring of industrial processes.

Radioactive boron beams produced by isotope online mass separation at CERN-ISOLDE

.We report on the development and characterization of the first radioactive boron beams produced by the isotope mass separation online (ISOL) technique at CERN-ISOLDE. Despite the long history of the ISOL technique which exploits thick targets, boron beams have up to now not been available. This is due to the low volatility of elemental boron and its high chemical reactivity which make the definition of an appropriate production target unit difficult. In addition, the short half-lives of all boron radioisotopes complicate tracer release studies. We report here on dedicated offline release studies by neutron capture and alpha detection done with implanted 10B in prospective target materials, as well as molecule formation and ionization tests, which suggested the use of multiwalled carbon nanotubes (CNT) as target material and injection of sulfur hexafluoride SF6 to promote volatile boron fluoride formation. Two target units equipped with an arc discharge electron impact ion source VADIS coupled to a water cooled transfer line to retain non-volatile elements and molecules were subsequently tested online. The measured yield of these first 8B ISOL beams increases in the series {}^{8}mathrm{BF}_{3} < {}^{8}mathrm{BF} < {}^{8}mathrm{B} < {}^{8}mathrm{BF}_{2}, reaching a maximum yield of 6.4 times 10^{4}8BF2+ ions per μC of protons.

MEMS ion source for ion mobility spectrometry

This paper describes a miniature, silicon-glass, electron-impact ion source, in which a field emission carbon nanotube cathode is separated from the external atmosphere by a semitransparent Si3N4 membrane. High vacuum necessary for the proper operation of the ion source is generated on a chip by an integrated micropump. The experiment revealed that it is possible to obtain and maintain high vacuum inside the structure; however, some problems associated with outgassing occurred during field emission measurements. Despite this fact, both the transmission of electrons through the thin membrane and the possibility of gas ionization have been confirmed.

Determination of 27 fragrances in cosmetics and perfume raw materials by gas chromatography-mass spectrometry

A chromatographic separation method for 27 fragrances in cosmetics and perfume raw materials was developed using gas chromatography-mass spectrometry (GC-MS). A weak polar capillary column and electron impact ion source were used in the GC-MS analysis, with methanol as the extraction solvent. The limits of detection were 1.2, 15, and 15 mg/kg for musk xylene, hydroxycitronellal, and hydroxyisohexyl-3-cyclohexene carboxaldehyde, respectively, and 3.0 mg/kg for the other 24 fragrances. The calibration line of the 27 fragrances presented a good relationship with correlation coefficients ≥ 0.996. At three spiked concentration levels, the relative standard deviations were <10% and the spiked recoveries were in the ranges of 73.3%-76.1% for musk xylene and 81.5%-118% for the other fragrances. Based on these determinations, one or more fragrance ingredients were detected in 69 perfume raw materials and cosmetics, the labels of which indicated that they contained fragrances. This method can be used to determine 27 fragrances in cosmetic and perfume raw materials.

EGT-A sensitive time-of-flight mass spectrometer for multielement isotope gas analysis.

The principles of operation and figures of merit of a novel, compact (324mm×Ø 114mm; volume approximately 1000cm3 ) reflectron-type time-of-flight mass spectrometer designed for simultaneous multielement isotope gas analysis is presented. The system, which consists of a pulsed electron impact ion source, is designed either to directly analyse gas samples collected and stored in a compartment or samples extracted from solids using a CW laser system (fibre-coupled diode laser, <75W, λ=808±10nm). In latter case, laser pulses are focussed onto the sample surface to spot sizes of approximately 400μm in diameter that allows for direct ablation and vaporisation of solid sample material and releasing of trapped gases. A cleaning and trapping system that consists of various cold stages and getters is used before the gas enters the mass analyser. Measurements on various gases were conducted for performance evaluation, ranging from standard gases (Ar, Kr, and Xe) to trapped gases extracted from a sample of the Millbillillie meteorite. At optimised instrument settings, mass spectrometric measurements can be conducted with a mass resolution m/∆m of up to approximately 1200 (16 O and CH4 can be resolved), with a dynamic range of approximately 6 orders of magnitude and a mass calibration accuracy of approximately 100ppm. The high detection sensitivity of the system allows the detection of gas species at partial pressures down to the low 10-16 mbar level (corresponding to <10 particles/cm3 at standard temperature and pressure, including an ion transmission of approximately 80%). Measurements using standard gases demonstrated that the isotope ratios for a given element can be measured with an accuracy at the per mill level (relative to terrestrial values). Measurements of Ar extracted from the meteorite Millbillillie gave a 36 Ar/38 Ar ratio of approximately 1.6, which is in good agreement with literature values.

Laboratory-size X-ray Microscope using Wolter Mirror Optics and an Electron-impact X-ray Source for Multi-energy Observation

Laboratory-size X-ray Microscope using Wolter Mirror Optics and an Electron-impact X-ray Source for Multi-energy Observation

Fast and environmental-friendly methods for the determination of polybrominated diphenyl ethers and their metabolites in fish tissues and feed

Environmental-friendly, cost-effective and fast methods were developed and validated for the analysis of seven PolyBrominated Diphenyl Ethers (PBDEs) and eight methoxylated PBDEs (MeO-PBDEs) in three distinct seafood matrices (muscle, liver and plasma) and feed using a Quick, Easy, Cheap, Efficient, Rugged and Safe (QuEChERS) extraction approach for solid samples and a Dispersive Liquid-Liquid Microextraction method (DLLME) for plasma. Instrumental analyses were performed with gas chromatography coupled to triple quadrupole mass spectrometry using electron impact source (GC-EI-MS/MS) and negative ion chemical ionization (GC-NICI-MS) to assess BDE-209. Statistical validation showed recoveries for all target substances near 100% with average Relative Standard Deviation (RSD) lower than 9% and recovery standards higher than 65% (average RSD below 20%). Average calculated Method Detection Limits (MDLs) were lower than 65 pg·g−1 wet weight (WW) for muscle, 5.35 ng·g−1 WW for liver, 4.50 ng·g−1 WW for feed, and 0.60 ng·mL−1 for plasma samples. Quality assurance and quality control practices were comprehensively described. Methods scored high in an analytical Eco-scale, thus being classified as “an excellent green analysis”. Finally, real seafood samples collected in local markets and local fishermen were analyzed. Positive samples presented both PBDEs and MeO-PBDEs in safe amounts (0.28–125.80 ng·g−1 WW) for human consumption.

Low-vacuum cylindrical ion trap mass spectrometry

ABSTRACTLow-vacuum mass spectrometry is desirable because it reduces the size, weight, cost, and power of the instrument by reducing the workload of the pumping system. To investigate low-vacuum mass spectrometry methods, a cylindrical ion trap (CIT) instrument was built. The platform used an electron impact source as the ionization source, a custom CIT as the mass analyzer, and an electron multiplier as the ion detector. The dimensions of the CIT were r0 = 10 mm and z0 = 8.98 mm. Aiming at low-vacuum conditions, its working parameters were optimized. By increasing the frequency of the radio frequency (RF) voltage, optimizing the electron impact source, using a higher voltage on the electron multiplier, and improving the current preamplifier, the mass spectrometry of methyl salicylate was successively performed at helium buffer gas pressures up to 2 Pa, which was dozens of times higher than the upper pressure limit of ion trap mass spectrometers. More importantly, this pressure can be obtained using a single pump, avoiding the use of a bulky turbo pump. In addition, we measured and analyzed the mass deviation of methyl salicylate with the changes in the background gas pressure and RF voltage frequency. The results experimentally verified the theory that the stability regions expand with increasing pressure for the first time. The methods we explored could be used to develop next generation hand-portable instruments and bring new applications to mass spectrometry.

Untangling the methane chemistry in interstellar and solar system ices toward ionizing radiation: a combined infrared and reflectron time-of-flight analysis.

Pure methane (CH4/CD4) ices were exposed to three ionizing radiation sources at 5.5 K under ultrahigh vacuum conditions to compare the complex hydrocarbon spectrum produced across several interstellar environments. These irradiation sources consisted of energetic electrons to simulate secondary electrons formed in the track of galactic cosmic rays (GCRs), Lyman α (10.2 eV; 121.6 nm) photons simulated the internal VUV field in a dense cloud, and broadband (112.7-169.8 nm; 11.0-7.3 eV) photons which mimic the interstellar ultra-violet field. The in situ chemical evolution of the ices was monitored via Fourier transform infrared spectroscopy (FTIR) and during heating via mass spectrometry utilizing a quadrupole mass spectrometer with an electron impact ionization source (EI-QMS) and a reflectron time-of-flight mass spectrometer with a photoionization source (PI-ReTOF-MS). The FTIR analysis detected six small hydrocarbon products from the three different irradiation sources: propane [C3H8(C3D8)], ethane [C2H6(C2D6)], the ethyl radical [C2H5(C2D5)], ethylene [C2H4(C2D4)], acetylene [C2H2(C2D2)], and the methyl radical [CH3(CD3)]. The sensitive PI-ReTOF-MS analysis identified a complex array of products with different products being detected between experiments with general formulae: CnH2n+2 (n = 4-8), CnH2n (n = 3-9), CnH2n-2 (n = 3-9), CnH2n-4 (n = 4-9), and CnH2n-6 (n = 6-7) from electron irradiation and CnH2n+2 (n = 4-8), CnH2n (n = 3-10), CnH2n-2 (n = 3-11), CnH2n-4 (n = 4-11), CnH2n-6 (n = 5-11), and CnH2n-8 (n = 6-11) from broadband photolysis and Lyman α photolysis. These experiments show that even the simplest hydrocarbon can produce important complex hydrocarbons such as C3H4 and C4H6 isomers. Distinct isomers from these groups have been shown to be important reactants in the synthesis of polycyclic aromatic hydrocarbons like indene (C9H8) and naphthalene (C10H8) under interstellar conditions.

Determination of 18 polychlorinated biphenyls in tea by gas chromatography-mass spectrometry coupled with dispersive solid-phase extraction

A method for the rapid determination of 18 polychlorinated biphenyls (PCBs) in tea by gas chromatography-mass spectrometry (GC-MS) coupled with dispersive solid-phase extraction (dSPE) was developed and evaluated. Carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) and primary secondary amine (PSA) were used as sorbents in pretreatment process. PCBs were ultrasonically extracted from tea samples with hexane-acetone (1:1, v/v), and purified with the mixed sorbents of MWCNTs-COOH and PSA after replacing hexane-acetone by toluene. The PCBs were determined by an electron impact ionization source and selected ion monitoring mode. The analytes were qualitatively confirmed from the retention time and relative abundance ratio of characteristic ions, and quantified by a matrix-matched standard solution. The chromatographic and MS parameters influencing separation and sensitivity were optimized, and major factors affecting the extraction and cleanup efficiency including type and volume of extraction solvent, extraction time, type and amount of cleanup sorbent, and clean-up time were investigated. The performance of the method was evaluated. Under optimum conditions, satisfactory linear relationship was observed with the content of PCBs ranging from 5 to 500 μg/kg, with correlation coefficients (r) no less than 0.9998. The 18 PCBs in three kinds of tea matrices including Biluochun tea, Tieguanyin tea and Pu'er tea were spiked at 5, 10 and 100 μg/kg to evaluate recoveries, which ranged from 90.7% to 115.2% with relative standard deviations (n=5) between 0.3% and 10.9%. The limits of detection and the limits of quantification were 0.3-1.7 μg/kg and 5 μg/kg for each PCB, respectively. The method is simple, rapid, accurate, sensitive and effective, and is suitable for the determination of the 18 PCBs in different kinds of tea.

Determination of 16 polycyclic aromatic hydrocarbons in marine sediments by gas chromatography-tandem mass spectrometry with accelerated solvent extraction

A method based on gas chromatography-tandem mass spectrometry (GC-MS/MS) with accelerated solvent extraction (ASE) was developed for the determination of 16 polycyclic aromatic hydrocarbons (PAHs) in marine sediments. The sediment samples were extracted with hexane-acetone (1:1, v/v). The extracts were dehydrated with anhydrous sodium sulfate, and concentrated with a termovap sample concentrator. The concentrated solutions were further cleaned up with Si SPE columns. The purified solutions were then isolated by HP-5MS column (30 m×0.25 mm×0.25 μm). The analytes were detected using electron impact source under multiple reaction monitoring (MRM) mode. The results showed that the 16 PAHs had good linearities in the range of 0.01-1.00 mg/L with the correlation coefficients (R) greater than 0.997. The spiked recoveries of the 16 PAHs were in the range of 75.8%-97.8%, and the relative standard deviations (RSDs) were less than 10%. The method detection limits (MDLs) of the 16 PAHs ranged from 0.048-0.234 μg/kg with 20.0 g sampling weight. This method is suitable for the determination of trace PAHs in marine sediments.

Determination of organochlorine and organophosphorus pesticides in soil by gas chromatography-tandem mass spectrometry with accelerated solvent extraction

A method based on gas chromatography-tandem mass spectrometry (GC-MS/MS) coupled with accelerated solvent extraction (ASE) was developed for the determination of eight organochlorine pesticides (OCPs) and five organophosphorus pesticides (OPPs) in soils. The soil samples were grinded after freeze-drying. Particles with diameters lower than 250 μm were chosen by ion-sieving. After this, 10.0 g soil mixed with 2.0 g diaomite was extracted with hexane-acetone (1:1, v/v). The extracts were dehydrated with anhydrous sodium sulfate and concentrated with a termovap sample concentrator. The concentrated solutions were further cleaned up with Si SPE columns and eluted with hexane-acetone (1:1, v/v). The purified solutions were then isolated by HP-5MS column (30 m×0.25 mm×0.25 μm) and detected using the multiple reaction monitoring mode at the electron impact source. It is observed that this method has good linearities in the range of 1.00-100 μg/L for the 13 compounds, and the correlation coefficients (R) were greater than 0.995. The spiked recoveries of the 13 compounds were in the range of 66.8%-88.4%, and the relative standard deviations were less than 10%. With 10.0 g of sampling weight, the method detection limits ranged from 0.02 to 0.04 μg/kg for the eight OCPs and from 0.06 to 0.12 μg/kg for the five OPPs. This method is suitable for the determination of trace OCPs and OPPs in soils.

Simultaneous Quantitative Determination of Synthetic Cathinone Enantiomers in Urine and Plasma Using GC-NCI-MS.

Development and validation of sensitive and selective method for enantioseparation and quantitation of synthetic cathinones is reported using GC-MS triple quadrupole mass spectrometry with negative chemical ionization (NCI) mode. Indirect chiral separation of thirty-six synthetic cathinone compounds has been achieved by using an optically pure chiral derivatizing agent (CDA) called (S)-(−)-N-(trifluoroacetyl)pyrrolidine-2-carbonyl chloride (L-TPC), which converts cathinone enantiomers into diastereoisomers that can be separated on achiral columns. As a result of using Ultra Inert 60 m column and performing slow heating rate (2°C/min) on the GC oven, an observed enhancement in enantiomer peak resolution has been achieved. An internal standard, (+)-cathinone, was used for quantitation of synthetic cathinones. Method validation in terms of linearities and sensitivity in terms of limits of detection (LODs), limits of quantitation (LOQs), recoveries, and reproducibilities has been obtained for fourteen selected compounds that examined simultaneously as a mixture after being spiked in urine and plasma. It was found that the LOD of the fourteen synthetic cathinones in urine was in the range of 0.26–0.76 µg/L, and in plasma, it was in the range of 0.26–0.34 µg/L. While the LOQ of the mixture in urine was in the range of 0.86–2.34 µg/L, and in plasma, it was in the range of 0.89–1.12 µg/L. Unlike the electron impact (EI) ion source, NCI showed better sensitivity by two orders of magnitude by comparing the obtained results with the recently published reports for quantitative analysis and enantioseparation of synthetic cathinones.

Mechanistic reaction pathway for hexafluoropropylene oxide pulsed plasma deposition of PTFE-like films

The pulsed plasma deposition of PTFE-like thin films using hexafluoropropylene oxide (HFPO) precursor has been investigated by time-resolved in situ mass spectrometry. Electron-impact and electron-attachment ionisation modes have been employed to monitor neutral plasma species; whilst electrical discharge negative and positive ions have been detected by switching-off the mass spectrometer electron-impact ionisation source. The relative concentrations of neutral versus ionic plasma species were found to rise for shorter pulse duty cycles. Mechanistically, HFPO molecules undergo pulsed plasma-initiated decomposition to form difluorocarbene (:CF2) radical species, and these can combine to form tetrafluoroethylene (CF2 = CF2) molecules, which subsequently polymerise leading to the growth of PTFE-like ([–CF2–]n) perfluoroalkyl chains. Shorter pulsed plasma duty cycles enhance the growth of linear PTFE-like perfluoroalkyl chains.

A GC-EI-MS/MS Method for the Determination of 15 Polybrominated Diphenyl Ethers (PBDEs) in Fish and Shellfish Tissues

This paper describes the development and validation of an analytical method to determine 15 polybrominated diphenyl ethers (PBDEs) in fish and shellfish tissues with gas chromatography coupled to triple quadrupole mass spectrometry using electron impact source (GC-EI-MS/MS). A QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe)-like procedure was applied; the extract was cleaned up on an acidic Extrelut-NT3 column connected on top of a silica cartridge and further purified with gel permeation chromatography (GPC). The SIM acquisition proved to yield better results than traditional MRM approach for octa-, nona-, and deca-brominated congeners. The validation study was carried out in inter-laboratory conditions. The average recoveries were in the range 66–118%, with relative standard deviations generally lower than or equal to 22%. Quality assurance and quality control practices were thoroughly described and highlighted. These long and tedious practices are fundamental to obtain reliable quantitative data during food monitoring programs and, therefore, to support daily intake assessments. Finally, the procedure was applied to the analysis of marine fish samples collected in local markets. Mackerel and mullet were the most contaminated species.

Elevated Concentrations of 4-Bromobiphenyl and 1,3,5-Tribromobenzene Found in Deep Water of Lake Geneva Based on GC×GC-ENCI-TOFMS and GC×GC-μECD.

We quantified the concentrations of two little-studied brominated pollutants, 1,3,5-tribromobenzene (TBB) and 4-bromobiphenyl (4BBP), in the deep water column and sediments of Lake Geneva. We found aqueous concentrations of 625 ± 68 pg L–1 for TBB and 668 ± 86 pg L–1 for 4BBP over a depth range of 70–191.5 m (near-bottom depth), based on duplicate measurements taken at five depths during three separate 1 month sampling periods at our sampling site near Vidy Bay. These levels of TBB and 4BBP were 1 or 2 orders of magnitude higher than the quantified aqueous concentrations of the components of the pentabrominated biphenyl ether technical mixture, which is a flame retardant product that had a high production volume in Europe before 2001. We observed statistically significant vertical concentration trends for both TBB and 2,2′,4,4′,6-pentabromobiphenyl ether in the deep water column, which indicates that transport and/or degradation processes affect these compounds. These measurements were enabled by application of a comprehensive two-dimensional gas chromatograph coupled to an electron capture negative chemical ionization time-of-flight mass spectrometer (GC×GC-ENCI-TOFMS) and to a micro-electron capture detector (GC×GC-μECD). GC×GC-ENCI-TOFMS and GC×GC-μECD were found to be >10× more sensitive toward brominated pollutants than conventional GC×GC-EI-TOFMS (with an electron impact (EI) ionization source), the latter of which had insufficient sensitivity to detect these emerging brominated pollutants in the analyzed samples. GC×GC also enabled the estimation of several environmentally relevant partitioning properties of TBB and 4BBP, further confirming previous evidence that these pollutants are bioaccumulative and have long-range transport potential.

Determination of six amide herbicide residues in soil by QuEChERS-gas chromatography-mass spectrometry

A method was developed for the determination of six amide herbicides, such as acetochlor, alachlor, metolachlor, metazachlor, butachlor and pretilachlor, by QuEChERS-gas chromatography-mass spectrometry. The soil samples were extracted by 10 mL acetonitrile, 10 mL deionized water and 4 g sodium chloride, then purified with 50 mg N-propyl ethylenediamine (PSA), 50 mg C18, 10 mg graphitized carbon black (GCB) and 100 mg MgSO4. Low-polar silica capillary column HP-5MS was used. The extracts were detected by electron impact ionization (EI) source in the selected ion monitoring (SIM) mode. The results showed that good linearities were obtained with the mass concentrations of the six amide herbicides in the range of 0.01-1.00 mg/L, and their correlation coefficients ranged from 0.9996 to 1.0000. Under the condition of adding contents of 0.025, 0.10 and 0.50 mg/kg, the average recoveries of the six amide herbicides ranged from 92.0% to 108%, and the relative standard deviations were 1.64%-8.25%. The limits of detection (LODs) were 0.002-0.006 mg/kg and the limits of quantification (LOQs) were 0.005-0.02 mg/kg. Comparing with the gas chromatography-electron capture detection (GC-ECD) which used to detect the amide herbicides, this method can improve the anti-interference ability, and get better separation of the six amide herbicides. The method has the advantages of simplicity, rapidity, and application value.

Simultaneous determination of 15 polycyclic aromatic hydrocarbons in cigarette filter by gas chromatography-tandem mass spectrometry

A method for the simultaneous determination of 15 polycyclic aromatic hydrocarbons in cigarette filter was developed by isotope internal standard combined with gas chromatography-tandem mass spectrometry. The cigarette filters were extracted with dichloromethane, and the extract was filtered with 0.22 μm organic phase membrane. The samples were isolated by DB-5MS column (30 m×0.25 mm, 0.25 μm) and detected using multiple reaction monitoring mode of electron impact source under positive ion mode. The linearities of the 15 polycyclic aromatic hydrocarbons (acenapthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, ben[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenzo[a,h]anthracene, benzo[g,h,i]perylene and indeno[1,2,3-c,d]pyrene) were good, and the correlation coefficients (R2) ranged from 0.9914 to 0.9999. The average recoveries of the 15 polycyclic aromatic hydrocarbons were 81.6%-109.6% at low, middle and high spiked levels, and the relative standard deviations were less than 16%, except that the relative standard deviation of fluorene at the low spiked level was 19.2%. The limits of detection of the 15 polycyclic aromatic hydrocarbons were 0.02 to 0.24 ng/filter, and the limits of quantification were 0.04 to 0.80 ng/filter. The method is simple, rapid, accurate, sensitive and reproducible. It is suitable for the quantitative analysis of the 15 polycyclic aromatic hydrocarbons in cigarette filters.