mass spectrometry has been widely employed for detection of environmental pollutants , illegal drugs , and explosives for its rapid , high sensitivity , and accuracy in qualitative analysis . however , traditional mass spectrometry used in lab can barely be applied for in situ detection due to its size and weight . the technology of portable mass spectrometry has been recognized as one of the most promising and widely used techniques in environment monitoring , field diagnosis , and process monitoring . researches on miniaturization of magnetic mass spectrometry , quadrupole mass spectrometry , ion trap mass spectrometry , and time - of - flight mass spectrometry ( tofms ) all have been reported in the literature [ 28 ] . compared with other types of mass spectrometry , tofms provides rapid scanning speed , full range scanning of the mass to charge , and simpleness in structure . hopkins et al . have achieved remarkable approaches in developing and application of field detecting tofms [ 911 ] . a tiny tofms analyzer was developed with a length of 20  cm and 500  g in weight . a coaxial reflecting tofms with maldi ion source was also developed , and the mass range is as high as 10,000 m / z ; sensitivity of picomole and resolution of 3001000 ( m/m ) were achieved . cornish and cotter   designed an end cap miniature tofms , this tofms featured with a laser ion source and a resolution of 210 at 1061 m / z . brinckerhoff et al .   explored a laser - focusing micro tofms with coaxial structure and a length of 20  cm , and multifield energy focusing technique was applied to this micro tofms . the micro tofms has a resolution of 1000  m/m with an upper mass range to 1000 m / z and was used for elemental and organic analysis on the surface of a planet . white et al . developed a mini tofms with membrane inlet and ei ion source . the mini tofms was 53  33  21  cm in size and 21  kg in weight with a resolution of 300 at 28 m / z . the experiment data was well linear fitted in the range of 501200 ppb for btx . a portable tofms investigated by syage et al . has a frequency of 200  hz , and the limit of detection for aromatic compounds was 10100 ppb with 10  s analysis time . in situ portable tofms prefers soft ionization source for rapid spectrum interpretation . due to the 70  ev high energy electrons , a large number of ion fragments could be present with traditional electron ionization source , and the overlapping peaks from fragments ions would bring much trouble in spectrum interpretation . a vuv lamp is a type of soft ionization source , and the analyte molecules can be ionized through single photon ionization ( spi ) with few fragments . the vuv lamp is compact and can be operated without chemical reagents . have been working on developing portable and miniature tofms with vuv lamp ever since 2002 , such as spi source with membrane inlet miniature tofms , magnetic enhanced photon ionization source tofms , single photon ionization combined with chemical ionization source tofms , and quasi - trapping chemical ionization source tofms [ 1619 ] . proton transfer reaction mass spectrometer ( ptrms ) is another widely used soft ionization source for online measurements of trace components with concentrations as low as a few pptv , which is developed on the basis of proton transfer reactions with reactant ion of h3o . but the ptr reaction is fulfilled at high pressure , and the vacuum system is much larger in the ptrms , so it is very hard for the miniaturization with ptrms [ 20 , 21 ] . the unique characteristics of the present portable tofms compared to existing technologies are the combination of novel dual spi / pei ionization source , versatile sampling inlet , differential pumping for high gas throughput , and low - power consumption . the details of the operating conditions and the applications of the portable tofms were demonstrated in the following sections . a schematic diagram and photograph that illustrate the features of the portable tofms are presented in figure 1 . the dimensions of the tofms are 40  31  29  cm , and it weighs about 27  kg including the battery and all the pumping system . the portable tofms consists of an ionization source , an orthogonal acceleration reflection mass analyzer , two sampling inlets , the pumping system , and the electronics for the tofms timing and data acquisition . the ions generated from ionization source are accelerated from the source region to orthogonal repelling region through a set of electrodes . the repeller then was applied with pulse voltage to converge the ions into acceleration region , and then the ions enter a field - free region . a cylindrical reflectron is used for focusing the ion beam and directing it back along the flight tube . when the ion beam reaches the detector , it is detected by a micro - channel - plate detector . the electronic signal from the detector is sent to a time - to - digital converter ( tdc ) based data acquisition system . mass spectrum histogram with 100  ps resolution is then passed to a laptop pc . data analysis and processing including data reduction are performed using home - made data processing software . the portable tofms employs a three - stage differential pumping system with an 80  l / s split - flow turbomolecular pump and a 10  l / s turbomolecular pump ; both of the turbomolecular pumps are backed by a 5  l / min diaphragm pump . the 10  l / s turbomolecular pump is used for the first - stage differential pumping of ion source and held the pressure at 1.5  pa . the split - flow turbomolecular pump provides about 3  l / s pumping for second stage of ion transmission region and 80  l / s pumping for the third stage of mass analyzer ( kept below 5  10  pa at working condition ) . vacuum gauges ( mks 925 ) were used to monitor the pressure of ion source , and a mks 972 vacuum gauge was used for mass analyzer . the portable tofms is equipped with two kinds of sampling inlet : a fused silica capillary ( 150  m in i.d . , 1.7  m in length ) based direct sampling inlet for high concentration analytes and a sheet pdms membrane ( with thickness of 50  m and 150  mm in area ) inlet for trace analytes . a stainless steel mesh was set under the membrane to keep the sheet membrane flat . volatile organic compounds ( vocs ) in the sample diffuse faster than air when passing through the membrane into the ionization region ; therefore , the vocs are relatively enriched . the flow rate of the samples through the surface of the membrane was optimized as 1  l / min and was controlled by a mass flow controller ( seven star electronics co. , ltd . , the electrodes configuration inside of the ion source and ion transmission region is shown in figure 2 . the ionization source consists of three parts : a vuv lamp , permanent magnetic ring , and dc voltage lens including electrode 1 , electrode 2 , and skimmer electrode . the vuv lamp is a commercial low - pressure krypton discharge lamp ( 10.6  ev , cathodeon ltd . , cambridge , uk ) , with a flux of about 1  10 photons  s , and is installed on the top of ionization chamber . the ion lens in the ion transmission region includes three cylinders : electrode 3 , electrode 4 , and electrode 5 . electrode 1 , electrode 2 , and skimmer are all tubular steel stainless electrodes ; the size is 5.5  mm in length , 16  mm i.d . , 28  mm o.d . , and with 4  mm central hole ( 1  mm central hole for skimmer electrode ) . these three electrodes are separated by insulator rings , and the distance from the vuv lamp light window to the surface of orifice electrode is only 20  mm . the permanent magnetic ring locates between electrode 2 and skimmer , the dimensions were 31  mm in inner diameter , 19  mm in outer diameter , and 6.5  mm in thickness , and it offered a maximum energy product ( bhmax ) of 1200  kj / m . the ion transmission region consists of three circular electrodes with inner diameter of 10  mm and thickness of 2  mm , 1  mm , and 2  mm , respectively . the insulated spaces among the skimmer , electrode 3 , electrode 4 , and electrode 5 are all 2  mm . the ion source was operated at two modes : single photoionization ( spi ) mode for analytes with ionization energy ( ie ) lower than 10.6  ev and photoelectron ionization ( pei ) mode for analytes with ie higher than 10.6  ev . the two different modes were controlled by the dc voltage applied on electrodes inside of ionization source . when 20  v , 17  v , and 5  v were applied to electrode 1 , electrode 2 , and skimmer separately , the ionization source operates under spi mode , and voltages on electrode 3 , electrode 4 , and electrode 5 were 20  v , 120 , and 40  v. the pei mode was originated from the electrons generated on the surface of skimmer by the photoelectric effect when vuv light irradiated on electrodes . the pei mode was activated when voltages on electrode 1 , electrode 2 , and skimmer were changed to 22.5  v , 21  v , and 44.5  v. the energy of the photoelectron was calculated as 67  ev by subtracting the voltage on the skimmer from the voltage on electrode 1 , and the voltages on electrode 3 , electrode 4 , and electrode 5 were changed to 9  v , 120 , and 0  v , accordingly . the total power consumption of the present ionization source was only 1.5  w with the vuv lamp , much lower than 50 w with traditional ei ionization source . the mass analyzer of tofms consists of four parts : the ion repeller region , ion accelerator region , field - free region , and the reflectron with a length of 6  mm , 23  mm , 210  mm , and 58  mm , respectively . typical parameters of the portable time - of - flight mass spectrometer were shown in table 1 . the two standard gas mixtures of 1  ppmv benzene , toluene , and p - xylene ( btx ) and 50  ppm so2 diluted with n2 ( 99.9993% purity ) were purchased from dalian special gas company ( dalian , china ) . the samples of btx and so2 with different concentrations used in the experiments were obtained by diluting standard gas with pure n2 within a ptfe sampling bag . the dilution process was as follows : a measured amount of n2 was blown into a ptfe sampling bag , and then a desired amount of sample was added to the bag . the amount of n2 and sample was controlled by 3  l / min and 100  ml / min calibrated mass flow controllers . the detection of 10 ppb benzene , toluene , and xylene ( btx ) was achieved in spi mode with the membrane inlet . the spectra shown in figure 3(a ) were collected during 50  s at a repetition rate of 20  khz . the linear response curve for of benzene , toluene , and xylene range is in the concentration range from 5  ppb to 400  ppm , and the linear dynamic ranges are greater than four orders of magnitude with a good linear correlation coefficient ( r 
  > 0.9900 ) . n = 3 ) , the limit of detection for btx is calculated to be 1  ppbv . this sensitivity is even much better than the reported results in the literature with more powerful vuv light source . so2 can rarely be ionized in spi mode for its high ionization energy ( 12.5  ev ) ; therefore , pei mode was selected for its analysis . the concentrations of so2 used in the experiment were 1  ppm , 2  ppm , 5  ppm , 10  ppm , 20  ppm , and 50  ppm , respectively . the spectra were collected during 75  s at a repetition rate of 40  khz . figure 3(b ) exhibits the mass spectrum and the linear response curve of so2 for 6 different concentrations . the linear correlation coefficient was calculated as 0.9965 ( r 
  = 0.9965 ) , and the limit of quantitation ( loq ) was determined as 1  ppm based on the criteria of s / n = 10  :  1 . the mass resolving power of the portable tofms , defined as m/m , where m is the peak center and m is the full - width half - maximum of the fitted gaussian peak , was greater than 1100 at 64 m / z ( so2 
 ) . the obtained resolution is higher than the reported 500 resolutions with similar size of 53  33  21  cm . in the laboratory , a mass resolving power of 1100 is routinely achieved for the range considered ( 10300  th ) . the stability of the portable tofms was investigated by comparing the tertian signal intensity of a 100  ppb btx , and the relative standard deviation ( rsd ) was less than 10% . the spectrum of vocs from wine with spi ionization is shown in figure 4 , and the components were interpreted as acetaldehyde ( m / z 44 , c2h4o ) , methanol ( m / z 32.04 , ch4o ) , n - propanol ( m / z 62.10 , c3h8o ) , n - butanol ( m / z 74.12 , c4h10o ) , ethyl acetate ( m / z 88.11 , c4h8o2 ) , pentanol ( m / z 88.15 , c5h12o ) , furfural ( m / z 96.08 , c5h4o2 ) , hexanol ( m / z 102.15 , c6h14o ) , ethyl butyrate ( m / z 116.16 , c6h12o2 ) , aldehyde acetal ( m / z 118.17 , c6h14o2 ) , ethyl lactate ( m / z 118.13 , c5h10o3 ) , and ethyl hexanoate ( m / z 144.21 , c8h16o2 ) . the main characteristic compounds inside of the wine were all detected by the portable tofms . the peaks for wine obtained with vuv lamp are characteristic with [ m  h ] , which is quite different with [ m + h ] peaks by ptrms . sulfur hexafluoride ( sf6 ) is widely used in gas - insulated switchgear ( gis ) and transformers due to its excellent insulating and arc - suppression properties . however , in the presence of partial discharge , sf6 decomposes into various byproducts , which leads to a significant decrease in the electric property of sf6 . for instance , so2 , sof2 , so2f2 , sf4 , sof4 , and s2f10o could be present in sf6 insulating devices with discharging fault . the capillary inlet was used and the analysis time was 13  s. the spectrum of the discharging products of sf6 is shown in figure 5(a ) . peaks of discharging products , so2f ( m / z , 83 ) , sif3 
  ( m / z , 85 ) , sof2 
  ( m / z , 86 ) , so2f2 
  or s2f2 
  ( m / z , 102 ) , and sof3 
  ( m / z , 105 ) , can be observed clearly in the figure . and the fragments and reaction products of sf6 such as sf ( m / z , 51 ) , of2 
  ( m / z , 54 ) , sf2 
  ( m / z , 70 ) , sf5 
  ( m / z , 127 ) , sf4 
  ( m / z , 108 ) , and sf3 
  ( m / z , 89 ) can be observed as well . peaks of o2 
  ( m / z , 32 ) and n2 
  ( m / z , 28 ) can be observed due to the air mixed with sf6 . the portable tofms was used for in situ detection of the real decomposition products of sf6 which were sampled from an insulation fault of a gis . the spectrum is shown in figure 5(b ) ; the peaks of m / z 64 and m / z 86 can be ascribed to so2 
  and sof2 
 , which originate from gaseous byproducts of so2 and sof2 , separately . the concentration of so2 is calculated as 400  ppm from the linear curve , and this result indicated that serious discharging has occurred inside the gis . a vuv lamp based portable tofms has been developed for field analysis of gas analytes . the portable tofms is equipped with two sampling inlets and dual ionization operating mode for different requirements . the tofms is 40  31  29  cm in size and 27  kg in weight with a favorable power consumption of 70  w. instrument performance was assessed using btx and so2 , a mass resolution of 1100 at m / z 64 has been obtained , and , under optimized conditions , the detection limits for btx were 0.005400  ppm by volume with linear dynamic ranges greater than four orders of magnitude . the portable tofms was used to detect so2 , the typical decomposition product of sf6 , for diagnosis of potential fault in sf6-insulated switchgear . the results reported here indicate that compact , high - performance tofms is ready for field development .
a vacuum ultraviolet lamp based single photon ionization- ( spi- ) photoelectron ionization ( pei ) portable reflecting time - of - flight mass spectrometer ( tofms ) was designed for online monitoring gas samples . 
 it has a dual mode ionization source : spi for analyte with ionization energy ( ie ) below 10.6  ev and pei for ie higher than 10.6  ev . 
 two kinds of sampling inlets , a capillary inlet and a membrane inlet , are utilized for high concentration and trace volatile organic compounds , respectively . a mass resolution of 1100 at m / z 64 has been obtained with a total size of 40  31  29  cm 
 , the weight is 27  kg , and the power consumption is only 70  w. a mixture of benzene , toluene , and xylene ( btx ) , so2 , and discharging products of sf6 were used to test its performance , and the result showed that the limit of quantitation for btx is as low as 5  ppbv ( s / n = 10  :  1 ) with linear dynamic ranges greater than four orders of magnitude . 
 the portable tofms was also evaluated by analyzing volatile organic compounds from wine and decomposition products of sf6 inside of a gas - insulated switchgear .