Abstract:
The disclosed method and apparatus couple a membrane interface directly to a mass spectrometer at atmospheric pressure. The membrane may be in capillary or sheet form and allows the introduction of a liquid or gaseous sample to one side of the membrane while the other side of the membrane is bathed with a solution that can easily be used in an atmospheric pressure ionization source. Volatile molecules permeate through a suitable membrane such as poly-dimethyl silicone (PDMS), mix into the appropriate solvent, and are ionized. Because of the rules governing abstracts, this abstract should not be used in construing the claims.

Description:
[0001]    This application claims priority from copending U.S. provisional application Ser. No. 60/664,095 filed Mar. 22, 2005 and entitled Membrane Interface Apparatus and Method of Analysis of Volatile Molecules by Mass Spectrometry, the entirety of which is hereby incorporated by reference 
     
    
     BACKGROUND 
       [0002]    The present invention is directed to membrane introduction devices coupled to a mass spectrometer to introduce volatile molecules in solution into the ion source. Most of that work has coupled a membrane interface to an electron ionization (EI) or chemical ionization (CI) source. As a result, the volatile molecules that permeate the membrane go under a pervaporization process on the other side of the membrane where they are desorbed from the membrane surface and can then be ionized in the gas phase. For more detailed information please see Mark Bier&#39;s Ph.D. Thesis, Purdue University 1988 (incorporated herein by reference). 
         [0003]    Membrane introduction mass spectrometry (MIMS) is a sensitive, selective technique for the analysis for many small organic compounds in water in real-time. In this technique, small molecules are selectively concentrated by adsorption and separated from the water by preferential permeation through a membrane. 
         [0004]    First used by Hoch and Kok (see A Mass Spectrometer Inlet System for Sampling Gases Dissolved in Liquid Phases.  Arch. Biochem. Biophys.  1963, 101(1), 160-170) as a method for monitoring the production and consumption of gases during photosynthesis, MIMS has since been developed as a versatile analytical method that is applicable to a wide variety of analyses. For example, MIMS has been used to monitor the ethanol content of a fermentation broth, biological reactions, the destruction of environmental contaminants, the chlorination of organic amines in water, kinetic and mechanistic aspects of chlorination of organic compounds in water, organometallic compounds in water, and volatile and semi-volatile organic compounds (VOC and SVOC) in a variety of matrices. Most VOCs have been analyzed by MIMS in air or in aqueous solution, but they have also been analyzed in soil and human breath. 
         [0005]    There are many methods available to analyze VOC and SVOC in water such as headspace analysis and purge-and-trap coupled to gas chromatography (GC) or gas-chromatography-mass spectrometry (GC-MS). Headspace analysis requires little time for sample preparation, but generally can be used only for contaminants at high concentrations. Purge-and-trap analysis is a recommended concentrating method by the Environmental Protection Agency (EPA), but it often has problems of sample carryover and contamination and it is also an expensive method that requires a bulky apparatus. Liquid-liquid extraction, and solid-phase micro extraction are commonly used for analyzing SVOCs. Liquid-liquid extraction methods generally require large quantities of solvents which makes them more expensive and less environmentally friendly. Solid-phase micro extraction, while fast compared to some other methods mentioned here, can require tens of minutes of off-line sample preparation prior to analysis and requires an individual solid phase concentrating cartridge per sample. 
         [0006]    Early MIMS systems were plagued with problems of irreproducibility, membrane memory effects, long response times and high detection limits (see Johnson, R. C.; Cooks, R. G.; Allen, T. M.; Cisper, M. E.; Hemberger, P. H. Membrane Introduction Mass Spectrometry: Trends and Applications.  Mass Spectrometry Reviews.  2000, 19(1), 1-37). To effectively analyze low level environmental contaminants, design improvements have been made to these devices. For example, a direct insertion membrane probe was built to place a capillary membrane directly inside the ionization source of a mass spectrometer located millimeters from the electron ionizing beam (see Bier, M. E.; Cooks, R. G. Membrane Interface for Selective Introduction of Volatile Compounds Directly into the Ionization Chamber for a Mass Spectrometer. Anal. Chem. 1987, 59(4), 597-601). The close proximity of the membrane to the ionization region allowed for rapid reproducible analysis of VOCs in water at low detection limits with minimal memory effects because the analyte was instantaneously ionized with little mixing rather than flowing inside a transfer line and the membrane was heated to increase permeation rates. In 1991, Silvon et al. developed a helium-purged hollow fiber membrane interface that allowed for detection at the sub part-per-billion (ppb) levels in the analysis of VOCs and SVOCs (see Silvon, L. E.; Bauer, M. R.; Ho, J. S.; Budde, W. L. Helium-Purged Hollow Fiber Membrane Mass Spectrometer Interface for Continuous Measurement of Organic Compounds in Water.  Anal. Chem.  1991, 63(13), 1335-1340). In Silvon&#39;s device, a capillary membrane is placed inside a flow cell with a helium purge running through the membrane to the mass spectrometer while the aqueous sample runs outside the membrane in the opposite direction. The best detection limit has come from Soni et. al. who detected 500 parts per quadrillion (ppq) of toluene with a S/N 11 with data point smoothing (see Soni, M. H.; Baronavski, A. P.; McElvany, S. W. Trace Analysis of Polyaromatic Hydrocarbons in Water Using Multiphoton Ionization-Membrane Introduction Mass Spectrometry.  Rapid Commun. Mass Spectrom.  1998, 12, 1635-1638). In this experiment, a stored wave form inverse Fourier transform (SWIFT) signal was applied to the end caps of a 3D quadrupole ion trap. The SWIFT waveform isolates desired analyte ions by ejecting only those contaminant ions at the frequencies in the SWIFT signal. The use of this waveform and other similar waveforms allowed for a significant concentration of the ions of interest. 
         [0007]    Finally, U.S. Pat. No. 6,360,588 to Ross et al. discloses an efficient and accurate method and apparatus for analysis of materials passing through a membrane. A sample is place on one side of a membrane and a carrier fluid from a reservoir flows past the other side of the membrane to carry any sample diffusing through the membrane to be detected. The disclosed method can allow for the accurate, precise, and specific real time measurements of compounds crossing a membrane. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0008]    According to one embodiment of the probe of the present disclosure, the probe is comprised of a first flow path, a second flow path defined by a membrane, the membrane separating the first and second flow paths, and a needle integrated with the membrane so as to form a part of the second flow path. In one embodiment of the method of the present disclosure, a method of operating a probe is comprised of flowing a material in a first flow path, flowing a fluid in a second flow path defined by a membrane, the first and second flow paths separated by the membrane, and transferring the fluid at the end of the second flow path to an ionization region of a mass spectrometer through a needle. 
         [0009]    In the disclosed embodiments of the apparatus and methods of the present disclosure, the carrier of the analyte to the ionization region is a liquid. The disclosed embodiments of the apparatus and methods allow for the coupling of a membrane interface directly to a mass spectrometer at atmospheric pressure. The membrane can be in capillary or sheet form and allows the introduction of a liquid or gaseous sample to one side of the membrane while the other side of the membrane is bathed with a solution that can easily be used in an atmospheric pressure ionization source. Volatile molecules permeate through a suitable membrane such as poly-dimethyl silicone (PDMS), mix into the appropriate solvent, and are ionized. The probe can be heated or cooled, various additives may be added to either side of the membrane, or a voltage potential may be impressed across the membrane to improve performance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]    For the present disclosure to be easily understood and readily practiced, the present disclosure will now be described, for purposes of illustration and not limitation, in conjunction with the following figures wherein: 
           [0011]      FIGS. 1A and 1B  illustrate embodiments of the present disclosure using a tubular capillary membrane fitted inside a stainless steel tube; mobile phase  1  travels within the space between the outside diameter of the membrane and the inside diameter of the steel tube while mobile phase  2  travels within the space within the center of the membrane. 
           [0012]      FIG. 2  illustrates an embodiment of a probe of the present disclosure using the tubular capillary membrane fitted within a stainless steel tube as shown in FIG  1 A, but including an electrospray needle; 
           [0013]      FIGS. 3A and 3B  illustrate the right and left Tees of the probe of  FIG. 2 , respectively; 
           [0014]      FIG. 4  illustrates a plot (using a probe constructed according to the teachings of the present disclosure) of ion current of m/z 9.1, 7 from aniline; at 1 minute an injection of HPLC grade water was made as a blank followed at 20 minutes by an injection of 1 μM (˜80 ppb) aniline solution; other parameters: 100 μL sample loop, 7 point smooth, 5 μL/min methanol flow rate, 100 μL/min water flow rate, 2.5 KV on spray needle; 
           [0015]      FIGS. 5A ,  5 B, and  5 C illustrate test results used to determine operating parameters; 
           [0016]      FIGS. 6A and 6B  are a comparison of the signal intensity of the mass chromatogram of 1 ppm aniline in water, m/z 94.1, using the probe of the present disclosure compared to the LCQ standard ESI source with no membrane, respectively; 
           [0017]      FIG. 7  is a plot demonstrating how the permeation of 1 μM aniline samples through the membrane at different pH values can be used to change the concentration of an analyte in the internal mobile phase. 
           [0018]      FIG. 8  illustrates a survey scan of a mixture of a five herbicide mixture each at 100 ppb with background subtraction using the ESI MIMS probe of the present disclosure and a rapid scanning technique; 
           [0019]      FIG. 9  is a mass spectrum from an analysis performed according to the teachings of the present disclosure showing the presence of four contaminants (boxed mass assignments) found in Buffalo Creek (Freeport, Pa.) water in negative ion mode; 
           [0020]      FIG. 10  illustrates another embodiment of the probe of the present disclosure in which a helical capillary membrane is supported by a wire frame for air analysis; 
           [0021]      FIG. 11  is a cross-section view of another embodiment of the probe of the present disclosure in which a capillary membrane is positioned inside a stainless steel tube, and a gas is forced through the probe for the analysis; 
           [0022]      FIG. 12  is an exploded view of another embodiment of the probe of the present disclosure which illustrates a spiral membrane air probe; 
           [0023]      FIG. 13  illustrates yet another embodiment of an air membrane probe according to the teachings of the present disclosure having a Peltier heating/cooling device. 
           [0024]      FIG. 14  illustrates how the teachings of the present disclosure may be used to monitor molecules in vivo using a capillary membrane probe coupled to an electrospray ionization mass spectrometer; 
           [0025]      FIG. 15  illustrates a probe constructed according to the teachings of the present invention having an adduct addition or photoionization; 
           [0026]      FIGS. 16A and 16B  illustrate embodiments according to the teachings of the present invention using multiple membranes with an EI/CI source in  FIG. 16A  and an ESI source in  FIG. 16B ; and. 
           [0027]      FIGS. 17A and 17B  illustrate results from using a MIMS probe constructed according to the teachings of the present invention for sampling air. 
       
    
    
     DETAILED DESCRIPTION  
       [0028]      FIGS. 1A and 1B  illustrate one embodiment of the device of the present disclosure. In the figures, a capillary membrane  10  in the shape of a tube is fitted within a stainless steel tube  12 . A space or channel  14  is formed between the outside diameter of the membrane  10  and the inside diameter of the steel tube  12 . Another space or channel  16  is formed inside of tubular membrane  10 . Mobile phase  1  (i.e. the phase that carries the analyte to the membrane) travels within space  14  as shown by the arrows  18 . Mobile phase  2  (i.e. the phase that carries the analyte away from the membrane to an ionization region) travels within space  16  as shown by arrow  20 . 
         [0029]    Volatile organics in mobile phase  1  are adsorbed onto and permeate through the membrane  10  and are then removed at the other side of the membrane  10  by mobile phase  2  that can easily be used in an atmospheric ion source. Such an ion source could be electrospray (ESI) ionization, or atmospheric pressure chemical ionization (APCI), among others. 
         [0030]      FIG. 1B  illustrates an embodiment similar to the embodiment shown if  FIG. 1A . However, in  FIG. 1B  two voltages are applied; voltage V 1  is applied to the steel tube  12  and thus mobile phase  1 , while voltage V 2  is applied to mobile phase  2 . The voltage differential may be used in certain circumstances to improve transfer rates through the membrane  10  in either direction depending on the polarity of the applied potential and charge on the molecular species. Such an embodiment could be used to improve the ESI signal by purifying molecules in mobile phase  2  by removing contaminants through an appropriate membrane material into mobile phase  1  or by concentrating analyte molecules into mobile phase  2  from mobile phase  1 . The voltage differential may be provided by any suitable means  21  such as a battery, power supply, multi-tap transformer or other power source, and maybe either AC or DC. 
         [0031]    One version of a membrane probe  28  constructed according to the present disclosure is shown in  FIGS. 2 ,  3 A and  3 B. The concept of the ESI membrane probe  28  was based on five main design considerations: i.) electrospray ionization rather than the more traditional EI/CI, although such other ionization techniques may be used ii.) two liquid mobile phases separated by the membrane rather than a liquid and gas phase, iii.) a capillary PDMS membrane, iv.) a short transfer line and v.) membrane heating. Design criteria four required the ESI needle to be incorporated into the membrane thus eliminating a transfer line and the fifth criteria of heating should improve the analyte permeation rate and thus signal strengths. 
         [0032]    The membrane probe  28  uses, for example, a tubular, PDMS capillary membrane  10  with dimensions of, for example, 0.025″ o.d. by 0.012″ i.d. and approximately 12 cm long. The capillary membrane  10  is inserted into a stainless steel tube  14  with dimensions of, for example, 0.0625″ o.d. by 0.040″ i.d. and approximately 10 cm long. The right end of the membrane  10  and steel tube  12  are inserted into a right Tee  30  (See  FIG. 3A ) while the left end of the membrane  10  and the steel tube  12  are inserted into a left Tee  32  (See  FIG. 3A ). The Tees  30  and  32  may have, for example, a bore having an i.d. of 0.0625″. The stainless steel tube  12  butts into each Tee  30 ,  32  and a seal is made with a standard HPLC PEEK fitting and 1/16″ ferrules  34 . 
         [0033]    A steel electrospray needle  36  (e.g. 28 G) with an outside dimension greater than 0.012″ (to allow for a good seal) is inserted into the membrane  10  at the extreme right side of the probe  28 , thus integrating the membrane  10  and the needle  36 . By “integrating” it is meant that the needle  36  forms a part of the mobile phase  2  flow path with little or no intervening mobile phase  2  flow path between the membrane  10  and needle  36 . The mobile phase  2  is immediately ionized at the needle&#39;s tip and is directly transferred to an ionization region  22  of a mass spectrometer. Burrs on the ends of the needle  36  were removed by dipping in an aqua-rega solution for approximately 1 minute and then immediately washing with water. The end of the membrane  10  carrying the needle  36  is inserted into a Teflon tube  38  (e.g. 0.625″ o.d.×0.030″ i.d.) that acts as a sleeve. A ferrule  40  seals the Teflon tube  38  to the membrane  10 , and the membrane  10  to the needle  36 . At the other end of the probe  28 , a capillary  42  such as a fused silica capillary coated with polyimide, is inserted into the membrane  10 , and this assembly is further inserted into a Teflon tube  44  (e.g. 0.625″ o.d.×0.030″ i.d.). A ferrule  46  seals the Teflon tube  44  to the membrane  10 , and the membrane  10  to the capillary  42 . 
         [0034]    Heat can be provided to the membrane  10  by adding a thermal mass  50 , such as a copper or aluminum heating block, around the exterior of the stainless steel tube  12 . A thermocouple  52  may be inserted into a hole drilled into one end of the thermal mass  50  block and the thermal mass  50  may be wrapped with heating tape (not shown) and driven by a standard temperature controller  54  to maintain the thermal mass  50  to ±1 degree Celsius. 
         [0035]    While the mobile phase  2  inside the capillary membrane flows to the right in  FIG. 2  where the needle  36  is attached, the mobile phase  1  flows in the opposite direction. This flow arrangement maximizes the concentration of the analyte from mobile phase  1  to mobile phase  2 . Additionally, the geometry of the capillary membrane  10  is also advantageous because it provides a means of concentrating the analyte by adsorption on a large surface area, (the outside of the membrane) followed by desorption from a surface area half that size, (the inner surface of the capillary membrane). 
         [0036]    Preliminary tests were performed using the probe  28  of  FIG. 2 . Samples were injected into a flow of water (or other solvent) using a standard 6-port injector and these sample plugs entered a Tee port  60  on the right Tee  30  that directed the water flow to the space  14  between the outside diameter of the membrane  10  and the inside diameter of the tube  12 . The analyte permeates from the outside to the inside of the capillary membrane  10  and is thus concentrated due to the geometry of the membrane  10 . The analyte is desorbed from the inner surface of the capillary member  10  where it enters the mobile phase  2 , i.e. a stream of methanol, ethanol, isopropanol or other suitable solvent for electrospray. An acid such as acetic or formic or other suitable acid can be added to mobile phase  2  to help with ESI and to charge the analyte so that it is less likely to permeated back through the membrane  10 . In addition, cations such as Li +  or Na +  or Ag +  can be added to mobile phase  2  to allow for enhanced ionization of molecules that do not typically ionize by proton addition. For negative ESI a suitable mobile phase without acid may be used. 
         [0037]    Preliminary results from the use of the probe the MIMS probe  28  are shown by a plot of ion current of m/z 9.1, 7 from aniline in  FIG. 4 . At 1 minute an injection of HPLC grade water was made as a blank followed at 20 minutes by an injection of 1 μM (˜80 ppb) aniline solution. Other parameters used in the analysis include: 100 μL sample loop, 7 point smooth, 5 μL/min methanol flow rate, 100 μL/min water flow rate, and 2.5 KV was applied to the spray needle; 
         [0038]    Additional tests were performed as follows. For mobile phase  1 , a HPLC grade water was used and for mobile phase  2 , HPLC grade methanol was used. When running in the positive ion mode, acetic acid was added to the methanol. Test samples of aniline, acetic acid and trifluoroacetic acid were from Sigma-Aldrich. Pyridine was from Fisher Scientific, and the environmental mixes of herbicides and phenols were purchased from Supelco (Bellefonte, Pa.). 
         [0039]    The electrospray ion trap mass spectrometer used in this study was an LCQ model from Thermo Electron Corporation (San Jose, Calif.). The software used on the LCQ was Xcalibur version 1.1. The scan modes used were full scan MS, SIM, MS/MS and the ‘turbo’ scan. The turbo scan is a rapid scan used to increase sensitivity with reduced mass resolution. For comparison, some experiments used the standard LCQ electrospray ionization source. For all of the MIMS experiments, the standard LCQ ESI source was replaced with the MIMS probe which incorporated its own ESI source. The on-board valve of the LCQ mass spectrometer was used for flow-injection analysis (FIA). 
         [0040]    To mount the probe assembly  28  to the mass spectrometer, the standard LCQ ESI probe head was removed and the MIMS probe  28  was positioned with the ESI needle  36  approximately 5-8 mm away from the LCQ heated capillary inlet. 
         [0041]    Four operating parameters of the MIMS probe  28  were optimized using continuous flow experiments: electrospray voltage, internal mobile phase flow rate, external mobile phase flow rate and membrane temperature. 
         [0042]    The MIMS probe  28  ESI needle  36  voltage was optimized using two separate methods. The stability of the Taylor cone and spray was viewed using an Olympus SZ-CTV 10-60X magnifying scope and the stability and signal strength of the mass spectrum was observed using the mass spectrometer. An ESI voltage of 2.5 kV was determined to be optimal. Higher voltages resulted in multiple Taylor spray cones and instability of the spray. The MIMS probe ESI voltage is less than the standard value used on the LCQ of 4-5 kV as expected given the smaller ESI needle dimensions used on our MIMS probe. No sheath gas was used to help with nebulization. 
         [0043]    For the internal mobile phase (i.e. mobile phase  2 ) and the external mobile phase (e.g. mobile phase  1 ) flow rate and temperature optimization experiments, the external mobile phase, which is normally water, was replaced with 1 μM aniline and the intensity of the [M+H] +  ion at m/z 94.1 averaged over 100 scans at each new setting was plotted versus the parameter of interest. The optimum setting was determined from the maximum signal recorded in these plots. To determine an optimal internal flow rate, the external flow rate was held at 60 or 100 μL/min while the probe was operated at 60° C. Each time the value of the internal flow rate was changed the probe was equilibrated for up to 10 minutes. The average counts for m/z 94.1 were plotted against the internal flow rate in  FIG. 5A . From this plot, 3.5 μL/min was selected as a suitable flow rate for the internal solvent. The plot shows that the signal was optimized at flow rates between 2-4 μL/min. Because ESI is a concentration, rather than amount dependent technique, a reduced signal must be due to either reduced analyte concentration or some other factor. At higher internal flow rates, it is expected that less exposure time of the desorbing sample to the internal mobile phase would result in a smaller concentration of analyte in the internal solution, however, at the low flow rates, only higher analyte concentrations would be expected until equilibrium was reached. At the lowest internal flow rates studied, the reduced signal may be the result of a poor spray which can also occur with the standard LCQ ESI source at less than 3 μL/min. Further optimization of the ESI voltage may have improved the spray at these low flow rates, but we did not undertake these experiments. 
         [0044]    The use of a two liquid interface where the internal flow rate may be reduced to allow the analyte more time to concentrate into the internal solvent is an advantage of our technique over the pervaporization techniques because in our liquid/membrane/liquid case, the analyte is not diluted by rapid gaseous diffusion. Gaseous diffusion in the pervaporization case would also be of concern in a flow injection analysis (FIA) experiment, however, with the ESI MIMS technique, the slow diffusion rates of analytes in the liquid prevent rapid dilution and limit peak broadening. 
         [0045]    To determine the optimal external mobile phase flow rate we set the internal flow rate to 3.5 μL/min and heated the probe to 60° C. The external flow rate was varied, and each time the setting was changed the system was allowed to equilibrate. The average counts for m/z 94.1 were plotted against the external flow rate. From this plot it was determined that 60 μL/min was the optimal external flow rate as shown in  FIG. 5B . 
         [0046]    To determine the optimal membrane temperature, the internal and external flow rates were set to 3.5 μL/min and 60 μL/min, respectively, and the temperature was varied from 20 to 75° C. After each change in temperature the system was allowed to equilibrate for 10 minutes. The average counts for aniline at 94.1 m/z were plotted against membrane temperature as shown in  FIG. 5C  with the error bars representing the standard deviation of the 100 scans collected. From 20-60° C. there is an increase in signal until a maximum is reached at 60° C. This signal increase is due to improved analyte permeation through the membrane at higher temperatures. Above 65° C., the boiling point of methanol, the signal becomes erratic due to the formation of gas bubbles disrupting the liquid flow to the electrospray needle. By 70° C. the signal was reduced by four fold. Other solvent systems with higher boiling points should be explored in the ESI MIMS probe to further enhance the analyte flux at even higher temperatures. Ethanol (b.p. 78° C.) should allow for a higher operating temperature and improve analyte permeation. Previous MIMS pervaporization experiments with liquid water showed an improved ion signal for 120 ppm benzene up to approximately 95° C. It should be noted that the entire length of membrane would not have been heated to 60° C. since the ends of the membrane, approximately 1 cm in length were not heated well. 
         [0047]    Flow injection analysis (FIA) experiments were used so that a sample plug could be injected inbetween segments of pure water. By using FIA we could flush the membrane with clean solvent between injections and also use background subtraction to reduce the chemical noise for our analysis. To obtain low detection limits we wanted to maximize the period for which the sample was exposed to the membrane; therefore a sample loop size was selected to allow for this maximal rise. A sample of 1 μM aniline was injected three times, with a different sample loop size each time. When the sample loop was increased from 100 μL to 250 μL we saw an increase in the peak height and area by a factor of about 2.5. However, when we went from a 250 μL loop to a 500 μL sample loop the peak broadened with no significant change to the signal height. To reduce the time for analysis we selected a 250 μL volume and that sample loop was used for all experiments. An external flow rate of 60 μL/min allows for a three min analyte exposure time. 
         [0048]    Admittedly, our determination of the optimum parameters was not exhaustively iterative and we also did not change design parameters such as the membrane length or wall thickness or perform extensive studies with other internal solvent systems. This methodology was used to reach reasonably good settings so as to efficiently explore the potential of the ESI MIMS probe as a useful analytical device. Further studies should improve the performance of the ESI MIMS probe. 
         [0049]    To demonstrate the performance of the probe, a figure of merit is shown in  FIGS. 6A and 6B  where we compare the improved signal intensity in the mass chromatogram of 1 ppm aniline in water, m/z 94.1, using the ESI MIMS probe  28  compared to the LCQ standard ESI source with no membrane. Three notable features are seen in this data. One, the signal is approximately 10× higher due to the enrichment of aniline in the methanol with the MIMS probe; two, the noise level is improved by approximately 7× which we expect is primarily due to the improve ESI performance by electrospraying methanol instead of 100% water; and finally, the width of the peak using ESI MIMS is about 20% greater and has significantly more tailing due to the permeation of the aniline through the membrane and the residue bleed from the PDMS membrane. The overall S/N improvement of the ESI MIMS method was 70× over direct ESI of a sample in water. 
         [0050]    The linear dynamic range was tested for aniline and pyridine, and it was determined that the linear dynamic range for aniline was from 100 pptr to 600 ppb and for pyridine it was 1 ppb to 600 ppb. Above this range the MIMS system was not quantitatively accurate. The drop off in signal may be due to the membrane nearing saturation and so the signal will no longer increase in the same proportion to concentration. The influence of ionization efficiency for a specific analyte may also change as the analyte composition changes and this effect should be explored at the concentration levels of interest for quantitative analysis work. 
         [0051]    The detection limits of the ESI MIMS probe were tested for four main classes of compounds: organic acids, organic bases, herbicides, and phenolic compounds. For these measurements the mass spectrometer was operated in SIM mode with a mass isolation width of ±2.5 Da. The detection limits for compounds in each of these classifications are shown in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Detection limits using the ESI MIMS probe and SIM scan mode. 
               
             
          
           
               
                   
                 Molecular 
                 Ion 
                 Limit of 
                   
               
               
                   
                 Formula 
                 (m/z) 
                 Detection 
                 S/N 
               
               
                   
                   
               
             
          
           
               
                 Positive Ions 
                   
                   
                   
                   
                   
               
               
                 Organic Bases 
               
               
                 Aniline 
                 C 6 H 7 N 
                 94.1 
                 75 
                 pptr 
                 8 
               
               
                 Pyridine 
                 C 5 H 5 N 
                 80.1 
                 1.0 
                 ppb 
                 14 
               
               
                 Herbicides 
               
               
                 Alachlor 
                 C 14 H 20 ClNO 2   
                 269.9 
                 5.0 
                 ppb 
                 94 
               
               
                 Atrazine 
                 C 8 H 14 ClN 5   
                 216.1 
                 250 
                 pptr 
                 22 
               
               
                 Butachlor 
                 C 17 H 26 ClNO 2   
                 311.9 
                 1.0 
                 ppb 
                 24 
               
               
                 Metolachlor 
                 C 15 H 22 ClNO 2   
                 283.9 
                 1.0 
                 ppb 
                 59 
               
               
                 Simazine 
                 C 7 H 12 ClN 5   
                 202.0 
                 500 
                 pptr 
                 24 
               
               
                 Negative Ions 
               
               
                 Organic Acids 
               
               
                 Acetic Acid 
                 C 2 H 4 O 2   
                 59.0 
                 1.1 
                 ppm 
                 16 
               
               
                 Trifluoroacetic Acid 
                 C 2 HF 3 O 2   
                 113.0 
                 150 
                 ppm 
                 64 
               
               
                 Phenolic Compounds 
               
               
                 2,4-Dinitrophenol 
                 C 6 H 4 N 2 O 5   
                 183.0 
                 1.5 
                 ppm 
                 41 
               
               
                 4-Chloro-3-methylphenol 
                 C 7 H 7 ClO 
                 141.0 
                 500 
                 pptr 
                 50 
               
               
                 Pentachlorophenol 
                 C 6 HCl 5 O 
                 265.0 
                 100 
                 pptr 
                 10 
               
               
                   
               
             
          
         
       
     
         [0052]    The detection limits are in the low ppb to high pptr range, with the exception of acetic acid, trifluoroacetic acid, and 2,4-dinitrophenol, all of which have detection limits in the ppm range. These acids have higher detection limits because a significant proportion of these molecules are dissociated in the external aqueous phase and so ion permeation through the nonpolar membrane should be poor. Trifluoroacetic acid (TFA) is a much stronger acid than acetic acid and its detection limit is two orders of magnitude higher, indicative that a much higher proportion of the TFA molecules are dissociated in the aqueous sample. Although 2,4-dinitrophenol is not significantly dissociated in water, the nitro groups are quite polar, and a lower permeation rate through the membrane is expected. 
         [0053]    To determine the amount of analyte that the probe extracts from the sample the quantitative properties of the probe were utilized. Aniline solutions of 0.1 μM, 0.5 μM, 1 μM, 1.5 μM, and 2 μM were run and the average peak area over three injections were plotted against the concentration of aniline in the solution to determine a calibration curve. Next, three injections of an aniline solution of 10 μM were made during which time the waste eluting from the probe was collected. The total volume infused was 3.00 mL. This waste solution was re-injected three times and the calibration curve was used to determine that the waste had a concentration of 1.5 μM aniline. The three injections of 10 μM aniline totaled 750 μL sample equal to 7.50×10 −3  μmoles. The 3.00 mL of 1.5 μM aniline waste equaled 4.5×10 −3  moles. This indicates that 3.00×10 −3  μmoles or 40% of the analyte molecules permeated the membrane. If we consider the small total volume of 175 μL of the internal mobile phase, methanol, used during the 50 minute experiment compared to the volume of external water used, we see that the probe does act as a good concentrator. During the experiment, the average concentration of aniline in the external solvent was 2.5 μM and the average internal concentration of aniline was 17.14 μM or 6.86 times higher. 
         [0054]    Additives in either the external or internal mobile phase can be used to improve permeability and/or the ionization efficiency of the analytes.  FIG. 7  shows how the pH of the aqueous sample can be adjusted with HCl to either enhance or suppress the permeation of aniline. A blank was injected at 5 minutes and samples of 1 μM aniline, at different pH values of 6.9, 2.9, 2.6, 2.3, and 2.0 were injected at every additional 15 minutes. At low pH a higher proportion of aniline would be protonated, which would inhibit permeation through the membrane. We still see a significant peak below the pKa=4.63, which is probably partially due to mixing of the sample plug with the external mobile phase, which would occur in the Tees. Although the internal mobile phase encounters negligible dead volume inside the device, the external mobile phase flows through a 90 degree bend through the tee and this is a site where mixing would occur. Future ESI MIMS probe designs should attempt to overcome this mixing region. 
         [0055]    Increasing the RF ramp during analysis is a rapid scan feature of the LCQ called a “Turbo scan”. This scan allows trapped ions to be scanned out of the trap much more quickly than in the normal scan rate. This scan significantly decreases the mass resolution, but it increases the signal height and in several cases can allow for determinations that were not possible in the full scan mode. The capabilities of the Turbo scan were demonstrated for the molecular weight determination of a protein. A mixture of five herbicides (alachlor, atrazine, butachlor, metolachlor, and simazine) each at a concentration of 100 ppb where injected. In the Turbo scan mode shown in  FIG. 8 , all five herbicides are observed. This MIMS example shows the advantage of first using the rapid scan for a survey analysis which could be followed by a mass spectrometry/mass spectrometry (MS/MS) scan of each analyte for identification. 
         [0056]    For a real life application of the MIMS ESI probe, a water sample was taken from the water treatment plant on Buffalo Creek a few miles upstream from Freeport, Pa. The 50 mL sample was collected from a pipe carrying the treated effluent that flowed into the creek. The sample was filtered with a 0.2 micron filter to remove any organic material that could dirty the membrane, injected into the MIMS probe and the permeate was electrosprayed in negative ion mode. As shown in  FIG. 9 , contaminant ions with masses shown in boxes were detected at m/z 199.5, 205.5, 227.5, and 253.4. The FIA mass chromatogram of one of the contaminants is shown in the inset where an injection of a blank between two injections of the effluent shows the presences of the molecule at m/z 199.5. Although not identified, the observation of the four contaminants shows the utility of the ESI MIMS probe for environmental analysis. 
         [0057]    According to another embodiment of the present invention, mobile phase  1  may be a gas. Volatile organics in the gas phase are adsorbed onto and permeate through the membrane and are removed at the other side by a liquid mobile phase that can easily be used in an atmospheric ion source. Examples are shown in  FIGS. 10-13 . 
         [0058]    Turning first to  FIG. 10 , an air membrane probe  66  is illustrated. The probe  66  is comprised of a capillary membrane  68  wound about a wire support frame  70  and having an integrated needle  36 . A gaseous mobile phase  1  is introduced into the probe  66  as shown by the arrow  72  while the liquid mobile phase  2  flows through the capillary membrane  68  as shown by the arrow  20 . The probe  66  can be temperature controlled so as to be cooler or warmer than ambient. 
         [0059]      FIG. 11  is a cross-sectional view of another air membrane probe  76  that uses ESI. The probe  76  is similar to the probe  28  of  FIG. 2A  except that the stainless steel tube  12  carrying the membrane  10  is wound about a heated support block  80 . Another difference is that mobile phase  1  is a gas as shown by the arrow  72 . A third difference is the use of a vacuum pump (not shown) which pulls a vacuum (e.g. 100-10,000 mL/min) as represented by the arrow  82  for the purpose of pulling the gaseous mobile phase  1  through the probe  76 . 
         [0060]      FIG. 12  is an exploded view illustrating another embodiment of an air membrane probe  86 . The probe  86  is comprised of a cover plate  88  having a spiral geometry  89  formed therein. The membrane is further comprised of a support member  90  having the same spiral geometry  89  formed therein. The spiral geometry shown in  FIG. 12  is one of many possible geometries. A sheet membrane  92  is sandwiched between the cover plate  88  and the support member  90  so that the sheet membrane  92  forms one of the walls of the flow path for mobile  2 . An integrated needle  36  is also provided. In this case, there is small gap between the wall formed by the sheet membrane  92  and the needle. The gaseous mobile phase  1  is represented by the arrow  72  while the liquid mobile phase  2  is represented by the arrow  20  as in the other figures. 
         [0061]      FIG. 13  illustrates another embodiment of an air membrane probe  96  having a Peltier heating/cooling device  100 . In  FIG. 13 , a membrane  98  is spiral wound (see top view) on one surface of the Peltier device  100 . The capillary membrane is cooled to condense out volatile organics onto the membrane surface; the membrane can then be heated to rapidly transfer the adsorbed analyte through the membrane into the internal mobile phase (i.e. mobile phase  2 ). This cooling and heating technique could also be used in a liquid/membrane/liquid configuration to improve analyte signal. 
         [0062]      FIG. 14  illustrates how the teachings of the present disclosure may be used to monitor molecules in vivo using a capillary membrane probe  106  coupled to an electrospray ionization mass spectrometer  107 . The probe is comprised of a tubular capillary membrane  108  attached on either end by non membrane tubes  108   a ,  108   b . The capillary membrane is inserted into a blood vessel  110  or other location in a living body. Mobile phase  1  is the fluid flowing in the living body as represented by the arrow  112  while mobile phase  2  is represented by the arrow  114 . 
         [0063]      FIG. 15  illustrates a probe constructed according to the teachings of the present invention. A first means  120  for inserting additives into mobile phase  1  and a second means  122  for inserting additives into mobile phase  2  are illustrated. The means may be any suitable device (e.g. syringe, pump, among others) suitable for inserting the additive into the mobile phase.  FIG. 15  illustrates the addition of various cations (e.g. H+, Na+, Ag+, Li+, Cl − , K+, among others) into mobile phase  2 . Various different means of ionization may be used with the probe of the present disclosure including, electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photo-ioization (APPI), inductively coupled plasma (ICP), among others. 
         [0064]      FIGS. 16A and 16B  illustrate embodiments according to the teachings of the present invention using multiple membranes with an ESI source. In  FIG. 16A  the gaseous analyte molecules are adsorbed onto the surface of membrane  1  and are transferred into the mobile phase  1 . A second step of analyte transfer occurs through membrane  2  and into the mobile phase  2 . The analyte pervaporates from membrane  3  and undergoes electron ionization (EI) or chemical ionization (CI). In  FIG. 16B  we show FIA with three concentric membranes. The analyte is purified in stages from the sample plug through the membrane  1  and into mobile phase  2 , and then from mobile phase  2  through the second membrane into the mobile phase  3 . From mobile phase  3 , the analyte passes through membrane  3  and into an organic mobile phase. Once in the organic mobile phase, the liquid solution goes under ESI. These embodiments can be used for additional stages of purification or selectivity if different membranes are used. 
         [0065]      FIG. 17A  illustrates the relative abundance of pyridine.  FIG. 17B  illustrates the results from sampling the air above a vial of 10 mM Pyridine. The signal at m/z 80 clearly goes up demonstrating the utility of the MIMS probe of the present disclosure for sampling air. 
         [0066]    The MIMS probe of the present disclosure uses a capillary membrane (PDMS). In certain embodiments, the membrane is a concentrator due to the geometry of the membrane (the sample travels from a larger exterior surface to a smaller interior surface) unlike in pervaporization techniques. The mobile phases flow counter to one another which also acts to increase sample concentrations. Mobile phase  2  has little dead volume due to the internal design of the probe, and thus flow rates are low. The low flow rates result in low volumes and higher analyte concentrations. 
         [0067]    The probe of the present disclosure integrates the ESI needle with the membrane. Thus, the ESI needle is part of the probe, reducing dead time and placing the ion source (both negative and positive ions are possible) near the membrane. Heating or cooling can be applied to the membrane to increase the permeation rate or adsorption rate, respectively. Liquids or gases can be used for mobile phase  1 . As a result, the liquid mobile phase  2  may be the same as or different from mobile phase  1 . 
         [0068]    The probe of the present disclosure can be used a real-time concentrator unlike the solid phase extraction columns. The probe of the present disclosure maybe used with different ionization species such as protons (e.g. H+, Na+ or Li+, K+, Cl −  among others). These supplemented species can be used to ionize molecules that are not typically ionized by proton addition which allow many more volatile molecules to be observed. 
         [0069]    Flow rates may be adjusted in both the external mobile phase (mobile phase  1 ) and the internal mobile phase (mobile phase  2 ) to affect the performance (such as by enhancing the sensitivity). A low internal flow creates an increased signal due to a concentrating effect. The ESI needle allows for a small ion point source. 
         [0070]    Voltage potentials can be added on either side of the membrane to cause an increase or decrease in permeation/migration of specific molecules in either direction across the appropriate membrane. Chemicals can also be added to mobile phase  1  to improve performance. Different solvent phases can be used on the two sides of the membrane. This is advantageous because one can use a good ESI mobile phase for mobile phase  2  such as acidic methanol while using an aqueous phase for mobile phase  1 . As a result, the operator avoids the direct ESI of water solutions which is not typically very good. Other membrane materials should be possible, for example, fluorocarbon polymer types. 
         [0071]    The probe of the present disclosure can be used for low level water analysis of semi-volatile organics, and works at atmospheric pressure unlike other MIMS probes for EI/CI sources. The probe of the present disclosure can also be used for air analysis. Finally, a multi-stage probe having more than one membrane is possible. 
         [0072]    While the present invention has been described in conjunction with preferred embodiments thereof, those of ordinary skill in the art will recognize that many modifications and variations are possible. The present disclosure is intended to cover such modifications and variations, and is intended to be limited only by the scope of the following claims.