Abstract:
An ion focusing member includes a ball having a surface with a plurality of dimples. The ion focusing member is adapted for being disposed in a mass spectrometer in a way that the ball is located at a spray path of analyte ions and located between a metal capillary and a mass analyzer. When the analyte ions pass through the ball, the analyte ions can be gathered at a downstream position of the ball, which in turn flow into the mass analyzer by a potential difference. Therefore, the ion focusing member of the present disclosure can effectively enhance the amount of the analyte ions entering into the mass analyzer, thereby improving ion transmission efficiency. As a result, a mass spectrometer equipped with the ion focusing member may have increased signal intensity of analyte, lowered limit of detection (LOD), and minimized detection error.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present disclosure relates generally to an ion focusing member and more particularly, to an ion focusing member adapted to be applied in a mass spectrometer for gathering analyte ions. The present disclosure further relates to a mass spectrometer using the ion focusing member. 
         [0003]    2. Description of the Related Art 
         [0004]    Recently, the mass spectrometer using electrospray ionization (hereinafter referred to as ‘ESI’) device has been widely used in the fields of identification of synthesized compounds, detection of environmental toxic substances, analysis of energy ingredients, development of drugs, biological metabolomics or pharmacometabolomics, analysis of natural products, food analysis, etc. 
         [0005]    In general, a mass spectrometer includes an ionization device, a mass analyzer and a detector.  FIG. 1  shows a schematic diagram of ionization mechanism of commercial ESI device  10 . The conventional ESI device  10  includes a metal capillary  11  having an open end  111  that opens toward a sample inlet  21  of a mass analyzer  20 . When the ESI device  10  is in use, an electric field, for example, a potential difference of 3,000 V to 5,000 V, is established between the open end  111  and the sample inlet  21  of the mass analyzer  20 . Subsequently, a sample solution  30  contained in the metal capillary  11  is forced out of the metal capillary  11  for traveling toward the open end  111 . The sample solution  30  forms a Taylor Cone T that is filled with electric charges when it passes through the open end  111  due to the electric field present between the open end  111  and the sample inlet  21  and the surface tension of the sample solution  30  at the open end  111 . As the electric field force overcomes the surface tension of the sample solution  30  at the open end  111 , liquid droplets  31  containing multivalent electric charges are formed, that is, a so-called electrospray phenomenon occurs. The solvent contained in the liquid droplets  31  vaporizes by a nebulization gas  40  to form analyte ions  33  which in turn travel into the mass analyzer  20  through the sample inlet  21 , a mass spectrum is thus obtained. 
         [0006]    As shown in  FIG. 1 , because the metal capillary  11  of the conventional ESI mechanism may create the plume-like analyte ions  33 , that is, the analyte ions  33  may form a dispersion area much larger than a sectional area of the sample inlet  21  of the mass analyzer  20 , at least 50% of the analyte ions  33  cannot flow into the mass analyzer  20 . As a result, the mass spectrometer equipped with the conventional ESI device has the problems that the signal strength of analyte is significantly decreased and the detection limit cannot be lowered. 
         [0007]    In order to improve the aforesaid problems, many methods for focusing ions to be transmitted into mass spectrometer by controlling electric field such as Field Asymmetric Ion Mobility Spectrometry (hereinafter referred to as ‘FAIMS’) have been developed. However, FAIMS has limited effect on ion-focusing under the influence of Maxwell&#39;s equation and has limited applicability due to its large volume and expensive price as well as it is not adapted to various mass spectrometers. 
       SUMMARY OF THE INVENTION 
       [0008]    In light of the above, it is an objective of the present disclosure to provide an ion focusing member, which can be directly applied to various mass spectrometers, has good applicability, and is capable of effectively enhancing the amount of analyte ions entering into the mass analyzer to improve the signal strength of analyte and lower the detection limit of mass spectrometer. 
         [0009]    It is another objective of the present disclosure to provide a mass spectrometer using the aforesaid ion focusing member. 
         [0010]    To attain the above-mentioned objective, the present disclosure provides an ion focusing member which is adapted to be applied in a mass spectrometer including a metal capillary for spraying analyte ions and a mass analyzer having a sample inlet. The ion focusing member includes a ball having a surface with a plurality of dimples. The ion focusing member is disposed inside the mass spectrometer in a way that the ball is located at a spray path of the analyte ions and adjacent to the sample inlet of the mass analyzer, thus the ball may have a front side facing toward the metal capillary and a back side facing toward the mass analyzer respectively. As such, when the analyte ions sprayed from the metal capillary flow toward the ball, the analyte ions can be very close to the surface of the ball because of the dimples and then are gathered at a downstream position of the back side of the ball where it is adjacent to the mass analyzer. The analyte ions gathered at the downstream position of the back side of the ball can thus enter into the mass analyzer due to a potential difference established between the metal capillary and the mass analyzer. 
         [0011]    In comparison with the conventional ESI device, because the plume-like analyte ions can be efficiently collected at a downstream position of the back side of the ball due to the principle of fluid dynamics and then pass into the mass analyzer, the ion focusing member of the present disclosure can effectively increase the amount of analyte ions entering into the mass analyzer, thereby successfully improving ion transmission efficiency. Accordingly, when the present disclosure is used in a mass spectrometer, the mass spectrometer may have the advantages of increased signal intensity of analyte, minimized detection error and lowered detection limit. 
         [0012]    In the ion focusing member of the present disclosure, each of the plurality of dimples on the surface of the ball may have a diameter of between 1 nm and 1 mm. 
         [0013]    In the ion focusing member of the present disclosure, each of the plurality of dimples on the surface of the ball may have a depth of 1 nm to less than a radius of the ball. 
         [0014]    In the ion focusing member of the present disclosure, it is preferable that the ball is made of a material which is resistant to acidic and basic solutions, organic solvents and a high temperature of 260° C. or more, so as to prevent damage to the ball or avoid causing erroneous analysis results. 
         [0015]    On the other hand, a mass spectrometer using the aforesaid ion focusing member is also provided in the present disclosure. The mass spectrometer includes a mass analyzer having a sample inlet, a metal capillary for spraying analyte ions, and the above-mentioned ion focusing member disposed between the mass analyzer and the metal capillary in a way that the ball is located at a spray path of the analyte ions. 
         [0016]    Accordingly, when the analyte ions are sprayed to the front side of the ball, the analyte ions will move along the surface of the ball and then be collected between the back side of the ball and the sample inlet of the mass analyzer. Because of the potential difference established between the metal capillary and the mass analyzer, the analyte ions collected at a downstream position of the back side of the ball can thus move toward the sample inlet of the mass analyzer to finally enter into the mass analyzer, such that the amount of the analyte ions entering into the mass analyzer can be enhanced effectively. As a result, the mass spectrometer of the present disclosure may have the advantages of high ion transmission efficiency, minimized detection error and lowered detection limit. 
         [0017]    A more detailed constructions and characteristics of the ion focusing member and the mass spectrometer of the present disclosure will be more apparent upon reading the following description in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The present disclosure will be described through the following embodiments with reference to the accompanying drawings wherein: 
           [0019]      FIG. 1  is a schematic diagram of ionization mechanism of commercial ESI device. 
           [0020]      FIG. 2  is a schematic diagram showing that an ion focusing member according to a preferred embodiment of the present disclosure is disposed inside an ionization chamber of a mass spectrometer. 
           [0021]      FIG. 3  is a partially enlarged cross-sectional view of the ball of the ion focusing member according to the present disclosure. 
           [0022]      FIG. 4  is a schematic diagram showing that the ion focusing member of the preferred embodiment of the present disclosure is used to gather analyte ions. 
           [0023]      FIG. 5  is a graph showing the ratios of signal intensities of 20 amino acids in oolong tea detected respectively by a mass spectrometer equipped with a conventional ESI device and a mass spectrometer equipped with a conventional ESI device in cooperation with the ion focusing member of the present disclosure. 
           [0024]      FIG. 6  is similar to  FIG. 5 , but showing the ratios of signal intensities of 20 amino acids in human urine. 
           [0025]      FIG. 7  is similar to  FIG. 5 , but showing the ratios of signal intensities of 20 amino acids in human serum. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0026]    It is to be noted that the drawings of the present disclosure are not drawn in actual scale but are exaggerated to make the construction clearly understandable. In addition, like reference numerals designate like elements throughout the specification. 
         [0027]    Referring to  FIG. 2 , an ion focusing member  60  according to a first embodiment of the present disclosure includes a ball  61 . 
         [0028]    The ball  61  has a surface  611  with a plurality of dimples  613 . Preferably, the ball  61  is made of a material resistant to acidic and basic solutions, organic solvents and a high temperature of 260° C. or more so as to prevent damage to the ball or avoid causing erroneous analysis results. For example, the ball  61  may be made of polyetheretherketone (PEEK), polyimide (PI), ceramic, or glass, and in the present embodiment the ball  61  is made of PI. Each of the plurality of dimples  613  may have, but not limited to, a cross section of a circle or an ellipse. In fact, the ball  61  may include some dimples  613  having a circular cross-section and some dimples  613  having an elliptical cross-section. Each of the plurality of dimples  613  may have a diameter A of between 1 nm and 1 mm and may have a depth D of 1 nm to less than a radius of the ball  61 . There is no specific limit in the pitches I formed between the dimples  613 , that is to say, the dimples  613  may be arranged equidistantly or non-equidistantly on the surface of the ball  61 . As shown in  FIG. 3 , the “diameter” A used herein means a length of an opening of each dimple  613  formed on the surface  611 , i.e. the distance along the longest axis or the shortest axis. The “depth” D used herein means a distance between the surface  611  and a bottom of the dimple  613 . The “pitch” I used herein means a shortest distance between two adjacent edges of two adjacent dimples  613 . 
         [0029]    In practice, as shown in  FIG. 2 , the ion focusing member  60  of the embodiment is placed inside an ionization chamber of a mass spectrometer equipped with a metal capillary  11  and a mass analyzer  20  having a sample inlet  21 . There is no specific limit in the method for disposing the ion focusing member  60  in the ionization chamber of the mass spectrometer. For example, the ball  61  may have a rod (not shown) inserted therein, and then one end of the rod can be fixed to a wall of the ionization chamber of the mass spectrometer so as to locate the ball  61  between the metal capillary  11  and the sample inlet  21  of the mass analyzer  20 . As such, a side of the ball  61  facing toward the metal capillary  11  is defined as a front side F and another side of the ball  61  facing toward the mass analyzer  20  is defined as a back side B which is adjacent to the sample inlet  21  of the mass analyzer  20 . 
         [0030]    As shown in  FIG. 4 , when the analyte ions  33  are sprayed from the metal capillary  11  and flow toward the front side F of the ball  61 , because the laminar flow near the surface  611  of the ball  61  is disturbed by the dimples  613  on the surface  611 , the plume-like analyte ions  33  can be very close to the surface  611  of the ball  61  when they flow through the ball  61  and then are gathered at a downstream position of the back side B of the ball  61 . Finally, the analyte ions  33  gathered at the downstream position of the back side B enter into the mass analyzer  20  due to a potential difference established between the metal capillary  11  and the mass analyzer  20 , a mass spectrum is thus obtained. With the principle of fluid dynamics, the analyte ions  33  that may be lost in the conventional ESI can be focused between the back side B of the ball  61  and the sample inlet  21  of the mass analyzer  20 , thus the amount of the analyte ions  33  entering into the mass analyzer  20  can be greatly increased. 
         [0031]    The present disclosure will further be clarified through the following Examples. However, it should be understood by those skilled in the art that the Examples are only used to illustrate the present disclosure without limiting the scope of the present disclosure. Various modification and variations can be made to present disclosure without departing from the spirit or scope of the invention. 
       EXAMPLE 1 
     Analysis of Amino Acids in Oolong Tea 
       [0032]    First, 1 mg of oolong tea leaves were pulverized and mixed with 10 mL of ultrapure water (resistivity: 18.2 MΩ·cm). The mixture was then heated in a 90° C. water bath for 5 minutes with stirring. After cooling to room temperature, the mixture was centrifuged at 14,000 rpm for 5 minutes at 4° C. to collect the supernatant. 
         [0033]    The mass spectrometric analyses were respectively conducted on a Finnigan TSQ Ultra EMR (purchased from Thermo Electron, San Jose, Calif., USA) equipped with only a conventional ESI device and with the conventional ESI device in cooperation with the ion focusing member of the embodiment. The results are shown in  FIG. 5 . The optimum parameters were as follows: 
         [0034]    Ion source temperature: 270° C.; 
         [0035]    Sheath gas flow rate: 50 arbitrary units; 
         [0036]    Auxiliary gas flow rate: 10 arbitrary units; and 
         [0037]    Spray voltage: 4.5 kV in positive scan mode. 
         [0038]    From the results shown in  FIG. 5 , it is clearly that all of the signal intensities of amino acids obtained from the mass spectrometer using ESI device in cooperation with the ion focusing member of the present disclosure are higher than those obtained from the mass spectrometer using only ESI device. In addition, Cysteine (Cys shown in  FIG. 5 ) cannot be detected by the mass spectrometer using only ESI device but can be detected by the mass spectrometer using the ion focusing member of the present disclosure. Further, for the other 19 amino acids, the signal intensities of amino acids obtained from the mass spectrometer using ESI device in cooperation with the ion focusing member of the present disclosure are increased about 2 to 51 times as compared to those obtained from the mass spectrometer using only ESI device. Therefore, it is apparently that the ion focusing member of the present disclosure can effectively increase the amount of analyte ions entering into the mass analyzer. 
       EXAMPLE 2 
     Analysis of Amino Acids in Human Urine 
       [0039]    1 μL of human urine was mixed with ultrapure water to a total volume of 500 μL. The urine sample solution was sonicated for one minute and then was filtered and centrifuged at 14,000 rpm for 10 minutes at 4° C. in a centrifuge tube with filter to collect the urine extract. 
         [0040]    The mass spectrometric analyses were respectively conducted on the aforesaid Finnigan TSQ Ultra EMR equipped with only the conventional ESI device and with the conventional ESI device in cooperation with the ion focusing member of the embodiment. The results are shown in  FIG. 6 . The optimum parameters were as follows: 
         [0041]    Ion source temperature: 270° C.; 
         [0042]    Sheath gas flow rate: 50 arbitrary units; 
         [0043]    Auxiliary gas flow rate: 10 arbitrary units; and 
         [0044]    Spray voltage: 4.5 kV in positive scan mode. 
         [0045]    From the results shown in  FIG. 6 , the signal intensities of amino acids obtained from the mass spectrometer using ESI device in cooperation with the ion focusing member of the present disclosure are apparently higher than those obtained from the mass spectrometer using only ESI device. In addition, Cysteine (Cys) cannot be detected by the mass spectrometer using only ESI device but can be detected by the mass spectrometer equipped with the ion focusing member of the present disclosure. For the other 19 amino acids, the signal intensities of amino acids obtained from the mass spectrometer using ESI device in cooperation with the ion focusing member of the present disclosure are increased about 11 to 2525 times as compared to those obtained from the mass spectrometer using only ESI device. 
       EXAMPLE 3 
     Analysis of Amino Acids in Human Serum 
       [0046]    1 μL of human serum was mixed with ultrapure water to a total volume of 500 μL. The serum sample solution was sonicated for one minute and then was filtered and centrifuged at 14,000 rpm for 10 minutes at 4° C. in a centrifuge tube with filter to collect the serum extract. 
         [0047]    The mass spectrometric analyses were respectively conducted on the aforesaid Finnigan TSQ Ultra EMR equipped with only the conventional ESI device and with the conventional ESI device in cooperation with the ion focusing member of the embodiment. The results are shown in  FIG. 7 . The optimum parameters were as follows: 
         [0048]    Ion source temperature: 270° C.; 
         [0049]    Sheath gas flow rate: 50 arbitrary units; 
         [0050]    Auxiliary gas flow rate: 10 arbitrary units; and 
         [0051]    Spray voltage: 4.5 kV in positive scan mode. 
         [0052]    From the results shown in  FIG. 7 , the signal intensities of amino acids obtained from the mass spectrometer using ESI device in cooperation with the ion focusing member of the present disclosure are significantly higher than those obtained from the mass spectrometer using only ESI device. In addition, Glycine (Gly), Alanine (Ala), Cysteine (Cys), Asparagine (Asn), Aspartate (Asp), and Arginine (Arg) cannot be detected by the mass spectrometer using only ESI device but can be detected by the mass spectrometer equipped with the ion focusing member of the present disclosure. For the other 14 amino acids, the signal intensities of amino acids obtained from the mass spectrometer using ESI device in cooperation with the ion focusing member of the present disclosure are increased about 31 to 1467 times as compared to those obtained from the mass spectrometer using only ESI device. 
         [0053]    From the results shown in the above examples 1 to 3, when the ion focusing member of the present disclosure is applied to the mass spectrometer equipped with conventional ESI device, the analysis of amino acids in complicated matrix can effectively be conducted even in the case where the concentration of the sample is low. As such, the ion focusing member of the present disclosure is contributed to the analysis of micro-biochemical sample. 
         [0054]    As described above, because the ion focusing member of the present disclosure can effectively gather the plume-like analyte ions in a location adjacent to the sample inlet of the mass analyzer, the amount of analyte ions entering into the mass analyzer can be significantly increased so as to improve ion transmission efficiency. Accordingly, when the present disclosure is used in a mass spectrometer, the mass spectrometer may have the advantages of increased signal intensity of analyte, minimized detection error and lowered detection limit. Furthermore, the present disclosure is widely used because it can be directly combined with the existing mass spectrometer.