Abstract:
An automatic solid-phase microextraction (SPME) sampling apparatus having a mobile sampler arm with a holder for a probe carrying a fiber 9 and an intermediate bracket for transferring the probe between a probe storage tray and the gas chromatograph injector. In the various probe handling stages, the probes are collected by a magnetic member attached to the end of the plunger in the holder, at the free end of the holder&#39;s body and at the lower end of the block for guiding the probe needle. The magnetic member interacts with a ferromagnetic connector attached to one end of the fiber and with a ferromagnetic flange attached to the end of the needle containing the fiber in order to place the probe on the transfer bracket or remove it therefrom. The apparatus can perform analyses on fibers that have already been exposed, extract analytes from samples, and subsequently transfer them for desorption in the gas chromatograph in a fully automatic way.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to the field of the instrumentation for chemical analyses, especially those carried out through gas chromatographic apparatuses. In particular, the invention concerns an automatic solid phase microextraction (SPME) sampling apparatus. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    As is known, solid phase microextraction (SPME) is an analytical method that uses a fused silicon fiber on which a fluid polymer phase or a solid adsorbent, or a combination of the two, has been immobilized. The coated fiber (hereinafter called the SPME fiber) is immersed in a liquid sample, or in the headspace above the sample, to absorb the analytes of interest, which are subsequently desorbed with the aid of heat in the gas chromatograph injector and thus transferred to the capillary column. The method&#39;s selectivity can be varied by changing the type of phase coating the fiber, or its thickness. This analytical method has become widespread in various sectors, and is used for biological and environmental analyses, toxicology and in the agro-food sector. 
         [0003]    An SPME unit essentially consists of two parts: a probe composed of coated fused silicon fiber attached at one end to a plastic connector and slidingly housed in a needle to enable the opposite end to be exposed, and a tubular container looking substantially like a syringe, complete with a plunger onto which the fiber&#39;s connector is screwed. The sliding action of the plunger makes a larger or smaller portion of the fiber extend from, or withdraw inside, the needle. The analyte can be extracted by exposing the coated portion of fiber in the headspace of a test tube, or by immerging it in a fluid (e.g. water, milk, fruit juices, wines, etc.), or by exposing the fiber to air (e.g. in a working environment where the concentration of airborne pollutants needs to be measured). 
         [0004]    Three types of probe container (hereinafter called holders) are available, i.e. holders for manual analyses, for field tests and for automatic samplers (autosamplers). An autosampler is a well-known apparatus equipped with a robotic arm, which enables the maneuvers for extracting the analyte (sampling) and injecting it in the gas chromatograph to be performed automatically, while the operator has to take action each time the coated fiber has to be replaced, depending on the class of compounds being assayed, which may call for different phases to be immobilized on the fiber. In such cases, the samples to be analyzed can be contained in test tubes inserted in suitable supporting plates accessible to the sampling head of the robotic arm. 
         [0005]    Using the SPME method, the automation of the extraction/injection procedure is limited because the operator has to take action every time the probe installed on the autosampler arm needs to be changed. There are situations, moreover, in which the probe has to be replaced after each injection in the gas chromatograph. For instance, automated analysis is not feasible for determining airborne pollutants when, during a given sampling campaign, the technician appointed to collect the samples is likely to return to the laboratory with a large number of probes whose fibers have been exposed to the environment being monitored. In this type of analysis, the number of fibers depends on the chosen number of sampling points in the area being monitored, and on the number of people working in the area. One or more probes may be attached to the workers&#39; clothes in order to assess the personnel&#39;s exposure to more than one pollutant. 
         [0006]    In environmental analyses, each SPME probe is the equivalent of a test-tube that operators must currently always change inside the holder before they can inject the sample into the gas chromatograph. Using the holders currently available on the market, SPME fibers can be used on some autosamplers, but several fibers cannot be used simultaneously in a single working session unless the autosampler is stopped and an operator is on hand to change the probe. For environmental analyses, moreover, where each fiber represents a given sampling point, there is no difference—in terms of timesaving—between injecting the analyte through the port by hand or installing the fiber in the holder in the autosampler. This represents a serious drawback of the SPME method because, if, for instance, there are two sets of samples on which two substances have to be assayed, which requires the use of two different SPME fibers, then the first set of samples has to be analyzed first, then the fiber has to be changed before the second set of samples can be analyzed. This problem is already important when it comes to assays on liquid or biological samples, but is further amplified in the case of environmental sampling, when the lack of a fully automated procedure considerably extends the time it takes to perform the analyses. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0007]    The object of the present invention is to provide an automatic sampling apparatus for use in solid phase microextraction (SPME) that enables the automatic replacement of the SPME probes on the autosampler arm. 
         [0008]    Another object of the present invention is to provide an apparatus of the aforesaid type that enables the SPME fiber on the autosampler arm to be changed in a manner that can also be readily integrated in sampling equipment already in use, simply by adding some further components. 
         [0009]    Another object of the present invention is consequently to provide an adapter kit containing the necessary components for converting a conventional semiautomatic autosampler into a fully automatic apparatus according to the present invention. 
         [0010]    A further object of the present invention is to provide an SPME probe suitable for use in an automatic sampling apparatus of the aforesaid type. 
         [0011]    These objects are achieved by the automatic solid phase microextraction (SPME) sampling apparatus according to the present invention, the main features of which are stated in claims  1 ,  10  and  15 . 
         [0012]    Further important characteristics are stated in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Other characteristics and advantages of the automatic solid phase microextraction (SPME) sampling apparatus according to the present intervention will become apparent from the following description of an embodiment thereof provided here as a non-limiting example with reference to the attached drawings, wherein: 
           [0014]      FIG. 1  is an overall perspective view of the apparatus according to the invention; 
           [0015]      FIGS. 2 and 3  respectively show exploded and assembled perspective views of a probe with an SPME fiber according to the present invention; 
           [0016]      FIG. 4  shows a perspective view of the holder for the probe with the SPME fiber; 
           [0017]      FIG. 5  shows a perspective view of the holder of  FIG. 4  complete with the probe and the SPME fiber; 
           [0018]      FIG. 6  shows a perspective view of a transfer bracket with a probe containing an SPME fiber installed thereon; 
           [0019]      FIG. 7  illustrates the positioning of the probe with the SPME fiber on the transfer station; 
           [0020]      FIG. 8  shows a side view of the step when the probe with the SPME fiber is loaded from the transfer station into the holder carried by the autosampler arm; 
           [0021]      FIG. 9  is a perspective view of the step illustrated in  FIG. 8 ; 
           [0022]      FIG. 10  is a perspective view, seen from below, of a guide block on the head of the autosampler arm; 
           [0023]      FIG. 11  is a perspective view of another embodiment of the probe with an SPME fiber according to the invention; 
           [0024]      FIG. 12  is a perspective view of an another embodiment of the transfer station suitable for use with the probe with the SPME fiber of  FIG. 11 ; 
           [0025]      FIG. 13  is a perspective view of the device for calibrating the probe; 
           [0026]      FIG. 14  is a perspective view of a protective container for the probe; 
           [0027]      FIG. 15  is an exploded perspective view of the protective container of  FIG. 14  with the probe housed therein. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The apparatus according to the present invention is schematically shown in  FIG. 1 , in association with a detector for gas chromatographic analyses (a mass spectrometer, for example) generically indicated by the numeral  1  and comprises a robotic autosampler arm  2  supported by the gas chromatograph, on which one or more trays  4  for supporting the probes containing the SPME fibers (indicated by the numeral  5 ) are neatly arranged. These probes may, for instance, come from a campaign of environmental analyses and are ready for gas chromatographic analysis. Alongside the trays  4 , there is a bracket for transferring the probes  5  (generically indicated by the numeral  6 ) that is described in more detail later on. The numeral  7  indicates two injectors of the gas chromatographic apparatus. 
         [0029]    With reference to  FIGS. 2 and 3 , the sampling probe  5  according to the present invention comprises a conventional probe consisting of a needle  8 , which slidingly houses an SPME fiber  9 . A plastic connector  10  is attached to one end of the fiber  9  and—in conventional systems—it is screwed to the probe holder&#39;s plunger. A brass spacer ring  11  is likewise attached to the adjacent end of the needle  8 . An adapter cap  12  made of a ferromagnetic material is attached to the connector  10 , while a flange  13  also made of a ferromagnetic material is fixed to the spacer ring  11 . In particular, the flange  13  has a radial discontinuity  13   a  and a portion  13   b  that is threaded both internally and externally. To assemble the flange  13  on the spacer ring  11 , the needle  8  is passed through the discontinuity  13   a  until it is coaxial to the flange  13 , which is screwed onto the spacer ring  11  by means of its threaded portion  13   b  until the two are juxtaposed. Then a fixing sleeve  14  is screwed onto the threaded portion  13   b  coaxially to the needle  8 . 
         [0030]    As shown in  FIGS. 4 and 5 , the numeral  15  identifies a holder of substantially conventional structure, suitable for attaching to the head  2   a  of the sampler arm  2  to carry the probe  5  with the SPME fiber  9 . The holder  15  comprises a tubular body  16  slidingly containing a plunger  17 , the stem  17   a  of which is connected to a moving device, not shown because it is of a type well-known to a person skilled in the art. The body  16  of the holder  15  has a longitudinal window  18  affording access to a chamber  19  containing the probe  5  during its movement, as shown in  FIG. 5 , so that the needle  8  extends axially therefrom. In particular, a magnet  20  is attached to the end of the plunger  17  inside the chamber  19  ( FIG. 4 ); likewise, a flange  21  carrying a circularly arranged row of magnetic elements  22  is attached to the free end of the body  16 . Both the flange  21  and the free end of the body  16  have a radial discontinuity to allow for the passage of the needle  8  when the probe  5  is placed inside the chamber  19  (as shown in  FIG. 5 ) or removed therefrom. In the condition shown in  FIG. 5 , the probe  5  is secured to the holder  15  by the two magnetic connections consisting, on one side, of the adaptor cap  12 , which becomes attached to the magnet  20  situated at the end of the plunger  17 , and, on the other side, of the flange  13 , which becomes attached to the magnetic flange  21  situated at the free end of the body  16 . 
         [0031]    As shown in  FIG. 6 , the bracket  6  for transferring the probes  5  carrying the SPME fibers comprises a base  23 , which can be fixed to the casing  3  of the apparatus, and an upright  24  rising the base  23 . First and a second supporting plates,  25  and  26 , extend from the upright  24  over the base  23  and are intended to engage the adaptor cap  12  and the flange  13  on the probe  5  carrying the SPME fiber  9  in their respective seats  25   a  and  26   a.  Between the first and the second supporting plates  25  and  26  a pair of arms  27  also extends from the upright  24 , the purpose of which will be explained later on. 
         [0032]    As shown in  FIG. 7 , the holder  15  is fixed in a substantially conventional manner to the head  2   a  of the autosampler arm  2  by means of a plate  28 . The head  2   a  of the autosampler arm  2  is also provided with two guide blocks  29   a,    29   b  for guiding the needle  8  of the probe  5 , in alignment underneath the holder plate  28  and supported by arms  30  extending from said plate  28 . Moreover, as shown in  FIG. 10 , the lower of the two guide blocks, indicated by the numeral  29   b,  carries a circularly arranged row of magnets  33  on its underside so that it can be magnetically coupled with the adaptor cap  12  on the SPME fiber  9 . 
         [0033]    The operation of the apparatus according to the invention is described below with particular reference to  FIGS. 7 ,  8  and  9 . Initially, the probes  5 —coming, for instance, from a campaign of environmental analyses—are placed vertically in suitable racks in the tray  4 , while the head  2   a  of the sampler arm  2  is in its resting position shown in  FIG. 1 . The sampler arm  2  is characterized by a Cartesian type of movement controlled by a computer (not shown), in which a map of the tray  4  with the position of each probe is stored, so that it can move into position over the probes  5  on the tray  4  and align the guide blocks  29   a,    29   b  over the required probe  5 . Then a downward movement begins until the lower guide block  29   b  magnetically engages the adaptor cap  12  of the prefixed probe, which can consequently be lifted from its seat in the tray  4  and carried to the transfer bracket  6 , where it is placed with the adaptor cap  12  and the flange  13  resting on the supporting plates  26  and  25 , in the respective seats  26   a  and  25   a.  The end of the guide block  29   b  is separated from the adaptor cap  12  by sliding the guide block  29   b  longitudinally away from the transfer bracket  6 . The cap  12  is kept in its seat by the plastic connector  10  projecting therefrom and is detached when the block  29   b  is moved away from the transfer bracket  6  and its magnets are no longer in contact with the cap  12 . 
         [0034]    Once it has been separated from the probe  5 , the head  2   a  of the arm  2  moves down to bring the window  18  in the holder  15  into line with the adaptor cap  12 . A subsequent forward sliding movement of the head  2   a  moves the holder  15  towards the upright  24  until the adaptor cap  12  comes to be inside the chamber  19  in the holder  15 , as shown in  FIGS. 8 and 9 . 
         [0035]    Then the plunger  17  brings its magnetic end  20  up against the adaptor cap  12 , which remains attached thereto, after which the head  2   a  on the arm  2  moves to bring its magnetic flange  21  into contact with the flange  13  on the needle  8 . The probe  5  thus becomes firmly attached to the holder  15  and can consequently be separated from the transfer bracket  6  and carried towards the injector  7 , in which the needle  8  ultimately engages. At this stage, the needle  8  also engages in the guide blocks  29   a,    29   b  to avoid it being bent during its insertion in the gas chromatograph injector  7 . 
         [0036]    Once the needle  8  is inserted in the injector  7 , the process continues in the usual way: the fiber  9  in the needle  8  is exposed to enable the desorption of the substance being analyzed and, when the necessary time has elapsed, the needle  8  is withdrawn from the injector  7  and the probe  5  is returned to the transfer bracket  6 , where it is repositioned as shown in  FIG. 9 . The probe is detached by raising the plunger  17  in the holder until the magnet  20  is separated from the adaptor cap  12  when it abuts against the top of the window  18 . Then the holder pulls the probe upwards until the flange  13  abuts against the arms  27  with sufficient force to overcome the magnetic force between the magnetic flange  21  and the flange  13 , so that the probe  5  drops freely down the bracket  6  until the cap  12  and the flange  13  fit into their respective seats  26   a  and  25   a  in the plates  26  and  25 . Then the previously-described sample collection steps are conducted in reverse order until the probe  5  is restored to its seat in the tray  4  and the autosampler arm  2  is ready to collect a new probe  5 . 
         [0037]    According to the present invention, there are essentially two different ways for the mobile sampler arm to collect the probe. The first mode involves the probe being connected to the head of the arm by means of its adapter cap  12 , which becomes magnetically attached to the lower end of the guide block  29   b,  and enables the positioning of the probe on the transfer bracket  6  and its detachment therefrom. The second mode involves the probe being connected to the holder  15  by means of the adapter cap  12  and the flange  13 , which are magnetically attached respectively to the end of the plunger  17   a  and to the free end of the body  16  of the holder  15 , and enables the transfer of the probe from the transfer bracket to the holder and its subsequent reinsertion in said bracket. The passage from one collecting mode to the other, due to the movements of the mobile arm  2  and the presence of the transfer bracket  6 , enables the procedure for analyzing a plurality of samples to be fully automated. 
         [0038]    The apparatus according to the invention can be used advantageously not only to perform analyses using probes containing already exposed SPME fibers, but also for the direct extraction of analytes of interest from samples contained in test tubes placed in the tray and their subsequent desorption in the gas chromatograph. In this case, according to a sequence that, in view of what has been explained above, is self-evident to person skilled in the art, the head  2   a  of the autosampler arm carrying a probe  5  that has yet to be exposed is brought over a given test tube in the tray  4  and inserts the needle in the test tube, by making the plunger  17  slide inside the holder  15 . Here again, the needle  8  is engaged in the two guide blocks  29   a,    29   b  to avoid any risk of it bending due to the force exerted by the plunger on the needle during its insertion in the test-tube. Once the SPME fiber is exposed inside the test-tube for the required time, it is withdrawn inside the needle, which is removed from the test-tube, then the probe is transferred to the gas chromatograph injector, where it is inserted and the desorption phase begins. 
         [0039]    If the analytes to assay belong to different classes of compounds and consequently require fibers with different phases, the apparatus according to the invention can automatically fit the appropriate unexposed probe, collecting it from the tray  4  after depositing the previously-used probe. The changeover is made using the transfer bracket  6  in the previously-described manner. 
         [0040]    In the above-described embodiment, the probe  5  is detached from the flange  21  by the pair of arms  27  up against which the probe&#39;s flange  13  is forced until the force developed exceeds the magnetic force between the latter and the flange  21 . In the alternative embodiment illustrated in  FIGS. 11 and 12 , the probe  5  is detached from the guide block  29   b  in the same way, while the flange  21  is separated from the flange  13  by the simple traction exerted by the head  2   a  in an orthogonal direction away from the needle  8 . For this purpose, the radial discontinuity  13   a  on the flange  13 , for inserting the needle  8 , has a widened opening section  31 . There is a corresponding reference tooth  32  on the plate  25  of the transfer bracket  6  for engaging in said widened opening section  31  so that, while the probe is kept in a constant position on the transfer bracket, it can be detached by radial traction without any risk of the probe sliding out because the flange  13  and the adapter cap abut laterally on their respective seats. This solution also avoids the probe being positioned by dropping it onto the transfer bracket, which could damage the probe. 
         [0041]      FIG. 13  shows a useful accessory for the apparatus according to the invention, for use in calibrating the probe, i.e. for adjusting the extent to which the fiber is withdrawn inside the needle, normally indicated as Z. This parameter determines the extent of exposure of the fiber and the consequent sampling range. The device comprises a plate  40  with a first block  41  fixed in a median longitudinal position, which has a first seat  42  shaped so as to contain the adaptor cap  12  on the probe, and a second movable block  43  consisting of two coupled parts  43   a  and  43   b  that, between them, define a second seat  44  shaped so as to contain the flange  13  on the probe and to prevent its axial sliding therein. At least one of the two blocks has a T-shaped appendage  45  engaged inside a slot  46  of corresponding shape formed longitudinally on the plate  40 . Once the probe is placed on the two blocks  41  and  43  so that it lies parallel to the plate  40 , with the adaptor cap  12  and the flange  13  engaged in their respective seats  42  and  44 , the block  43  can be made to slide in relation the block  41  to obtain the required adjustment. The extent of the probe&#39;s withdrawal can be read on a suitable scale  47  provided on the plate  40 , as schematically shown. 
         [0042]      FIG. 14  shows a protective container for use with the SPME fiber probe and the apparatus according to the invention, which enables sampling to be done in the field and on workers without any risk of damaging the SPME fiber. This container comprises two cylindrical elements  50  and  55 , which surround the fiber and prevent any movement of the flange  13  or adaptor cap  12  so as to avoid the length Z of the fiber changing during the sampling procedure. The element  50  engages inside the element  55  so that the flange  13  rests up against the bottom of the latter, and the end of the element  50  comes up against the flange  13 . The element  55  partially engages in a further tubular element  51 , which protects the needle during the fiber sampling and transport. The element  51  has slits  54  to enable the fiber to come into contact with the flow of air in the environment being analyzed, without creating a second diffusion chamber (which would falsify the value of the sampling range). There are also caps  52  and  53  to use after completing the sampling procedure in order to interrupt the fiber&#39;s absorption and enable its transportation in a safer way. 
         [0043]    The above description clearly shows that the apparatus according to the invention fully achieves the previously-stated objects. In particular, it enables the full automation of the SPME analytical procedure, both when the gas chromatograph has to be fed with probes already exposed in other environments, and when the analyte has to be extracted from the contents of test tubes accessible to the sampler arm. This enables a drastic reduction in the action of the operator on the apparatus, also minimizing the risk of error. In addition, the time it takes to complete the analyses are reduced and the apparatus can even work continuously for several days without the operator needing to intervene. 
         [0044]    Finally, the full automation of the procedure is enabled by means of simple structural changes to the conventional equipment, which means that all the necessary components can be supplied in the form of a kit for converting partially-automated apparatus already in use into the fully-automated apparatus according to the present invention. 
         [0045]    A further advantage of the sampling apparatus and the relevant probe according to the invention consists in that they can be used not only to automatically feed exposed SPME fibers to a gas chromatographic apparatus, but also to perform automatic sampling in an unattended sampling station, e.g. for air analysis, wherein, at a certain time, a probe must be exposed in a sampling window and, after a prefixed time, stored back in a rack awaiting to be analyzed in a remote laboratory later on. 
         [0046]    Various modifications and alterations to the invention may be appreciated based on a review of the disclosure. These changes and additions are intended to be within the scope and spirit of the invention as defined by the following claims.