Patent Application
20100006502
Chemical and biological separations are routinely performed in various industrial and academic settings to determine the presence and/or quantity of individual species in complex sample mixtures. There exist various techniques for performing such separations.
One particularly useful analytical process is chromatography, which encompasses a number of methods that are used for separating ions or molecules for analysis. Liquid chromatography (‘LC’) is a physical method of separation wherein a liquid ‘mobile phase’ carries a sample containing a mixture of compounds or ions for analysis (analytes) through a separation medium or ‘stationary phase.’ Stationary phase material typically includes a liquid-permeable medium such as packed granules (particulate material) or a micro-porous matrix (e.g., porous monolith) disposed within a tube or similar boundary. The resulting structure including the packed material or matrix contained within the tube is commonly referred to as a ‘separation column.’ In the interest of obtaining greater separation efficiency, so-called ‘high performance liquid chromatography’ (‘HPLC’) methods often utilizing high operating pressures are commonly used.
Often an electro-spray system is used as in the interface between the LC device and a mass spectrometer. In electro-spray systems, a voltage is applied to the mobile phase to charge the mobile phase. As the fluid comprising the mobile phase and analytes exits a tube or channel, a Taylor cone is formed and the fluid forms a stream, which, in a short distance, will start to breakup into small droplets. The mobile phase droplets have a charge and, as the mobile phase begins to evaporate, the charge can be transferred to the analytes.
In electro-spray systems there is no need to account for the accuracy of the flow rate, or changes in flow rate due to the composition of the mobile phases, or changes in mobile phase composition during a mobile phase gradient program.
The majority of ions formed by the electrospray process are mobile phase or solvent ions. Because a limited number of charged molecules can be accepted by the mass spectroscopy (MS) inlet a charge competition between the ions of interest and the mobile phase ions can result.
Because of the shortcomings of known electro-spray systems, LC/MS interfaces in which the mobile phase is not charged have been investigated. In particular, the fluid effluent is transformed into the gas-phase, and only the analytes are ionized. Unfortunately, prior attempts to form a gas or vapor phase of the mobile phase and analytes have certain drawbacks.
One such drawback results from the inconsistency of the volume of the drops of fluid provided. Various factors can impact the volume of the drops, including but not limited to, flow rate of the fluid from the LC column and the composition of the fluid, which can vary depending on the selection of the mobile phase. As should be appreciated, the flow rate can be consistent, but inaccurate, due to design and manufacturing variations in the pumping systems. In addition, during a run, the composition of the mobile phase can change in a programmed gradient where the percentage of one type of mobile phase changes with respect to another type of mobile phase, such as starting from a mobile phase of 100% methanol and 0% water, and over time, changing the mobile phase to 0% methanol and 100% water.
What is needed, therefore, is a drop formation device for dispensing fluid from an LC column that overcomes at least the drawbacks of known devices described above.
The present teachings are best understood from the following detailed description when read with the accompanying drawing figures. The features are not necessarily drawn to scale. Wherever practical, like reference numerals refer to like features.
It is to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting.
As used in the specification and appended claims, the terms ‘a’, ‘an’ and ‘the’ include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, ‘a device’ includes one device and plural devices.
In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings:
The term ‘LC’ as used herein refers to a variety of liquid chromatography devices including, but not limited to, HPLC devices;
The term ‘flow-rate’ as used herein refers to a volume of a fluid per unit time;
The term ‘drop rate’ as used herein refers to the number of drops that are formed by the discrete drop dispensing device per unit time; and
The term ‘drop dispensing rate’ as used herein refers to the number of drops of a fluid dispensed per unit time multiplied by the volume per drop.
In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. Descriptions of known systems, devices, materials, methods of operation and methods of manufacture may be omitted so as to avoid obscuring the description of the example embodiments. Nonetheless, systems, devices, materials and methods that are within the purview of one of ordinary skill in the art may be used in accordance with the representative embodiments.
The mass spectrometer 100 includes an ion source 101, a mass analyzer 102 and a detector 103. The ion source 101 will be described in more detail below. The mass analyzer 102 may include a conduit such as a sleeve, transport device, dispenser, capillary, nozzle, hose, pipe, pipette, port, connector, tube, orifice, orifice in a wall, coupling, container, housing, structure or other apparatus used to transport ions from the ion source 101 to the detector 103. Such apparatuses are known to one of ordinary skill in the art and are not described in detail to avoid obscuring the description of representative embodiments. The detector 103 may be a known ion detector used to detect the analyte ions that are collected and transported by the mass analyzer 102. The detector 103 may also include known hardware, software or firmware, or a combination thereof useful in detecting analytes.
Illustratively, the oscillator 305 comprises an actuator or an electro-mechanical transducer and a source of an alternating drive waveform. In a specific embodiment, the electromechanical transducer comprises a piezoelectric element. The oscillator 305 is disposed either annularly about the disk or at diametrically opposed locations along a side of substrate 301, beneath surface 304 (as shown) or over surface 303 and may comprise a piezoelectric oscillator connected to appropriate driver circuitry to cause oscillation. One illustrative example of such an oscillator and its arrangement is described in U.S. Pat. No. 6,540,153 to Irvi, the disclosure of which is specifically incorporated herein by reference.
The number and size of the orifices 302 are determined considering the flow rate of the fluid from the LC and the composition of the fluid. As described in greater detail below, a greater flow rate requires a greater drop dispensing rate by the discrete drop dispensing device 201. Two ways to control the drop dispensing rate is by controlling the number of drops dispensed per unit time and the volume of the drops dispensed. Therefore, larger orifices will provide larger drop volumes and more orifices will provide more drops. It follows that fewer orifices, or smaller orifices, or both, will reduce the drop dispensing rate. It is noted that other factors can influence the volume of the drops and the rate of their dispensing.
The volume of the drops dispensed by the discrete drop dispensing device 201 is influenced by the composition of the fluid and such parameters as surface tension, viscosity, and temperature. The drop volume can also be varied by the amplitude of the drive waveform applied to the oscillator used to vibrate the disk 300. The drop rate can also be controlled by changing the duty cycle of the drive waveform applied to the oscillator. Normally, the discrete drop dispensing device 201 is operated at or near a resonant frequency of the disk. In an example, the oscillator frequency was 128 kHz. The drop rate can be reduced by discontinuing the drive waveform applied to the actuator/transducer of the oscillator for a percentage of the time.
In
As described above, a mismatch of the flow rate of fluid 402 from LC column 401 and the drop dispense rate of drops 202 can be caused simply by difficulty matching the consistent, but often inaccurate, continuous flow rate from the LC column to the discrete drop dispensing rate of the discrete drop dispensing device 201. Additionally or alternatively, it is possible for the drop dispensing rate to change during an LC analysis due to a gradient program where the mobile phase composition changes over time. The change in mobile phase composition is likely to cause a change in the volume of the drops formed by the discrete drop dispensing device 201. This can result in the occurrence of the conditions shown in and described in connection with
If the volume per unit drop decreases due to a change in the mobile phase composition, but the drop rate remains the same, the drop dispensing rate decreases. This can cause an increase in the volume of fluid 402 accumulated in region 403 and the occurrence of
If the volume per unit drop increases due to a change in the mobile phase composition, and drops per unit time remains the same, the drop dispensing rate increases. This can cause a decrease in the volume of fluid 404 accumulated in region 403 and the condition shown in
The ion source 101 of the presently described embodiment comprises a drying tube 505. The drying tube 505 may be a separate component or may be integrated with the housing 500. The drying tube 505 is positioned adjacent to the lower surface 304 of the discrete drop dispensing device 201 for receiving the drops 202, which are aerosolized and dried as described above. The drying tube 505 may be heated by a heater (not shown). The heater may include, but is not limited to, an infrared (IR) lamp, a heated surface, a turbo spray device, and a microwave lamp.
Alternatively or additionally, the drying tube 505 may be heated by flowing a hot inert carrier gas 506 through the drying tube 505. When flowed through the drying tube 505, the carrier gas also serves to guide or direct the aerosolized fluid drops towards an outlet 507 of the drying tube 505. The drying tube 505 turns the aerosolized fluid drops into a gas-phase mobile phase and a gas-phase analytes 509. The heat input required to dry the mobile phase of the fluid may be calculated based on the drop volume of the discrete drop dispensing device 201 and the mobile phase composition. Beneficially, the gas-phase mobile phase is substantially without any charge since the aerosolized fluid is not affirmatively subjected to any electric potential. In the present embodiment, the ionizer 205 comprises a light source 508, such as at least one ultraviolet (UV) lamp. The light source 508 may also include, but is not limited to, a krypton light source, an argon light source, and a helium light source. In a specific embodiment, the light source 508 may be a microplasma UV source such as described in commonly-owned U.S. patent application Ser. No. 11/932,835, entitled “Micro-plasma Illumination Device and Method” to Viorica Lopez-Avila, et al. and filed Oct. 31, 2007. The disclosure of this application is specifically incorporated herein by reference.
In such an embodiment, the light source 508 may be inside drying tube 505 with a gas line provided through the wall of the drying tube 505 and the wall of the housing. Providing the light source 508 inside the drying tube allows its windowless properties, which enable the emission of comparatively very short wavelength UV radiation, to be exploited. The light source 508 may be positioned in a number of locations downstream from the discrete drop dispensing device 201 adjacent to a portion of the drying tube 505 where the aerosolized fluid has dried sufficiently to transform into the gas-phase mobile and gas-phase analytes 509. This portion of the drying tube 505 defines an ionization region 511. The wavelength of the LV light generated by light source 508 is selected such that the UV light at least substantially ionizes the gas-phase analytes 509 to produce analyte ions without substantially ionizing the gas-phase mobile phase. In other words, usefully, the gas-phase analyte 509 is strongly ionized while the gas-phase mobile phase is minimally ionized, if at all.
The mass analyzer 102 may include the conduit 501 or any number of capillaries, conduits or devices for receiving and moving the analyte ions from the ionization region to the detector 103. The conduit 501 is disposed in the housing 500 downstream from the discrete drop dispensing device and illustratively opposite to the light source 508. The conduit 503 comprises an orifice 510 that receives the analyte ions and transports them to the detector 103. Optionally, a gas conduit 512 may direct a drying gas 513 toward the ions in the ionization region. This drying gas 513 interacts with the analyte ions in the ionization region to remove any mobile phase remaining from the aerosolized fluid provided from the discrete drop dispensing device 201.
At 601, the method comprises providing a fluid comprising analytes at a flow rate to a discrete drop dispensing device 201, such as from an LC column 401. At 602 the method comprises dispensing drops (e.g., drops 202) of the fluid comprising a substantially similar volume from the discrete drop dispensing device at a drop dispensing rate. Beneficially, the flow rate is substantially identical to the drop dispensing rate.
A discrete drop dispensing device, an ion source comprising the discrete drop dispensing device, a device for performing liquid chromatography and a method are described in conjunction with various representative embodiments. The discrete drop dispensing device provides drops at a drop dispensing rate that is substantially equal to the flow rate of the fluid from the LC. If the LC flow rate is slightly less than the drop dispensing rate, the drop dispensing rate will decrease; and if the LC flow rate increases, the drop dispensing rate will also increase in a self-correcting manner. This way, the drop dispensing rate will self regulate, dispensing a little more or a little less fluid for a short period. This self-regulation occurs rapidly and the average drop dispense rate will be substantially constant over the time periods of the measurement. Having multiple orifices on the disk also allows a wider dynamic range of LC flow rates to be dispensed. Additionally, and as described, changes in mobile phase composition result in changes in the drop dispensing rate so that the flow rate and drop dispensing rate will remain substantially the same; and the average drop dispense rate remains substantially constant over time periods of measurement.
In view of this disclosure it is noted that the methods and devices can be implemented in keeping with the present teachings. Further, the various components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.
