Patent Application
20050236565
The apparatus and method of the invention utilize a two-photon resonance-enhanced multiphoton ionization (REMPI) instrument for trace species analysis. The invention is directed to a method and apparatus for utilizing a continuous flow of a liquid sample to detect and to identify trace organic substances in the sample. As REMPI is fundamentally a gas phase method the invention combines REMPI with membrane introduction mass spectrometry (MIMS), whereby organic compounds are extracted into the gas phase from a polar solvent such as water. A significant feature of MIMS is the simultaneous introduction of all organic analytes into the mass spectrometer. In many MIMS applications, the mass spectrometer is a standard quadrupole instrument, although both ion traps and triple quadrupole devices have also been used. Most of the studies using MIMS utilize electron impact or chemical ionization. However, the application of conventional ionization methods such as electron impact can make analysis of complex mixtures more difficult due to extensive molecular fragmentation. Accordingly, the invention combines MIMS with REMPI as the laser photoionization method, the latter of which may be adjusted so as not to produce photofragmentation. The combination of MIMS and REMPI provides sensitive and rapid analysis without prior separation or sample preparation and without deconvolution of multiple mass peaks.
While many of the analytes of interest which pass through the membrane to the photoionization zone may be photoionized using REMPI, there remains in the liquid sample analytes which either do not pass through the membrane. In particular, there may be analytes which are retained within the liquid sample flowing past the membrane that remain in solution. As a further embodiment, the liquid sample, after contact with the membrane, may be introduced into a capillary inlet tube which directs the liquid sample as droplets to the photoionization zone at subatmospheric pressure. Analytes in these droplets may be photoionized by REMPI.
As a further embodiment, it is realized that not all of the analytes, particularly the analytes which are not permeable to the membrane, may be readily photoionized by REMPI. Accordingly, both a radiation source for performing REMPI and a second source of radiation for performing single photon ionization (SPI) are provided. The two sources of radiation are selectively directed to the photoionization zone by a system of reflecting surfaces so that radiation from either source may be selected.
As yet another embodiment of the invention, there is a third source of analytes from the liquid sample, that is, compounds that are adsorbed or absorbed onto and into the membrane, but which do not pass through the membrane at the sampling temperature. Subsequent to photoionization and mass spectrometrical analysis of the other analytes, the analytes adsorbed/absorbed onto or into the membrane may be released therefrom by applying heat to the membrane or by running a different solvent to the membrane. This latter process would require halting the continuous flow of sample to the membrane, so it is preferred that heat be applied. These analytes will then pass through the membrane into the photoionization zone where they may be analyzed by REMPI or single photon ionization, as appropriate.
The present method and apparatus are applicable for detecting and identifying organic compounds in water samples without interference from the bulk water solvent. Thus, water samples such as ultrapure water for semiconductor processing, ground water, surface water, biological fluids, and potable water may be analyzed in real time for the presence of volatile organic compounds (VOCs), such as benzene, toluene, and xylene; for explosives, nitro compounds, organic molecules containing halogen, inorganic compounds such as metal and heavy atoms, aromatic ketones, large biomolecules, and the like. Because of their short-lived excited states, such molecules often cannot be detected using conventional nanosecond pulse-duration laser ionization sources. Typical detection ranges for the method according to the present invention using either the membrane or capillary inlet systems are in the range of about 1 ppb to about 1 ppt and the range of about 1 ppb to about 1 ppm of analyte in a sample.
Since sample preparation is not required, location of the apparatus need not be confined to a laboratory. A compact and portable analytical unit for sensitive and selective detection, identification, and quantification of trace organic chemicals and toxic compounds in water is provided by the invention.
A method is provided for identifying analytes at low concentration in a liquid sample by mass spectrometry comprising the steps of
A method is provided for identifying analytes at low concentration in a liquid sample by mass spectrometry comprising the steps of
An apparatus is provided for identifying analytes at low concentration in a liquid sample by mass spectrometry comprising a zone of ionization for ionizing gaseous or liquid analytes; a membrane impermeable to the solvent and permeable to at least a portion of analytes contained in the liquid sample, whereby the permeable analytes are deliverable to the zone of ionization; a capillary tube adapted for receiving the portion of the liquid sample impermeable to the membrane containing other analytes not retained on the membrane, the tube directed to introduce the liquid sample and other analytes from the membrane to the zone of ionization; a first source for providing radiation for performing resonance enhanced multiphoton ionization of the analytes; a second source for providing radiation for performing single photon ionization of the analytes; a system of reflecting surfaces for selectively directing radiation either from the first source or the second source to the zone of ionization; a mass spectrometer for determining the m/e ratio of ions formed in the zone.
The apparatus may further comprise a component for driving analytes initially retained on the membrane through the membrane into the zone of ionization.
An apparatus is provided for identifying analytes at low concentration in a liquid sample by mass spectrometry comprising a membrane impermeable to the solvent and permeable to at least a portion of analytes contained in the liquid sample; a zone of photoionization for analytes passing through the membrane; a source for irradiating the zone for performing resonance enhanced multiphoton ionization of the analytes; a mass spectrometer for determining the m/e ratio of ions formed in the zone.
An apparatus is provided for introducing analytes from a liquid sample into an ionization zone for analysis by mass spectrometry comprising a membrane impermeable to the solvent and permeable to at least a portion of analytes contained in a polar liquid sample, the permeable analytes being capable of delivered to a zone of ionization; a capillary tube adapted for receiving the portion of the liquid sample impermeable to the membrane containing other analytes not retained on the membrane, the tube capable of introducing the liquid sample and other analytes from the membrane to the zone of ionization.
An apparatus is also provided for photoionizing analytes for analysis by mass spectrometry comprising
The liquid sample may comprise ultrapure water for semiconductor processing containing trace organic compounds, potable water, or any aqueous sample containing organic contaminants in trace amounts.
The present invention provides an apparatus comprising a membrane inlet system that uses a selective permeability membrane to admit organic compounds and reject water and other polar solvents, and a photoionization source to photoionize the compounds to be analyzed by residence enhanced multi photon ionization and time-of-flight mass spectrometry. These components may be utilized as a compact and portable analytical instrument since there is no requirement for sample preparation and the equipment need not be confined to a laboratory. Referring to
The type of membrane can be varied by those of ordinary skill in the art depending upon the types of molecules that are of interest to be studied or analyzed. Each membrane rejects different compounds, thus allowing for the deduction of a wide variety of molecules. Membranes may be selected which allow for the selective fusion of the analyte of interest preferentially over the aqueous matrix or other possible interfering compounds. This separation at the sample inlet enhances the sensitivity of the instrument.
Typical membrane temperatures are between about 50° C. and 80° C. At higher temperatures, more sample passes through the membrane into the ion source of the mass spectrometer, which may result in higher sensitivity. However, as more water also diffuses through the membrane at higher temperatures, this increases the background pressure in the ion source and could lead to deteriorated performance of the detection system. However, by using selective photoionization, background components such as water with a relatively high ionization potential are not ionized.
A preferred membrane material is silicone, which tends to exclude polar molecules since polar molecules are not soluble in silicone and therefore not absorbed on the membrane surface. Higher molecular weight species tend to adhere to the surface of the membrane and do not evaporate into the vacuum space of the photoionization chamber. An advantage of the membrane inlet system is that the membranes may be easily replaced and this allows for the examination of alternative materials and membrane geometries, such as thinner walled membranes.
Referring to
For analytes such as aromatic ring containing compounds, the resonant excitation step can operate close to optical saturation, so that a sizable fraction can be elevated to the excited state using REMPI. From the excited state, the ionization step is estimated to operate at between 10 and 100 percent efficiency, thus the overall yield can reach up to about 10 percent. This is several orders of magnitude better efficiency than typically found by using electron impact ionization apparatus. When operated at low to medium laser intensity, the REMPI process produces solely or primarily the parent molecular ion structure which greatly simplifies the interpretation of the mass spectrum because of lack of fragmentation.
Referring to
Alternatively, a fixed wavelength from the laser may be extracted to provide single photoionization at the photoionization zone 34. In this case, the radiation is sent through a gas cell 38 to rectify the beam to the desired wavelength and then is reflected off surfaces 36D and 36C to the photoionization zone 34. A preferred wavelength for SPI is 118 nm. The surface 36C represents a moveable mirror whereby radiation reflected from 36B or 36D can be selectively directed to the photoionization zone 34. The REMPI and SPI radiation sources may be single or multiple lasers and the REMPI radiation may be provided from a different laser or set of lasers from the laser or set of lasers that provide the SPI radiation. One or more sources of radiation may also be provided such that along the path from the respective laser to the zone of ionization, the beam is tuned to result in either REMPI or SPI-suitable radiation.
The liquid sample, which is not absorbed/adsorbed in the membrane probe 31, exits the probe and is directed to a separator 37 which directs most of the water sample to the water return and takes a small sample to a direct liquid injection capillary probe 39. Separator 37 may be a differential pump that drives a portion of the sample to water return and portion to the probe 39. A capillary probe is described in published PCT Application WO 2004/097891-A3, published Nov. 11, 2004, which is incorporated by reference herein. The liquid sample is directed as fine droplets 40 to the photoionization zone 34 where they may be ionized by either REMPI or single photon ionization as described above.
The water sample at the continuous flow water inlet 30 may be heated, for example by a flow of heated air 41, to an appropriate temperature optimized for membrane permeability of the desired analytes. Also, the sample may be heated to a higher temperature to drive through any absorbed/adsorbed analytes on the membrane which were retained on the membrane but did not permeate the membrane at a lower sample temperature.
It is also considered to be within the scope of the present invention an apparatus for introducing analytes from a liquid sample into an ionization zone for analysis by mass spectrometry comprising a membrane impermeable to the solvent of the sample and permeable to at least a portion of the analytes contained in the sample; and a capillary tube adapted for receiving the portion of the liquid sample impermeable to the membrane containing other analytes not retained on the membrane. The capillary tube is capable of introducing the liquid sample and other analytes from the membrane to a zone of ionization.
Also within the scope of the invention is an apparatus for photoionizing analytes for analysis by mass spectrometry comprising:
The first and second source of radiation may be the same source, i.e., the same laser, or they may be different sources, i.e., different lasers.
The apparatus according to the invention may be utilized to detect trace organic compounds in water sources such as ultrapure water for semiconductor processing, ground water, surface water, biological fluids and potable water. For example, contaminants found in ultrapure water may be due to the source of the water itself, for example, municipal water, from the water purification systems used to purify the water such as ion exchange resins, and from semiconductor processing chemicals found in reclaimed water. Such specific contaminants include but are not limited to, trimethylemine, benzene sulfonic acid, isopropyl alcohol, urea, glycidol, tetremethylammonium hydroxide (TMAH), 1-3 dichloro-2-propanol, and ethylene glycol. Other contaminants that may be found in various water sources are siloxanes, low molecular weight alcohols, organic nitrogen compounds, organic sulfur compounds, organic surfactants, organic acids, chlorinated or brominated hydrocarbons, phthalates and silicones. Ultrapure water is required not only in semiconductor manufacturing, but also in areas such as pharmaceuticals, biotechnology products, optoelectronic products, the food and beverage industry, the power industry (steam boilers), and the like.
The following examples are presented for purposes of illustration and are not intended to limit the invention in any way.
A membrane introduction device was obtained commercially from MIMS Technology (Palm Bay, Fla.), consisting of a flow injection module and a heated membrane tip. The device contained a membrane of pharmaceutical grade platinum-cured silicone tubing (HelixMark) manufactured with Dow Corning Silastic Q7-4750. The sample is loaded into the flow injection module which maintains a constant flow of water through the membrane tip. As the analyte solution passes across the inner surface of the membrane, the target organic molecules diffuse through the membrane and evaporate into a REMPI mass spectrometer ionization chamber. The temperature of the membrane is controlled by varying the temperature of the water flow. As the temperature is increased, the analyte diffusion rate through the membrane increases, thus reducing the measurement time. However, diffusion of organic compounds is hampered as the temperature reaches 100° C. due to formation of bubbles in the water. Tests were performed initially with the temperature varied between 30° C. and 90° C. in order to determine the minimum response time in combination with the most sensitive response.
The membrane probe is inserted into the inlet of the vacuum chamber through a standard ½″ probe lock, forming a vacuum-tight seal. The VOCs (volatile organic compounds) flow effusively into the vacuum chamber from the exit of the membrane tip which is approximately 2 cm from the laser ionization region. VOC molecules that cross the laser beam path are ionized, extracted using ion optics, and their mass analyzed by a time-of-flight mass spectrometer. A schematic of the laser photoionization mass spectrometer is shown in
A second group of tests was conducted to evaluate reproducibility and limits of detection (LOD) for the membrane inlet/laser photoionization/mass spectrometer combination described in Example 1. For this purpose, sample concentrations of 10, 1.0, 0.1, 0.01 and 0.001 μL/L were prepared through serial dilution of benzene, chlorobenzene, and o-xylene in deionized water. Toluene was not used in these low concentration tests because of its residual background due to initial spiking at high levels in the reservoir. In these tests, the 2 L reservoir was filled with deionized water then spiked with 20 μL of benzene-d6 to provide a constant reference at m/z 84 throughout the experiments. This reference compound permitted normalization of intensities of the analyte without concern for small changes in the operating characteristics of the combined membrane/laser/spectrometer system. To evaluate reproducibility, 10 mL of a 1.0 μL/L o-xylene solution was injected in the input of the flow injection controller and measurements made 1-second averaged peak signal intensities at 80° C. for both this compound and the deuterated benzene. The results are summarized in Table 2. The intensities are given in units that are arbitrary (volts) but the same for each peak. The absolute o-xylene intensity differs from the benzene-d6 because of differences in the total ionization efficiency and permeability of the membrane for these two compounds. While there is a 19% standard deviation for deuterated benzene in these three runs, there is less than a 4% standard deviation for the o-xylene. The lower value is more characteristic of this system.
For the LOD determinations, 10 mL injections were made using the five sample concentrations noted above. Averaging time was increased to 5 s to improve the statistics. Typical results are shown in
These results show that the advantages of laser photoionization detection include the speed of response and high sensitivity with good chemical selectivity. There is no need to deconvolute mass peaks either experimentally or mathematically because the parent ion peak is directly proportional to the absolute analyte concentration. Variants of the fixed wavelength REMPI photoionization scheme can also be employed. Using a jet inlet to entrain and cool the VOCs in a supersonic flow and a narrow-band tunable laser increases both the sensitivity and selectivity of the system (e.g., easily distinguishing ethylbenzene and the three xylene isomers), but at the expense of added complexity owing to the tunable laser source and pulsed inlet valve. To make a single photon laser, the initial 1.064 μm Nd:YAG fundamental frequency can be tripled to 355 nm using nonlinear, solid state crystals, and then tripled again in a gas cell containing Ar and Xe to produce 118 nm photons. These 10.5 eV photons are capable of directly ionizing many VOCs, again producing the parent ion with no fragmentation. This renders accessible many other important, but non-aromatic VOCs such as chloroform and trichloroethylene that are important in environmental problems involving ground water contamination.
Priority is hereby claimed under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 60/564,087, filed on Apr. 21, 2004, by Oser, et al. and entitled, “METHOD AND APPARATUS FOR THE DETECTION AND IDENTIFICATION OF TRACE ORGANIC SUBSTANCES FROM A CONTINUOUS FLOW SAMPLE SYSTEM USING LASER PHOTOIONIZATION-TIME-OF-FLIGHT MASS SPECTROMETRY,” which is incorporated by reference for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 60564087 | Apr 2004 | US |
