Patent Application: US-93099211-A

Abstract:
a photochemical vapour generator is disclosed having a reaction chamber with a liquid sample inlet , a carrier gas inlet and a carrier gas outlet . a liquid sample support surface for supporting a film of a liquid sample is in the reaction chamber . a source of ultraviolet radiation is positioned in radiative contact with the liquid sample support surface . also described is a process for detecting and / or quantifying an analyte by providing a liquid film of a sample containing an organic acid or a salt thereof and a precursor compound of the analyte , exposing the liquid film to ultraviolet radiation to generate a volatile species of the analyte , separating the volatile species from the liquid film to form a vapour containing the volatile species , and directing the vapour to a detector to detect and / or quantify the analyte in the vapour . amalgamation of photochemical reactions with thin film generation and gas - liquid phase separation achieves the benefits of all of these principles in a single unit , providing a simple and effective way of enhancing generation of an expanded suite of elements .

Description:
high purity 18 mω - cm deionized water was generated using a mixed bed ion - exchange system supplied with reverse osmosis feedstock ( nanopure , model d4744 , barstead / thermoline , dubuque , iowa ) and used for preparation of all solutions . stock solutions ( 1000 mg ml − 1 ) of as ( iii ), sb ( iii ), bi ( iii ), se ( iv ), te ( iv ), hg ( ii ), ni ( ii ), fe ( iii ) and co ( ii ) were obtained from scp science ( montreal , pq , canada ). a 1000 mg stock solution of fe ( ii ) was prepared from dissolution of high purity iron powder with sub - boiling distilled hydrochloric acid and dilution in boiled , degassed deionized water . the stock solution of i ( i ) ( 1000 mg ml − 1 ) was prepared by dissolving high - purity ki ( sigma - aldrich ) in boiled , degassed deionized water . both solutions were stored in screw - capped polypropylene bottles securely sealed with ptfe tape in an effort to minimize any oxidation . reagent acids , including hcl , hno 3 , h 2 so 4 and low molecular weight ( lmw ) organic acids , including formic ( 88 %, gfs , canada ) and acetic acid ( 6 . 3 m , bdh ), were purified in - house prior to use by sub - boiling distillation of the reagent grade feedstocks in a quartz still . working standards of lower concentration were prepared daily by dilution of the stocks using deionized water . environmental grade ammonia ( 20 - 22 %, v / v ) from anachemia science ( montreal , pq , canada ) was used to adjust the ph of the fe solutions as required . high purity ar was obtained from praxair products inc . ( mississauga , on , canada ). national research council canada certified reference materials dorm - 3 ( fish protein ) and dolt - 4 ( dogfish liver tissue ) were used to validate the accuracy of the methodology . fig1 provides a schematic drawing of a first embodiment of a thin - film photochemical vapour generator of the present invention in vertical orientation . with reference to fig1 , the thin - film photochemical vapour generator 1 integrates both vapour generation and gas - liquid separation functions utilizing a 6 mm o . d . vertical central quartz rod 2 onto which a liquid sample containing a precursor compound of an analyte is pumped through sample inlet 3 to yield a thin liquid film on the surface of the rod . the surface of the rod is fashioned into a screw design 5 to increase surface area of the rod in order to perturb what would otherwise be an unencumbered flow of liquid down a smooth surface under the influence of gravity , thereby enhancing sample residence time in the generator . the thin liquid flow facilitates rapid escape of any generated volatile species from the condensed phase . the rod is housed within a concentric 13 mm o . d . quartz tube 7 . a flow of argon carrier gas is passed from gas inlet 9 through the quartz tube to strip and transport the generated volatile species to gas outlet 11 and thence to a detector where the presence of the analyte in the gas flow is detected . concentric quartz tube 7 is itself surrounded by a 78 - turn 0 . 5 m long quartz coil low pressure mercury discharge lamp 13 operating at 20 w . the coil lamp was custom fabricated by jelight co . ( irvine calif .). the top of the generator is fitted with a first poly ( tetrafluoroethylene ) ( ptfe ) cap 14 which supports rod 2 via a series of gas - tight “ o ”- rings 15 a , 15 b , and which also supports concentric quartz tube 7 via a series of gas - tight “ o ”- rings 16 a , 16 b . first cap 14 is provided with sample inlet 3 as well as gas outlet 11 . in a similar fashion , the bottom of the device is closed with a second ptfe cap 17 containing gas inlet 9 and liquid waste outlet 19 , and sealed with gas - tight “ o ”- ring 18 . second cap 17 comprises inclined wall 20 to facilitate draining of liquid waste through outlet 19 while preventing liquid waste from entering gas inlet 9 . a contoured interior surface of the concentric quartz tube also helps prevents the ar gas from passing through any pooled liquid phase thereby reducing aerosol production . the lower tip of central quartz rod 2 terminates approximately 2 cm from the surface of the second cap . fig2 provides a schematic drawing of a second embodiment of a thin - film photochemical vapour generator of the present invention in horizontal orientation . with reference to fig2 , the thin - film photochemical vapour generator 101 integrates both vapour generation and gas - liquid separation functions utilizing a 6 mm o . d . horizontal quartz sample support element 102 having channel 104 grooved therein into which a liquid sample containing a precursor compound of an analyte is pumped through sample inlet 103 to yield a thin liquid film on the inner surface of the channel . flow rate of the liquid sample through the sample inlet is regulated to enhance sample residence time in the generator . the thin liquid flow facilitates rapid escape of any generated volatile species from the condensed phase . the channel is housed within a concentric 13 mm o . d . quartz tube 107 . a flow of argon carrier gas is passed from gas inlet 109 through the quartz tube to strip and transport the generated volatile species to gas outlet 111 and thence to a detector where the presence of the analyte in the gas flow is detected . concentric quartz tube 107 is itself surrounded by a 78 - turn 0 . 5 m long quartz coil low pressure mercury discharge lamp 113 operating at 20 w . one end of the generator is fitted with a first poly ( tetrafluoroethylene ) ( ptfe ) cap 114 which supports sample support element 102 via a series of gas - tight “ o ”- rings 115 a , 115 b , and which also supports concentric quartz tube 107 via a series of gas - tight “ o ”- rings 116 a , 116 b . first cap 114 is provided with sample inlet 103 as well as gas outlet 111 . in a similar fashion , the other end of the device is closed with a second ptfe cap 117 containing gas inlet 109 and liquid waste outlet 119 , and sealed with gas - tight “ o ”- ring 118 . second cap 117 comprises inclined wall 120 to facilitate draining of liquid waste through liquid waste outlet 119 while preventing liquid waste from entering gas inlet 109 . a contoured interior surface of the concentric quartz tube also helps prevents the ar gas from passing through any pooled liquid phase thereby reducing aerosol production . tip 121 of quartz sample support element 102 terminates approximately 1 cm from the surface of the second cap and is inclined toward inclined wall 120 to facilitate draining of liquid waste from channel 104 into liquid waste outlet 119 . fig3 illustrates how the thin - film photochemical vapour generator of fig1 is utilized in association with an icp - oes detector . a similar configuration can be utilized with the tf - pvg of fig2 . referring to fig3 , generator 1 is housed in a large outer glass tube ( not shown ) through which a flow of nitrogen or argon is passed to eliminate oxygen and avoid ozone generation . the outer glass tube is wrapped with aluminum foil to reflect uv radiation from the lamp and to protect the operator . a four channel minipuls 2 peristaltic pump 21 ( gilson , middleton , wis .) is used to transport samples to the generator through line 23 and to evacuate the liquid waste from the bottom of the generator through line 25 . generated volatile species are separated within the generator and directed through line 26 to a separate in - line small ( 10 ml internal volume ) gas - liquid separator ( gls ) 27 whose waste outlet is sealed . the gls is immersed in ice bath 29 to minimize water vapour carryover and to ensure no condensed liquid droplets are transported to icp 31 . the mass flow controlled ar nebulizer gas from the icp - oes serves as the carrier gas , which is carried to the generator through line 33 . the outlet of the gls was connected to the icp torch via a 75 cm length of ptfe tubing ( 2 . 5 mm i . d .× 4 . 0 mm o . d ). in the examples below , an inductively coupled argon plasma optical emission spectrometer ( icp - oes ) serves as the detector ( perkin - elmer optima 3000 radial view icp - oes instrument ). operating parameters of the icp - oes are summarized in table 1 and compared with parameters for normal liquid nebulization . once the ar plasma and the uv lamp of the tf - pvg of example 1 are stabilized , the pvg generator is brought on - line and volatile species are generated when analyte standard solutions containing formic or acetic acid at varying concentrations are delivered to the tf - pvg . gaseous products , simultaneously separated from the liquid phase within the generator , are flushed by the ar carrier gas to the icp torch for optical emission measurements . all experimental conditions are optimized for maximum response for each analyte . a blank is measured before each run using solutions containing only sub - boiling distilled lmw acids ( ammonia and the lmw organic acid in the case of iron ). no blank signal is generally detected ; however , it is noteworthy that analytical grade formic acid produces a very high blank for iron and nickel before purification . test samples of nominal 0 . 25 g of crms dorm - 3 and dolt - 4 are weighed into teflon ™ digestion vessels ( cem acv type ) and 7 ml of nitric acid and 200 μl ( 30 %) hydrogen peroxide added . the vessels are capped , and the samples digested using a model mds - 2100 microwave digestion system ( cem corp .). the digested samples are cooled and reconstituted to a final volume of 25 ml final volume using diw and stored in pre - cleaned polyethylene screw - capped bottles . ni ( co ) 4 and fe ( co ) 5 are extremely toxic , and additional unknown gaseous products may be generated during exposure of the tested solutions to uv irradiation . essential safety considerations must be taken during all manipulations and an adequate ventilation / exhaust system must be used . optimization of icp - oes plasma and auxiliary gas flow rates as well as rf power for the resultant “ dry ” plasma conditions are quickly performed by monitoring analyte intensities while supplying 100 ng l − 1 solutions of each analyte element to the tf - pvg . values of 15 l min − 1 , 0 . 5 l min − 1 and 1250 w for these parameters were selected as overall compromise conditions . carrier gas flow rate ( surrogate for the nebulizer gas in this configuration ) through the tf - pvg - gls system significantly affected response as it influenced analyte concentration in the carrier gas , analyte transport efficiency as well as analyte residence time in the plasma ( hence atomization / excitation efficiency ), as shown in fig4 a and 4 b . efficient gas - liquid separation and quantitative transport of the volatile species to the plasma could not be achieved using carrier gas flow rates lower than 0 . 45 l min − 1 . a response plateau was obtained for all elements except iodine in the range 0 . 70 to 0 . 80 l min − 1 which likely reflects a simple balance between dilution of analyte with excess ar flow and increased transport of the hydrophobic analyte from the liquid phase due to the concentration gradient ; higher argon flow rates caused significant dilution effects and decreased response . no loss of signal was encountered when up to a 5 - m length of ptfe transfer tubing was used to connect the gls to the icp , facilitating the hyphenation and suggesting that the volatile products arising from these analyte elements are very stable . of the variety of possible low molecular weight organic acids typically used to elicit pvg , only formic and acetic acids were examined in detail in this study , although other lmw organic acids are expected to have utility as well . in agreement with earlier reports , the pvg efficiencies were strongly dependent on the type and the concentrations of organic reducing reagent as well as the analyte element . compared to formic acid , acetic acid is more favourable for the pvg of hydride - forming elements and iodine . no signals could be detected for bi and i when generation was attempted with formic acid . the effects of acetic acid concentration on the intensity of atomic emission for a solution containing 100 ng ml − 1 as ( iii ), sb ( iii ), bi ( iii ), se ( iv ), te ( iv ) and 1000 ng ml − 1 i ( i ) are shown in fig5 a . intensities for all elements increased with increasing concentration of acetic acid in the range of 0 %- 40 % ( v / v ). for the purpose of achieving simultaneous determination of these elements , 50 % ( v / v ) acetic acid was used for subsequent experiments . in contrast to the acetic acid system , the formic acid system is more convenient for the generation of volatile fe , co and ni species . fig5 b shows that response for cobalt can be detected at acid concentrations as low as 2 % ( v / v ). however , the optimum concentration of formic acid is more than 50 % ( v / v ) for fe and ni and no response is achieved for fe below 20 %. pvg of hg is very efficient with optimum response achieved at concentrations below 10 % ( v / v ) of either formic or acetic acid . results generated with the present system are similar to those reported earlier wherein generation efficiencies for transition metals are largely determined by the ph of the sample solution . for trace and ultra - trace elemental analysis , many analytical grade solid reagents such as formate , acetate and phosphate cannot be used to adjust the ph of a solution because they contain high levels of transition metal impurities . for this study , environmental grade ammonia containing & lt ; 10 ng l − 1 transition metal impurities was used to adjust the ph of the sample solution prior to pvg . no signals from hydride - forming elements were detected when the ph of the solution was higher than 5 . although a similar response for each of as , sb , bi , se and te could be obtained in the ph range 0 to about 5 , this was completely different from that of each of the transition elements which required an optimum ph range for pvg . at ph less than 0 or higher than 4 , almost no response was obtained for fe but a more than 40 - fold enhancement could be achieved when using ammonia to adjust the ph to the range of 2 . 5 to 3 . 0 in the presence of 50 % ( v / v ) formic acid . based on the well - known fact that co reacts with metallic ni to form gaseous ni ( co ) 4 , one may predict that the mechanism for pvg of transition metal species may involve an initial reduction of the analyte ion to free atoms in solution concurrently with the generation of and attack by organic radicals . the resultant volatile species then transfers to the gas phase . if the reaction is too acidic , redissolution of the transition metal may immediately occur or a metal hydroxide may form when the solution is too basic . response from all elements was achieved only in the presence of the uv source , as neither volatile species nor any aerosol containing the sample was transported to the icp in its absence . for many elements , response ceases immediately after the uv lamp is turned off . radical generation will halt immediately as the uv field is removed and any lingering signal arises because of the finite washout time of the generator - gls system convoluted with the finite phase transfer of the volatile species to the ar carrier gas . volatile species of some of the tested elements , such as hg and i , can be generated within less then 5 s irradiation time whereas tens of seconds are required for the hydride - forming elements and that for the transition metals can take several minutes . ideally , analyte response should increase linearly with sample introduction flow rate until saturation occurs due to plasma processes or detector function . however , the multiple processes that sequentially occur in the pvg are all kinetically controlled and the slowest step in the sequence will determine the overall rate and thus the impact of the sample introduction rate on response . thus , for a continuous mode of sample introduction the response curve is expected to exhibit an optimum , determined at low flow rates by the flux of sample to the reactor and at high flow rates by the residence time of the solution in the uv field which will influence the extent of the reaction as well as the efficiency of the phase separation . lengthening the central quartz rod or altering its surface topology can be used to extend the irradiation time for a given sample flow rate . to provide sufficient irradiation time , especially for transition metals , the longest central quartz rod possible with the current design was used ( 58 cm ) and its surface was modified by adding a threaded topology . the effect of sample flow rate on response is shown in fig6 a and 6 b . signals for hydride - forming elements increased with sample flow rate in the range 0 . 2 to 2 . 3 ml min − 1 , reaching an optimum at about 2 . 3 ml min − 1 , corresponding to an irradiation time of 15 s . in this region the increased sample flux is the main factor determining response . beyond 2 . 3 ml min − 1 signals decreased due to insufficient irradiation time . as hg and i are readily converted to their volatile species , less than 10 s irradiation time is sufficient , permitting flow rates up to 3 . 5 ml min min − 1 for hg and higher for i . however , for fe , no optimum irradiation time could be achieved and even 150 s ( the longest available with this system ) appears insufficient . one method used to estimate the overall pvg efficiencies uses the ratio of their signals ( aas or afs ) to that arising from conventional hg based on the nabh 4 - acid hydride generation system under optimized conditions ( guo 2004a ). this assumes that the efficiency of the latter is 100 %. unfortunately , such an approach could not be used in this study as it is too difficult to generate hydrides of fe , co , ni and i using the nabh 4 - acid system . consequently , the pvg efficiencies were estimated from an assessment of the relative concentrations of analyte in the feed and remaining in the waste solution after the sample was subjected to pvg . based on such data , overall generation efficiencies achieved under optimum conditions are summarized in table 2 . efficiencies for as ( iii ), sb ( iii ), bi ( iii ), se ( iv ), te ( iv ), fe ( iii ), ni ( ii ) and i ( i ) using the tf - pvg are typically found to be more than 3 - fold greater than those reported with other pvg devices ( grinberg 2009 ; zheng 2009a ; zheng 2009b ). the pvg efficiency obtained for mercury in this study is not different from that reported for earlier systems ( zheng 2005 ), probably because it can be readily reduced to its cold vapor in various cvg reactors . other factors also come into play , as for example when it is noted that yin et al . ( yin 2008 ) reported that mercury can be reoxidized by uv irradiation when long exposure times are used for the pvg , resulting in decreased generation efficiency . additionally , feng et al . ( feng 2003 ) demonstrated that many such generated species are soluble and stable in water , with the result that the pvg efficiencies of the overall measurement process ( ε cvg ) are determined by the efficiencies of formation ( ε for ), separation ( ε sep ), transport ( ε tra ) and photo - decomposition ( ε dec ) of the analyte , i . e ., the design features of the tf - pvg are such as to optimize the efficiencies for generation , separation and transport in that a thin film serves to maximize uv intensity throughout the sample while promoting rapid phase separation into an efficient small dead volume transport system . the remaining drawback associated with the finite size of the current system may possibly be circumvented by increasing the lamp power so as to reduce the irradiation time for elements such as fe . the efficiencies estimated using the above technique are consistent with the enhanced signal intensities arising from pvg sample introduction as opposed to conventional solution nebulization wherein a sample introduction efficiency of typically 2 % can be assumed . analytical figures of merit for as , sb , bi , te , fe , ni , co and i were evaluated under optimal experimental conditions . the linear correlation coefficients for calibration curves were better than 0 . 99 at the trace concentration level and began to deviate from linearity at concentrations & gt ; 5 mg l − 1 due to the decreased pvg efficiencies at higher concentrations . such a phenomenon has been reported earlier with the formation of an amorphous se ( 0 ) precipitate deposited onto the inner surfaces of a photo - reactor when high concentrations of selenium ( 100 mg l − 1 ) were photolysed ( guo 2003 ). this may arise because of the high transient concentrations of reduced se atoms promote collisional flocculation before radical attack can produce a stable volatile species . it is clear that optimal application of this technique will lay with the analysis of these elements at trace concentration levels . table 3 summarizes the sensitivities and limits of detection achieved with the tf - pvg for sample introduction as compared to those arising from conventional solution nebulization obtained using the manufacturer &# 39 ; s suggested conditions . significant improvements are notable : 108 -, 117 -, 127 -, 251 -, 124 -, 231 -, 77 -, 1 . 3 -, 16 and 31 - fold enhancements in sensitivity ( slope of calibration curve ) were obtained for as , sb , bi , se , te , hg , ni , co , fe and i , respectively . these enhancements may be accounted for in part by the increased sample flow rates used with pvg , i . e ., 2 . 3 ml min − 1 for as , sb , bi , se and te , 2 . 9 ml min − 1 for hg and i and 1 . 9 ml min − 1 for ni and co as opposed to the 1 . 5 ml min − 1 used for solution nebulization ( note that , in contrast , a pvg flow rate of only 0 . 2 ml min − 1 was used for fe ). additionally , the drier plasma arising from pvg sample introduction should provide an improved environment for enhanced analyte emission as a consequence of higher local plasma excitation temperature . the lod , defined as the analyte concentration equivalent to 3 s of 11 repeat measurements of a blank solution , was improved 114 -, 143 -, 170 -, 266 -, 200 -, 300 -, 155 -, 2 . 7 -, 50 - and 44 - fold for as , sb , bi , se , te , hg , ni , co , fe and i , respectively compared to those arising from conventional solution nebulization . the lod and sensitivity may potentially be further improved if a longer generator was used to increase sample radiation time or a higher power lamp was applied since no significant blank arises with use of high - purity reagents . conventional cvg processes based on the nabh 4 - acid reductant system generates significant hydrogen , evolved in the form of bubbles , which perturbs plasma detection systems not only because it is difficult to sustain the plasma in the presence of excess hydrogen , but the bubbles enhance the noise and degrade precision . the nature of the pvg process and the smooth and continuous phase separation available with the tf - pvg preclude these problems , leading to lower rsds and enhanced lods . fig7 illustrates steady - state signals obtained for a number of analytes using the tf - pvg , yielding precisions of less 0 . 5 % rsd ( based on 11 measurements ). the accuracy of the proposed methodology was validated by the determination of two representative elements , fe and ni , in dorm - 3 ( fish protein ) and dolt - 4 ( dogfish liver tissue ). direct analysis of these analytes in the digested samples was not possible because of serious interferences arising from the high concentration of residual nitric acid and thus an evaporation step was undertaken to remove it . for the determination of fe in these two samples , 1 ml of the digested sample was evaporated to near dryness on a hot plate under a clean hood and then diluted to 25 ml using 50 % formic acid . ammonia was used to adjust the ph to 2 . 5 - 3 . due to the low level of ni 2 + present in the samples , 10 ml volumes of digest were evaporated to near dryness and then diluted to 5 ml using 50 % formic acid . analytical results are summarized in table 4 . it is clear that the proposed tf - pvg is capable of generating accurate and precise results for these elements in real samples when calibration is performed using external standards . it is now well appreciated that detection of volatile transition and noble metal species following hydride generation , as well as reduction in potential interferences arising from transition and noble elements , can be achieved under conditions facilitating rapid gas - liquid phase separation of the product ( ding 1997 ; tao 1999 ). earlier studies have reported that optimum irradiation times for uv - pvg are dependent on the lwm acid and the analyte element : i , co and hg are easily reduced to their volatile species within 5 seconds ( zheng 2005 ; grinberg 2008 ; grinberg 2009 ). however , vapour generation of se or ni requires some 60 s of irradiation time ( guo 2004a ; zheng 2009a ), whereas up to 250 s is needed for pvg of iron . a universal pvg generator as described herein which incorporates both vapour generation and gas - liquid separation functions accommodates these varied requirements . thus , in the present examples , a novel tf - pvg was coupled with icp - oes to demonstrate its successful application to the generation of volatile species of the classical hydride - forming elements , hg , several transition metals and non - metals . compared to conventional pvg , the present thin - film device significantly improves sensitivities , precision and lods due to its inherent advantageous features which include a large reaction surface area to promote photo - generation as well as efficient phase separation of the products and flexible irradiation time . lods could be further enhanced by several orders of magnitude by coupling the generator to icp - ms instrumentation or in situ trapping electrothermal atomic spectrometry . the contents of the entirety of each of which are incorporated by this reference asfaw a , wibetoe g . 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( 2009b ) microchem . j . in press , doi : 10 . 1016 / j . microc . 2009 . 09 . 010 . other advantages that are inherent to the structure are obvious to one skilled in the art . the embodiments are described herein illustratively and are not meant to limit the scope of the invention as claimed . variations of the foregoing embodiments will be evident to a person of ordinary skill and are intended by the inventor to be encompassed by the following claims .