Patent Application: US-201514810311-A

Abstract:
the present invention provides a means of greatly reducing interferences from mercury vapor , uv - absorbing compounds and water vapor in the measurement of ozone by uv absorbance . a heated graphite scrubber destroys greater than 99 % of ozone passing through it while reducing biases from typical atmospheric uv - absorbing interferents by large factors compared to conventional ozone scrubbers . substitution of a heated graphite scrubber in place of traditional ozone scrubbers such as hopcalite , metal oxides and heated silver scrubbers , results in a more accurate measurement of ozone by reducing the responses to uv - absorbing interferences and water vapor . the heated graphite scrubber also may be used in combination with other ozone sensors , such as electrochemical and hmos sensors , to provide a reference measurement with ozone selectively removed and thus greatly reduce contributions from interfering species in those measurement devices as well .

Description:
fig1 is a schematic diagram ( 100 ) of a single beam ozone monitor based on absorbance of uv light . other than the ozone scrubber , the ozone monitor is the same as prior art devices and therefore its operating parameters will not be discussed in detail . in general , sample gas such as ambient air is drawn into the instrument through inlet 1 by pump 8 . valve 2 alternately directs the gas sample directly into detection cell 7 ( path a ) or through an ozone scrubber 3 ( path b ) and into the detection cell . in a 2b technologies model 202 ozone monitor , valve 2 switches every two seconds , for example . from the absorbance cell , the sample gas passes through pump 8 and exits the instrument at outlet 9 . the sample gas flow rate is typically 0 . 5 to 3 l / min , with 1 l / min typical for single beam instruments and 2 l / min typical for dual beam instruments . the temperature of the detection cell is measured with temperature sensor 6 , and pressure within the detection cell is measured with pressure sensor 4 . lamp 5 produces uv light , which passes through the detection cell and strikes detector 10 to produce a signal proportional to the light intensity . the lamp is normally a low pressure mercury lamp . detector 10 is normally a phototube or photodiode . a band pass filter centered near 254 nm is commonly placed in front of the detector or built into it in the case of a photodiode to limit detection to the 253 . 7 nm emission line of mercury . the ozone concentration is calculated , in units of molecules / cm 3 for example , using the beer - lambert law , where i o is the detector signal when air is passing through the ozone scrubber ( no ozone in detection cell ) and i is the detector signal when the sample gas bypasses the ozone scrubber ( ozone present in detection cell ). here σ is the absorption cross section for ozone at 253 . 7 nm ( 1 . 15 × 10 − 17 cm 2 / molecule ) and / is the length of the detection cell ( 15 cm for the 2b technologies model 202 ozone monitor ). the measurements of pressure and temperature allow calculation of the total air molecule concentration within the detection cell using the ideal gas law , so that the concentration of ozone may be expressed as a mixing ratio ( mole fraction ) in units such as parts - per - billion by volume ( ppb ). note that any species that absorbs light at 253 . 7 nm will be detected if its concentration is reduced when passing through ozone scrubber 3 . however , the values of the light intensity i o ( sample gas passing through scrubber 3 ) and i ( sample gas bypassing scrubber ) will be identical if the uv - absorbing species is not removed by the scrubber , and the ozone concentration calculated from equation 1 will be zero . thus , selectivity against potential interferences from uv - absorbing species other than ozone is achieved if those compounds partially or completely pass through scrubber 3 without being destroyed or removed . the ideal ozone scrubber 3 would destroy or remove all ozone but quantitatively pass all other uv - absorbing compounds . a heated graphite scrubber is a more nearly ideal scrubber than solid - phase ozone scrubbers currently used in ozone monitors . fig2 is a schematic diagram showing typical heated tubular and packed graphite scrubbers . the upper drawing shows a cross - sectional view along the longitudinal axis of a graphite tube ( 200 ) housed in a typical temperature - controlled heating block consisting of a metal ( typically aluminum or stainless steel ) or ceramic block ( 201 ) to provide thermal mass , with cartridge heater ( 202 ), temperature sensor ( 203 ) and insulation ( 204 ). the lower drawing shows a cross - sectional view along the longitudinal axis of a temperature - resistant tube ( typically aluminum , stainless steel , alumina , glass , quartz or graphite ) packed with graphite particles ( 205 ) within the same heating block . this tube could also be an open tube with the interior coated with graphite . typically a temperature controller monitors the temperature sensor and varies the current through the cartridge heater to maintain a chosen temperature within a narrow range of typically 1 ° c . or better . many methods of maintaining a constant elevated temperature of the graphite scrubber are known and can be used with either tubular or packed graphite scrubbers . in particular , a current can be passed directly through the graphite tube so that the graphite tube simultaneously serves as the heating element , as is done in graphite furnace atomic absorption spectroscopy . note that multiple graphite tubes or multiple tubes packed with graphite particles may be used in parallel in order to enhance the flow conductance of the scrubber . in that case , the multiple tubes may be inserted into the same heating block ( 201 ). individual tubular or packed graphite scrubbers may be used in series to increase the surface area to which ozone is exposed , but at the cost of reduced conductance . fig3 compares the abilities of different ozone scrubbers to destroy ozone . experiments were performed with a 2b technologies model 202 ozone monitor with its normal hopcalite scrubber installed and with tg - 1 , tg - 2 and tg - 4 scrubbers installed at temperatures of 25 ° c . and 100 ° c . tg - 1 is a single graphite tube 6 inches in length , tg - 2 consists of two 6 - inch long graphite tubes in parallel , and tg - 4 consists of four 6 - inch graphite tubes in parallel . the graphite tubes were obtained from ohio carbon blank , inc . ( edm - af5 ) and are ¼ - in o . d . and ⅛ - in i . d . graphite tubes are widely used in the tooling , mold making and general manufacturing industries . the ozone monitor was exposed to known concentrations of ozone in the range 0 - 800 ppb . the hopcalite scrubber is known to destroy & gt ; 99 % of the ozone and serves as a control . rounding to three significant figures , the slopes of the linear regressions using the hopcalite scrubber are 1 . 01 at 25 ° c . a slope of 1 . 00 is expected if the instrument and ozone source are both perfectly calibrated . at 25 ° c ., the tg - 4 scrubber provides a regression slope of 1 . 01 as well , showing that it provides the same degree of ozone scrubbing (& gt ; 99 %) as the traditional hopcalite scrubber . the tg - 2 scrubber , however , provides a downwardly curved plot at 25 ° c ., indicating that it is not an adequate ozone scrubber at room temperature . at 100 ° c ., hopcalite , tg - 2 and tg - 4 scrubbers all provide regression slopes of 1 . 01 , indicating that tg - 2 and tg - 4 scrubbers both destroy ˜ 100 % of the ozone . at 100 ° c ., the tg - 1 scrubber provides a slope of 0 . 987 over the range 0 - 800 ppb and a slope of 1 . 01 over the range 0 - 400 ppb , showing that a single graphite tube is an adequate scrubber for ozone concentrations up to 400 ppb but not up to 800 ppb . these results are consistent with the expectation that ozone destruction is more effective at higher temperatures and for higher surface areas and longer residence times of exposure . fig4 shows the effect of varying the relative humidity of air passing through a 2b technologies model 202 ozone monitor with its internal dewline ™ ( nafion ® tube ) removed and using either the standard hopcalite or a tg - 2 ozone scrubber . with the dewline ™ removed , there is a & gt ; 20 ppb increase in the offset of the ozone measurement as relative humidity is increased from 15 - 90 % using the standard hopcalite scrubber . however , when the hopcalite scrubber is replaced with a tg - 2 scrubber at 100 ° c ., the effect of humidity on the offset is seen from fig3 to be very small , with only a ˜ 2 ppb decrease while varying relative humidity over the range 18 - 90 %. the same very small effect of water vapor also was seen when 100 ppb of ozone was present . this experiment demonstrates that the dewline ™ can be eliminated if the standard hopcalite scrubber is replaced with a heated graphite tg - 2 scrubber . however , the dewline ™ can be used in combination with the heated graphite scrubber as further assurance that any humidity interference is removed . fig5 is a plot of the relative responses to a source of mercury vapor using different internal scrubbers in a 2b technologies model 205 dual beam ozone monitor . a diffusion source of mercury was used and mercury concentration varied by changing the bath temperature ( 13 to 160 ° c .) of the mercury reservoir . measured equivalent ozone values are plotted for different internal ozone scrubbers vs values obtained using a standard hopcalite scrubber . a tg - 2 scrubber at 100 ° c . gave the smallest response ( least interference ) of all the scrubbers tested , which also included internal scrubbers taken from a teledyne - api model 400e ( t - api ) ozone monitor and silver wool ( ag wool ) scrubber heated to 100 ° c . ( similar to the ozone scrubber used in horiba model 360 ozone monitors ). silver wool provides the largest interference , removing ˜ 100 % of the mercury vapor , followed by hopalite which removes ˜ 93 %. the heated tg - 2 scrubber was found to make the ozone monitor ˜ 30 times less sensitive to hg than the convention hopcalite scrubber . fig6 shows the response of a 2b technologies model 202 ozone monitor fitted with different internal ozone scrubbers to measure concentrations of the common uv - absorbing atmospheric interferent p - xylene . ozone monitors with hopcalite , teledyne - api and thermo scrubbers responded identically within error to p - xylene in dry air . the linear regression slope of 0 . 053 to the combined data for these scrubbers indicate that ˜ 19 ppb of p - xylene will provide a false signal equivalent to 1 ppb of ozone . the linear regression to the heated tg - 2 scrubber yields a slope of 0 . 008 , indicating that it requires ˜ 125 ppb of p - xylene to provide a response equivalent to 1 ppb of ozone , an improvement of a factor of 6 . 5 over those commercially available scrubbers . fig7 shows the response of a 2b technologies model 202 ozone monitor fitted with different internal ozone scrubbers to known concentrations of the common uv - absorbing indoor air interferent phenol . the tg - 4 ozone scrubber heated to 80 ° c . and the tg - 2 scrubber heated to 100 ° c . both gave linear regression slopes of 0 . 05 , meaning that it requires 20 ppb of phenol to provide a response equivalent to 1 ppb of ozone . this is an improvement of a factor of ˜ 4 compared to the t - api , thermo and hopcalite scrubbers . birks , j . w . ; andersen , p . c . ; williford , c . j . 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