Apparatus for trace sulfur detection using UV fluorescence
A method and system for performing low level sulfur UV fluorescence measures including an UV interference reduction system which removes or destroys interfering nitrogen oxides. The preferred nitrogen removal systems include introducing ozone into the system in sufficient quantities to destroy any produce NO and optionally a nitrogen sparge or similar nitrogen gas removal system.
1. An apparatus for performing low level sulfur UV fluorescence detection comprising:
an oxidation or combustion chamber including:
a sample inlet,
an oxidizing agent inlet,
an oxidation zone, and
an oxidized sample outlet,
where the oxidation chamber is adapted to convert substantially all oxidizable components of a sample into their corresponding oxides;
a transfer tube connected to the oxidized sample outlet adapted to transfer the corresponding oxides to an UV chamber;
an UV interference reduction system adapted to introduce an UV interference reduction agent into the sample before during and/or after oxidation, where the UV interference reduction agent is capable of reducing or eliminating nitrogen oxides that interfere with sulfur UV fluorescence detection; and
a detector/analyzer system including:
an excitation light source adapted to generate excitation light,
the UV chamber having:
an excitation light port in optical communication with the light source adapted to excite SO 2 molecules in the oxidized sample into electronically excited SO 2 molecules,
an oxidized sample inlet connected to the transfer tube,
an oxidized sample outlet for exhausting the oxidized sample from the chamber after irradiation from the excitation light, and
a fluorescent light port oriented at an angle to the excitation light port, where the angle is sufficient to reduce or eliminate excitation light from entering the fluorescent light port;
a fluorescent light detector in optical communication with the fluorescent light port adapted to detect fluorescent light emitted by the electronically excited SO 2 molecules passing through the fluorescent light port and to convert the detected fluorescent light into an electrical output signal, and
an analyzer in electrical communication with the detector and adapted to convert the electrical output signal into a concentration of sulfur in the sample,
where the UV interference reduction agent is introduced in an amount sufficient to convert interfering nitrogen oxides into non-interfering nitrogen oxides thereby lowering a sulfur detection limit to sulfur concentrations of less than 100 ppb.
2. The apparatus of claim 1 , further comprising:
a nitrogen filter connected to the oxidizing agent inlet to remove trace amounts of nitrogen gas (N 2 ) in the oxidizing gas prior to the oxidizing agent entering the oxidizing agent inlet of the combustion chamber.
3. The apparatus of claim 1 , wherein the UV interference reduction agent comprises a nitric oxide reactive species selected from the group consisting of ozone and hydrogen peroxide.
4. The apparatus of claim 1 , wherein the amount of the UV interference reduction agent is sufficient to lower the sulfur detection limit to sulfur concentrations below 50 ppb.
5. The apparatus claim 1 , wherein the UV chamber further includes:
an optical filter associated with the fluorescent light port and
wherein the fluorescent light detector comprises a photo-multiplier tube (PMT).
6. The apparatus of claim 1 , wherein the oxidizing agent comprises an oxygen containing gas selected from the group consisting of oxygen, oxygen in argon, ultra-pure oxygen, ultra-pure oxygen in argon, or ultra-pure oxygen in ultra-pure argon.
7. The apparatus of claim 1 , wherein the UV interference reduction system comprises:
an ozone generator and
wherein the UV interference reduction agent comprises ozone.
8. The apparatus of claim 7 , wherein the ozone is introduced into the oxidizing agent inlet of the combustion chamber.
9. The apparatus of claim 7 , wherein the ozone is introduced into the oxidizing zone through an ozone inlet.
10. The apparatus of claim 7 , wherein the ozone is introduced into the combustion chamber at its distal end through an ozone inlet.
11. The apparatus of claim 7 , wherein the ozone is introduced into the transfer tube through an ozone inlet.
12. The apparatus of claim 7 , wherein the transfer tube includes:
an ozone chamber having an ozone inlet and
wherein the ozone is introduced into the ozone chamber through the ozone inlet.
13. The apparatus of claim 7 , wherein the UV chamber further includes:
a first sub-chamber having an ozone inlet and
wherein the ozone is introduced into the first sub-chamber through the ozone inlet.
14. The apparatus of claim 7 , wherein the ozone generator generates variable concentrations of ozone to simultaneously minimize interfering nitric oxide fluorescence and ozone absorption of excitation light and/or SO 2 fluorescent in the UV chamber.