Antares Analytical Technical Bulletin for
November 2002
Subject: Baird Spectrometer Optical Components
Relevant Models: All Baird Spectrometers
Relevant Matrix: Not Applicable
Background: All Baird spectrometers, actually all OES spectrometers … the manufacturer is irrelevant, all use the same type basic optical components. There will be differences from Baird model to model and where there are difference these will be noted.
Most of the optical components are made of pure quartz for optimum light transmission.
The majority of element wavelengths used on OES (Optical Emission Spectrometers) are between 2000 and 4000 Angstrom (200-400 nm). At wavelengths below 4000 A, glass acts as a barrier and doesn’t allow the lower wavelengths to pass so pure quartz is used.
Exceptions are:
Basic Components: For simplicity the following components will be listed in the order of A) Entrance Optics components, B) Gratings & C) Exit Optics components
Entrance Optics: All Baird vacuum spectrometers (DV series, FSQ, all ICP including 2070) all use the same basic components which are:
· Slide Stand Window – used to separate the argon purged sample stand from the argon purged entrance optics chamber. On the DV6-SN this is Mag Fluoride.
· Heated Purge Window – This is used on the DV6 (Quartz) and DV6-SN (Mag Fluoride) to separate the vacuum cell from the purged entrance optics. The DV6 & DV6SN differ from earlier DV models, ICP & FSQ models because of the straight in optics, not the entrance mirror configuration other models use. The heater is design is to help cut down on “UV Burn” (explained later). This heater can not be retrofitted to earlier spectrometers.
· Vacuum Window - This window is part of the vacuum cell door (DV Series, ICP) or imbedded in the vacuum cell wall (FSQ, ICP 2070).
· Entrance Mirror (DV 1-5, ICP & FSQ) – deflects the light coming into the vacuum cell through the entrance slit and onto the diffraction grating.
Diffraction Grating: The grating is the “heart” of the optical system and the spectrometer.
· The grating is a series of rulings, essentially a series of prisms in rows, which diffracts (breaks up) the light into it’s component spectra.
· Gratings are replicas of Master gratings which are made on a very precise ruling engine. Because the rulings number as many as 4000 per millimeter, a master can take as long as 1 week to produce.
· After replicating the master onto a “blank” it is then coated, tested for reflectivity, band pass and light transmission loss to be sure it meets specifications. Only then is it ready to be placed in a spectrometer
Exit Optics: After the light is diffracted into it’s component element spectra, the individual element lines are transmitted to their respective exit slits for light intensity detection by the PMT. Before it gets to the PMT it can go through several exit optic components.
· Deflection Plate – (used on MOA / FAS, later FSQ, DV6 / SN models … not used on DV1 to 5 or ICP) before the element line gets to it’s exit slit it may be put through a deflection plate which is mounted in front of the exit slit strip
1. Best analytical results require maximum amount of the element line go directly through the center of the exit slit. Sometimes to achieve this the element line must be “bent” and “deflected” from it’s current path into a new path so light goes straight into the exit slit.
2. These deflection plates (when used) were installed & aligned when the unit was built and final alignment checked again during installation. If a spectrometer is moved from plant to plant for example, a “slit scan” should be done and plates adjusted as necessary. This should be done by a qualified Service Engineer.
· Exit Mirror – whether a mirror or series or mirrors is used to direct a particular element to it’s correct PMT will depend on the number and arrangement of the PMT in the vacuum cell. All Baird spectrometers used exit mirrors. Where possible an element line was aligned directly into it’s PMT without mirrors.
· Exit Filter – depending on the element wavelength and the spectral order the wavelength is transmitted in, an optical filter will be used to block certain wavelengths from reaching the PMT.
1. Without going to deep into the physics, Baird spectrometers will use either 1st, 2nd or 3rd order lines. When a 2nd order wavelength is used, it is possible (and probable) overlapping 1st or 3rd order wavelengths could be detected by the PMT
2. This will lead to an inaccurate result because the PMT will be picking up an unwanted wavelength, in addition to the correct element wavelength being measured in that PMT.
3. The correct exit filter will block these unwanted “other” order lines allowing only the correct element line to be detected by the PMT.
· Lens- The last optical element is the lens which is mounted onto the PMT housing and is used to focus the element line onto the center of the PMT grid.
· PMT – used to detect the element light level and convert the light intensity to an analog signal which is sent down to the readout electronics. For a more complete explanation of PMT please see the Antares Technical Bulletin, June 2002, which discusses PMT detectors in more detail.
Fiber Optic Cables: (MOA, Foundrymate, MS3-MAX II, DV6 with Pistol Option): In some Baird spectrometer designs it is necessary to take the light from the sample stand and direct it to the entrance optics using a shielded fiber optic cable. Fiber optic cables are used only in air path (non vacuum) spectrometers where transmitted wavelengths are above 2000 Angstroms.
· The MOA has the optical system at the rear of the spectrometer so an F/O cable is used to direct the light to the entrance optics.
· The FoundryMate, who’s basic design is based on the MOA optical system, uses an F/O cable to direct the light to the rear optical cell and uses a direct light path for the ˝ meter vacuum option. The ˝ meter vacuum option is used when UV elements C, P,S, As, Se, B Sn need to be analyzed, usually in a Fe matrix.
· MAX II, MS3, DV6 with Pistol Option because of the movement flexibility needed for the pistol, an F/O cable needs to be used.
· The main disadvantage to a F/O cable is the cable material, usually made of a PCS / quartz or similar material, which does not have the ability to transmit element wavelengths below 2000 Angstroms. This make is unusable for any application where UV elements C, P, S, B, Sn, As ,Se and several other are needed.
Problems, Solutions and Maintenance of Optical Components:
NOTE: Magnesium Fluoride components used on the DV6/SN can not be cleaned using the methods described below. Because of the special nature of these optical components, a special procedure is required or damage WILL result. Please contact Antares Analytical if you need a copy of this procedure.
NOTE: The Grating in ANY spectrometer is NEVER touched or cleaned by the customer using ANYTHING. The grating ruling are very delicate and ANYTHING used will damage them. The end result will be a useless grating and, depending on the spectrometer, will mean a grating replacement. Over the years companies have made a lot of money replacing gratings after the customer tried to “clean” it.
UV Burn: The most common problem with all optics is UV (Ultraviolet) light burn. Because of the intense light generated by the sample sparking, strong UV light is generated and will … over time …“burn” the optical components. This is a common problem with ALL spectrometers and it can not be “designed” away.
· The 1st symptom of UV burn is a gradual decrease in the intensity signal of the UV elements, much more rapidly than the higher wavelength elements such as Ni, Cr or Cu.. Optical UV burn gradually “absorbs” the lower (less than 2000 A) wavelengths at a much higher rate than higher wavelengths.
· This can usually be detected because the Standardization Factors for elements P,S, C, B etc will change at a much faster rate than others:
Example: Assume all Standardization Factors are at a 1.00. Over time the factors for C, P, S, all fall to about 0.50 but Ni, Cr, and Cu are still between 0.90 and 1.00. This is a good indication you have UV burn starting. There may be other causes, such as an argon problem, but UV burn is the 1st to look at.
· UV burn is not always visible to the naked eye but the UV deposit is on the window. In severe cases you will see a coating which looks brown or gray on the Entrance Window. This is UV burn.
· This tells you it’s time to clean the Sample Slide or Entrance window in the sample stand / gun. A periodic cleaning (daily or more) of the window will help decrease this burn rate. Usually optical cleaner will solve the problem, but in cases where brown deposit is seen, Acetone will usually take it off.
· While UV burn deposits on the surface it also “burns” the quartz material itself so that, eventually the only solution is to replace the entrance mirror and / or entrance & vacuum windows. Antares Analytical supplies the complete range of entrance optics components when you will need them.
Solarization: This will affect only the F/O optical cables and is directly related to UV burn. Solarization is another name for UV burning of the F/O cable and this can not be cleaned. In many cases adjustments can be made to high voltage setting to decrease some of the affects of solarization but eventfully, the F/O cable will need to be replaced
· This is most common on the MOA where the lower wavelength elements Zn @ 2138A and Cd @ 2288A will be affected much faster. The standardization or “N” factor will decrease at a rate faster than the visible elements Na @ 5889 / 5895A & K.@ 7664A.
Dirt, Oil etc: Normal cleaning should solve these problems:
Cleanable / Replaceable Components:
· Entrance Window / Lenses: The entrance windows / lenses on the sample stand slide, vacuum window are customer replaceable. Please contact us for replacement windows / lenses and the procedure to replace them. When replacing these components, it’s also best to replace the omni seals / o rings at the same time.
· FSQ Entrance Window: Since it is embedded into the vacuum cell wall, this window takes more care to replace. Please contact us for more information about replacement.
· Entrance Mirrors (FSQ, DV1-5, ICP,) The entrance mirror can not be cleaned and UV burn will eventually cause you to replace this mirror also. Correct alignment of this entrance mirror when replacing is CRITICAL and should be done by a qualified Service Engineer during a scheduled Maintenance Call. We will have the correct tools and experience to do this. An entrance mirror not replaced correctly will make the spectrometer unusable because you have lost your critical entrance mirror to entrance slit alignment.
· Grating: The Grating is NEVER cleaned.
· It is okay to use argon ONLY to blow dust off but the argon line must NEVER come in contact with the grating. In the case of MOA / FAS where argon is not available, a can of compressed air, available to electronic stores, is permissible. Electronic component cleaner is NEVER to be used.
· In the event of vacuum pump blowback (see Antares Bulletins for September / October 2002 for more explanation) there is ONE method only, to be performed ONLY by a qualified Service Engineer which can be done.
· Some times the only solution is to replace the grating. This can be done onsite (FSQ / MOA) or in Antares facility (MOA).
· F/O Cables: This will be required in the MOA / Foundrymate eventually (meaning after some years of use) depending on use. There is a method described in the MOA manual but, unless this is done exactly as described, using the exact tools described, and alignment is done perfectly you will lose entrance slit alignment. The F/O cable on the MS-3 / MAX II is not designed to be replaced. It is best to let a qualified Service Engineer replace this cable during a scheduked Maintenance Call.
· Exit Mirrors / Deflection Plates / Filters / Lenses:
· Except in case of breakage these almost never need replacing. Cleaning is using a cotton swab with window cleaner, Windex window cleaner is adequate, and CAREFULLY swabbing the plates, filters & mirrors.
· Deflection plates must be done with most care as these are delicate quartz plates and WILL break. That happens and you’ve lost that particular element’s “line to slit” alignment. Result will be inaccurate, if any, analytical result for that line.
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This Technical Bulletin is written by Gerry Smith, Technical Services Manager of Antares Analytical. Having spent 22 years with Baird in various capacities, including many years as Final Test / Calibration Manager and Senior Service Engineer, International Operations, he or any of our Technical Support Staff, are well qualified to answer any Technical or Applications problems you may have.
For further information or access to past monthly Technical Bulletins covering a variety of subjects, please visit our website at www.antaresanalytical.com
For more information, to be added to this list, or help with other subjects including Tech Support, spare & replacement parts, or on-site Service by qualified Antares Analytical Service Engineers, please contact us at info@antaresanalytical.com
This bulletin is meant for informational purposes only and it not meant as a substitute for Service performed by a qualified Service Engineer. Any use of this information is done strictly at the discretion of the user. Antares Analytical is not repsonsible for any misfortune which results as the result of use of these Technical Bulletins, in place of service by a Qualified Service Engineer.