Antares Analytical Technical Bulletin for June 2002

Subject: Photomultiplier Tubes (PMT)

Relevant Models: All Baird Spectrometers

Relevant Matrix: All Matrices

Background:

All Baird spectrometers use PMT for element detection and to send the correct signal level to the spectrometer electronics for correct percent calculation. Each element channel will have one PMT dedicated to only that element.
A PMT is a photonic (optical / electronic) device which detects the element light level coming from the grating / exit slit, and accurately sends an electrical current to the measuring electronics.
A PMT is a transducer (a component which changes one type of energy into another ie: light energy into electrical energy) type analog device. Since all PMTs are analog devices, this "PMT signal" is measured and converted, through an analog to digital (A/D) converter, into a number the computer (a digital device) can use for accurate element concentration reporting.
A faulty or failed PMT can and will generate an inaccurate electrical signal which will cause the computer to report an incorrect element concentration. The good thing is that PMTs are a very stable, reliable device and many customers will probably never see a problem. With average use the life expectancy of the average PMT is measured in years, though with time they will weaken, but again this is usually over years.

PMT Characteristics: The following general characteristics are basic to all PMTs used in a Baird Spectrometer:

All PMTs use a certain voltage from the high voltage power supply to work.
This voltage is normally within the -450 to -1000 VDC range and the correct voltage setting depends on the PMT sensitivity (discussed below), the sensitivity of the element line being measured, and the relevant matrix. This is normally factory selected and can be adjusted by the customer using thumbwheels, or from the computer for spectrometers using an MC20. The exception is for the old Baird spectrometers using the RS3, RS4 or RS6 readouts (DV3 .. certain DV5 and certain ICP models) which use a plug-in resistor board for HV adjustment.
The newest DV6E and OneSpark also use this plug-in resistor type adjustment board.
On some other type spectrometers (Labtest for example) this high voltage is controlled by adjustable "attenuators" but the idea is the same.
All PMTs have a spectral response outside the wavelengths listed below, but for optimum response, should only be used for the ranges listed.
Electrical current is "induced" when element light hits the the PMT "grid." This grid is the part of the PMT which looks like a window screen and is generally made of platinum or rhodium. It is this induced current which is sent down to the measuring electronics / computer where it is measured and a percent concentration is calculated.
There is a direct relationship between the amount of element in a sample and the amount of light which hits the grid of the PMT and the amount of current sent to the electronics. More sample = more light (photons) = more current at a fixed high voltage.
PMTs of a certain type (R1414 for example, but the same criteria for all PMTs) have different sensitivities based upon the anode sensitivity of the individual tubes. Generally the more anode sensitivity the more light the PMT can detect. this is very important when analyzing certain very low element concentrations. Examples would be Boron in Steel or Phosphorous in Aluminum. For higher concentrations (examples are Nickel is Stainless Steel or Zinc in Gunmetal) or when the line is used as a reference line you want a lower sensitivity line.
Tubes were graded by sensitivity by Baird and marked with either a number or color. Some tubes (R928, R889, 1P21, R166 among several others) were not given a sensitivity rating. Please see "Sensitivity Ratings" below for more details.
When your Baird spectrometer was configured at the Baird factory (closed by Thermo Electron in 2000) all these factors were taken into account when choosing exactly which tubes for which lines in your particular spectrometer. This is why you should replace a PMT with a closely equivalent PMT, however any PMT of the same type will work but adjustments in high voltage setting or optical attenuation may be necessary.

PMT Selection Criteria: The correct PMT's are selected based on the element wavelength being measured and the relevant matrix. The Baird spectrometers are designed to work in the 170 nm to 800 nm (1700 to 8000 Angstroms) range. There is an exception for the DV6S/N which has been configured to measure Nitrogen, Oxygen and Hydrogen in the > 160 nm range but this is a special case requiring special MgFl optics, grating and PMT.

1. Element Wavelength: Generally PMT's are selected for the wavelength being measured along these general criteria. This will be the same criteria for all Baird spectrometers, regardless of type.

170 nm to 240 nm (approximate) will use an R427, R1657, or R166
240 nm to about 440 nm which will use an R300, R1414 or IP28
440 nm to 800 nm which will usually use an R889, R928 or 1P21
Some element lines of an element (example: Fe 259.9 nm which is used in the .001 to 2.50% range), will be much more sensitive than others of the same element (example: Fe 271.4 nm which is used as a reference line in Ferrous matrix). In the this case

2. Matrix: Generally the specific Matrix will determine which analytical lines will be used. The choice of tubes is dependent on the wavelength and concentration range for the desired element.

3. Baird Spectrometer Model: Generally the older the model spectrometer HA10 series, FAS Series and early DV2-3 series will use only larger type tubes (R166, R928, 1P21 and 1P28). Later models of the DV series (4 and 6), all ICP series, and HA12 will use a combination of large and small (R1414, R1657, R300, R427, R889) type PMT. DV5,MOA, FSQ and all mobile spectrometers (MAS MAXII, MS3) will use only the small PMT.

4. PMT Sensitivity Grading: PMT sensitivity is graded according to the anode sensitivity and varies within tube type. This sensitivity is NOT depend on an element wavelength: Generally as follow:

1P21, R166, R928, R889 - no grading
1P28 originally with a -1 to -5 with -1 being least sensitive and -5 being most sensitive.
R427, R300: only graded with a standard sensitivity or a -1 sensitivity for a more sensitive tube:
R1657, R1414: 2 grading systems were used over time for these tubes. a 5 color system replaced with an 11 color system being based on the Intl standard color coding for resistors with black being least sensitive to white / white being most sensitive.

5. PMT Replacement Availability:

1P21, R166, R928, R1259 are still available.
R427, R300 are no longer available on the open market. Antares Analytical does have a good supply in stock for customers needing a replacement
R1414, R1657, R889 are no longer available. These have been replaced by a 6350 to 6358 series, again dependent on wavelength and sensitivity. They are direct replacement for the R1414, R889 and R1657. Please contact Antares Tech Support for advice & availability of these replacement PMT.
1P28 are still available but only in the 1P28-1 and 1P28-5 versions.

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