The Tekran Model 2500 Mercury Vapor Detector provides a continuous measurement of ultra-trace amounts of elemental mercury in an inert carrier gas. This document covers both the base model which uses a ball flowmeter/needle valve carrier flow adjustment and the Option 001 version which features an electronic mass flow controller.
The detector is suitable for a wide range of laboratory applications, and, with proper sample preparation, can detect sub-picogram amounts in virtually any type of sample matrix.
The instrument is a Cold Vapor Atomic Fluorescence Spectrophotometer (CVAFS). Elemental mercury (Hg0) atoms in an inert carrier stream are excited by a source of ultraviolet radiation. The atoms fluoresce as they return to ground state. Both the excitation and fluorescence occur at a wavelength of 253.7 nm.
The Model 2500 contains a number of features that provide enhanced stability.
- Optical path stabilization to prevent baseline and sensitivity shifts.
- The entire instrument is powered from an internal DC power supply, rendering it immune to line voltage fluctuations.
- Excitation source is kept at a constant temperature in a thermostatically controlled block.
- An optical feedback system provides constant intensity over the life of excitation source.
- Ultra low noise electronics provides a high quality analog output.
Selectable Output Range
The analog output can be set to provide either a 1, 5 or 10 Volt full scale output.
Selectable Time Constant
A four position jumper allows adjustment of the time constant to optimize response time versus baseline noise.
Carrier Gas Flow Adjustment
The detector is available with two types of carrier flow control.
Standard: Ball Flowmeter/Needle Valve
In the standard configuration, the detector is equipped with a glass tubed ball flowmeter and adjustable needle valve.
Option 001: Mass Flow Controller
An optional mass flow controller (MFC) allows the flow of carrier gas to be set via either a front panel adjustment knob or via a rear panel 0 - 5 V input. The MFC provides accurate and precise control of the carrier gas flow for those applications where the carrier flow significantly impacts analytical performance.
The rear panel input allows the flow to set for optimum performance during the various steps of a measurement cycle. In addition to improving sensitivity and repeatability, this feature conserves carrier gas. Any external 0 to +5 V source may be used to set the flow.
Option 010: Separate Purge and Carrier Gas Inlets
Normally, the same gas inlet fitting is used to supply both the analytical carrier gas and the detector purge gas. Option 010 provides a separate gas inlet on the rear panel for the purge gas. It is thus possible to use two different gases, or the same gas at different supply pressures, for the two functions. See the flow diagram for details.
Principles of Operation
Cold Vapor Atomic Fluorescence Spectrophotometry (CVAFS)
CVAFS is rapidly replacing more traditional techniques such as Cold vapor Atomic Absorption Spectrophotometry (CVAAS or AA) for low level mercury detections. The technique has the following advantages over AA.
- It is linear over 4-5 orders of magnitude.
- It is less prone to both positive and negative interferences.
- It offers greater sensitivity.
In Tekran detectors, the ultraviolet source provides excitation at 253.7 nm. Elemental mercury that is present in the detection cell will fluorescence at the same wavelength as the excitation. The fluorescence is omnidirectional and can be detected with a photomultiplier tube. The excitation path and viewing path are perpendicular to each other to allow separation. See the block diagram of a CVAFS detector.
The major interference mechanism with CVAFS is quenching of the excited mercury atoms prior to fluorescence. Virtually any molecular species in the carrier gas suppress the output signal. Although the detector can be run using nitrogen as a carrier, sensitivity drops by 50 to 75% compared to argon. If using air or oxygen, sensitivity is typically 50 to 100 times less than with pure argon.
Even small amounts of polyatomic species will cause suppression. For example, it has been reported that 10 ppm of residual air in the argon carrier can reduce sensitivity by 50%.
Quenching can be eliminated by ensuring that the sample preconcentration methodology results in a minimum of polyatomic species entering the argon carrier gas stream alongside the mercury vapor. Any trace of atmospheric gases should be thoroughly flushed out of the system before a measurement is taken.
The argon carrier gas itself should be ultra high purity (UHP) grade in order to minimize quenching due to trace amounts of O2, water vapor, etc.
There are no known positive interferences caused by atomic or molecular fluorescence, however improper sample preparation can result in false readings due to light scattering caused by turbidity. The main culprits are water vapor or organic solvents which form small droplets while in the cell or condense out on the cell walls. This phenomenon usually manifests itself as very broad peaks or as a shift in the baseline.
Steps may be taken to reduce the likelihood of this phenomenon causing problems.
The sample preconcentration method should ensure that offending compounds are not sent into the detector. If it is not possible to do this, the offending compounds can be vented around the detector. For example, during the first heating step in the dual step thermal desorption procedure for ambient air cartridges, a bypass solenoid valve may be installed to vent the effluent released during heating of the sample cartridge. Any mercury released would be trapped on the second (analytical) cartridge, but the potential problem compounds would vent to atmosphere rather than contaminating the detector's cell and sample lines.
Instrument Flow Diagram
The Lamp Stabilizer system maintains the excitation source at a constant intensity over the life of the source.
Option 010: Flow Description
Detectors equipped with the Separate Purge Gas Inlet, Option 010, have an additional rear panel fitting labelled PURGE GAS. This fitting replaces the internal tee splitter and is used to provide purge gas to the detector through a restrictor. Using this option, it is possible to use helium for the carrier gas with argon or nitrogen used as the purge gas. Refer to the flow diagram.