home - facilities - publications - people - news - photo gallery
|CO2 Laser and Laser Disk
Individual grains are loaded into pits within a copper laser disk. Our CO2 laser is which has a zinc selenide laser port, isconnected to a video camera, enabling us to coordinate the laser on an x-y stage, thus allowing our sample measuremets to be automatic. We use a CO2 laser to fuse each sample individually; this liberates the argon trapped inside the crystal lattice.
This is a photograph of a copper laser disk after the grains have been analyzed for argon ; in each pit is a small glass bead, the remains of the mineral grain after the laser has fused the mineral to release the gas.
A KBr plate is placed on top of the laser disk; the disk is then placed inside the disk chamber (see below) and the chamber is then heated so that it is evacuated, with the air removed.
CO2 Laser mounted above the laser disk.
A closer look at the laser disk inside the disk chamber.
The Laser Extraction Line is what delivers the argon that we want to measure in our sample from the laser disk to the mass spectrometer. As the gas travels through this line, it is scrubbed of gases that we don't want to measure, but which are released during fusion of the mineral, such as hydrogen and carbon dioxide, by two getters.
Parts of the Laser Extraction Line
The purpose of getters is to remove reactive gases that are unwanted during analysis. They consist of a reactive surface that will react with these gas molecules, thus separating them from the gases you are interested in. Gases released from the heating of samples by the CO2 laser are scrubbed of reactive gases such as heavy water, carbon dioxide, and carbon monoxide by exposure to Zr–Fe–V and Zr–Al sintered metal alloy getters.
An ion gauge is a gauge sensitive enough to measure low pressures (high vacuums). This type of gauge measures pressure indirectly; the gas is bombarded with electrons, which creates ions. These ions are then measured and used to determine the pressure.
An ion pump is a vacuum pump that employs a cloud of electrons to ionize incoming gas molecules. The presences of a strong electric potential then accelerates these ions towards a solid electrode.
The air pipette is used for measuring the atmospheric ratio of 40Ar/36Ar. Since we know the ratio in the atmosphere is 295.5, by measuring the ratio in the mass spec we can correct for any mass fractionation that may occur during the analysis.
A cold finger is used to condense potential contaminants in the gas you are measuring, e.g., carbonon dioxide and water. Water, dry ice or liquid nitrogen are often used as the cooling agent. As the gas passes through the cold finger, these contaminants condense, allowing the remaining, uncondensed gas molecules to continue throught the extraction line. The coldfinger is later warmed, releasing the condensed molecules, and then evacuated.
The source of a mass spectromter uses an electron beam to convert the gas molecules into charged particles (ions), and to then focus these ions into an ion beam. Below is a schematic of the source in the VGS5400.
Electrons emitted by the filament are attracted to the source block by the electron voltage potential and further collimated by a magnetic field (magnetic intensifiers). The electrons pass through a slit into the source ionization region and are collected on the opposite side, after passing through another aperture, by a trap held at a fixed voltage relative to the source. This electron beam lies just behind the ion exit slit so that ions, which are formed by the impact of electrons on the neutral molecules, can be efficiently drawn out of the chamber by the penetration field created by the Y focus plates.
The ion extraction field is modified by the presence of the ion repeller plate inside the source. The ion repeller is normally operated at a negative potential to ensure that the ions are formed in a region of relatively low electric field gradient. The ionising electron beam is constrained in its passage between the filament and trap by the presence of two collimating magnets (magnet intensifiers) which produce a strong magnetic field parallel to the required electron beam axis.
The ions extracted from the ionization region pass between the Y focus plates and are brought to a focus in the region of the defining slit (see Collectors Schematic). The image formed is normally smaller than the width of the sit. This reduces mass discrimination in the source due to the presence of the magnetic field from the source magnets.
Photograph of the source housing
The basic principle of mass spectrometry is that if pacharged particles are accelerated around a curve (the flight tube) while in a magnetic field, they will separate along this path according to their mass to charge ratio, with the heavier mass on the outside of the curved path, and the lightest mass on inside of the curve. After the ion beam is created in the source it is accelearted through the curved flight tube, passes through a magentic field , and separates according to the mass to charge ratios, producing beams of the isotopes that we wish to measure i.e., 40Ar, 39Ar, 38Ar, 37Ar, 36Ar.
In this photograph, the charged particles are accelerated from left to right through the flight tub (from the source to the collectors).
After the ion beam has traveled through the flight tube and been separated into the different argon isotopes according to the mass to charge ratio, it enters the collector housing, where the signal is measured as a current. While the Faraday measures the current by using a resistor (amplifier) to convert the signal from a current to a voltage reading, the electron multiplier measures the signal simply as a current in nano-amps.
The ion beam passes through one of two defining slits upon entering the colllector housing. On the high mass side is the deifning slit for the high mass Faraday collector; the axial defining slit is for the electron multiplier.
High Mass Faraday Cup
Behind the defining slit is the suppressor plate, followed by the ground plate. The suppressor plate is operated at a negative potential relative to the high mass bucket to prevent the escape of secondary electrons. The bucket has electrical contact (through a ceramic feedthrough) to outside the vacuum. An evacuated head attached to the flange contains an electrical circuit which amplifies the ion beam current and converts it to voltage.
In the axial position the beam passes through a duct after the ground plate. The electron multiplier system is located behind this duct. A few centimeters in front of the multiplier are two deflection plates. When the multiplier is in use, one of these plates is used to adjust the position of the ion beam incidence, and acts as a fine tuning of the multiplier peak shape. If the the electron multiplier is not in use, positive voltage applied to one of the plates is used to deflect the beam away from the multiplier.
Once we have finished analyzing all of the grains in a pit, it is time to look at the data and go through a data reduction in which we throw out bad data and correct for background values and mass fractionation.
A photograph of the laser disk after all of the samples have been fused by the CO2 laser.
When we are running a disk of samples, we perform three different types of measurements: 1) “unknown” (this is either a sample or a monitor standard) 2) "blanks" and 3) "airs".
1) The sample or unknown is analyzed as described above in the section on mass spectrometry.
2) The "blank" measures the background values of each of the argon isotopes in the mass spectrometer. Blank runs are performed just like unknown runs, except no sample is heated up. This allows us to estimate how much Ar is in the system with the amount of time the extraction line is not pumped in order to release and clean up the samples. The background value on each of the isotopes is subtracted from airs and unknowns. Blanks are run periodically through the run, and the frequency at which we run them depends on the required precision of the correction for the type of analysis. For provenance studies, we require 1-2% precisions on the ages, and we typically run a blank every 4 samples. For samples where high-precision is required, and especially if the gas yields are relatively low, we tend to run at least one blank between unknown or air measurement.
3) The "air" routine takes a small aliquot of air (~5e-14 moles of Ar) and measures the ratio of 40Ar to 36Ar in an aliquot of atmospheric air. This measurement is made frequently and the value and reproducibility allows us to the mass fractionation that occurs in the analysis, as well as the uncertainty on that correction. The 40Ar/36Ar in the atmosphere is known to be 295.5 (McDougall and Harrison, 1999); when we measure the ratio in the mass spectrometer, we typically record a value of ~285. By knowing the actual ratio in the atmosphere we can correct for the fractionation. Airs are measured throughout the run sequence, with frequency depending on the precision requirements of the analysis.
We use the steps in the following to reduce our data:
1. To get an age we can trust, we first evaluate the run evolutions on the isotopes. The isotopes are collected consecutively by “peak hopping” with the magnet. The value of each isotope is estimated at time zero. Time zero is the time when the sample gas has entered the mass spectrometer (in our case, time zero is 20 seconds after we open the valve to let the sample gas into the mass spectrometer. At this time the signal has reached a maximum, and we use this value for the “equilibration time”)
2. After fitting the isotope evolutions. We make a time series of the “blank” analyses for each of the isotopes. We either use the preceding blank or we fit the time series with a function and use this equation to interpolate values for the blank or background, and subtract this from both our “unknowns” and our “airs”.
3. Next, we look at the 40Ar/36Ar values of the ‘Airs’. In the ideal case, there are no trends in the time series and we calculate the mass discrimination based on the average and standard deviation of the air pipettes run within a disk sequence. In some cases, there are trends and we either group the discrimination data or fit with a time series equation to estimate the corrections for individual analyses.
4. In order to calculate an age, we need to determine the J value.
n + 39K --> 39Ar + p
We also have monitor standards (minerals of known age, e.g., Mhb, which is a hornblende from southern Colorado, and has an age of 520.4 Ma (Samson and Alexander, 1987) (more recently estimated to be 525.1, Renne et al., 1998), which we subject to the same neutron flux (or “co-irradiate”).
There is really no way to measure the neutron flux directly. The following equation is used to describe the amount of 39Ar generated during an irradiation:
where the duration of the irradiation, the neutron flux density and the capture cross section of 39K at energy e all determine how much 39Ar is created. From the basic decay equation of 40K to 40Ar* we have:
Where λe/λtot is the decay constant of 40K to 40Ar*, over the total decay constant of 40K (89% of 40K decays to 40Ca).
If we divide the second equation by the first we have:
To simplify this equation, we say:
where J takes into account the neutron flux, and the K isotopes. We can now simplify equation (3) to :
This equation (5) applies to both the unknowns (samples) and the monitor standards with known ages, thus we have two equations with two unknowns, J and t. Since we know a priori what the age of the monitor standard should be, we take the measurements for the monitor standards and use this to solve for the J value:
Once we have calculated J, we can then insert this value into equation (5) using the isotope measurements from the sample, to calculate the age of the sample: