System Validation

With all components of the system setup and validated, it is then necessary to check the system as a whole.

Vacuum Readbacks

It is essential that the vacuum is at a level sufficient for analysis. With the inlet connected, the Nupro valve open and the source inlet valve (SIV) set to the appropriate inlet you can observe the measured vacuum by viewing the system status. This can be done by either clicking on the ‘view status’ link at the top right corner of the connected system page or by selecting ‘system status’ icon under the ‘layout’ tab (Figure 6-1). As standard the connected system page comprises of the task list, data view and data analysis windows and a ribbon bar on the top showing contextual content. This layout is fully configurable and can include more information by selecting an array of windows from the layout tab of the ribbon (Figure 6-2).

Within the system status window, under centrION, the SIV position can be changed using the dropdown list. For normal operation (allowing the sample to be transferred from the inlet) it should be in the correct inlet position. To isolate the source from the inlet, set the SIV to either Waste or All Off. The low and high vacuum readings from the Pirani and cold cathode ionization gauges are displayed as low vacuum and high vacuum under the isoprime precisION section of the status. Measured in mbar, the low vacuum reading should be between 1e-2 and 7e-2 mbar and the high vacuum between 2e-6 to 5e-6 mbar; if the vacuum readings fall outside these recommended values, please refer to the Error Reporting.

Ion source tuning and background check

Before measuring the background gas levels, it is necessary to tune the ion source.

Tuning

Under the isoprime precisION tab, select, for example CO₂, from the dropdown list in the species section. From the tuning section you can then load a particular tune file, again from a dropdown list (Figure 6-3). By selecting the manage icon, the tuning parameters for each tune file can be observed and manually adjusted (Figure 6-4). Tuning parameters with recommended values are displayed within Table 6-1, however this is only coarse and fine tuning using the Autotune method is required before analysis.

Automatic Peak Detection

If the coarse tuning is significantly ‘off-peak’, i.e. with the monitoring gas on (centrION  > monitoring gas on), no increase in the ion beams are observed, then it may be necessary to run the Auto Detect Peak method as part of the fine tuning protocol.

First add a new task to the task list (Tasks > Add) and then select Auto Detect Peak from the method dropdown and press start. This Auto Detect Peak method introduces a monitoring gas into the IRMS and then scans the accelerating voltage over a wide range of voltages until the ions are detected at the Faraday collectors. Figure 6-5 displays the Auto Detect Peak scan for the CO₂ monitoring gas. The accelerating voltage is ramped between 1000 to 5000 V, while all other tuning parameters remain unchanged. The mass 44 beam is visible in the major collector at around 4000V, while the beams corresponding to masses 45 and 46 are evident in the Minor 1 and Minor 2 collectors at the same time.

Autotune

To run the Autotune method, select it from the method dropdown in a new task and press start. The method comprises of four different scans that are run sequentially to fine tune the ion source; these are an initial auto peak center (Figure 6-6), an auto half plate (HP) differential scan (Figure 6-7), an auto Z-plate (ZP) scan (Figure 6-8) and a final auto peak center; each scan is shown within the data display window.

Similar to auto peak detect, the auto peak center method scans the accelerating voltage (AV), but over a shorter range of 70 V (180 V for H₂ measurements). It is important that all ion beams are fully focused into their respective collectors over a range of AV values, this is illustrated in a peak center scan by the coincidence of the flat tops of the three ion currents, in this example, over the 3980 to 4005 V range. With the AV set to the middle of this range, any minor fluctuations in the magnet field and/or accelerating voltage should not affect the stability of the signal.

The purpose of the HP differential scan and the Z-plate voltage scan is to steer the ion beam along the y-axis and z-axis, respectively. The differential voltage range between the two HP electrodes is -250 to +250V and between the ZP electrodes is -150 to +150V and are automatically set so the signal has maximum intensity.

Refer to the centrION / isoprime precisION troubleshooting section if the peak shape from the peak center scan is not symmetrical with steeply rising sides and does not have a flat top of at least 10 V. Similarly refer to the troubleshooting section if the HP and ZP scans show non-coincidence.

Other tuning parameters

Depending on the sensitivity required from the instrument it may be necessary to alter the trap current (TC), with the higher trap current corresponding to higher sensitivity. The electron emission from the filament is trap regulated, therefore a higher trap current results in more electrons and consequentially more ionization in the ion source block.

The extraction voltage may also require optimizing. Following an ‘auto EX scan’ the software automatically sets this to 10% less than the maximum, this can be set manually, but should never be set with a higher % V than the maximum.

Background gas levels

The levels of background gas in the system can be evaluated by running a method called ‘background scan’ through the task list. During this method the magnet current is altered stepwise from 1500mA to 4500mA and by detection of the ion beams sweeping across the Faraday collectors the background gas levels can be determined; for the best resolution it is advisable to view the scan using the Minor 1 collector (Figure 6-9).

If the background gas levels exceed the recommended levels displayed in Table 6-2 then please refer to Error Reporting. Briefly, if m/z 28 (N₂) and m/z 40 (Ar) are higher than their recommended values, then this may be indicative of a leak somewhere in the system. If m/z 18 (H₂O) is high then perhaps the instrument is not ready for analysis after some maintenance or perhaps the Nafion drying membrane is off. If the background scan has more peaks than those illustrated in Figure 6-9, then this maybe a due to contamination of the system; again, please refer to the troubleshooting section.

After running the background scan method or altering the magnet current in any way, it is important that the magnet is cycled to remove hysteresis; this can be done by clicking the Cycle Magnet icon under the isoprime precisION tab (Figure 6-10). The cycle magnet procedure involves setting the acceleration voltage to zero, followed by setting the magnet current to maximum, then minimum and then back to its original setting and before finally restoring the acceleration voltage.