isoprime precisION IRMS and centrION

Evaluation of the system by analyzing monitoring gases that are introduced through the centrION. Depending on which mode the system is set up as determines which gases will be used for evaluation, e.g. in NCS mode, system performance testing is required to be carried out using N₂, CO₂ and SO₂ gases. Similarly, in OH mode, CO and H₂ gases are required.

Tuning

Before performing any performance checks, it is advisable to run the Autotune method . Please refer to the fine tuning section within system setup.

Monitoring gas peak height calibration

The analyst can specify a desired monitoring gas peak in nA, by selecting the centrION tab and typing in a value in the ‘monitoring beam’ box (Figure 6-11). If by switching the monitoring gas on the visible beam height is not what was selected then the monitoring gas may require peak height calibration; this is achieved automatically by running the ‘Calibrate Monitoring Gas‘ method through the task list.

The Calibrate Monitoring Gas method involves the introduction of monitoring gas pulses at different pressures into the constant helium carrier stream which typically has a flow of between 3.0 and 5.0 ml/min. The monitoring gas is introduced into the helium flow via a MOVPT valve and its pressure is controlled by a mass flow / pressure controller. The software then automatically compares the peak heights achieved relative to the gas pressures to achieve calibration.

Monitoring gas stability

In order to evaluate the stability of the IRMS, the ‘stability’ method is run through the task list. By default, this method consists of 10 consecutive pulses of monitoring gas with 30 seconds between each pulse, however these parameters can be altered by selecting the ‘methods’ icon under the ‘task’ tab (Figure 6-12). The input parameters for the default method as shown in Figure 6-13 reveal that the currently selected species and tune file will be used, there will be 10 pulses of monitoring gas (‘number of pulses’), a 30s delay at the end of the analysis (‘end delay’), 30s between peaks (‘pulse delay’) and a 30s pulse time (‘pulse duration’). The method is set to run only once (‘repetitions’), however this parameter is editable in the task list.

If the ‘editable in task list’ checkbox is selected in the method, the parameters can be specified directly within the task list. If a parameter is specified in the task list, this will override the parameter specified in the method, if left blank in the task list or the checkbox has not been ticked then the default value specified in the method will be used.

A CO₂ monitoring gas stability acquisition run is shown in Figure 6-14. In the task list, a ‘2’ was entered under the ‘repetitions’ column, therefore two stability methods were analyzed as part of the one task. The traces can be viewed within the ‘Data view’ window, with the two tabs, each named ‘Acquisition (CO₂)’ corresponding to the two runs. 

The ‘data analysis’ window, under the ‘peaks’ tab, shows data from each of the individual peaks (Figure 6-14). The display is fully customizable by selecting the required information from the field chooser in the top left corner of the window. Options can include peak id, retention times, peak area, peak heights and the isotope ratios, which in the case of CO₂ would be 45/44 and 46/44.

A good way to quantify the stability of the instrument is to measure the ‘standard deviation of the fit’ for the isotope ratios (i.e. 45/44 and 46/44 for CO₂) expressed in per mil. lyticOS calculates this automatically at the end of each stability run and is displayed in the data analysis window under the results tab (Figure 6-15). In the case of the 45/44 ratio, a straight line is fitted to the ten 45/44 ratios obtained from the ten consecutive pulses of monitoring gas by the method of least squares fit. The standard deviation of the standard errors of the points from this best fit line is calculated and reported. If a stability level of less than 0.06 per mil (0.20 per mil for H₂) cannot be achieved then please to refer to the troubleshooting section of this manual.

Monitoring gas linearity

After establishing that the system is stable through analyzing monitoring gas, it is essential to evaluate if the instrument behaves with linear characteristics when different peak sizes are introduced into the IRMS. The measuring dynamic range, defined in the specifications for the isoprime precisION IRMS, is for ion currents between 1 and 10 nA, however, this range should be adjusted if your samples cover a different dynamic range.

Similar to ‘stability’, the ‘linearity’ method can be run through the task list and can be edited in ‘manage methods’ by selecting the ‘methods’ icon under the ‘tasks’ tab. The input parameters for the default method, as shown in Figure 6-16, reveal that the currently selected species and tuning file are used, the peak heights range from 1nA (‘start beam’) to 10nA (‘end beam’), with the ‘number of steps’ between the start and end being 5 and there being 2 pulses at each peak height (‘number of pulses’).

 

An example CO₂ monitoring gas linearity acquisition run is shown in Figure 6-17; similar to the stability run, 2 ‘repetitions’ were analyzed as part of this one task. The ‘data view window illustrates the differences in the beam heights for the peaks, while in the ‘data analysis’ window under the ‘results’ tab, the software automatically compares the ratios obtained relative to the peak heights and calculates the linearity in per mil / nA.  If calculated linearity values are greater than the acceptable levels given in Table 6-3, please refer to the troubleshooting section of this manual.

H₃⁺ Factor for hydrogen

Specific only to hydrogen, a H₃⁺ factor is required to be applied to the stable isotope measurements. H₃⁺ ions are formed within the ion source as a result of ion / molecule collisions and is proportional to the pressure of hydrogen:

When measuring stable hydrogen isotope ratios (3/2) the molecules of interest are HD (m/z 3) and H₂ (m/z 2), however as H₃⁺ ions have the same mass as HD⁺ this will interfere with the ratio measurements, which must be corrected for by applying the H₃⁺ factor.

To measure the H₃⁺ contribution, select the ‘H3 correction’ method through the task list and press start. This method is essentially the same as the ‘linearity’ method described above except the software automatically calculates the H₃⁺ factor in ppm / nA. The input parameters can be adjusted in the same way by opening the ‘manage methods’ window (‘tasks’ tab > ‘methods’ icon). The H₃⁺ factor is calculated automatically and is applied to subsequent sample runs by default. Please refer to the troubleshooting section if the H₃⁺ factor is greater than 8.0 ppm / nA or shows greater variability than 0.03 ppm / nA / hr.

Quick tasks

A quick task is a means to add multiple tasks into task list in one step. These quick tasks are fully configurable and can be setup to run immediately or delayed for a specific time or date. This function is briefly introduced here to illustrate that the tuning, stability and linearity tasks could all have been analyzed using a single pre-defined quick task.

A pre-defined quick task can be added to the task list by selecting from the ‘quick task’ section under the ‘Tasks’ tab (Figure 6-18). To manage quick tasks, either to edit existing ones or create new, select the ‘tasks’ tab and the ‘quick tasks’ icon in the manage section (Figure 6-19). The ‘manage quick tasks’ window is shown in Figure 6-20, and displays a simple quick task that includes an Autotune, 2x stability runs and 2x linearity runs. Under the ‘add behavior’ tab, there are choices to edit the quick task prior to addition to the task list, to specify to start the analysis immediately or just populate the task list and introduce a delay to the analysis by selecting a specific start time (Figure 6-21).

More comprehensive information on quick tasks are provided in Quick Tasks.