Sent August 23, 2005 Here in the department of geological sciences at Arizona State University we are using a MAT 253 (instrument serial number 8632) for hydrogen measurements from a dual inlet, a GasBench, a GC/TC and soon from a TC/EA. For ourselves and our customers, it is essential that we know how the numbers generated by isodat are determined. At the very least, I need to understand how the H3 factor is determined by Isodat. I have read the literature concerning how the H3 factor can be determined and applied but this tells me nothing about how Isodat does it. The following slides are intended to explain and justify my need to know how Isodat determines and applies the H3 factor.

To determine the H 3 + factor in continuous flow mode, I run a series of pulses with different m/z 2 beam intensities. Isodat then calculates the H3 factor from these results in an unknown way. Figure 1 shows a typical data file used to determine the H3 factor. Figure 1: Data file used to calculate the H3 factor on a MAT 253. Isodat calculated a value for the H3 factor of ppm/nA from this data.

Measure mean ratio between extremities? Measure mean ratio during 5 seconds of the pulse? To determine the H3 factor it is necessary to observe the variation in the ratio of the m/z 3 and m/z 2 ion currents (I 3 and I 2 respectively) as a function of the m/z 2 ion current. This is not straightforward. How does Isodat determine the ratio of I 3 /I 2 from a square pulse? Two possible ways are shown below. There are a wide variety of way to do this that are not referenced in the literature. Figure 2: Expanded view of two reference pulses. The ratio of m/z 3 to m/z 2 will have to be averaged over some part of the top of the square peak.

The H3 factor is the slope of the line obtained by plotting the ion current ratio (I 3 /I 2 ) as a function of the m/z 2 beam intensity (I 2 ). In the first plot of Figure 3, the H3 factor is ppm/nA (the slope of the line). From the plots on the right, it can be seen that the H3 factor is not stable through a wide variety of beam intensities. Here, the linearity degrades below about 4 or 5nA. This is seen by the far greater linear fit when using only square pulses with amplitudes above 5nA (as seen in the lower plot on the right). I exported the raw trace data from the file shown in Figure 1 and then used it to generate the data in Table 1 and Figures 2 and 3. Figure 3: Manual calculation of the H3 factor using all pulses (top plot) and using only pulses with amplitudes great than 5nA (lower plot) Table 1: Averaged m/z 2 beam intensities (I 2 ) and I 3 /I 2 ratios from 10 reference gas pulses.

What I would like to know is how Isodat goes about measuring the H3 factor from a continuous flow file and then how Isodat makes the correction in real data files. For example: Isodat first does a timeshift correction on each individual peak, then a background correction, then a 5-second region at the top of the reference peak is averaged for intensity at m/z 2 and for isotope ratio. A regression is then performed on the collection of sets of these two points. The slope of the line is used as the H3 factor. The H3 correction is then made on real continuous flow data files by… I would like to be able to simulate Isodat as much as possible to see how the numbers are processed. This is important to me as someone who provides numbers to users. I realize that you are probably insanely busy, however, this issue of making clear how the numbers are obtained, is essential for users to at least have access to. Many thanks for whatever help you may offer to shed light on this matter. Anthony Michaud