EPA Method 200.8 is an established and widely used ICP-MS method. It describes the analysis of water and wastes for 21 elements. Included in these 21 elements is arsenic, which has a large potential polyatomic interference from 40Ar35Cl. It is common for analysts to face a high bias in their data for 75As. To correct for this potential 40Ar35Cl interference EPA Method 200.8, Table 5, prescribes using the correction equation below:
What is the Purpose of This Interference Correction Equation?
This correction equation subtracts the counts of 40Ar35Cl from the total counts recorded for mass 75. This is done by measuring the counts from a similar polyatomic: 40Ar37Cl. In theory, the counts for 40Ar35Cl and 40Ar37Cl should be the same if you correct for the isotopic abundance differences between 35Cl and 37Cl.
Defining Each Component of the Arsenic-75 Interference Correction Equation
Understanding these correction equations can be difficult, so let’s review each part of the equation to understand how 75As results are corrected: the factor of 3.127 is the isotopic abundance ratio of 35Cl/37Cl - this should normalize your counts for 40Ar37Cl to that of 40Ar35Cl.
Mass 77 is measured because that is where the polyatomic interference of 40Ar37Cl will fall on the spectrum. However, the counts on mass 77 need to be corrected to remove any counts coming from 77Se. This is done by measuring the counts on mass 82 from 82Se and using the isotopic abundances of Se isotopes to convert counts of 82Se to counts of 77Se.
The natural abundance ratio of 77Se/82Se is 0.874. EPA Method 200.8 uses a ratio of 0.815, but why? It’s possible that there was data generated during the writing of EPA Method 200.8 that provided evidence that a ratio of 0.815 was better suited than the natural abundance ratio of 0.874. EPA Method 200.8 has a footnote for this correction equation stating that you must correct the counts on mass 82 to remove any potential 81Br1H (bromide hydride) interference. It is possible that this ratio of 0.815 was used due to possible hydride interferences. EPA Method 200.8 was written in 1994 and does not include the use of modern interference removal technologies such as kinetic energy discrimination or collision cell. There may have been more odd interferences present on ICP-MS instruments of the time when EPA Method 200.8 was authored.
Possibilities for Bias in Arsenic-75 Concentration Results
With an understanding of how the correction equation works, it is important to review how 75As concentrations could have a high bias – there are three possibilities:
First, counts on mass 77 may be artificially low. It’s usually hard to have an artificially low signal as interferences are additive to the instrument signal, especially when looking at raw counts. So, this option is likely ruled out as a cause for high 75As concentrations.
Second, this interference equation could give a high bias to 75As concentration is if the counts on mass 82 are artificially high. This is more likely as it points to possible interferences. Possible interferences on 82Se include 82Kr, 66Zn16O, 40Ar42Ca, 47Ti35Cl, 45Sc37Cl, 164Dy++, and 164Er++. Given the sample types measured using EPA Method 200.8 of water and wastes, you should not expect interferences from 47Ti35Cl, 164Dy++, or 164Er++. The potential interference of 82Kr should also be ruled out since if present as a contaminant in your Ar gas supply, then it would be eliminated by blank correcting your data. The other potential interferences of 66Zn16O, 40Ar42Ca, and 45Sc37Cl are all much more common. Zn and Ca are expected in water and wastes samples and Sc can be added as an internal standard.
Finally, high bias in the 75As concentration data may be due to the isotopic ratio of 77Se/82Se. If the isotopic ratio of 77Se/82Se is too high, then the correction equation will give a result for 75As that is artificially high. The reverse situation also holds true, as an isotopic ratio of 77Se/82Se that is too low will result in a low bias on the 75As concentration.
Modifying the Arsenic-75 Interference Correction Equation
After reviewing all the possible causes of high bias on 75As, it is possible to troubleshoot ways to correct the 75As correction equation. First, run a high purity sample of Se in a matrix of HNO3 and calculate the instrument’s isotopic abundance ratio of 77Se/82Se, making sure to blank correct the data.
When measuring mass 77 and 82 to calculate the isotopic abundance ratio be sure to also monitor 66Zn and 42Ca. Compare the counts for 66Zn and 42Ca against all method blanks. If counts are elevated in these blanks, then take action to correct the counts on mass 82 from 66Zn16O and 40Ar42Ca before using those 82Se counts in the correction equation for 75As.
Need Additional Support with EPA Method 200.8?
Still have more questions about EPA Method 200.8? Inorganic Ventures is here to help! You can schedule a meeting with our Technical Support Team at your convenience or send an email to [email protected]. Inorganic Ventures offers all Certified Reference Materials you need to run EPA Method 200.8, ranging from calibration standards, internal standards, QC checks, and EPA Method 200.8 tuning solutions.
If you are new to ICP-MS instrumentation, then be sure to review our ICP Operations Guide and Trace Analysis Guide. These resources and many others offered on our website are great tools to help new and experienced analysts alike. If you are looking to gain more confidence in your analysis skills, try listening to our weekly Bench Boost podcast for new analysis tips every Tuesday! Finally, we also offer ICP instrument training through our online learning platform, IV Ignite, and our yearly ICP Conference!