EPA 515.4 wierdness
Posted: Fri Dec 23, 2011 1:25 pm
I am having an issue with the method surrogate ( 2,4-dichlorophenylacetic acid).
In the past I was getting very low recoveries which I found was due to my not preparing my diazo methane correctly. That issue has been corrected.
Here's the strangenesss.
In the last week I had 2 samples run , one in duplicate and one in triplicate in which the surrogate failed. the best recovery was about 55%. Other samples run at the same time were fine in regards to the surrogate.
I spiked each of the failing samples, 1 at 1 ppb and the other at 10ppb. Both spike recoveries were within acceptance, and the surrogate were fine . The recoveries for the surrogates in the spikes were 88 and 108%.
The internal standard recoveries for the two spikes were 105 and 109 %!!
So what am I missing?? Anyone doing this method come up with a better surrogate?
In the past I was getting very low recoveries which I found was due to my not preparing my diazo methane correctly. That issue has been corrected.
Here's the strangenesss.
In the last week I had 2 samples run , one in duplicate and one in triplicate in which the surrogate failed. the best recovery was about 55%. Other samples run at the same time were fine in regards to the surrogate.
I spiked each of the failing samples, 1 at 1 ppb and the other at 10ppb. Both spike recoveries were within acceptance, and the surrogate were fine . The recoveries for the surrogates in the spikes were 88 and 108%.
The internal standard recoveries for the two spikes were 105 and 109 %!!
So what am I missing?? Anyone doing this method come up with a better surrogate?