[Golden_4_Life]

We are trying to develop a method for quantifying 4 clorophenols in river water systems that are adjacent to a landfill and a pulp paper mill.  Plan to extract them with methylene chlor, dry down, and reconst. with mobile phase (ACN:water:0.1% glacial acetic). Wanting to assess the extraction efficiency, recovery, and quantity of clorophenols in 1 liter samples taken within a set of quadrants that near the mill and landfill--also will test crustacean and fish liver samples.  Questions: 1) do I add the ISTD to the 1 liter gross river water sample or add the ISTD to the 100 mL aliquot that is destined for solvent extraction?
2) at what conc'n should the ISTD be prepared as?  We expect levels of clorophenols in the parts per thousand range--should the ISTD be prepared to mimic this conc'n or preppe'd at a conc'n that is lower or higher than for the analyte?
3) peak area of analyte standard/peak area of ISTD = ratio value for a mg/mL conc of STD.
4) compare this ratio value to the ratio obtained by "sample peak area/ISTD peak area".

Thanks in advance for any assistance. :delta:

[JGK]

Add the ISTD to the Aliquot to be extracted.

As a general rule I would prepare my ISTD (Stock) solution such that after adding to the sample the concentration of ISTD is in the mid range of the analyte levels expected. For example if your analyte levels are within the 20 - 200 ppt range set the ISTD at 100 ppt. However, check with solution STDs first to make sure the peak responses are OK.

[Golden_4_Life]

Thanky JGK: we were wondering about this issue because if the ISTD peak area were tiny or exorbitantly large with respect to the analyte peak area, then the chromatogram plots would look "awkward".  I guess, too small or too large to evaluate recovery.  So, it seems like, per your suggestion, that we ought set the ISTD prep so that the peak area value will be about the Midpoint in peak area relative to the samples.  This helps.

[Golden_4_Life]

How can I be sure that the compound I choose for an ISTD is not itself an analyte in the test samples?

[JGK]

You need to compare a range of extracted samples (without ISTD) against extracted (with ISTD) samples to check for interfering peaks at the ISTD RT. if there is a consitent interference you may have to review your ISTD selection.

[Golden_4_Life]

From what I can gather the ISTD is selected based on its boiling point (for Gas Chromatography work) in comparison to the boiling pts for the test analytes no?
So, we want to select and ISTD that has a Bpt different than that of the analytles no?