3 Replies Latest reply on Dec 17, 2018 3:18 PM by evanclay

    ICP-MS (7800) Method Validation Elemental Impurities

    eme091

      So I have been getting conflicting advice on my work on the ICP-MS system.  I will try and give as much information as possible on my situation and hopefully it makes sense.  So I'll start with our sample prep.  We use a microwave digestion system.  We use 0.5g product in 5 ml of HNO3, once digested we add 1 ml HCl and dilute to 50 mls with H2O.  This gives us a 100 fold dilution for our sample.  As an example our Spec is 30 ppb so when performing a spike recovery I would assume we would spike prior to digestion(to incorporate the whole sample prep).  My spike for this I thought should be at 0.3 ppb to better represent what we would see out of a sample that would contain 30 ppb of the element and then be diluted 100 fold.  I was recently told to spike post digestion, so prep my solution and just add a standard to make the 50 ml solution contain 30 ppb of the element and leave the multiplier as 1.  This doesn't seem to be correct to me because if my sample is diluted by 100 fold the instrument will see my sample at .3ppb then multiply the result by 100(volume/weight).  Should I be performing my spike recoveries by spiking at the spec or at 100 fold below the spec and then multiplying by 100?

       

       

      Note: a calibration curve is performed 7 points across this range prior to running.

       

      Thanks in advance for any help I appreciate it.

        • Re: ICP-MS (7800) Method Validation Elemental Impurities
          evanclay

          First can I just point out that there are different reasons for spiking the sample before digestions versus spiking after digestion - so it may also depend
          a little on what your goal is.

           

          Spiking the sample before digestion is a test you can use to check for potential loss of analyte or contamination during digestion - as well as checking the effect of the sample matrix on analyte recoveries. This is commonly referred to as a Matrix Spike or Laboratory Fortified Sample Matrix.

          The post digestion spike will only check the effect of the sample matrix on analyte recoveries. So this type of spiking will help confirm if there are interferences present which may require correction techniques or potentially, use of alternate masses for quantification.

           

          If you refer to the US EPA requirements (using Method 200.8 as an example), the recommendation is that the pre-digest spike should have a concentration ranging from 40-100 μg/L for each analyte, except selenium and mercury (in the sample).

          A post digest spike would normally be at a concentration that is 20-100 times the MDL.

           

          Hope that helps you,
          Eric

            • Re: ICP-MS (7800) Method Validation Elemental Impurities
              eme091

              Hey,

                 Thanks that does explain a little of where the trainer was coming from.  We are following the USP's specifications and have historically done pre digestion spiking and this is most likely how we will continue at least for validation processes.  Do you have any opinion on my spiking and dilution question?

                • Re: ICP-MS (7800) Method Validation Elemental Impurities
                  evanclay

                  For a pre-digest spike, the logic you outlined in your original message makes sense. If the sample prep. involves a 100x dilution, you need to allow for this when deciding what concentration to add to the sample.

                  Ideally, you want the spike to be a similar concentration to the expected analyte concentration in your sample. If you add a much larger spike
                  concentration, this may mask the matrix effects. And using a spike that is an order of magnitude different (smaller) to the sample analyte concentration may mean you do not see or recover the spike added concentration. So your suggestion to spike at the spec (around 30 ppb) is reasonable.

                  As noted in the earlier reply, the US EPA methodology does also recommend a spiking concentration of 40-100 μg/L for each analyte, except selenium and mercury. That is also a good guide (a little lower than your planned spike concentration of the spec of 30ppb).

                  In terms of the maths, you will need to account for the 100 fold dilution applied during preparation to both the spike and the sample - since you will calculate the recovery with respect to the original spike added concentration (before dilution - or your spec. of 30 ppb).

                  Hope that helps you,
                  Eric