21 Replies Latest reply on Apr 12, 2018 4:06 AM by gchaplain

    RSD% failure - PAHs

    jerivera

         I please ask if you can verify what I am doing wrong. My analysis is 8270 PAHs using a 7890A/5975C inert XL MSD. I currently use 30m x 0.250mm x 1.00µm DB-5MSUI (Part # 122-5533UI) Agilent column. The attachment below shows my RSD% for three of my four groups of analytes over 15% with noticeable peak deviation in the final CC concentrations. I have made the calibration curve with the desired concentrations more than once with the same instrument results. I will greatly appreciate your help.

       

      Current column settings:

      1) 50°C (1min)

      2) 15 °C/min to 300° - hold 4.5min

      3) 25 °C/min to 320° - hold 12.5

      Total time 35.467min

       

      Current GC settings:

      MS Source 230°C, MS Quad 150°C, Aux Temp 310°C, HiVac 1.14e-05

      Inlet:  Temp=280°C, Pressure=10.052psi, Flow=24.20mL/min, Constant Flow

      Splitless injection at 50mL/min at 1min

        • Re: RSD% failure - PAHs
          ryoboyle

          I moved this to the Environmental Applications area where it will be more likely to received an informed answer to your question.

          • Re: RSD% failure - PAHs
            ron_tackett

            Jerivera,

            This looks to be a linearity issue that could be due to the dynamic range. Try injecting less, turn EM down a little or making 2 curves. We cannot give up much on the bottom end of the curve but a little is all we may need. If still having issues we would recommend that you contact you local Agilent support and they most likely will need the raw data files and the method to evaluate.

              • Re: RSD% failure - PAHs
                jerivera

                Hello Ron. I saw an article this weekend with a colleague having a similar issue as mine and he believed it was the EM. After autotuning he did lower the voltage as you are suggesting. My method EMV had a gain factor, then a relative and now running with an absolute trying to maintain voltage constant. Thank you. I will follow through and proceed.

                • Re: RSD% failure - PAHs
                  jerivera

                  No change after changing replacing the EM, lowering the EMV, injecting less from 50mL/min to 15mL/min nor changing the acquisition settings from constant flow to constant pressure (trying everything). The higher the concentration analyzed the farther the curvier the linearity. Still going...

                • Re: RSD% failure - PAHs
                  jerivera

                  After cutting 10 inches from my previous column and after changing to a new column I still have a upward curved in both procedures. My MMI inlet is untouched since I lack the abrasive brush which I will need to order. Only the glass-wool liner and o-ring has been changed. I have only to re-clean my ion source and rerun my CC. If this doesn't work I would only be able to use quadratic regression to have an acceptable r² coefficient.

                  • Re: RSD% failure - PAHs
                    trifecta

                    Hi jerivera,

                    I could not tell from your attachment whether the PAH curves were curving "up" with higher concentration or flattening out.  Flattening out is easiest to fix as it is almost always due to too much material in the source or too high of a gain.  Either increase the split ration or lower the gain.  Curving up is a bit more difficult to fix.  It has the appearance of more response than expected at the higher concentrations but it is really lower response than expected at the lower concentrations.  This has been studied extensively and a good summation can be found in Agilent publication

                    5991-3003EN.pdf.  I am not suggesting that you try the hydrogen addition as detailed in that note.  The problem is about 30% inlet related and 70% source related.  At a minimum try these steps: Use a 9mm drawout lens, part# G3440-20022. Ron suggested the 6mm earlier, which will work if you have already purchased it.  The 9 will give a bit better linearity with a slight S/N loss vs the 6.  Source temp to 350C.  MMI to 330C.  Pulsed injection either split or splitess. Liner with wool is a must for either injection to transfer heat to the heaviest PAHs that stick to everything.

                    Best regards

                    1 person found this helpful
                      • Re: RSD% failure - PAHs
                        jerivera

                        Hello trifecta,

                        Yes my curve is curving upwards. I have not used a source temp over 230C or an MMI temp over 280C as I will give it go. I will order the drawout lens as Ron and you suggest. I really appreciate your time to ease my concerns.

                        Warmest regards

                      • Re: RSD% failure - PAHs
                        timstrutt

                        Hi jerivera,

                        If you are still having problems, a couple points to consider:-

                        Your instrument, when it was new, would have been capable of detecting 10pg on column (in scan mode for 8270) for many PAHs (generally the late eluters would be worse). The top range should be in the order of >10ng on-column giving you a calibration range of ~1000:1 - Assuming you are using a 1-2µL injection and your bottom calibration of 0.5 corresponds to ppm concentration, that implies that you are injecting 0.5-1ng on column - The top range is 140-280ng? - This at the top capacity of your 1µm column and is close to saturating the system, suggesting that you inject less. Your low level calibration responses are <20,000, so if you do inject less, you might want to put the gain up. Allowing for that, I think that most of your problems are at the MS end of the analysis. As you know, 8270 extracts are often very dirty and contamination can quickly build up inside the injector, column and MS. If you ignore your lowest calibration point, the higher points can be plotted as reasonably straight line (These lines would cross the origin at ~+0.3 for 1-methylnaphthalene and benzo[k]fluoranthene and ~+0.1 for chrysene). This can often be caused by background chemical noise ("dirt").

                         

                        Are you only analysing PAHs in these runs? They are very stable and the source temperature can (and should be) higher - As trifector posted, 350C is normal for the source temperature for this work, and you could put the Quad temp up to at least 180C which will help with background contamination build up. If you can put the EI Energy above 70eV it should also help as it will give normal spectra for the stable PAHs, but more fragmentation with background ions.  Like trifector, I would avoid hydrogen carrier gas on an older instrument  - If you do try it, it tends to remove existing contamination from inside the system and it might take days before you get a clean background. If you are analysing a lot of dirty samples, to help keep the source clean, you might consider using a post-column backflush fitting and turning the backflush on a minute or so after benzo(g,h,i)perylene elutes.

                         

                        I suggest: 1) Cleaning the source; 2) Inject less sample - This will also put less contamination into the system (Hot split injection as suggested by Ron Tackett, but >=10:1) and turning the detector gain up - If the curve got worse, it indicates possible background contamination.

                        If you still can't get a good linear calibration, as you say, try quadratic: The new 8270E method, recently promulgated by the EPA, states in Section 7.7.1 "ICAL standards must be prepared at a minimum of five different concentrations from the secondary dilution of stock standards or from a premixed certified solution. Include a minimum of five different concentrations in the calibration for average response factor or linear (first-order) calibration models and six different concentrations for a quadratic (second-order) model with the low standard at or below the lower limit of quantitation (LLOQ)". Visual inspection of your curves indicates that you should get reasonable quadratic calibrations...

                         

                        Regards, Tim

                        • Re: RSD% failure - PAHs
                          gchaplain

                          Just noticed that your curve is forcing through 0.

                          Remove this and have it fall naturally. Because it's forcing through 0 its taking it a true definative point, you may find that at the lower levels anyway that the graphs are not striaght line linear but in fact polynomial once noise effect the signal.