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stdev ☆ Greece, 2012-09-12 00:00 (4643 d 07:19 ago) Posting: # 9181 Views: 6,038 |
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Dear everyone, I have an 8-point calibration curve and have established my LLOQ after multiple successful runs. However, I am now noticing the following symptoms in my MS system: The 2-3 lowest calibrators exhibit approximately the same, near zero or zero signal, then all the others appear to be normal, for standards in mobile phase and matrix. Any ideas about what could be causing the problem? |
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ElMaestro ★★★ Denmark, 2012-09-12 12:47 (4642 d 18:32 ago) @ stdev Posting: # 9182 Views: 5,257 |
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Hi Stdev, ❝ I have an 8-point calibration curve and have established my LLOQ after multiple successful runs. However, I am now noticing the following symptoms in my MS system: The 2-3 lowest calibrators exhibit approximately the same, near zero or zero signal, then all the others appear to be normal, for standards in mobile phase and matrix. Any ideas about what could be causing the problem? An old column? S:N too high for the peak algorithm to quantify properly? — Pass or fail! ElMaestro |
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Helmut ★★★   Vienna, Austria, 2012-09-12 16:40 (4642 d 14:39 ago) @ stdev Posting: # 9185 Views: 5,247 |
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\(Hi\;\sqrt{\sigma^{2}},\) are you working with freshly (=daily) prepared CCs? Do you also see low/no response of your lowest QC sample? — Dif-tor heh smusma 🖖🏼 Довге життя Україна! ![[image]](https://static.bebac.at/pics/Blue_and_yellow_ribbon_UA.png) Helmut Schütz ![[image]](https://static.bebac.at/img/CC by.png) The quality of responses received is directly proportional to the quality of the question asked. 🚮 Science Quotes |
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stdev ☆ Greece, 2012-09-12 21:10 (4642 d 10:09 ago) @ Helmut Posting: # 9191 Views: 5,282 |
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Hello Helmut, nice way to write my nickname by the way  Yes, I am working with daily prepared calibrators and QCs and I see low/no response for my lowest QC sample as well. |
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Ohlbe ★★★ France, 2012-09-12 21:21 (4642 d 09:58 ago) @ stdev Posting: # 9192 Views: 5,222 |
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Dear stdev, The next question would then be, how often do you prepare your working solutions ? I suppose you tried to re-prepare fresh solutions after you started to get this problem ? Otherwise adsorption could be an explanation. Did you also see a decrease in the peak areas at higher concentration levels, compared to the response you had initially ? Regards Ohlbe — Regards Ohlbe |
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stdev ☆ Greece, 2012-09-12 23:58 (4642 d 07:21 ago) @ Ohlbe Posting: # 9194 Views: 5,210 |
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@Ohlbe: I prepare my working solutions once a week, but have confirmed the bad response with several batches of working solutions so I am sure they are not the problem. To tell the truth, adsorption seems a reasonable explanation. Yes, I have seen a 40-70% decrease in the peak areas at higher concentration levels, compared to the response I had initially. I vented the instrument and cleaned some easily accessible components of the mass spectrometer (not as deep as the optics/mass analyzer though), but saw no improvement. @Helmut: I had to inject several hundreds of matrix extracts to optimize the method, so this could be the case. Contamination was one of the first scenarios that occured to me as well but when cleaning did not work, I focused in other scenarios. Thank you very much for your suggestions, they were really valuable feedback! Any other possible causes that you may think would be of great help! stdev |
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Ohlbe ★★★ France, 2012-09-13 00:32 (4642 d 06:46 ago) @ stdev Posting: # 9195 Views: 5,270 |
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Dear stdev, ❝ Thank you very much for your suggestions, they were really valuable feedback! Any other possible causes that you may think would be of great help! OK, let's try, though you have probably already thought of all of these. The method worked initially. Was there any change afterwards in:
Did you try with a new HPLC column ? If you still have the same problem it is not due to plasma dirt getting slowly released from it. Do you have a second LC/MS/MS of the same make and model ? If you do, do you have the same issue on the second instrument ? Finally, was there any re-calibration of your MS in between ? Did you try to re-do some fine tuning of your source and MS conditions ? Regards Ohlbe — Regards Ohlbe |
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Helmut ★★★ Vienna, Austria, 2012-09-12 21:26 (4642 d 09:52 ago) @ stdev Posting: # 9193 Views: 5,290 |
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Hi stdev! ❝ nice way to write my nickname by the way Could not resist. ❝ Yes, I am working with daily prepared calibrators and QCs and I see low/no response for my lowest QC sample as well. OK, that fits into the picture. I was hoping that you are working with a batch of deep-frozen QCs and prepare only calibrators daily. Have you considered adsorption? Does your system require multiple (to many) injections of matrix-extracts in order to achieve good sensitivity (typical for some LC/MS-MS conditions) or did it work “right away”? Maybe the system is overloaded with matrix components which chews up low concentrations. Only if your system does not require “pre-loading” consider cleaning (I know, I know…). We once had a stupid case of adsorption in the sample loop! — Dif-tor heh smusma 🖖🏼 Довге життя Україна! Helmut Schütz The quality of responses received is directly proportional to the quality of the question asked. 🚮 Science Quotes |
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stdev ☆ Greece, 2012-09-13 18:45 (4641 d 12:34 ago) @ Helmut Posting: # 9202 Views: 5,171 |
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Thanks to all of you, all of your points are very useful to me, I will look into everything you posted! Best regards, stdev |
Ing. Helmut Schütz