Dear Victor,

The first thing you have to remember is that an internal standard is some compound that you add to your sample at some point time in the sample processing method (beginning, in the middle or at the end). An external standard is some compound that you don´t add to your samples. Having said that, surrogates are usually added before sample extraction and normally use to monitor your method recoveries. You could also add additional standards to monitor specific steps of you processing method, for example in the clean-up steps. At the end of your processing step, when you have your extract in your chromatographic vial, you need to add another standard to quantify your native compounds and your surrogates (which give you idea of your recoveries). This last standard is very often known as internal standard (I never understood why, because from the analytical point of view there are all internal standards to my understanding). One other thing you have to bring in mind is that any kind of standard you add to your samples must be different that your native compounds. Seems silly, but I have seen people mistaking this point. There are other ways of working, but this is one of the most common one. Labeled standards are better but more expensive. Since there many PCB congeners in environmental samples, I would recommend using 13C-labelled PCBs for a more precise determination. Of course you have to calibrate your GC/MS making different levels covering the range of expected contractions (amounts) and containing both native standards and internal standards. But this is another issue. I would recommend reading some environmental analytical chemistry chapter on this, or maybe the EPA methods for PCB analysis. Hope being helpful

Regards

Dear Javier,

Thanks for the explanation, really appreciate your time for this. Yes, it really helps me out to better understand those terms though not fully. But at least you provided me a better picture on the usage of standards. Having explained, can I safely say that surrogates and internal standards is recommended to use 13C-labelled PCBs, while external standards are calibration standards?

Dear Victor,

Here there is a "terminology issue". Briefly, The calibration standard composition will depend on your quantification method. If you quantify your target PCB using the "internal standard method" your calibration solution (not added to the sample) just to calibrate your GC/MS should contain your target PCBs and both the surrogates and internal standard used. A careful reading of the EPA protocol for PCB determination or similar should help. For an accurate PCB determination I would recommend using 13C-labelled PCBs as internal standards for quantification.