An internal standard should be ud when performing MS quantitation. An appropriate internal standard will control for extraction, HPLC injection and ionization variability. In a complex matrix it is not uncommon for two different standard levels in SRM integrated plots, at the lower end of the standard curve, to give nearly an identical respon. It is only when an internal standard is ud that the two points can be differentiated. Some rearchers attempt to prepare standard curves and run samples without an internal standard and find moderate success. Often without an internal standard % RSDs of replicates can be as high as 20%. Using an internal standard the % RSDs can be brought down to approximately 2%. We run triplicates at each level of our standard curve.
How do I choo an internal standard?
The best internal standard is an isotopically labeled version of the molecule you want to quantify. An isotopically labeled internal standard will have a similar extraction recovery, ionization respon in ESI mass spectrometry, and a similar chromatographic retention time. If you are performing non-clinical PK quantitation it may be difficult to justify such a sta
ndard since a special synthesis of an isotopically labeled standard can be expensive and time consuming. Often if you are working with medicinal chemists they will have a library of compound analogs that can be ud as internal standards. The analogs were made in the evolution of the compound to be tested and will be similar to the compound to be quantified and more importantly will be slightly different by parent mass. Try to avoid using de-methylated (-14) or hydroxylated (+16) analogs as internal standards since the are the most common mass shifts obrved in naturally occuring metabolites of the parent compound. A common internal standard is a chlorinated version of the parent molecule. A chlorinated version of the parent molecule will commonly have a similar chromatographic retention time which is an important characteristic of an internal standard. We have found that one of the most important characteristics of an internal standard is that it co-elutes with the compound to be quantified.
How do I u an internal standard?
空中英语
First of all an internal standard should be added at the beginning of the sample work-up, typically before the plasma crash or solid pha extraction. The internal standard should be added at the same level in every sample including the standards. An internal standard should give a reliable MS respon. Care should be taken that the amount of the internal standard is well above the limit of quantitation but not so high as to suppress the ionization of the analyte. "How much internal standard should I add?", this is an important question. It pays to know roughly how much compound is in your sample. This can be ac
complished by making trial analys of an early, middle and late time point with perhaps one or two standard points. This information will be very valuable when building an appropriate standard curve and in knowing how much internal standard to add. If you were trying to quantify samples in the range of 100 fg to 25 pg and the limit of detection was 100 fg you might add 5 to 10 pg of internal standard to every sample. A good rule of thumb is to target the internal standard to the lower 1/3 of the working standard curve. This is a range that will give a comfortable respon without interfering with the ionization of the analyte.
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什么叫内标法?怎样选择内标物?
内标法是一种间接或相对的校准方法。在分析测定样品中某组分含量时，加入一种内标物质以校谁和消除出于操纵条件的动摇而对分析结果发生的影响，以提高分析结果的准确度。
北京舞蹈教学
内标法在气相色谱定量分析中是一种重要的技术。使用内标法时，在样品中加入一定量的尺度物质，它可被色谱拄所分离，又不受试样中其它组分峰的干扰，只要测定内标物和待测组分的峰面积与相对响应值，即可求出待测组分在样品中的百分含量。采取内标法定量时，内标物的选择是一项十分重要的工作。理想地说，内标物应当是一个能得到纯样的己知化合物，这样它能以准确、已知的量加到样品中去，它应当和被分析的样品组分有基底细同或尽可能一致的物理化学性质(如化学结构、极性、挥发度及在溶剂中的溶解度等)、色谱行为和响应特征，最好是被分析物质的一个同系物。当然，在色谱分析条什下，内标物必须能与样品中各组分充分分离。需要指出的是，在少数情况下，分析人员可能比较关心化台物在一个复杂过程中所得到的回收率，此时，他可以使用一种在这种过程中很容易被完全回收的化台物作内标，来测定感兴趣化合物的百分回收率，而不必遵循以上所说的选择原则。筷子英语
在使用内标法定量时，有哪些因素会影响内标和被测组分的峰高或峰面积的比值?
emc是什么意思影响内标和被测组分峰高或峰面积比值的因素主要有化学方面的、色谱方面的和仪器方面的三类。
mary says
由化学方面的原因发生的面积比的变更经常在分析重复样品时出现。
化学方面的因素包含：分析测试百科网u"f?6BL(lNb4{\4q[#}"dv3l01、内标物在样品里混合欠好；分析测试百科网;apWq~.{~k(j分析测试百科网@P0f(DIm1k.J$O
2、内标物和样品组分之间发生反应，
V/VZ;n1Q0(u1y;m^)hD pc0G&}03、内标物纯度可变等。
