importance of internal standard in lcms analysis

[pliva]

hi everybody,
i have some questios in my mind
1)what is the use of internal standard in lcms analysis
2)what is the criretia of its selection
3)will it do to make standard curve without internal standard

i am going to work in lcms so please help me.

[tom jupille]

1)what is the use of internal standard in lcms analysis

To correct for fluctuations in sample introduction or detector response. The idea is that these fluctuations will affect the analyte peak and the internal standard peak in the same proportion, and so will cancel out when the ratio of peak areas is calculated.

2)what is the criretia of its selection

It should have similar response tot he analyte and elute at the same time or just after. The "best" internal standards are isotope-labelled analyte.

3)will it do to make standard curve without internal standard.

In many cases, yes. If the precision without using IS is good, then an internal standard can actually make things worse (having to measure two peaks can add error).

[DR]

Internal standards are particularly useful when you have considerable "sample clean-up" to do. Use of an IS will prevent sample to sample variations in extraction efficiency from ruining your precision and accuracy. A good IS is also useful in regular LC or GC when derivitization is required.