[misstry]

hi, i would like to ask about the quantitative in the GC and LC.

1) How to calculate the concentration of sample using internal standard method?
2) Do i have to put in the preservatives in the standard? this is because in the sample, we add in.
3) what is the concentration we use to calculate the detection limit?
4) do we inject the pure standard for the calibration curve or the extracted standard after SPE of SPME?

Thanks in advance.

[Alpha-Omega]

This is huge.  All of this should be in your Analytical Text and Instrumental Analysis Text

Skoog and West is a good Resource:  Analytical Chemistry 8th Edition, and Principles of Instrumental Analysis by Douglas A.Skoog.


1) The method of internal standards is used to improve the precision of quantitative analysis. An internal standard is a known concentration of a substance that is present in every sample that is analyzed. Internal standards can be used with either the calibration curve or standard addition methods, although the former is probably more common.
The purpose of the internal standard is to behave similarly to the analyte but to provide a signal that can be distinguished from that of the analyte. Ideally, any factor that affects the analyte signal will also affect the signal of the internal standard to the same degree. Thus, the ratio of the two signals will exhibit less variability than the analyte signal.
Internal standards are often used in chromatography, mass spectroscopy and atomic emission spectroscopy. They can also be used to correct for variability due to analyte loss in sample storage and treatment.

Please see the attached PDF document.

2)   This sounds like you are asking about matrix matching.  What you add to the samples you add to the standards.  Treat your samples and standards the same way.


3)   If you are referring to MDLs also referred to as LDLs, they are left up to the analyst.  You have to do what is known as an MDL study.  This is part of Method Development.  You have to make up a range of standards to determine the instrument response.  This can be a very arduous procedure. EPA has a number of procedures available on their site in the form of  PDFs for this determination. The EPA defines the MDL as the "minimum concentration of substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero, and is determined from analysis of a sample in a given matrix containing the analyte".

4)   http://www.sigmaaldrich.com/Area_of_Interest/Analytical__Chromatography/Sample_Preparation/SPME/FAQ.html

Links to MDLs and Detection Limits

Analytical Detection Limit Guide:  http://www.dnr.state.wi.us/org/es/science/lc/OUTREACH/-Publications/LOD%20Guidance%20Document.pdf

Setting Meaningful Detection Limits:  http://www1.dionex.com/en-us/webdocs/52823_LPN%201926_Quantitation.pdf

[missaturn]

Thanks Mebecker

Another question is does all the calibration curve has to start from the point (0,0)? If not, how can we explain the curve when the concentration is 0? Thanks.

[Alpha-Omega]

NO...never force your function through 0.  That is very bad chemistry.  That is called "Forcing The Function." And only use offset if making multiple injections of one standard.  Make sure your standards lie in a linear range if you are restricted to using a linear fit.  If you have a very wide concentration range your data can deviate from a linear fit.  You fit the function to the data...not the data to the function.

You use error analysis to explain why your line does not pass through (0,0).  Statistical analysis...that is all in "Analytical Chemistry," Skoog and West, 8th Edition and "Principles of Instrumental Analysis," Douglas A. Skoog.

[enahs]

Yes, never force it through zero. If you carefully setup your experiment and your calibration curve does not go through zero, depending on how you setup your experiment it (i.e. conditions, not just quality) the non-zero y-intercept can tell you a lot of information about the matrix, etc.

[SemiAuto]

This topic came up at work.  When is using 'zero' or the 'origin' appropriate in calibration curve?  A lot of our methods seem to either 'include' or 'force' zero.  We do Internal Standard and External Standard methods mostly.

[Alpha-Omega]

It is not a good idea....because...

Never fit a calibration curve thru the origin as even when there is no detectable analyte present, there will still be a very very small signal present (ie noise). It is the "size" of this signal that is used to determine the limit of detection for your system, simply by the LOD rule of 3 times background noise.

If you are quantifying samples with very low analyte concentration, then you must prepare very low conc. standards to accuratley describe the linear relationship at such low signal levels. If there is no signal (ie area o), then 3 times 0 is 0 and your LOD is thus 0, impossible, thus noise is alway present in your chromatograms.