Surrogate and internal standard

[allene]

Hi everybody,

I'm new in GC. I can't understand difference between internal standard and surrogate. Why does surrogate add to the sample and how can I use it in my calculation? Should I add surrogate in standard solution? Should I make surrogate standard solutions at different concentration and make a calibration curve for it?

[chromatographer1]

Surrogates are organic compounds that are similar in chemical composition to the analytes of interest and spiked into environmental and batch QC samples prior to sample preparation and analysis. Surrogate recoveries for environmental samples are used to evaluate matrix interference on a sample-specific basis. However, in order for this approach to be viable, the surrogates must behave in the same manner as the corresponding target analytes that are native to the matrices of interest (e.g., must partition between various phases in the same manner as the
native target analytes).

The most representative surrogate will typically be an isotopically-modified version of the target analyte. Therefore, when evaluating surrogate results, how well the surrogate represents should always be taken into account.

An internal std is just another analyte added to the extraction after the matrix is processed which allows variances of injection size to be compensated.

Rod

[allene]

So, should I make a calibration curve for surrogate? If I need to calculate the recovery of sample preparation method, I need to calculate the surrogate concentration in the processed sample first.

Should surrogate response be divided to internal standard response?

So far as I know, the analyte-spike method is also used for evaluating matrix interference. Do the surrogate standard and analyte-spike method make the same role for evaluating matrix interferences? Can I use analyte-spike method instead of surrogate standard?


Thanks so much.

[chromatographer1]

So, should I make a calibration curve for surrogate?

You should ascertain if your surrogate parallels the recovery of the analyte first.

If I need to calculate the recovery of sample preparation method, I need to calculate the surrogate concentration in the processed sample first.

By making a synthetic standard solution with an internal standard you can calculate the response of analyte versus int std. Repeat for your surrogate.

Then when you add the internal standard to the processed sample which also contained the (spiked) surrogate, you can calculate the amount of recovery of your surrogate and hopefully, if you have a good surrogate, the amount of your analyte in your sample, assuming the recovery is the same as the surrogate.

Can I use analyte-spike method instead of surrogate standard?

If you spike at three levels and do a linear regression you should be able to calculate the amount of analyte as long as you do not have an interference with the analyte in your analysis.

best wishes and I hope this makes sense, because the hour is late and my mind may be a little fuzzy.

Rod

[allene]

Dear Rod,
Thanks for your response

I don't understand yet if I could use "analyte-spike" method instead of "surrogate" or not? Why is in EPA standard methods, both 'analyte-spike" and "surrogate" recommended, whereas they do the same work.

As I found from your explanation, I should have surrogate compound in my calibration standard solutions at different concentration to get a calibration curve to calculate surrogate compound concentration in the final processed sample for recovery calculation. Do I understand that correctly?
for example, assume I have three standard solution an one sample, so:

Standard 1 contains: 1 ppm analytes + 1 ppm surrogate + 10 ppm IS
Standard 2 contains: 5 ppm analytes + 5 ppm surrogate + 10 ppm IS
Standard 3 contains:10 ppm analytes + 10 ppm surrogate + 10 ppm IS

and the final processed sample contains :

X ppm analyte + X' ppm surrogate + 10 ppm IS
(surrogate was added before sample preparation and IS added after sample preparation)

Where analytes and surrogate concentration are calculated from calibration curve.

Is this procedure correct?

Thanks so much for the patience.

[chromatographer1]

It looks good. But remember,

"If you spike at three levels and do a linear regression you should be able to calculate the amount of analyte as long as you do not have an interference with the analyte in your analysis."

You might want to run a sample without a surrogate, for comparison.

Rod