Hello Hong,

I did this a long time back. So, could provide some rough idea. When you do pesticide residue analysis in a biological sample, even after careful extraction there could be several chemicals present in the sample which would be injected into the system (HPLC/GC). Therefore, the peaks would be definitely more if you compare it to a standard run. For knowing the composition of this peak, you could either search the literature or may be run more standrads in your lab, if you are doubtful of cross-contamination with other pesticides/chemicals, thus comparing the time-point at which peak appears. However, point of worry is only when this peak is big enough.

Hope I could help somewhere.   

It is worth to do GC-MS/HPLC-MS to know the unknown peaks

First at all, You should check your standards. Many of currently used pesticides are chiral and some of them gives detector response as few peaks. Also when You applied a non chiral column. Some of the standards can degradate during storage (especially pyrethroids and triazoles). Moreover, when You analyzed extract (after various extraction techniques) from natural samples, You can have a matrix effects. You should inject first standards, matrix, enrichment matrix and check obtained peaks. When You work with selective detector, You should confirm your results on another column with different stationary phase. I recommend to use MS detector or tandem MS. You will have confirmation in ions (m/z) and retention times at the same time. Please read my papers on my profile. I hope will be helpful.

Kindly regards

there may be different reasons for the appearance of the unexplained peaks

1. Due to unknown impurities or artifacts in the HPLC or GC system.

2. Due to matrix effects which may not be eliminated during the clean-up process.

3. Due to unstable detector response which may give false peaks.

4. Use of impure solvents

Confirm the purity of the standards, confirm the column specifications so that you use the right column for the samples, ensure that the column is clean and run blank samples in between sample runs. Only standards of high purity can confirm  m the presence of the compounds in samples.

Hope above information is of help to you.

Kind Regards

Dear Hong

In addition to the comments already made, please consider this:

- What kind of samples are you dealing with? In case of environmental samples or biological experiments: think of metabolites and degradation products (hydrolysis, photolysis).

- If you are really sure that sample preparation or matrix do not contribute to your unknowns and you are dealing with commercial pesticide preparations rather than pure substances: check for additives.

- On your final question "Then how do we know whether composition of these peaks is toxic or not ?": chemical analytics is not able to tell you. This is subject to toxicological studies. Once you know the substance, you either will find respective data published (hopefully) or you got to get such studies done. Eventually you may try to estimate from similar substances but that's a unsteady and questionable path.

In any case it is essential to apply mass spectrometry to identify your unknowns, as previously suggested. In case your samples can be analyzed by GC (maybe after derivatization), I'd strongly suggest to use GC-MS. The spectra mostly show sufficient fragmentation to allow interpretation from scratch, but in most cases a proper library will do the job.

I hope it helps

Peter

A lot of pesticides produce degradation products. One of the products I run, trichlorofon, will produce peaks both before and after it as it degrades, in as quick as a few days. The peaks will grow larger with time. I've also noticed degradation with some herbicides.

Many of my analytical standards must be refrigerated to maintain integrity.

Thanks all of you!