PAHs with GC/MS

[brightsun]

Hello everybody,

Now i'm running PAHs with GC/MS, although i used standard PAHs to determinate qualitative compounds, but when i compare peaks with library, they are not PAHs...

I use EPA 8270 to analyze PAHs...

What the matter with my compounds ?

(I have begun this machine for 3 months, so there are somthing i can't understand

Thank you for replying !

[Peter Apps]

If I understand correctly you inject a standard mixture of PAHs and the MS library search identifies the peaks as compounds that are not PAHs ?

What identies does the MS library search give you ?

Peter

[Don_Hilton]

There is most likely a parameter in the mass spectral searching set up that lets you select the threshold below which background noise in the spectrum is ignored. You want to set that threshold to ignore noise - but not good spectral data.

Tell what instrument software you are using and someone may recognize the approprate control.

[mungss]

If yur are biggner of GC/MSD analysis, it's not easy to idenfiticate of PAHs mixture. Cause structure of PAHs compound is very similr, and mass spectrum is also.
So if you want to identificate each compound, it will be better that analyse each compound you wnat to find.

[Don_Hilton]

I would suggest looking at the list of PHA's on the standard mixture and getting the retention order for PAH's on the particular column (available on the internet, if needed). The peaks will fall in order in the chromatogram and will be identifiable by order and molecuar ion. You can have them all identified with even one injection.

[brightsun]

Thanks Don_Hilton, mungss and Peter Apps !

I have solved this problem, the chromatogram cannot detect peaks of PAHs because their concentration are lower threshold of method ( i have standard but i haven't their concentration, and i diluted so much)

Standard consists of 16 compounds. But when seeing the chromatogram, there are some peaks which cannot seperate:





I think there are 2 compounds, how to seperate them clearly ???

[Don_Hilton]

Step one is to check to see what the vendor of your standard used for chromatography - and how well it woked. There should be a reference chromatogram for the lot of standard. Typically these are available on the vendor's web site. Take a look. If the chromatography the vendor had is adequate, copy it as exactly as you can. Use not only the same type of column, but even the same brand. (While there are not major differeces between the 5% phenyl columns - there can be small differences where the small change can be the important difference. And, the same is true for many of the other commonly used kinds of staionary phases.)

There are several possibilities. For a couple of PAHs, the relative retention times are not going to change, so all you can do with a specific stationary phase is increase the resolution by making the peaks sharper - and narrower. So, check to be sure that you have optimum column flow and that the column is properly installed (correct lemgth of ends into the inlet and detector and inlet), square cut ends and such. Be sure you are using a clean liner - and one appropriate for the method. The inlet needs tobe hot enough to move the analytes quickly onto the column. These things affect the shaprness of the peak.

Ocasionally you will find a couple of compunds where the separation changes a bit with temperture or heating rate of the column. But with the chemical similarity of PAHs, I dont expect it - and have never looked for it. Use a temperture program that is slow enough to allow the compunds time to separate, but is fast enough that you do not suffer from excessive band broadening on the column.

If you are already close to optimum on all of these, you will have to pick a different column to get better separation. Check the web sites for the various column manufacturers and see what they have to offer. Some vendors make specialty columns that have optimized stationary phases for PAHs, pesticides, specific EPA methods and such.

[brightsun]

Thanks so much Don_Hilton...

Although i read many different documents, books...but when i analyze these compounds, there are something which happens in the process.

Now, it's different between the solvents in the extract and the standard.

The solvent of extract is DCM/ Acetone (1:1 v/v) , the solvent of standard is ACN.

When i run PAHs standard with GC/MS, i determined all compounds in the standard, i registered them in the MS library.

But when i run extracts with the same method, they cannot detect PAHs in this method.

I think there are two problems here:

Firstly, there is different polarity in the solvent
Secondly, the extracts are destitute of PAHs.

And, is any solution to solve the first problem ? ( different polarity ??? )

Thanks everybody !

[Peter Apps]

Are you sure that there are PAHs in the samples that you are extracting ? If there are no PAHs in the sample, or there are PAHs only below the detection limit of the method they will not be detectable in the extract.

Peter

[Don_Hilton]

Unless you have the PAHs eluting on the solvent tail, the change of solvent you describe should have no effect on retetnion times or detectabillity of your PAH's.

Following on what Peter says. A couple of things: 1) part of what an analllytical method is for is to tell you when something is not present at a level at which you can see it. 2) you need to test your extraction procedure to see if stuff is just gettting lost along the way.

Add a spike of PAH solution to your sample before you begin the extration. Calculate to be sure that you will have enough material to clearly see a response, even if half of the material is lost. Extract the sample and look for the result. If you get the theoretical result for your sample you know that you can see a sample at that level. If there is no reponse in the analysis of a spiked sample, you know that the prepartion steps are giving you a problem. If you see only 10 or 50% of the expected result, you know that you have losses in the sample preparation and the procedure needs some adjustment.

[brightsun]

Thank you so much everybody !

I have determined qualitative compounds in PAHs standard.

And now, i'm running the calibration curve.

The concentration of my standard after diluting is 1061ng/ml. I used this concentration to run the calibration curve.

The volumes i have used are 1microlit, 2microlit, 4microlit by hand (with 1061ng/ml). My method is based on the weight of volumes which i used. Then i used peak area and weight of above volumes to create the calibration curve.

Some compounds in this curve are linear but others are not.

The range of my concentration is not linear ???

Must i dilute this concentration ???

Someone has experiences to determine the range of linear concentration ???

Regards !

[Peter Apps]

You cannot calibrate by injecting different volumes. You need to make up solutions of different concentrations, and inject the same volume of each, and that volume has to be the same is the volume of extract that you will inject when you run samples. Also, preferably, the standards and samples have to be in the same solvent.

Peter