Single Quad MS peak

[lusi]

Hope someone can help me with this.

I am doing some work on using a single quad MS (Thermo MSQ Plus) to detect pyrethroid pesticides.

Anyway I am getting fairly good peaks, except I am getting two dominant weights close to the expected weight but not the expected weight e.g. If I am supposed to get 232 due, the weights I see are 231 and 233.

Can someone please offer some advice?

Thanks

[tom jupille]

Is your analyte one of the chlorinated ones?
Chlorine has two stable isotopes: 35 & 37, with a relative abundance of about 3:1 (if memory serves).

[lusi]

Hi Tom,
It is cypermethrin and alpha-cypermethrin.
Also I have had a similar experience with 2,4D and diuron.

[AA]

So, why are you looking at 232 when the molecular wt of cypermethrin is 416?

In negative ion, I would expect to see ions at 415 and 417.

[lusi]

Sorry for the Mix Up. I was giving an example with those figures.

The cypermethrins are giving 415 and 417.

Its the diuron that is giving the 232 though.

[AA]

Diuron has a MW of 233.1, so in negative ion (you did not state pos mode or neg mode) I would expect M-H at 232 and a large ion (from the Cl isotope) at 234.

[lusi]

Sorry once again AA, I should have been more precise.
This is what I am getting.
As you rightly point out diuron is 233.1
So in negative ionisation you would expect to see 232.
I am getting 231 and 233 instead.
And for the cypermethrins - 414 and 416.
I tried water and ACN, ammonium buffer, ammonium acetate buffer, acetic acid buffer, formic acid buffer. Want to retry the ammonium acetate.
I get a nice peak but when I check the spectrum these are the weights I see.
Now I am wondering if its a calibration prob. However, I did pop in a deltamethrin (505.24) in there and got 503.93 in neg ion mode. So I figure that was fine.
Thanks for the info. I didnt know that was the trend with neg ion mode. Is it with all compounds?

[AA]

The first thing I would do is re-calibrate the MS. Then I would run whatever Thermo recomends as a system check or performance qualification compound and confirm that the masses are as expected.

[lusi]

I did the mass recalibration and the instrument indicated that the tune passed. I am guessing that means that everything is ok. However, I did notice it missed a peak.

I accepted the results. The manual did not suggest anything after that.

SO I ran a few samples.

The results are still similar. I did 2,4D (221.04) and got 218.89 and 220.89.

The diuron (ACN 60: H2O 40) give a small 'peak' 322.86, 324.86 and a large peak 230.96, 233.06.

The standards are a few months old though.

Any other suggestions?

[lmh]

lusi, if you are in negative mode, these are exactly the masses you would expect, aren't they?

Don't even look at the molecular weight of any of these compounds. It's irrelevant because it's the average mass of all isotopologues at natural abundance. Find the mono-isotopic (35Cl-only!) mass and deduct one to get the lowest-mass peak.

The monoisotopic mass of diuron is 232 (not 233), so 231 is the correct mass for its negative ion, and 233 the first 37Cl isotope peak.

[Jetjamnong]

Here are isotopic pattern (accurate mass) of those compounds with Negative mode
Diuron (C9H10Cl2N2O)
m/z (Abundance)
231.008645 (100.000)
232.011591 (10.614)
233.005787 (64.637)
234.008683 (6.821)
235.003103 (10.673)
236.005855 (1.108)

2,4-D (C8H6Cl2O3)
m/z (Abundance)
218.961026 (100.000)
219.964411 (8.824)
220.958195 (64.881)
221.961542 (5.703)
222.958829 (10.832)
223.958829 (0.941)
224.960453 (0.100)

Cypermethrin (C22H19Cl2NO3)
m/z (Abundance)
414.065825 (100.000)
415.069114 (24.485)
416.063346 (67.407)
417.066358 (16.016)
418.061595 (12.473)
419.063955 (2.736)
420.066524 (0.375)

[lusi]

Jetjamnong, thanks for the information. Where can I pick up this information?

Just for clarification, what I am seeing on the chromatographs is what I should be getting?

[Jetjamnong]

I use my MS tool to generated this pattern. However, you can generate your analyte formula to Isotopic pattern by using free tool like this
http://www.sisweb.com/mstools/isotope.htm
(Plus 1 H when using +ive mode and minus 1 H when perform with -ive mode)

For this case, LC/MS (Low Res) analysis of some pesticides by Negative mode you can use the following ion (Deprotonated ion if these compounds can be ionize by -ive mode).
Diuron
m/z
231.01 +/- 0.2 amu. (M-H)
233.01 +/- 0.2 amu. (Cl Isotope)

2,4-D
218.96 +/- 0.2 amu. (M-H)
220.96 +/- 0.2 amu. (Cl Isotope)

Cypermethrin
414.06 +/- 0.2 amu. (M-H)
416.06 +/- 0.2 amu. (Cl Isotope)

The use of isotope ion as qualifier ion is not good, for SQ LC/MS you can increase ISCID energy to get their fragment and use that for qualier ions.

[lusi]

Thanks so much.

What is the ISCID?
Is this done by increasing the cone voltage?

[lmh]

lusi, are you using Xcalibur?
If so, there is an isotope tool within it that will predict the information Jetjamnong gave you.