Compound shows up using the UV detection but not the MS

[lusi]

Our system is routed to the UV detector and then the MS. A compound came off through the UV detector but it is not showing up in the MS. In fact all I am getting is a bunch of noise with some small pikes and negative peaks. (At least that is what is looks like to me). Also the weight of the compound - 450 is not showing up but 454 is quite dominant.

Also I had a situation with another compound and a peak did show up, but the dominant mass was 229 and the compound is 221 (which is show up, but not as large).

I do not have methods for the compounds, I am just trying to see whether I can get.

Any advice???

[paddle1234]

May related to ionization efficiency.

[lusi]

Is there a way to increase this.

[lmh]

can you tell us a little more? What sorts of compounds are these? What sort of solvent mix are you using?

[Widebore]

I've seena few compounds like this. Simple fact is that they don't form ions , adducts or undergo proton transfer reactions in the source. You could try APCI to try to force ionisation or APPI. Alternatively, it might be a mobile phase problem.

[lusi]

I am working with pesticides. I tried negative ESI for three and was able to get better results, in that I see a clear peak. However, I did notice that the dominant mass for two was like two less than the expected mass. e.g. compund weight is 220, mass I expected in -ve ESi is 219, this is clearly seen in the peak; but 217 is more abundant. I have no idea if this is the norm.

However, for some pyrethroids - alpha-cypermethrin, cypermethrin, lambda-cyhalothrin I am getting really poor results.
Never used the APCI, as you guessed I very new to the whole HPLC analysis. Was considering doing so, but I was looking for methods.

The solvent mix is acetonitrile and water.

Unfortunately I do not have a list of those compounds which are likely to ionise and not being a chemist does not help. I tried searching for a list but was not successful.

[H.Thomas]

However, for some pyrethroids - alpha-cypermethrin, cypermethrin, lambda-cyhalothrin I am getting really poor results.

I have analyzed cyhalothrin, you need to put some ammonium formate in the mobile phase and you will get a nice response for m/z 467 and 469 [M+NH4]in positive ESI. The same for cypermethrin (m/z 433 and 435).

What are the other compounds you cannot detect?

[lusi]

That's so much for the method. Now I know they can be done. However, I do not have ammonium formate, can I used ammonium acetate instead? Also do you do like a 1% for the mobile phase?

I have quite a list of chemicals that I have not had the time to try on the machine, some I do not currently have standards for. I will attach the list below and maybe you can scan thorough it sometime and indicate whether you did any of those. I have no MS methods for them, and for some I dont even have proper HPLC methods.

Can you tell me if I got the calculations for these methods right?:
:5 g sample + 10 mL water + 10 mL MeCN + internal standards
:shake for 1 h
:add 4 g MgSO4 + 1 g NaCl, etc
Given that the MgSO4 is supposed to take out the water, would it be 5g in 10ml (+ 100ul internal standards)? So I need to just multiply my chromatograph concentrations by 2?

And this one:
:15g sample + 15ml 1% acetic acid in MeCN + 75uL TPP
: add 6g MgSO4 + 1.5 g NaAc
:shake, centrifuge
: take 1ml top layer, clean with the PSA etc
:vertex, centrifuge, evaporate to dryness
:add 0.9ml Hexane/Acetone.
I guess the sample will be the same concentration since its 15g sample in 15ml solvent. So essentially I am taking 1g sample and drying transferring to to hexane/acetone, so no calculations needed for my chromatograph concentrations?

I have attempted methods and got some results for 2,4 D, fipronil, carbendazim, hexazinone and imidacloprid.

The ones I have to work on include:
Abamectin (Avermectin)
Acetamiprid
Alpha Cypermethrin
Aluminium Phosphide
Ametryn
Asulam
Azoxystrobin
Bispyribac Sodium
Boscalid
Brodifacoum
Bromadiolone
Carbaryl
Chlorfenapyr
Chlorothalonil
Chlorpyrifos
Cypermethrin
Cyromazine
Deltamethrin
Diafenthiuron
Diazinon
Difenacoum
Diquat Dibromide
Dimethomorph
Diuron
Endosulfan
Ethephon
Ethoxysulfuron
Fenamidone
Fenoxaprop-p-ethyl
Flocoumafen
Fluazifop-p-butyl
Fosetyl-al
Glufosinate ammonium
Glyphosate
Imazapyr
Indosulfurom methyl
Indoxacarb
Isoxaflutole
Lambda Cyhalothrin
Lufenuron
Malathion
Mancozeb
Methamidophos
Methomyl
Metsulfuron Methyl
Monocrotophos
Oxamyl
Paraquat Dichloride
Pendimethalin
Pirimiphos methyl
Propanil
Propaquizafop
Propiconazole
Propineb
Pyraclostrobin
S-Metolachlor
Terbutryn
Thiamethoxam
Thiodicarb
Triazophos
Trifloxystrobin