NPD

[emsman]

Hello,

Can anybody help me about parameters for NPD detector of Agilent 6890?

I try to detect PCl3 and esters of PCl3 and OPP (o-Phenylphenol). FID is fine for all the esters and OPP, but I have to determine PCl3 (and eventually POCl3) as well.

With 30 pA I get huge signals (until 5000 pA) which more or less correspond with parallel FID analysis. But it seems like the detector is overloaded.
When using little bit lower Offset (25pA) sensitivity is nearly gone (signal max. 100 pA, same sample!!)
Additionally, baseline seems dropping from run to run.

- The main problem is, which value shall I take for the offset?
- Principally, can I use the same concentration for NPD as for FID?
- Am I right, that the instrument doesn't make automatically adjustment after every run of a sequence?

Thanks in advance for Your answers
Emsman

[AICMM]

Emsman,

I cannot access my notes on NPD at the moment but I do know that you do not want the "auto-ignite" feature of the Agilent turned on for the NPD. When I get back in the office I will try to find best practice on NPD from DET (Paul Patterson's company.) Your results make perfect sense for the NPD. First, you turn up the current on the NPD until it ignites the hydrogen chemistry (big jump in standing current.) Then you have sensitivity to P compounds. You should be able to leave it there for a period of time. Just a little bit lower bead current and there is not enough heat for reactive hydrogen chemistry and thus, poor/no response.

However, your other result is working against you, that is that NPD's hate chlorinated compounds. It completely messes up the bead chemistry. That is probably why you are seeing the drop in signal from injection to injection.

ECD or PID or AID are all alternatives....???

Will post more NPD advice when back in office.

Best regards.

[AICMM]

Emsman,

I cannot access my notes on NPD at the moment but I do know that you do not want the "auto-ignite" feature of the Agilent turned on for the NPD. When I get back in the office I will try to find best practice on NPD from DET (Paul Patterson's company.) Your results make perfect sense for the NPD. First, you turn up the current on the NPD until it ignites the hydrogen chemistry (big jump in standing current.) Then you have sensitivity to P compounds. You should be able to leave it there for a period of time. Just a little bit lower bead current and there is not enough heat for reactive hydrogen chemistry and thus, poor/no response.

However, your other result is working against you, that is that NPD's hate chlorinated compounds. It completely messes up the bead chemistry. That is probably why you are seeing the drop in signal from injection to injection.

ECD or PID or AID are all alternatives....???

Will post more NPD advice when back in office.

Best regards.

[emsman]

Hello,

thanks for Your answers.

The alternative detectors You mentioned: Is one of them as easy to build into an existing GC as FID and NPD?

Regards
Emsman

[AICMM]

Emsman,

PID will fit into FID slot and use FID electronics (typically.) OI has one that will use Varian or Agilent electrometer. Hnu, I think, uses their own electrometer since they do positive bias instead of negative bias.

AID (my detector) uses Varian or Agilent electrometer as well. Can be operated as an helium ionization detector if you cannot see low enough on TCD to see CO but is easier to operate as an AID since HID requires higher purity specifications. (Basically any time someone can operate as AID instead of HID I recommend it to them because it is less fussy.)

I should be able to get to my NPD notes today and send you operating recommendations.

Best regards.

[AICMM]

According to Patterson:

On the Agilent NPD "Chemistry ignition is indicated by a sudden increase in detector output singal as the Bead Voltage is systematically increased in magnitude. To check on this "Chemistry ignition was verified by momentarily turning the H2 flow Off and then back On. With no H2 flow, the NP chemistry was extinguished and then re-ignited when H2 was re-established."

Also, "The 6890/7890 NPD electronics include a feature called Adjust Offset in which the Bead Voltage is automatically adjusted in order to produce a detector output singal of pre-determined magnitude. We have long recommended that the Adjust Offset feature NOT be used because it leads to overheating the ion soure and greatly shortening the source's operating lifetime. " Patterson suggests adjusting voltage to ignition and leaving it until the sensitivity drops significantly then slightly adjusting again until ignition, etc....

Hope this helps.

Best regards.