PID response

[pueblito]

Dear friends.
I´m trying to analyse VOCs using a HP5890 and a 4430 O.I. analytical, PID detector not in use for a long time. the carrier was set at 12psi and the UV lamp at 1.3mV ( a level of 7 for this model ) for a DB624 75m, 0.45mm, 2.55u ( helium for carrier and make up).
I ran a 4ng VOC standard and I got sharp peaks but the problem is the response which seems too low.
For example
0.5ng Benzene gives area counts 18000, height 4000.
does it have enough sensitivity?
what´s the proper column isertion lenght into the detector port?
Does the sweep gas flow affect the sensitivity? what about the make up flow.

many thanks.

[AICMM]

Pueblito,

PID is a concentration dependent detector. So, yes, sweep flow and make-up flow will certainly affect sensitivity. On the other hand, you want enough flow through the detector so that you get good peak shape without significant tailing. At 0.5 ng, what is the signal to noise look like (can you post the chromatogram?) Likewise, if that were in 5 mL's of water you would be below 1 ppb so is that really all that bad?

Best regards.

[JI2002]

Pueblito,

At 0.5 ng, what is the signal to noise look like (can you post the chromatogram?) Likewise, if that were in 5 mL's of water you would be below 1 ppb so is that really all that bad?

Best regards.

To be exact, it's 0.1 ppb for 5 mLs. Do you have to get that low?

[pueblito]

AICMM and JI2002 thanks for the reply.
Actually, one of the problems is the sensitivity specified in the 4430´s manual:
2 picograms benzene.
It´s difficult to get to 2 picograms. I dont know what are the capabilities of the PID detector ( I work with a GC-MS sistem) or I´m maybe misuderstanding the concept of "sensitivity" or my UV lamp is getting older , I dont know, because my only reference is the manual.
On the other hand, How do I post the chromatogram?

many thanks.

[AICMM]

Pueblito,

Your lamp is getting on in age at a setting of 7. Did you have to put it there to get it to light and can you go down from there now that it is lit? Also, when was the last time it was cleaned. Lots of people I know raise the current to maintain sensitivity rather than clean to maintain sensitivity. Unfortunately, this affects lamp life adversely.

PID's are concentration dependent so I am a bit surprised that the manual gives you a mass and not a concentration.

I have asked Tom if the instructions for posting chromatograms has come over with the server change in the Around the water cooler section.

Best regards.

[pueblito]

AICMM,
This lamp wasn´t in use for four years ( another analyst was performing BTEX many years ago).
So, I disassembled the detector according to manual´s instructions but I didn´t polish the lamp window ( a crystal disc at the lamp´s bottom?) I only swept it with agilent swabs an methanol.
And yes, I can go down from 7 to 5 now that it is lit.

many thanks.

[pueblito]

These are the 0.5ng VOCs std, lamp intensity 6.



.

many thanks.

[AICMM]

pueblito,

First, lamps age somewhat with time. Which is why everyone wants to order them just as they are dying.... Second, your bleed is going to kill you. More of that than target compounds and it all goes splat on the window. Third, it is advisable to clean the window when you get the opportunity. Fourth, and to the point (finally) you have quite a bit of room to go lower in terms of on-column amount. You are not down near the noise of the detector yet, probably not by a factor of 10 at least. But, do you really want to work that low. It makes integration and calibration quite difficult working near the noise. I would also point out that your internal standard being so high and your target compound being so low is probably going to create some reproducibility problems.

Best regards.

[Ron]

You definitely need to clean the lamp, but it will probably have to be replaced. Lamps eventually go bad even if they are not used regulary, and I would not expect a lamp that has been used and then has sat for 4 years to give a very good response.

[pueblito]

AICMM, JI2002, RON.

Thanks a lot for your help.

I have to clean or replace the lamp.
I will probably change the column too ( due to the bleed) and replace the oxigen trap.
I will always keep lamp at 4 or 5.
I think, I will start my calibration around 2ng ( 0.4ppb). I need that concentration because in my country´s legislation ( recently changed) Hexachlorobutadiene has a value of 0.0006mg/L.

good bye.

[Ron]

I'm not sure the column is bad, the bleed looks high, but if you can run at a lower setting with a new lamp the bleed may not be that bad. You are running a long, thick phase column, so you will see more bleed than you would from a 30 m thin phase column. Has the column also been sitting for a few years without use? if so, it should be conditioned using the manufacturers procedures.

I saw something similar to this over 10 years ago, an environmental lab was running GRO analysis around petroleum storage tanks. The samples contained diesel range organics as well as the gasoline range, and the heavy hydrocarbons caused significant background at the end of the run, and built up on the window and killed sensitivity.

[pueblito]

Ron.
You are right, about conditioning the column, it is one that I seldom use, I prefer a VOCOL but it´s dedicated to a GC-MS sistem. So, I will condition the DB624 for four or more hours instead of the short time I conditioned it before the PID test.With regard to the lamp window, I will have to polish it ( I´m afraid about polishing crystal surfaces but it´s the only way).

many thanks.

[Ron]

If you have the very fine aluminum oxide powder that is used to clean GCMS source parts it will work well to polish the window. Moisten a wooden cotton swab with methanol, dip it into the powder, then lightly polish the window using a circular motion. It doesn't require much pressure, and only a few seconds of polishing should clean the window.

[pueblito]

Ron.
I´m going to polish the window as you suggest.
Thank you very much.

[Georguslat]

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