Chromatography Forum

How many types of sensitivity unit in chromatogram of GC? What are they? Why they different?
I am confusing.


Thank you
K.M.P

You ask practical problem or a history of scale?

From a practical point of view - the signal to noise ratio. Should be more than three for quality and over 10 for quantitative analysis. How to count - let FDA and EuDQM debate, IMHO, " you may be say me "you is pot", but don't put in the oven"

Scale - what kind of tester for prototype design half a century ago was on hand, such a scale he used. Was voltmeter-get volts, the ammeter was-get amps, and if there was a capacious capacitor, resister, clock and calculator -hertz ...

How many types of sensitivity unit in chromatogram of GC? What are they? Why they different?
I am confusing.


Thank you
K.M.P

Indeed you are slightly confusing ..... this group

Generally speaking chromatogram is graphical representation of detector signal as a function of time. Thus no sensitivity units can be found on chromatograms.

I guess you meant different units of signal magnitude.
For ECD unit is Hz, for FID it's pA.
And their differences are based on different principles of operation of detectors and different electric quantities measured respectively by electrometers.

Yes, I meant signal magnitude.
I am sorry to confuse all of you.

Thanks again
K.M.P

(...)
I am sorry to confuse all of you.
(...)
K.M.P

I was only kidding.

My pleasure if helped you.

I run pesticides OP mixed standard by GC-FID. My signal magnitude showed mV. But in my reference sheet from standard manufacture, they use GC-FID also their signal magnitude unit is pA.
So please kindly let me know, the pA vs mV.
I meant 1 pA is equal how many mV??

Help me,
K.M.P

pA is a unit of current, mV is a unit of voltage. They are not comparable, in other words you cannot express one in terms of the other.

In an FID a current flows through the flame when a compound elutes from the column. This current is very small, and is measured in pA. The electrometer of the GC converts the pA current to a mV signal that is sent out to the integrator. How many mV you get for 1 pA depends on the electometer design and its operating settings. You might be able to find this information in the GC operators manual - or drop an e-mail to the manufacturers.

Peter

I run pesticides OP mixed standard by GC-FID. My signal magnitude showed mV. But in my reference sheet from standard manufacture, they use GC-FID also their signal magnitude unit is pA.
So please kindly let me know, the pA vs mV.
I meant 1 pA is equal how many mV??

Help me,
K.M.P

Ohm Low
http://en.wikipedia.org/wiki/Ohm's_law

If you know amplifair/ attenuator design- you may calculate mV to pA. But you don"t know electonic construction . Who know this - don't asked similar question.
Not because he knows how to calculate, but because he knows that in this case, the scale does not play a role. Chromatogram does not describe an electric current, but describe the mixture => through the ratio of the peak area on chart.
Therefore, he is not interested "units" but interested the chromatogram graphics. He must see the RRT, the ratio of peak areas and signal to noise ratio on peak.

DSP007: Ohm's law simply provides a means to calculate the voltage drop across a resistor for a given current through it. Since an electrometer is not a resistor Ohm's law does not apply here.

As far as I can make out KMP needs to compare his GC signal in pA with a specification sheet for a pesticide standard that gives signal values in mV. What this has to do with "the RRT, the ratio of peak areas and signal to noise ratio on peak" is beyond me.

Peter

This is due to the so-
Units which expressed the scale does not play any practical role.
KMP must find to the original chromatogram, for which validated his paper . And on the chromatogram to calculate the actual chromatographic characteristics .

How to obtain this chromatogram is the question more interesting.
Perhaps it was preserved in the archives of the developer. He may be lucky and someone with a similar device to make him accept the standard analysis.
In the end, he can spit and revalidate method for his device, not their brains about the dimension.

Regarding whether "calculate" - theoretically knowing the amplifier circuitry can calculate all with sufficient accuracy. Including through Ohm's law, if the divisor is represented by a resistance cassette.
Once this has no practical meaning of the magnitude of the electronic signal "as an abstract unit" has no practical meaning.

Older versions of HP (Agilent) GCs used microVolt as the Y axis parameter. More recent versions use pA as the parameter of scale for the Y axis.

I believe if you divide the 'counts' or mV by 2048 you will approximate the output in pA.

4100 counts or microV equals approximately a 2 pA signal.

I hope this is useful as well as accurate. My memory is not as good as it used to be.

best wishes,

Rod

KMP,

The use of mV is, in my opinion, a hold over from the bad old days when everything was reported using the 0-1 V or 0-1mV scales for external plotters and, God forbid, strip chart recorders. I would try Rod's calculation (never knew that Rod, thank you) but I would also suggest a simple experiment if you have a 6890 or a 5890. Sit down and watch a peak come off and look at the maximum pA reported under Sig1 and then compare it to your peak height in your chromatogram in mV. Crude but effective.

There is a very good table in the detector chapter of "Process Gas Chromatography" by Annino and Villalobos that explains the different signals from different detectors and their noise levels and their MDQ's.

Just out of curiosity, how much luck are you having with OP pest since FID is a non-selective detector? (Better done with FPD on NPD but we use the tools we have...)

Best regards,

AICMM

Thanks to all.

Regards,
K.M.P