Disput between PDA & U.V. detector

[AMIT KUMAR JAIN]

Dear Sir

I think there i no problem with your sample preparation or method. Only problem is there PDA V/S UV detector. Practically if u see, UV detector are more sensetive compare to PDA as in PDA light passes through the diodes. Intensity of the light is less for PDA compare to UV detector.

Could u tell me what wavelength range you are using for PDA. If u r using wider range i.e. 200-400nm, mke it narrow by having only +/- 10nm,u will find difference in responses.

Regards

AKJ

[AMIT KUMAR JAIN]

Dear Sir

I think ur problem is related to detector only. As far as I know PDA detectors have less response compare to UV detector. In PDA detector light passes through several diodes, so intensity of the light is less compare to UV detectors.

Could u share what wavelength range u r using i.e. 200-400nm. If u r using wider range pl. make it narrow i.e. +/- 10 nm of working wavelength.

Regards

Amit Jain

[HW Mueller]

There is the inclination here to kindly suggest to you, Amit Jain, to read this chain again, especially the contributions of DR, JM, and me. This will bring you nearer a solution of your problem voiced in another chain.

Now, organic chemists used to be warned not to trust a chromatogrphic results unless it is veryfied by a different chromatographic or other technique. The other way around, this means that if the chromatography is done correctly different methods will jive (and have to, otherwise it would be pointless to pursue analytical chemistry).

[Alex Buske]

Hallo,

Just a few remarks:
-PDA are not scanning.
-usually you can compare VWDs and PDAs.
-PDAs can have the same sensitivity as VWDs
-light doesnt pass through the diodes
-to the original poster: knowing the wavelength and the reference compound is essential to answer the question.

While PDAs and VWDs are usually comparable, we have noticed in the past differences under certain circumstances:
-Using low wavelengths (i.e.210nm for MeOH or lower for ACN)
-references and impurity have sligtly different UV-spectra,
-at least one of the compounds (reference or impurity) has a slope at the selected wavelength.
Then differences can occurr, even if linearity and sensitivity for bothe compounds are established.

Alex

[Mark Tracy]

"Sensitivity" is simply the slope of the calibration line. This can be influenced by flow cell path length, spectral bandwidth and spectral calibration, and in the case of PDA, the choice of reference wavelength. There is no a priori reason for an PDA to be inferior to a VWD, and in general there is not much difference.

Detection limit is determined by signal/noise ratio, and here there may be greater differences. The diode array usually has slightly greater electronic noise than the PMT or single diode in a VWD. In practice, the photon detector's intrinsic noise is usually not the dominant source of noise, rather it is one of many sources including the mobile phase, pumps, lamp, sample matrix, etc.

In the case at hand, may I suggest a careful look at the calibration lines and their intercepts? A very small offset in the calibration can have a major impact on the quantification of small peaks.

[wanda50]

I have done many validations with my PDA vs a VWD for ruggedness, and they have never been out of the limit due to the detector differences. One place for a source of error is the data collection system and the vagaries of human interaction when integrating peaks. With some methods we have had to write a protocol for how to integrate specific peaks.

should your spec be NMT or equal to 0.2%? Do you have a rounding SOP if the spec is <0.2%

[AICMM]

[quote="Mark Tracy"]"There is no a priori reason for an PDA to be inferior to a VWD, and in general there is not much difference.

Detection limit is determined by signal/noise ratio, and here there may be greater differences. The diode array usually has slightly greater electronic noise than the PMT or single diode in a VWD.

Okay, so you are talking to a GC guy and not an LC guy but I am very interested in the subject of single wavelength versus multi-wavelength so I am enjoying this thread. I would think a VWD would beat a PDA in sensitivity all the time. My reasoning is this, the VWD has (probably) got a broader bandwidth and certainly a larger diode or (even more sensitive) a PMT. This should easily beat the diodes on a PDA unless the band width on the PDA is very broad, correct? Also, don't you normally subtract some other signal in a PDA (thus the reason to look at multiple signals to begin with) in order to correct for other vagaries such as lamp or pump or whatever.

Very interested in the reply since I am working on just such a project with bandpass filter and diode and I am thinking about a PDA spectrometer and they are telling me it may not be sensitivie enough.

Best regards.

[Mark Tracy]

Keep in mind that "sensitivity" is the derivative of the curve of output versus input. When speaking casually, we often mix up the terms sensitivity with detectability. Detection limits are determined by signal to noise. All other factors being equal, the PDA has the same "sensitivity" as a VWD, but it may have greater noise, hence the detection limit is higher for the PDA.

The noise contributed by the physics of the photon transducer is usually not the dominant source of noise in a real analysis. For the reasons cited, the PDA does have greater transducer noise than the VWD. However, the design of the flow cell, thermal gradients, refractive index effects, lamp quality, bandwidth settings, use (or non-use) of background correction, stray light, pump noise, and sample matrix interferences all contribute noise. Some of these you can influence, some are designed in. Of course, there are well-designed PDAs and poorly-designed VWDs, so no general rule can be made.

Also, we never really see the "sensitivity" of the photon transducer. Internal signal processing converts the signal from the photodiode to absorbance units and that is the value reported to the data system. We don't see the raw photocurrent data. In this context, the "sensitivity" of the PDA is lower than the VWD, but its hidden.

In practice, the instrumental detection limit using a PDA is roughly twice the IDL of a VWD under best-case conditions.

In the original question, there was a small discrepancy in the calibrated results. There could be many reasons for that, but blaming PDAs in general is not warranted.

[Mark Tracy]

To address a couple other points, few VWD designs allow for adjustable bandwidth; it is a mechanical adjustment and expensive to implement. Typically, the BW is about 2-10 nm. The PDA can adjust the bandwidth cheaply in software to just about anything; setting the bandwidth to the match the width of the spectral peak can do wonders for the noise.

Background subtraction is optional with the PDA. It can be beneficial for reducing refractive index noise or temperature drift. It can really hurt noise if the reference wavelength/bandwidth is in a low-light part of the lamp's output spectrum. If RI noise is a problem for your VWD, there is no trick like background subtraction to help you.

Despite my defense of the PDA, my bread-and-butter detector is a VWD. Why? It is half the price, easier to use and does what I want about 80% of the time.