Multidimensional wrote:
This is a training issue, not an instrument problem. As already noted, using an HPLC method at UV = 198 nm (198 nm has little value as no selectivity can be obtained at such a low UV wavelength where everything absorbs) coupled to a mobile phase containing acetic acid (absorbs strongly at low UV wavelengths) is by itself, invalid and does not confirm to good chromatography practices. The idea that the use of the HP 1050 system may be to blame for any instability or problems is irrational as the instrument is not the issue. Setting up and using the system with a wavelength of 198nm is. Additionally, you used the word "Reference Wavelength" in your post. *I hope that you are not turning on the MWD's "[b]Reference Wavelength" feature as that will subtract all data collected at the "Reference"value from your original signal (destroys the original data forever!).[/b] Never turn on the "Reference Wavelength" and please get some formal training before using any of the advanced features.
To develop high quality HPLC methods using the instruments you have available, ask your employer to higher a professional chromatographer. Please contact someone with demonstrated industrial experience in developing HPLC methods, on the systems you use, so a valid method can be developed that is selective for your analytes of interest and follows good chromatography guidelines.
Thanks for the great info!
I did not use the term 'reference wavelength', I wrote 'reference signal'; I used an MP signal at 258nm which yields a nice, flat, zero signal as a comparison to the bouncing 198nm signal.
I am 'the employer' in this case. These samples are run on one of my LC systems at home as I do not have an HPLC in my day-job lab; working on that. Current methods are UV/Vis; one method based on a ratio of standard to sample, the other utilizes a 'fudge factor' to account for inaccuracies in the ratio method. Neither are remotely accurate.
Hiring a professional chromatographer is a fine suggestion, but would be a significant waste of salary, and a waste of their developed skillset. Our analyte is a singular organic additive which is measured on a frequent basis. Same method, day in, day out. Additives are replaced with a new product very infrequently due to the broad scope of environmental testing required to validate and approve the 'new' process.
The 198nm is specified by the mfg as the WOI, I have no control over that. The alternate 269nm wavelength provides very nice chromatograms, but calculated concentrations are 15-20% lower than reported at 198nm. I'll run both wavelengths and track the concentration deltas, if consistent enough I can then run at 269nm and apply a calculated factor for the final concentration, either manually or built into the method parameters.