Clarification needed on reference spectra Foss NIR Rapid Liquid Analyser

dangdk's picture


I would like to ask a question on the reference spectra I have obtained using the FOSS Rapid Liquid Analyser. The reference spectra were obtained by running scans with only the spacer present in the instrument and no vials or cuvettes. However, the absorbance values are very high (between 0.5000 and 0.6000 approximately for the entire spectrum). In addition, another reference was run after the removal of the spacer as well, giving absorbance values more in the range I was expecting (0.1500 approximately). Does anyone know what might cause such high absorbance results for the scan of just the empty spacer? Thank you very much for your time. 

dwhopkins's picture

Hi dangdk,

I'm not familiar with the particular instrument you have, but I do have a Foss 6500 with transmission measurements of cuvettes.  I assume that there is no glass in the optical path.  Is the spacer tight fitting, so that it cannot twist and block some more light than was measured in the reference scan?  It should be blocking the beam in a reproducible way, so that light cannot reach the detector through the sides of the vials without going through the samples.  I think the best way to measure your reference scans is with the empty vial or cuvette, and the scans should be very close to zero.

Best wishes,



td's picture

Hi  Dangdk,

I am in the same state of ignorance as Dave. I am not familiar with the optics but I suspect it would require a cell to be in place. How does FOSS recommend the instrument is used?

Do you have access to CCl4 ? It does not absorb NIR nad so might be a usefull blank but it is toxic so you need to have the instument in  a fume cupboard. If no CCl4 then you could try using wine and see if your spectrum is similar to published spectra. What liquids are you interested in measuring?

Best wishes,


craigwilk's picture

Are you talking about the reference or the blank?  The reference for the RLA is air and it's measured with the sample draw open, so is independant of the sample presentation.  A blank is taken with the spacer (cuvette holder) inserted and no cuvette present.

hlmark's picture

Dangdk - CCl4 is definitely considered toxic and its use should be avoided whenever possible. However, there are very few other materials that do not have absorptoin bands in the NIR spectral region. There are several freons that contain no hydrogen, but most of them are too low-boiling to be of practical use for spectroscopic applications. Two of them that you might consider, however, are:

Freon-11L CCl3F: B.P. = 75 deg C

Freon-113: CFCl2CF2Cl: B.P. = 118 deg C


Theoretically they sbould have no absorbances, but I've never seen any spectral data about them. If you can find out, and especially if you can measure their spectra, it would be very useful to know about, and I'd apprecuate your letting us know, here on the forum.

And I think Tony would, too.







hlmark's picture

Correction - that should say Freon 11