Pathlenght appropriate for vinegars in XDS-RLA

roireboredo's picture

I am starting a calibration for wine vinegars and I need to buy the appropriate vials. I am thinking in buying quart cells of 1 mm pathlenght and spacer,  but I am not sure whether these would best performing ones. Should I try the 0,5 mm ones?.
Does anyone have experience with vinegars or maybe wine?
Which ones do you suggest?.

Kspanman's picture

I am now working with rice vinegar. I use 0.1 mm depth reflection probe with vial. Still there is over absorption around the combination peak of water.


DSanbornTec5's picture

Hello Kspanman,
Water is the principle constituent in vinegar (probably ~95%), and it's O-H bonds strongly absorb NIR energy at approximately 1930 nm, 1450 nm, 1160 nm, etc. It is the magnitude of these absorbance peaks that will generally "saturate" your signal* and define the maximum effective pathlength (typically, 0.5 mm is used for aqueous samples for this reason; the 1930 nm and 1450 nm peak "saturate" the signal*). If you are utilizing peaks in those 1930 nm or 1450 nm regions, you may be constrained to a 0.5 mm pathlength. However, looking at the absorption profile, water is generally relatively "flat" (i.e., it does not have characterstic absorptions) in certain regions (such as the first overtone alkyl [i.e., CH(x) region in the vicinity of 1700 nm]), where other compounds (like the alkyl groups in acetic acid) have characterstic absorptions useful for analysis. So, you could potentially choose a larger pathlength, and the regions centered around 1930 nm and 1450 nm will be totally unusable, but if you don't need/include them in a calibration or identification method (instead, using a portion of the 1600-1800 nm range), this doesn't really present a problem.
[*] - "Saturate" the signal: typically, aqeuous solutions are measured in transmission (or transflectance), so the measured signal is transmission (which is subsequently transformed logarithmically into absorbance). What is actually occuring at say 1930 nm, is that so much light is absorbed by the O-H bonds in the water the structure of the peak is distorted or flattened; you are essentially just seeing the electronic noise in the detector.

roireboredo's picture

Thanks, I think I will try the 0,5 mm then and see.

dangdk's picture

Where do you get the 0.5mm cells? Are they available in bulk? (so that we don't have to wash and clean between every sample)

roireboredo's picture

No. They are not disposable. It is the quartz cell that I would have to clean by myself for every analysis. My budget only allows to buy one, so...
Cuvette Open Top Pathlength 0,5 mm     Code: 60013911
Spacer for 0,5 mm:    60008312