We have difficulties in measuring very low ortho-phosphate concentrations. Maybe you know of a method or a laboratory that can help.
The questions we have are: 1) Is there a reproducible, accurate method to measure ortho phosphate values below 10 ppb in tap water, fresh water and/or seawater? 2) What is the lowest phosphate concentration that can be measured in water? We search for a method able to measure 1 ppb and preferably lower values.
The rationale to measure very low phosphate concentrations in water is to be able to evaluate to what extent the phosphate needs to be removed from water for prevention and control of biofouling of membrane systems. Biofouling is a major problem caused by biofilm formation in spiral wound membrane systems used for production of high water quality (from a broad range of water sources, such as e.g. seawater).
US EPA Method 365.3 says it can reach 10 ppb (we have never tested that statement). The method uses a 1-cm path length. If one could use a longer path, that would be possible to get lower detection limits. Where will your samples come from? One problem would be contamination. Phosphorus is everywhere and most systems have 1 ppm in phosphorus. You would have to use glassware dedicated for phosphorus only.
Dear Kenneth, Incremental dosage with low concentrations did not result in satisfactory results. Claimed low detection limits of a laboratory were not supported by the results of control samples.
Dear Y. Jun, dedicated glassware that was thoroughly cleaned was used. The cleaning solution was used as control sample as well.
Thanks for the feedback. Below is some additional information that may be useful for providing suggestions.
Earlier studies showed that low phosphate concentrations in the feed water restricted the biomass accumulation, even at high substrate (organic carbon) concentrations. Vrouwenvelder et al., Water Research 44 (2010) 3454 - 3466
Using radioisotopes we were able to show the phosphate removal efficiency from aqueous solutions down to residual concentrations at the picomolar level by thermostable ferritin enzyme from Pyrococcus furiosus. However, this technique is not suitable to determine phosphate levels in water. Sevcenco et al., Water Research 76 (2015) 181 - 186.
I suggest the following method for estimating micromolar concentrations of P in water.
Subba Rao,A.,Sammi Reddy,K.and Takkar,P.N.1997.Malachite green method compared to ascorbic acid for estimating small amounts of phosphorus in water,0.01 M calcium chloride and Olsen soil extracts. Commun.Soil Sci.Plant Anal.28,589-601.
I suggest the following method for estimating micromolar concentrations of P in water.
Subba Rao,A.,Sammi Reddy,K.and Takkar,P.N.1997.Malachite green method compared to ascorbic acid for estimating small amounts of phosphorus in water,0.01 M calcium chloride and Olsen soil extracts. Commun.Soil Sci.Plant Anal.28,589-601.
1 chang 3-cm path length 2 I am not sure which method you used. Try "malachite green-heteropoly molybdophosphoric acid spectrophotometry"。It was said the lowest detection limits is 1 ppb.
Using LWCC long flow cell technique can lower detection limit to 0.5 nM (ES&T 2002 , 36, 1048-1053)
Hans, we use the "tried and true" Murphy and Riley Method (i.e. molybdate blue ascorbic acid method) on an old Bausch and Lomb spec, not a flow-injection system like a Lachet. This involves manually placing the test tubes into the spec, as this is not automated. Our lowest standard P solution is 1 ppb (20 mm pathlength), although we could probably go lower if we pushed it. The optics in the older specs are superior to the modern instruments. Go on Ebay and pick an old Bausch and Lomb or Milton Roy Spec 20 or 21. They are very inexpensive.
I agree with Chad. The older instruments had better optics which allowed the use of longer path lengths but in the end the limit of detection will be limited to the variability of the blank and that really depends on the cleanliness of the glassware and the quality of the reagents.
I agree with Mr Penn and Mr Alan. The path length 50 mm is better to measure lower concentration, but most important source of contamination is chemicals and source of distilled water.
The maximum permissible concentration of phosphate ions in surface water is 1 mg/l (Dojlido and Best, 1992).
We used a continuous flow analysis system with 200cm liquid waveguide flow cell and we can achieved a detection limit of 0.5 nM in seawater. (see ES&T 2002 , 36, 1048-1053)
Our lowest fosfate-analysis is validated for ultrapure water and drinking water to a level of 1 ug/l P.
We have a bioassay which measures biological available phosphate below 1 ppb (P) which is directly related to the hypothesized application of phospahte limitation to minimize biofouling of membranes.
Dear all, many thanks for the feed back and suggestions. Most likely we will try both the analytical tool (Hilde Prummel) able to measure down to 1 ppb and the bioassay (Talis Juhna and Wim Hijnen) able to measure below 1 ppb. Best regards, Hans
We use Photometer 8000 for chemical analysis and the lowest measurement ranges between 0-4mg/L.
I would be interested to know how folks use the molybdate blue method with high silicon waters. We had issues with false positives because of silicon interference. On good days, with a Bausch and Lomb, we could get down to 10 ppb. Currently, we have a IC, but that only gets down to 100 ppb (50 ppb if we really push it). We also have an OI Analytical autoanalyzer that theoretically should get down to 1 ppb, but again we have had issues with silicon interference. Our water typically has 5-9 ppm Si4+ as measured with ICP-AES.