I am attempting to quantify total arsenic content in my L. sativum (garden cress) digests via hydride generation MP-AES (HG-MP-AES), however, for all my samples I receive negative intensity values. I am almost certain that my samples do have arsenic in them as they have been exposed to arsenic for 120 h, prior to digestion in HCl. Furthermore, a previous non-HG MP-AES analysis yielded positive intensities. What could be a reason for this?
- Standards used: 0, 5, 10, 15, 20 and 25 ppm.
- Matrix: 1% HCl 1% KI
- Reductant: 1.5% NaBH4 in 0.2% NaOH
P.S. I added selenium internal standard (10 ppm) to everything and the instrument was unable to determine its concentration.
Paul Milham They all yielded positive intensities, from 852.86 (blank) to 35605.01 (25 ppm). Calibration 'curve' was not linear, rather an obvious curve from 20 ppm upwards. Polynomial R = 0.9991, linear R = 0.879.
If standards yielded positive results, it seems that the instrument works properly.
Try a different procedure, e.g. ISO 17239:2004 specifies a hydride generation atomic absorption spectrometric method for the determination of the arsenic content of fruits, vegetables and derived products.
You said that your standards didn't give a linear trend and to you it seems obvious.
To me seems odd because the nature of the signal comes from the emission at your standards wavelenght and should be linear.
Why do you think it's different on your case ?
Have a look at this paper in which they used a single element standar for As
http://hpst.cz/sites/default/files/attachments/5990-8914en-appnote-4100mp-aes-soils.pdf
it is better to upload the working condition of the machine. Is vacuum pump working properly? please check it. Lab temp? you want to determine total conc. or speciation of different species?
i have faced the same problem but later i found that the vacuum pump is not in proper condition. in which mode do you want to measure? STD mode or KED mode.
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