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ANIMAL NUTRITION |
Department of Animal Science, University of Wyoming, Laramie 82071-3684
Abstract
A procedure was developed for the rapid analysis of titanium dioxide (TiO2) concentrations in feed and fecal samples. Samples were digested in concentrated H2SO4 for 2 h, followed by addition of 30% H2O2, and absorbance was measured at 410 nm. Standards were prepared by spiking blanks with increasing amounts of TiO2, resulting in a linear standard curve. Complete analysis using this procedure can typically be accomplished within 4.5 h. This procedure was compared to a previously published dry-ash procedure for the analysis of TiO2 in bovine fecal samples. Three sources of OM devoid of TiO2 (a forage sample, a bovine fecal sample without Cr2O3, and a bovine fecal sample containing Cr2O3) were spiked with graded amounts (0, 2, 4, 6, 8, or 10 mg) of TiO2. With our procedure, TiO2 recoveries averaged 96.7, 97.5, and 98.5%, for the three OM sources, respectively, vs. 74.3, 83.8, and 53.1% for the same samples analyzed using the dry-ash method. These results suggest that our procedure is a rapid and accurate alternative to dry-ash procedures for the determination of TiO2.
Key Words: Analytical Procedure • Digestibility • Markers • Titanium Dioxide
Introduction
The use of titanium dioxide has been explored in recent years as an alternative to more commonly used digestibility markers, such as chromic oxide. Titanium dioxide offers advantages over Cr2O3 in that it can be legally added to an animal’s diet (Titgemeyer et al., 2001
), and its use avoids concerns regarding potential carcinogenic properties of Cr2O3 (Peddie et al., 1982). Titanium dioxide has been reported to be a viable marker for total-tract digestibility in studies with rats (Krawielitzki et al., 1987), pigs (Jagger et al., 1992), chickens (Short et al., 1996), dairy cows (Hafez et al., 1998), and beef steers (Titgemeyer et al., 2001).
The use of TiO2 as a digesta flow marker necessitates a rapid, accurate analytical procedure for quantitative analysis. Upon reaction with H2O2, TiO2 produces an intense orange color (Muhlebach et al., 1970), and researchers have exploited this observation to quantify TiO2 concentration. Njaa (1961) analyzed TiO2 in rat feces using wet-ash digestion (Kjeldahl digest) of sample followed by addition of H2O2. Short et al. (1996) outlined a procedure for analyzing TiO2 in chicken excreta based on modifications of the procedure of Leone (1973) for determination of TiO2 in cheese. The Short et al. (1996) procedure was later modified by Titgemeyer et al. (2001) for analysis of TiO2 in bovine fecal samples. However, the procedure of Short et al. (1996), with or without the modifications of Titgemeyer et al. (2001), failed to produce consistent and accurate results in our laboratory. Therefore, our objective was to develop a rapid and accurate analytical procedure for preparation and determination of TiO2 concentrations in fecal samples for use as a digestibility marker for ruminants.
Materials and Methods
Procedure Development
An outline of our analytical procedure is presented in Table 1. Briefly, the procedure includes wet-ash digestion of sample (Njaa, 1961), followed by addition of H2O2 as described by Titgemeyer et al. (2001) to produce an orange/yellow color that was subsequently read at 410 nm using a UV/Vis Spectrophotometer (DU 640, Beckman Instruments, Inc., Fullerton, CA). A standard curve was prepared by adding 0, 2, 4, 6, 8, and 10 mg of TiO2 to tubes without OM, and analyzed as described for samples. The standard containing 0 mg of TiO2 is used to zero the instrument. We observed that the background readings (sample blanks) were slightly greater for fecal samples than for other sources of OM, such as forage samples. Therefore, we suggest that the same type of OM (devoid of any known TiO2) as that being analyzed be used for background correction. Alternatively, standards could be prepared in the same matrix as samples rather than including sample blanks for background correction, as long as independent standards for each type of sample being analyzed are prepared. However, including sample blanks as described herein will further adjust for any potential variation in background between assays. Samples are analyzed gravimetrically (100 g) as opposed to a volume basis because gravimetric comparisons in our laboratory (data not shown) produced more accurate and repeatable results than volumetric measurement. Although any variation in residual ash content of samples (or ammonium sulfate from digestion of protein) will contribute to the final weight, this effect should be negligible and less than the variation observed in samples analyzed volumetrically. Figure 1 depicts a typical standard curve established using this procedure. Linearity was maintained up to 10 mg TiO2/100 g, the maximum concentration we studied. Higher levels of TiO2 were not tested because the 0 to 10 mg/100 g range included the expected range of TiO2 in fecal samples used in the validation experiment. Mean absorbance for the 10 mg/100 g standard was 1.036.
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as modified by Titgemeyer et al. (2001). For analysis, the dependent variable (analyzed concentration) was regressed against the independent variable (TiO2 added) using the REG procedure of SAS (SAS Inst. Inc., Cary, NC). For each TiO2 procedure, the six tubes containing 0 mg of TiO2 per tube were used for background correction, and thus only 30 samples for each OM to which TiO2 was added were included in the regression. The slopes of the three regression lines of both procedures were then tested for difference from 1.0 using the TEST statement within PROC REG. Recoveries of TiO2 (% of TiO2 added) from tubes with added TiO2 were analyzed as a completely randomized design with a factorial arrangement of treatments using the GLM procedure of SAS (SAS Inst. Inc.). The model included the effects of analytical procedure, source of OM, and the procedure x source of OM interaction; least squares means were separated using Fisher’s protected LSD. Results and Discussion
Procedure Development
Digested samples had a blue-green color immediately following removal from heat that faded upon cooling to room temperature. This color presumably reflects mineral (CuSO4) of the catalyst used in the digestion process. Upon addition of 30% H2O2, reagent blanks remained blue, but spiked samples ranged from a medium intensity green for lesser amounts of TiO2 to an intense orange color with a green hue for higher amounts of TiO2. A substance is said to have an absorption spectrum in the region in which it is absorbing light (Robyt and White, 1990). In performing a wavelength scan using a 10 mg/100 g standard, we were able to determine a maximum absorbance at 406 nm, which is slightly lower than the 410 nm wavelength specified in the dry-ash procedure. However, the average difference in absorbance when the same standard was read at 406 vs. 410 nm was 0.0017, which is within the normal analytical variation associated with our spectrophotometer. Accordingly, a wavelength of 410 nm was used for our procedure in an effort to maintain consistency with the dry-ash procedure. The presence of green in the solution should not interfere because orange and green absorb light at opposite ends of the visible spectrum. Moreover, any interference should be obviated by preparing the standards in the same manner as the samples. Mixing of TiO2 and H2O2 most likely forms H4TiO5 (pertitanic acid; Snell and Snell, 1949). However, Vogel et al. (1989) suggested that the TiO2(SO4)2-ion was formed, and both compounds may be present with the initial reaction forming H4TiO5 that dissociates into the TiO2(SO4)2-ion, forming the orange color. Muhlebach et al. (1970) reported that the color of the TiO2/H2O2 reaction is orange below pH 1, turns yellow around pH 3, and gradually dissipates as pH rises into the alkaline region. The pH of samples analyzed using our procedure ranged from 0.30 to 0.44. We found that the color remained stable for at least 9 wk; this agrees with Snell and Snell (1949), who reported that the color is stable for 2 yr. This increases flexibility in laboratory protocol, as samples can be read immediately or stored for several weeks.
Procedure Validation
Results of our validation experiment are shown in Figures 2, 3, and 4. For each source of OM, our procedure closely quantified the amount of TiO2 present. The slopes of the regression lines approaching 1.0 with our procedure confirmed earlier work in our laboratory indicating that this procedure correctly determined the concentration of TiO2 in a sample. This high degree of linearity was not observed when samples were dry ashed. Regression lines had lower slopes when using the dry-ash procedure; this was expected based on previous work in our laboratory in which we were unable to obtain accurate and repeatable measurements of TiO2. However, slopes differed (P 
0.006) from 1.0 for our procedure, but only because the CV associated with our regression lines were quite small (1.3 to 2.4%). The dry-ash procedure gave a slope for the fecal sample without Cr2O3 that did not differ (P = 0.06) from 1.0, but the CV associated with that slope was large (36.0%). Slopes for the other two OM samples analyzed with the dry-ash procedure differed (P 0.008) from 1.0, and similarly had large CV. Likewise, the y-intercepts with our procedure were much closer to the ideal of 0, averaging 0.057 ± 0.049, whereas y-intercepts with the dry-ash procedure were higher and more variable, averaging 0.188 ± 0.902. For our procedure, analytical precision appeared to diminish as the amount of TiO2 increased (8 or 10 mg of TiO2). Averaged across the three sources of OM, the standard deviation across the six observations increased from 0.061 for samples containing 2 mg added TiO2 to 0.188 for those containing 10 mg added TiO2. Although this increase in variation remained within acceptable limits (less than 5% CV), ideal sample concentrations may be below 6 mg/100 g for optimum precision.
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. Our procedure provided acceptable recoveries of TiO2 (average recovery = 97.56%), whereas the dry-ash procedure yielded lower (P < 0.001) and more variable recoveries (average recovery = 70.39%). Furthermore, the source of OM did not influence (P < 0.74) the recovery of TiO2 when using our procedure. However, recoveries using the dry-ash procedure were lower (P < 0.001) for fecal samples containing Cr2O3 than for the other two sources of OM. The lower and more variable recoveries obtained using the dry-ash procedure may be related to our inability to completely solubilize the ashed sample in H2SO4. In the modified procedure described by Titgemeyer et al. (2001), samples were boiled for approximately 1 h over medium heat until completely digested (E. Titgemeyer, Kansas State University, personal communication). However, we experienced difficulty getting the samples to boil, and most often samples would only boil for approximately 20 to 25 min. Increasing the temperature in an effort to enhance boiling proved ineffective and increased evaporative loss. The inadequate boiling and solubilization of minerals may be related to the high altitude (2,200 m above sea level) of our laboratory, hindering the ability of samples to boil as necessary. Thus, incomplete extraction of TiO2 may explain our low recoveries and the large variation between samples.
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reported that Cr interfered with colorimetric analysis of TiO2. However, our procedure gave similar regression lines (Figures 3
and 4) and recovery values (Table 2) from fecal samples with or without Cr2O3 present. This agrees with Snell and Snell (1949), who reported that Cr in amounts up to 1% (which is below typical Cr concentrations in digesta samples from animals dosed with Cr2O3) would not interfere with the assay. Our procedure should permit researchers to use both markers simultaneously to estimate digestibility in research animals.
In addition to an accurate analysis, our procedure provides researchers with rapid analysis for TiO2. The average time to complete analysis is under 4.5 h in our procedure, with 2 h for digestion, 1 h for cooling, and 1 to 1.5 h for processing and reading. The Short et al. (1996) procedure, as modified by Titgemeyer et al. (2001), requires dry ashing for 13 h, a 1-h digestion, and subsequent processing of samples. Our procedure increases flexibility in laboratory practices by not requiring that samples be ashed the night before analysis.
Implications
The procedure outlined herein provides researchers with a rapid analytical method for the analysis of titanium dioxide concentrations in digesta samples. As compared to previous digestion and analysis procedures, this method facilitates further exploration of the use of titanium dioxide as an alternative marker for site and extent of digestion studies with ruminants.
Footnotes
1 We gratefully acknowledge the assistance of E. Titgemeyer and his laboratory, at Kansas State University, for providing technical assistance with their analytical procedure, and J. Hess for assistance with manuscript preparation. 
2 Correspondence—phone: 307-766-4213; fax: 307-766-2355; e-mail: ludden{at}uwyo.edu.
Received for publication July 31, 2003. Accepted for publication September 22, 2003.
Literature Cited
Hafez, S., W. Junge, and E. Kalm. 1998. Estimation of digestibility with an indicator method in comparison to the "Hohenheimer-futterwert-test". Arch. Tierernaehr. 38:929–945.
Jagger, S., J. Wiseman, D. J. A. Cole, and J. Craigon. 1992. Evaluation of inert markers for the determination of ileal and faecal apparent digestibility values in the pig. Br. J. Nutr. 68:729–739.[Medline]
Krawielitzki, K., R. Schadereit, E. Borgmann, and B. Evers. 1987. 51Cr2O3 and TiO2 as markers for estimating passage rate and protein digestibility in rats. Arch. Anim. Nutr. 37:1085–1099.
Leone, J. L. 1973. Collaborative study of the quantitative determination of titanium dioxide in cheese. J. AOAC 56:535–537.
Muhlebach, J., K. Muller, and G. Schwarzenbach. 1970. The peroxo complexes of titanium. Inorg. Chem. 11:2381–2390.
Njaa, L. R. 1961. Determination of protein digestibility with titanium dioxide as indicator substance. Acta Agric. Scand. 11:227–241.
Peddie, J., W. A. Dewar, A. B. Gilbert, and D. Waddington. 1982. The use of titanium dioxide for determining apparent digestibility in mature domestic fowls (Gallus domesticus). J. Agric. Sci. 99:233–236.
Robyt, J. F., and B. J. White. 1990. Biochemical Techniques Theory and Practice. Waveland Press, Inc., Prospect Heights, IL.
Short, F. J., P. Gorton, J. Wiseman and K. N. Boorman. 1996. Determination of titanium dioxide added as an inert marker in chicken digestibility studies. Anim. Feed Sci. Tech. 59:215–221.
Snell, F. D. and C. T. Snell. 1949. Titanium dioxide by hydrogen peroxide. Pages 438–441 in Colorimetric Methods of Analysis, 3rd ed. D. Van Norstrand Co., Inc., New York.
Titgemeyer, E. C., C. K. Armendariz, D. J. Bindel, R. H. Greenwood, and C. A. Loest. 2001. Evaluation of titanium dioxide as a digestibility marker in cattle. J. Anim. Sci. 79:1059–1063.
Vogel, A. I. 1989. Vogel’s Textbook of Quantitative Chemical Analysis. 5th ed. G. H. Jeffrey, J. Bassett, J. Mendham, and R. C. Denney, ed. Longman Scientific and Technical, Essex, England.
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) and the dry-ash procedure (
) were y = 0.002(± 0.032) + 0.971(± 0.005)x, and y = 0.290(± 0.848) + 0.685(± 0.128)x, respectively. The r2 values were 0.999 for the new procedure and 0.506 for the dry-ash procedure.
