Immunoassays for Trace Chemical Analysis - American Chemical

Chapter 14

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Field Evaluation of Immunoassays for Aflatoxin Contamination in Agricultural Commodities Douglas L. Park, Henry Njapau, Sam M . Rua, Jr., and Karen V. Jorgensen Department of Nutrition and Food Science, University of Arizona, Tucson, AZ 85721

Four commercially available aflatoxin test kits based on immunochemical technology were used to monitor aflatoxin contamination in whole cottonseed under non-laboratory conditions. The kits, utilizing monoclonal antibodies to measure aflatoxin levels, can be divided into two categories: enzyme-linked immunosorbent assay (ELISA) and affinity columns. Assays were performed at cotton gins, dairy farms, feed mills, ammoniation plants and cotton oil processing plants under varying environmental conditions. Resultsfromthese locations were compared to those obtained in the laboratory using the same kits and thin layer chromatography (TLC). Preliminary results show good agreement between on-site and laboratory kit performance. Agreement between kits was 92% on-site and 83% in the laboratory. These results indicate that the kits are suitable for use under non -laboratory conditions. Aflatoxin stability in aqueous methanol extracts was determined using the immunochemical kits and TLC. Aflatoxin concentration decreased by almost 40% after 4 days. Natural toxicants occurring in human foods and animal feeds present a potential health hazard to man. The significance of the risk due to mycotoxin contamination is dependent on the toxicological properties of the compound (acute, sub-acute or chronic toxicity, reproductive, mutagenic or teratogenic) as well as the extent of human exposure (occurrence, incidence and level of contamination). Aflatoxins, characterized as B B , G„ and G , are potent hepatocarcinogens and toxins. Their metabolites and reaction products, including aflatoxin M,, a metabolite of aflatoxin B, found in milk of dairy cows exposed to aflatoxincontaminated feed, have also demonstrated toxic potentials (1). Prevention of exposure to such toxic agents, which is dependent on rapid screening of food and feedstuff's, is more desirable than undertaking curative measures after toxicity has occurred. Such prophylactic measures are often in the form of surveys or monitoring programs which involve assaying large quantities of samples. On a global scale, surveys and monitoring programs have had limited effectiveness due to various constraints, among them: the need for a laboratory set-up (often only one facility exists in a developing economy), transportation and methodology requiring expensive equipment. Thin layer (TLC) and liquid chromatographic (e.g. high performance liquid chromatography (HPLC)) methods have been developed, validated and are still used in a variety of situations (2.3). These methods are unsuited for rapid screening because they involve expensive and sophisticated equipment, extensive sample cleanup limiting the number lf

2

0097-6156/91/0451-0162$06.00/0 © 1991 American Chemical Society Vanderlaan et al.; Immunoassays or Trace Chemical Analysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Field Evaluation ofImmunoassays for Aflatoxin

of assays that can be performed and the need for a laboratory set-up. A number of immunochemical assays using aflatoxin-specific polyclonal antisera have been developed (4.5). The advantages offered by these assays include high specificity and sensitivity, relatively inexpensive equipment, and the capability to routinely screen large numbers of samples due to a reduction in assay time. The potential of immunochemical methods, particularly the enzyme-linked immunosorbent assay (ELISA) and affinity columns to rapidly assay for low levels of aflatoxin in milk, corn, cottonseed, peanuts and peanut products has been evaluated. In these studies their performance has been comparable to, and often more sensitive than, TLC (6), HPLC © , and radioimmunoassay (8). Commercial production of stable, solid-phasecoupled, aflatoxin antibody kits (Aflatest-10, Vicam Corp.; Afla-20 Cup, International Diagnostics Systems Corp.; Agri-Chek, IDEXX Corp.; and Agri-Screen, Neogen Corp.) has greatly increased the potential of immunoassays as a rapid screening tool. The performance of some of these kits (Afla-20 Cup, Aflatest-10 and Agri-Screen) under laboratory conditions has been demonstrated in inter-laboratory validation studies for corn, peanuts, and cottonseed (9-11). The United States Department of Agriculture (USDA) has adopted the use of these techniques for screening aflatoxin in corn and peanuts (12). Studies evaluating the performance of these immunological based methods under non-laboratory conditions, which would enhance the efficiency of surveillance programs, have not been conducted. This study focused on the applicability of the commercially available immunochemical test kits to adequately screen for aflatoxin in cottonseed under non-laboratory conditions by comparing their performance to laboratory results. On a global scale, TLC and immunochemical methods would have highest potential use. TLC was used to confirm the presence and identity of aflatoxins in the test samples. Materials and Methods Sample Collection and Preparation. Cottonseed samples of unknown aflatoxin contamination levels were collected from cotton gins, ammoniation plants (ammonia is used to decontaminate aflatoxin-contaminated seed), a dairy farm, cottonseed oil processing plants, and feed mills in the areas of Stanfield, Casa Grande and Tucson, Arizona. At each location six to ten three-pound samples of whole cottonseed were collected using a 3-inch vacuum probe (Probe-A-Vac) (Park, D.L. J. Assoc. Off. Anal. Chem.. in press). Approximately 1 Kg of each sample was dehulled in a ultracentrifugal mill (Retsch, Brinkmann, ZM 1, Westbury, NY) with the grinding screen removed, followed by fine grinding in the same mill with a 2.0 mm grinding screen installed such that the ground material passed a No. 18 sieve. The entire ground sample was mixed in a twin shell blender for 20 minutes prior to taking a test portion. The test portion (50g) was extracted with 250 mL methanol:water (80+20) for 1.0 minute at high speed in a Waring blender. The mixture was gravity settled for 15 minutes and filtered through Whatman No. 4 filter paper. This common filtrate was used for all immunochemical and TLC analyses. Aflatoxin Analysis. Immunoassays were performed at the various locations where samples were collected and repeated approximately 24 hours later in the laboratory (Food Toxicology Research Laboratory, Department of Nutrition and Food Science, University of Arizona). The four commercially available (US market) test kits, listed below, were used: 1. Aflatest-10 - Vicam Corp., Somerville, MA 02145. 2. Afla-20 Cup - International Diagnostic Systems Corp. St. Joseph, MI 49085 3. Agri-Chek - Idexx Corp. Portland, ME 04101. 4. Agri-Screen - Neogen Corp. Lansing MI 48912. Each test kit is available with reagents and accessories sufficient for analysis. Upon receipt, the Afla-20 Cup, Agri-Chek and Agri-Screen test materials were kept refrigerated (4° C) until

Vanderlaan et al.; Immunoassays or Trace Chemical Analysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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IMMUNOASSAYS F O R TRACE CHEMICAL ANALYSIS

use. The AflatesMO kit does not require refrigeration. The assays were conducted according to manufacturers' specifications (as summarized in Table I) with the relative concentrations of methanol in the extract adjusted accordingly. Assay readings were classified as negative or positive depending on whether they were below or above 20 ppb, respectively. The AgriScreen test kit used in this study was Version I. Version II, currently available commercially, has improved sensitivity and specificity and is currently under study.


Table I. Comparison of Procedural Steps of Immunochemical Test Kits for Aflatoxin AflatesMO (Affinity Column)

Immunochemical Kit Afla-20 Cup Agri-Screen (ELISA) (ELISA)

Agri-Chek (ELISA)

MeOHiHjO Ratio

70+30

80+20

55+45

70+29"

Extract Amount Used in Analysis

20 mL

100/iL

100 μΐ

100 jiL

Incubation Time (minutes)

NA

1.0

5.0

30.0

Washing

b

2 χ 10 mL HjO

Color Develop ment Time

d

15-30 secs.

1.5 mL reagent*

10 χ 0.5 mL Hfi

1.0 min.

5.0 min.

10 min.

10

10

Total # of Analytical steps Assay Time (minimum) Nature of Test

12 min.

5 min.'

semi quantitative

positive/ negative

f

15 min.

semiquantitative

5 χ 0.5 mL Hp

45 min.

f

semi quantitative

"Methanohwaterrdimethyl formamide (70+29+1) *ΝΑ = not applicable 'Washing reagent supplied with kit Reaction time with dilute bromine developing solution •Simultaneous analysis of up to three samples possible 'Simultaneous analysis of up to 10 samples possible (with multichannel pipette) TLC analysis was performed in the laboratory approximately 24 hours after extraction in the field due to long distances from on-site locations (25-100 miles). The assays were based on adaptation of the method of Thean et al., (13) and AOAC sections 26.031 and 26.083. Fifty (50) mLfromthe common extract was mixed with 50 mL of 0.6M (NH^SOj and 5g of Celite 545, then stirred for 2 minutes and filtered throughflutedfilter paper. The filtrate (lOOmL) was placed in a 250 mL separatory funnel and partitioned twice into 5 mL portions of chloroform. The solution was evaporated to dryness under nitrogen and reconstituted in 0.5 mL chloroform. After dilution with 0.5 mL hexane the material was applied to a 1.0 cm (i.d.) column (4 cm bed, 37-75 /on Porasil A silica gel). The column was washed sequentially with 2 mL hexane and 2 mL ether. Aflatoxin was eluted from the column with 5 mL



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Fidd Evaluation of Immunoassays for Aflatoxin 165

chloroformrethanol (95+5) and evaporated to dryness. The dried extract was dissolved in 200 fiL of benzene:acetonitrile (98+2) and varying amounts of sample and aflatoxin standard were spotted on a pre-coated TLC plate (Kieselgel 60, E. Merck) and developed in chloroform:acetone (9+1). Quantities were estimated visually under long wave UV light (365 nm). Aflatoxin identity was confirmed by trifluoroacetic acid derivatization (AOAC 26.083). In order to determine whether the aflatoxin content of the 80% methanol extracts remained stable over the 24 hour period between extraction and laboratory analysis, 3 extracts from naturally contaminated cottonseed were stored at 4° C for a period of 10 days. The extracts were analyzed on days 0, 1, 2, 3, 4, 5, 6, 7, and 10 by both TLC and immunochemical methods. Reductions in aflatoxin concentration were recorded as a percentage of that obtainedfromday zero (Figure 1). Results and Discussion Results in Table II demonstrate that the performance of all kits was comparable under both conditions. There was an apparent difference in the performance of the individual kits, with Agri-Screen showing the highest variation. This discrepancy may be attributed to the underlying principle of each test kit and the ability of the analyst to perceive changes in color development. The detection phase of the AflatesMO affinity column is based on the measurement of thefluorescenceof a reaction product between aflatoxin and bromine. For this specific test, extract age and site conditions may account for the variation in readings since afluorometerwas used on-site and in the laboratory. The other three immunochemical kits are based on color development. For the Afla-20 Cup, blue coloration implied an aflatoxin concentration below 20 ppb. This distinct end point greatly simplified the reading of the results hence rnmimizing analyst error. The Agri-Chek and Agri-Screen kits also employ color development but in a graded fashion where a small variation in the amount of color produced was dependent on the amount of aflatoxin in the extract. This compromised visual estimation as it was sometimes difficult to determine whether a sample was equal to, less than, or slightly above the concentration of the standard it was being compared to. There were no apparent difference between the individual kits because comparable results were observed onTABLE II. Comparison of On-Site and Laboratory Results of Immunochemical Methods Immunochemical Kit

Number of Analyses

Percent (%) Agreement 1

Afla-20 Cup

72

85

AflatesMO (affinity column)

62

94

60 46

67 70

51 47

96 85

2

Agri-Screen (visual ) (reader ) 3

2

Agri-Chek (visual ) (reader ) 3

1 2 3

Agreement between on-site and laboratory results Analytical result determined visually Analytical result determined with Titertek Multiscan III

site (92%) and in the laboratory (83%). This observation may be attributed to the fact that although antibody specificity of the kits varies, they all have high reactivity with aflatoxin B„ the major contaminant in cottonseed. Antibodies used in these kits cross-react with other



166

IMMUNOASSAYS F O R TRACE CHEMICAL ANALYSIS

Figure 1. Aflatoxin recovery versus time from a methanohwater (80+20) extract of cottonseed meal.


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Field Evaluation ofImmunoassays forAflatoxin

aflatoxins to varying degrees; therefore, inter-kit variation may be different in commodities with higher levels of aflatoxins G„ or G . Ammoniated cottonseed samples were also analyzed in this study. Ammoniation is a process that has successfully reduced (>99%) anatoxin levels in corn, peanut and cottonseed products Q4). Arizona uses this process to treat aflatoxm-contaminated, whole cottonseed and cottonseed meal. This process had no apparent effect on the function of any of the immunochemical test kits. Manufacturer's specifications for the kits included an operating temperature range of 2329° C (room temperature). Temperatures at the testing sites varied from 26° C to 38° C; only the laboratory conformed to the recommended range. As the results in Table II indicate, these differences do not seem to affect kit performance to a great extent, as long as kits requiring refrigeration are kept at low temperature, i.e. in an ice chest, until an hour before assay time. Reports by other workers indicate that portions of immunochemical assay (enzyme-conjugate incubation) could be carried out at 37° C (15.16). An investigation into the effect of prolonged exposure to such high temperatures was outside the scope of this study. However, Koeltzow and Tanner (12) reported that the Afla-20 Cup, AflatesMO and Agri-Screen performed equally well at 18, 24 and 30° C. Compared to TLC values, the kits seem sparingly accurate, i.e., 67% on-site and 68% in the laboratory (Table III). The laboratory performance of the kits used in this study has been evaluated by several investigators. Dorner and Cole (17) and Trucksess et al., (2) reported the satisfactory performance of the Afla-20 Cup test kit in assaying for anatoxin in corn, cottonseed and peanuts. This kit has been adopted as officialfirstaction by the AOAC for corn (^30 ppb) and cottonseed and peanut butter (^20 ppb). Similarly the effectiveness of the affinity column test kit for monitoring total aflatoxin in corn, peanuts and peanut butter, has been validated through a collaborative study, and was found suitable for levels of 10 ppb or more (Trucksess, M.W. J. Assoc. Off. Anal. Chem.. in press). This kit has also been designated interim first action by the AOAC. In other validation studies, Park et al., (10.11) evaluated the performance of the Agri-Screen test kit, which is also AOAC approved, and reported a 35.2 ± 15.9 mean when compared to a TLC value of 36 ppb in corn and peanuts. The difference between our results and those reported by other investigators (preceding discussion) could be attributed to the design of each study. We tested cottonseed of unknown, naturallyincurred contamination levels whereas spiked (known concentration) samples were used in the other studies. In this report inherent errors and variations in readings in both TLC and immunoassays (Table IV) were ignored because TLC was not intended to provide the correct aflatoxin concentrations to which the immunoassays were to be compared. TLC was solely used to confirm the presence and identity of aflatoxin in the samples. The kits are therefore more accurate than is reflected in this report Because of long distances between the laboratory and on-site locations, laboratory analyses had to be performed a day later. We investigated the effect of extract storage on the values obtained in the laboratory. Our results show that aflatoxin content decreased with time, to about 60% after four days, in methanohwater (80+20) when stored at 4° C (Figure 1). Contrasting findings have been reported by other workers. Dorner and Cole (17) reported that aflatoxin extracted into methanolfrompeanuts remained stable for at least three months. On the other hand, Kiemeier and Meshaley (18) reported aflatoxin M, decreased by 11-25% at 5° C after three days and by as much as 80% after six days at 0° C. Although we did not specifically investigate the changes occurring to the aflatoxin molecule, it was interesting to note that the decrease observed using immunochemical methods was less than TLC (Figure 1). We postulate that there may be co-extractants that bind the aflatoxin molecule making it less available for TLC but still recognizable by antibodies.


2

Summary and Conclusions Current emphasis in aflatoxin monitoring programs is on large-scale screening. Using these immunochemical methods on-site offers the advantage of speed and prompt "go/no go" decisions. The kits are versatile and easily transported to testing locations with accessories


168

IMMUNOASSAYS FOR TRACE CHEMICAL ANALYSIS

Table III. Percent (%) Agreement of Immunochemical Results When Compared to TLC

Anatoxin Kit Mean % Concentration Afla-20Cup AflatesMO Agri-Screen Agri-Chek Agreement Range Qig/kg) (per site)

Testing Location


On-site

Laboratory

2

20

78(18)

56(18)

78(18)

59(12)

Mean

70

71

62

65

20

78(18)

50(18)

Mean

68

69

Overall Kit Agreement 1 2 3

1

67

3

72 (39)

3

67 (12)

3

59 (12)

73

61

69

72

62

67

3

64

Modified Thean et aL (1981) and AOAC CB method (26.031). Number of analyses in parentheses. Based on visual estimates only. 1

Table IV. Percent (%) False Readings by Immunochemical or TLC Methods on Analyses Performed in the Laboratory 2

False Positives Afla-20 Cup

8.7

4.3

AflatesMO

0

0

Agri-Screen Visual Reader

0 34.8

8.8 0

Agri-Chek Visual Reader

0 17.4

0 0

TLC

4.3

21.7

3

4

3

4

1 2 3 4

False Negatives

Based on one method (kit or TLC) in disagreement with the rest, Comparisons based on 23 samples. Analytical result determined visually. Analytical result determined using Titertek Multiscan III.


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169

and solvents necessary for analyses easily fitting the trunk of any vehicle. With these immunoassays, the longest technique takes about one hour to run 10 samples (excluding sample preparation and extraction). The same number of samples would require 6-8 hours by TLC. The cost in terms of analyst wages is therefore greatly reduced. The preliminary resultsfromthis study demonstrate that the immunoassay kits can be satisfactorily used in varying non-laboratory environments for semi-quantitative screening of cottonseed for aflatoxins, provided extraction and analysis are done on the same day. For strict enforcement purposes, official quantitative methods should be used to confirm positive samples.


Acknowledgments This project was partially funded by the National Cottonseed Products Association, Office of the State Chemist (Arizona), Arizona Cotton Research and Protection Council, Food and Drug Administration (1-RO1-FD01461-01) and the following commercial organizations in Arizona: Walker's Cottonseed (Stanfield), Arizona Grain Inc. (Casa Grande), Casa Grande Oil Mill (Casa Grande), Producers Cotton Oil Co. (Phoenix), Valley Industries (Peoria), Western Cotton Services (Phoenix) and Wilber-Ellis Co. (Chandler). Literature Cited 1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Lee, L.S., Dunn, J.J., Delucca, A.J. and Ciegler, A. Experientia 1981, 37, 16-17. Park, D.L. and Pohland, A.E. In Foodborne Microorganisms and their Toxins. Developing Methodology, Pearson, M.D. and Stern, N.J. Eds.; Marcel Dekker New York, 1986; 425-438. Sheppard, M.J. In Modern Methods in the Analysis and Structural Elucidation of Mycotoxins. Cole, R.J. Ed.; Academic, New York, 1986; 294-334. Chu, F.S. In Modern Methods in the Analysis and Structural Elucidation of Mycotoxins. Cole, R.J. Ed.; Academic, New York, 1986; 207-239. Ram, B.P., Hart, L.P., Shortwell, O.L. and Pestka, J.J. J. Assoc. OffAnal. Chem. 1986 69, 904-907. Kawamura, O., Kajii, H., Nagayama, S., Ohtani, K. Chiba, J. and Ueno, Y. Toxicon 1989, 27 887-897. Warner, R., Ram, B.P., Hart, P.L. and Pestka, J.J. J. Agric. Food Chem. 1986, 34 714717. Lee, R.C., Wei, R-D. and Chu, F.S. J. Assoc.Off. Anal. Chem. 1989, 72, 345-348. Trucksess, M.W., Stack, M.E., Neshiem, S., Park, D.L. and Pohland, A.E. J. Assoc. Off. Anal. Chem. 1989, 72, 957-964. Park, D.L., Miller, B.M., Hart, P.L., Yang, G., McVey, J., Page, S.W., Pestka, J.J. and Brown, L.S. J. Assoc. Off. Anal. Chem. 1989, 72, 326-332. Park, D.L., Miller, B.M., Nesheim, S., Trucksess, M.W., Vekich, Α., Bidigare, B., McVey, J. and Brown, L.H. J. Assoc. Off. Anal. Chem. 1989, 72, 638-643. Koeltzow, D.E. and Tanner, S.N. Comparative Evaluation of Commercially Available Test Methods. USDA Grain Inspection Service. 1989. Thean, J.J., Lorenz, D.R., Wilson, D.M., Rodgers, K. and Gueldner, R. J. Assoc. Off. Anal. Chem. 1980, 63, 631-634. Park, D.L., Lee, L.S., Price, R.L. and Pohland, A.E. J. Assoc. Off. Anal. Chem. 1988, 7l, 685-703. El-Nakib, O., Pestka, J.J. and Chu, F.S. J. Assoc. Off. Anal. Chem. 1981, 64, 10771082. Sing, P. and Jang, L. Intl. J. Food Microbiol. 1987, 5, 73-80. Dorner J.W. and Cole, R.J. J. Assoc. Off. Anal. Chem. 1989, 72, 962-964. Kiermeier, F. and Meshaley, R. Z. lebensm. unters Forsch. 1977, 164, 183-187.

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Immunoassays for Trace Chemical Analysis - American Chemical

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