LC-(DMS)-MS/MS Analysis of Emerging Food Contaminants. Quantitation and Identification of Maleic Acid in Starch-Rich Foods  

Recent findings (in May 2013) of maleic acid in foods, such as tapioca starch, tapioca balls, rice noodles, and hotpot ingredients, caused the recall of many starch-based food products in Asia [1-3].
Maleic acid is usually not used in manufacturing of food products, and it is an unapproved food additive.
Occasional consumption of maleic acid at low levels does not pose any significant health risk; however, long-term consumption of high levels of the compound can cause kidney damage.
The substance has been traced to a modified starch containing maleic anhydride, a chemical used in the production of food packing materials.
Reliable analytical methods are needed to detect maleic acid in foods to identify potential trace contamination in food production, processing, and packaging and to ensure consumer health.
Maleic acid is cis-butenedioic acid (Figure 1) and is closely related to fumaric acid (trans-butenedioic acid). The LC-MS/MSbased method presented here can be used to confidently identify and accurately quantify maleic acid even in presence of fumaric acid.

Experimental
Sample Preparation
Simple liquid extraction of food samples was performed using the following procedure developed by the Taiwan FDA [4]:
 • Weigh 1g of homogenised samples into polypropylene centrifuge tubes (50mL)
 • Add 25mL of 50% methanol
 • Shake vigorously for 30 min using a shaker
 • Add 20mL of 0.5 N KOH
 • Vortex and let stand for two hours
 • Add 3mL of 5 N HCl and bring to a final volume of 50mL with deionised water
 • Vortex and centrifuge
 • Transfer an aliquot of 100μL of the extract into an autosampler vial and dilute with 900μL of water resulting in a total dilution factor of 500
Further dilution of the extract might be necessary if the sample is heavily contaminated
LC
Maleic acid and fumaric acid were analysed using an Agilent 1260 system with a gradient on a Poroshell EC C18 column (150 x 3.0mm, 2.7μm) and a mobile phase of water containing 0.1% formic acid (A) and methanol containing 0.1% formic acid (B). The flow rate was set to 0.3mL/min. Gradient details are listed in Table 1. A sample volume of 10μL was injected.
Figure 2. LC-MS/MS analysis of maleic acid and fumaric acid
Table 1. LC gradient used for the separation of maleic acid and fumaric acid

MS/MS
The AB Sciex QTRAP® 5500 was used with the Turbo V™ source and an Electrospray Ionisation (ESI) probe. The mass spectrometer was operated in Multiple Reaction Monitoring (MRM) mode using negative polarity. Two selective MRM transitions were monitored using the ratio of quantifier and qualifier ion for identification (Table 2).
In addition, SelexION™ differential mobility separation was investigated to increase selectivity, improve Signal-to-Noise (S/N), and increase confidence in identification.
LC-MS/MS data were processed using the MultiQuant™ software version 2.1.

Table 2. MRM transitions and retention times of maleic acid and fumaric acid
Results and Discussion
An example chromatogram of the detection of maleic acid and fumaric acid is shown in Figure 2.
First, the limit of quantitation (LOQ), linearity, and repeatability were evaluated using injections of maleic and fumaric acid standards ranging from 0.5 to 200 ng/mL and spiked matrix samples.
Both compounds had LOQ values in the sub ng/mL range, allowing a sample extract dilution to minimise possible matrix effects. Linearity was excellent with a regression coefficient of 0.999 for quantifier and qualifier transitions. The accuracy values ranged from 89.6 to 107.6% across the linear dynamic range (Figure 3).
Figure 3. Chromatograms of the quantifier and qualifier transition of maleic acid of the blank sample and at concentration of 0.5, 1.0, and 2.0 ng/mL (top) and calibration lines from 0.5 to 200 ng/mL (bottom)
Repeatability was evaluated using 7 injections at 5 ng/mL. The coefficient of variation (%CV) was 2.9% for the quantifier transition (115/71) and 1.8% for the qualifier transition (115/32).
A number of food samples were analysed for maleic and fumaric acids, including noodles, tapioca starch, and processed foods. The analysis of a 20 ppb spiked blank extract gave 91.9% recovery.
Table 3. Maleic acid findings in different food samples
Table 3 and Figure 4 show quantitative and qualitative results. MRM ratios were calculated using the ‘Multicomponent’ query in MultiQuant™ software.
Figure 4. Results for maleic acid in different food samples, the ‘Multicomponent’ query in MultiQuant™ software was used to identify target analytes based on their MRM ratio
In a last experiment we investigated the use of SelexION™ differential mobility separation (DMS) to increase selectivity and confidence in identification.
SelexION™ uses a planar differential mobility device that attaches between the curtain plate and orifice plate of the QTRAP® 5500 system (Figure 4). An asymmetric waveform, called Separation Voltage (SV), combined with a Compensation Voltage (CoV) is used to separate ions based on difference in their mobility [5-6].
Chemical modifiers, like isopropanol, methanol, or acetonitrile, can be introduced into the transport gas via the curtain gas to alter the separation characteristics of analytes.
Figure 5. SelexION™ differential mobility separation (DMS)
SV and CoV were optimised for maleic and fumaric acids to separate these two isomers with identical MRM transitions. Best separation and highest selectivity was achieved using an SV of 3600 V and CoV of -8.0 V and -10.5 V, respectively (Figure 6). The added selectivity resulted in reduced background interferences. The presence of an MRM signal in combination with an optimised CoV value can also be utilised as an additional ‘identification point’ to increase confidence in data quality.
Figure 6. Compensation voltage (CoV) ramps for maleic and fumaric acid, best separation and highest selectivity was achieved using CoV of -8.0 V and -10.5V, respectively
Figure 7. Selective detection of maleic acid and fumaric acid using LCDMS-MS/MS, the added selectivity resulted in lower background noise and interferences and increased confidence in identification
Summary
The method and data presented here showcase the fast, easy, and accurate solutions for the analysis of maleic acid and fumaric acid in starch-rich foods by LC-MS/MS and LC-DMSMS/MS. The AB Sciex QTRAP® 5500 systems provide excellent sensitivity and repeatability for this analysis, with minimal sample preparation allowing maximised throughput for the analysis of many samples in a short time period.
Maleic acid was quantified in different food samples. MRM ratio calculations in MultiQuant™ software used for compound identification. SelexION™ differential mobility separation was also used successfully to further increase selectivity and to clearly differentiate between isomeric species adding another ‘identification point’ and increased confidence to the results.
References
1 http://www.fda.gov.tw/EN/newsContent.aspx?ID=9918&chk=454d1df8-1f26-43a3-9a85-5540dc07caae
2 http://www.ava.gov.sg/NR/rdonlyres/9253E7B2-E57D-4992-982C-1304E73748D6/26074/Pressrelease_RecallofstarchbasedproductsfromTaiwan.pdf
3 http://www.fda.gov.ph/advisories/food/76474-fda-advisoryon-maleic-acid
4 http://www.fda.gov.tw/TC/siteList.aspx?sid=3503
5 B.B. Schneider, T. R. Covey, S.L. Coy, E.V. Krylov, E.G. Nazarov: Int. J. Mass Spectrom. 298 (2010) 45-54
6 B.B. Schneider, T. R. Covey, S.L. Coy, E.V. Krylov, E.G.Nazarov: Anal.Chem. 82 (2010) 1867-1880