rare earth elements ( rees ) , referred to as industrial vitamins , are of particular importance and have extensive applications in various fields , especially in material science and industry , mainly due to their special chemical properties and luminous characters based on their 4f shell electrons . recently , along with large amounts of exploitation of rare earth resources and the discard of the rees contained materials , rees accessed inevitably environment , food chain , and then the human body . despite this , the positive or negative effect of rees on the biological body is completely unclear till now .
clearly , a high - sensitive analysis method will promote the researches in this field and give an explicit pattern .
similarly , the potential application fields of rees are also increasing in current society , and the demand of high - sensitivity analysis methods for rees is rising gradually .
for instance , for biological samples , the sensitivity is general at the level of ng / g . in environmental analysis field , for soil , air , lake water , and so forth , the requirement detection level must at least achieve the magnitude of g / l , while for sea water and other high technique rees - containing materials , the according detection level should be the level of ng / l or pg / g . therefore
, developing the sensitive analytical methods for rees is of vital significance and becomes one of the hot points in analytical science .
because the chemical properties of the 17 rees are similar , the key point of rees analysis is the mutual separation of rees , thus the detection of rees is usually accompanied by extraction and preseparation .
the development of the high - sensitive analytical methods also advances these new separation techniques .
currently , numerous analytical techniques have been applied in the detection of rees with the aid of extraction and separation , including atomic absorption / fluorescence spectroscopy ( aas / afs ) , atomic emission spectroscopy ( aes ) , x - ray fluorescence spectroscopy ( xfs ) , neutron activation analysis ( naa ) , inductively coupled plasma optical emission spectrometer ( icp - oes ) , and inductively coupled plasma mass spectrometer ( icp - ms ) [ 9 , 10 ] . among them , icp - ms has become one of the most commonly used techniques because of its multiple advantages , including extremely high sensitivity , high analytical speed , broad dynamics range , and synchronous analysis of various elements .
the sensitivity of icp - ms has reached the level of 0.01  ng / g for most rees and only 10  ng / g for icp - oes under the same conditions . however , restricted by its large and expensive equipment , the sample injection technique , matrix effect , and the difficulty in the direct analysis of solid sample , icp - ms is difficult to be applied in the fast field analysis of rees .
ambient ionization techniques are a revolution in the mass spectroscopy , which enabled the ionization of samples in their native surrounding with minimum sample pretreatment and brought the breakthrough in the application of ms to high - throughput analysis [ 11 , 12 ] . there are more than 10 direct ionization techniques based on the atmospheric plasma that have been invented , including dielectric barrier discharge ionization ( dbdi ) , desorption atmospheric pressure chemical ionization ( dapci ) , desorption corona beam ionization ( dcbi ) , plasma - assisted desorption ionization ( padi ) , flowing atmospheric pressure afterglow ( fapa ) [ 18 , 19 ] , and microplasma discharge ionization ( mdi ) . usually , in these methods , the electronic field employed to produce plasma under atmospheric pressure is high discharge voltage ( usually several kilovolts ) and low frequency ( up to 850  hz ) .
the temperature of plasma is low ( usually several hundred kelvin in excitation temperature ) , so that the plasma is weak and even invisible .
the microwave plasma torch ( mpt ) is a kind of plasma generator at atmospheric pressure by means of the high - frequency ( 2450  mhz ) microwave field , which was developed initially at jilin university , and substantial improvements were made at indiana university .
the mpt can be sustained in a variety of supporting gases , including ar , he , n2 , ne , and air and produce a stable flame - like plasma , similar to icp . based on its multiple merits , for instance , small size , low power consumption , low cost , easy operation , and high excitation efficiency , mpt has broad applications in aes portable spectrometer , supercritical fluid chromatography ( sfc ) , and liquid chromatography ( lc ) as an excitation light source for elemental analysis [ 24 , 25 ] .
owing to the excellent ionization efficiency , mpt has also been employed as the ion source in mass spectrometry mainly for the elements analysis and the detection of halogenated hydrocarbons separated by a capillary gas chromatography .
recently , zhang et al . studied the direct desorption / ionization approach of mpt on a linear ion trap ( ltq ) mass spectrometer to analyze a series of small organic molecules and showed that mpt is a useful alternative ambient ion source .
however , mpt has been rarely used as the detection of metal elements in mass spectrometry .
when the analyte is inorganic molecule , the desorption or ionization efficiency is bare due to the ultralow vapor pressure .
moreover , in the usual direct desorption / ionization approach , plasma crossing sideling the sample surface restricts the efficient transformation of the energy from the plasma to the analytes due to the short interaction length . but
this obstacle can be overcome by the approach of sample injection via the central tube of mpt .
some previous researches   had demonstrated that the mpt - ms is sensitive enough for the demand of field analysis of metal elements in aqueous liquid at the level of 0.021  ng / ml . in this study , an ar - assisted mpt coupled with the ltq mass spectrometer was used for the sensitive detection of trace levels of ree ions in water .
the nebulized ree aqueous solution , without any pretreatment , flowed into the plasma through the central tube of the mpt to increase the interaction time of the plasma and the samples .
for the slat solution of single rare earth elements , including yttrium , lanthanum , cerium , praseodymium , neodymium , samarium , and europium , the generated complex ions in plasma were detected in both positive and negative ion modes and further characterized in collision induced dissociated ( cid ) experiments .
we further examined the semiquantitative capability of this method for the direct detection of rees in different aqueous liquids .
the 2450  mhz microwave generator ( yy1 - 50  w-2450 ) and the coaxial line ( sfcj-50 - 9 ) were purchased from the nanjing electronic technology co. , ltd .
lacl3 , cecl3 , prcl3 , eucl3 , and ndcl3 ( analytical grade ) were purchased from j&k chemical technology co. , ltd .
yttrium , lanthanum , cerium , praseodymium , neodymium , samarium , and europium standard substance ( 10  mg / l in 1.0  mol / l
the water used was deionized water provided by the chemistry facilities in the east china institute of technology ( ecit ) .
we used pure water and rees standard substance to prepare a series of solutions with concentrations from 0  ng / ml to 500  ng / ml for the calibration curve measurements .
for these rees , the maximum concentrations for the calibration curve measurements are different based on the practice signal intensities ; that is , the linear ranges are discriminating .
for instance , for samarium , the maximum concentration is 500  ng / ml , but for yttrium and praseodymium the corresponding values are 100  ng / ml . the practical water sample was gathered in a well in ganzhou , jiangxi province , pr china .
this practical aqueous was directly analyzed with any other pretreatment , except for the necessary dilution with deionized water .
two mass spectrometers were employed in this study , including a commercial linear ion trap mass spectrometer ( thermo fisher scientific , san jose , ca , usa ) and a home - built quadrupole mass filter mass spectrometer .
most of mpt - ms experiments were carried out by the ltq xl mass spectrometer ( see ) , and the quadrupole mass spectrometer is only used in some assisted experiments . to detect the metal ion in solution , a desolvation unit is combined with the mpt ion source and the ltq mass spectrometer . 
 the desolvation unit .
water samples , without any pretreatments , were pneumatically nebulized and generate the aerosols . the aerosols flow by carrier gas through a desolvation system to reduce the loss of the microwave power due to the absorption by water .
the heated tube is 25  cm long and is wrapped with heating tape which is monitored with a thermocouple and a heater controller ( xmt61x , beijing huibang science technology co. ltd . , china ) .
a 500 ml flask was filled with concentrated sulfuric acid ( 98% ) to further remove the h2o molecules from the aerosol . at
last , the dry aerosols were introduced into the plasma through the central tube of the mpt ( see figure 1 ) .  
 the mpt ion source .
the mpt was described previously [ 29 , 30 ] , so only a brief summary of the main features is given here .
the mpt consists of three tubes : ( 1 ) the outer tube , ( 2 ) the intermediate tube , and ( 3 ) the central tube .
the central tube is made of quartz with an outer diameter of 3  mm and inner diameter of 2  mm .
the intermediate tube ( 5.5  mm o.d and 4.5  mm i.d . ) and outer tube ( 26  mm o.d . and 22  mm i.d . ) are made of copper . the supporting gas flows in the intermediate tube . and
the working gas is divided into two routes , one flows directly in the central tube and the other carries the dried salt in the argon aerosol into the central tube .
this structure is called the dual - flow system and is beneficial to optimize the plasma jet shape . two adjustable rotameters ( 1001000  ml / min )
are controlled and optimized the two argon flow rates . a radio - frequency ( rf ) voltage is applied to the mpt .
a 2450-mhz voltage with a maximum power of 100  w is applied to the intermediate tube by the center conductor of a coaxial cable .
a simple tool from a paper clip , a rubber band , and a sheet of a4 paper is employed .
the paper is wrapped around the paper clip and is secured with the rubber band .
about 5  mm of the paper clip extends beyond the clip - paper junction .
the tool was touching the torch between the intermediate tube and the inner tube to create a short circuit between the two tubes , then a cone - shaped plasma jet was generated on the top opening of the mpt .
the horizontal distance between the opening end of mpt and the ion inlet of ltq was about 30  mm . 

the ltq mass spectrometer was set for the rees analysis with mainly parameters including the capillary voltage of (020 )  v and the tube lens voltage of 40  v. the temperature of the heated capillary was 150c .
ions of interest were isolated with a mass - to - charge window width of 0.8 units for collision induced dissociation ( cid ) experiments by applying a collision energy ( ce ) of about 30% ( arbitrary unit defined by the ms manufacturer ) .
in general , mpt possesses high excitation efficiency , because the excitation temperature of the plasma is relatively high , about 8002000  k , depending on the microwave power and the working gas .
but the ionization degree of mpt is only 0.01% , far less than that of icp , 0.2% .
thereby some groups with bright characterization can be as reagent ions and make the mass spectra easy assignment .
these groups include no3 
 , nh2 
 , h2o , and oh , since the no3 
  is electrophilic , while metal ions are general electron donor ; the metal element m appears in the mpt mass spectra in the form of m(no3)n pulsing several h2o molecules or oh groups .
depending on the number n , the result group can be cation or anion , so that the rees can be detected in positive and negative modes . in this study ,
the characteristic mpt mass spectra of some rees , including yttrium , lanthanum , cerium , praseodymium , neodymium , samarium , and europium , had been obtained in both positive and negative modes . 

figure 2(a ) showed a typical mpt mass spectrum collected from an experiment using 10  mg / l la standard solution in a full scan positive mode with background subtraction .
lanthanum has two natural isotopes : la with a natural abundance of 99.91% and la with a natural abundance of 0.89% .
combined with the empirical molecular formula mentioned above , the peak at m / z 317 and 334 can be assigned tentatively to [ la(no3)23h2o ] and [ la(no3)23h2ooh ] , respectively . to further confirm the assignment of m / z 317 ,
figure 2(b ) showed the mpt ms mass spectra , n up to 6 , in which the precursor ion m / z 317 lost successively three h2o groups to generate the peak at m / z 299 , 281 , and 263 in ms , ms , and ms spectra , respectively .
the subprecursor should contain two no3 
  groups ; in further cid experiments , the main fragment was that of m / z 233 by the loss of no in ms spectrum .
the ultimate fragment ion is that of m / z 155 , which was produced from the disaggregation of the ion of m / z 233 by throwing away a group of no4 and would not be dissociated further in cd experiments .
using the lacl3 aqueous solution with the equal concentration , we can get the same results with the final fragment ions of m / z 155 .
the results demonstrated again that the relevant anion groups no3 
  in the mpt mass spectra originated from the background in the plasma but not the sample solution . to further confirm this assignment
, we repeated the same experiment on the quadrupole mass filter mass spectrometer with mpt as the ion source ; the straightforward ions of m / z 155 were gained , as shown in figure 2(c ) .
it is worthy to pay serious attention to the precursor ion of [ la(no3)23h2o ] and [ la(no3)23h2ooh ] in which la element is still kept in its + 3 valence state , same as that in its original solution .
this is a common character for rees in mpt - ms , because of the low atomization efficiency in the plasma produced by mpt .
therefore , mpt - ms may gain the information on the original valence states of metallic ions and have a potential as a supplement of icp - ms .
relative to the almost single isotope of lanthanum , neodymium is a representative multiple isotopic element , which has seven major natural isotopes : nd with a natural abundance of 27.2% , nd with a natural abundance of 12.2% , nd with a natural abundance of 23.8% , nd with a natural abundance of 8.3% , nd with a natural abundance of 17.2% , nd with a natural abundance of 5.7% , and nd with a natural abundance of 5.6% .  
figure 3 is a typical mpt mass spectrum collected from the experiments of 10  mg / l neodymium standard solution . as shown in figure 3(a ) , there were two parallel spectral - shape seven - peak bands with central positions at 320 and 338 .
empirically , these two mass spectral bands can be assigned tentatively to [ nd(no3)23h2o ] and [ nd(no3)24h2o ] , respectively . here again , nd remains in original + 3 valence state .
the inset in figure 3 showed the amplified view of the mass spectral region between m / z 310 and 350 .
the intensity ratio of major seven peaks at m / z 320 , 321 , 322 , 323 , 324 , 326 , and 328 was 1  :  0.365  :  0.667  :  0.246  :  0.465  :  0.106  :  0.097 , which was integral lower than but yet approximately agreed with the neodymium natural isotopic ratio ( 1  :  0.448  :  0.875  :  0.305  :  0.632  :  0.209  :  0.206 ) .
although further studies are necessary , one of the possible reasons is the complicated ionization mechanism for metal ions in mpt plasma .
in fact , in mpt plasma , the phase transition for metal ions from liquid phase to gas phase may play an important role , as well as the electron collision and redox reactions in plasma .
the ms spectra of these precursor ionic peaks exhibited analogous fragmentation patterns , that is , consecutively losing three h2o molecules , no and no4 to produce ( ndo ) , analogous to that of la(no3)2 
 3h2o .
for simplicity , only ms ( n up to 5 ) spectra of m / z 320 , 322 , and 324 were shown in figure 3(b ) , which possesses vivid isotopic characterization . likewise
, similar discussions on the mass spectral band of 338 can give the assignment of [ nd(no3)24h2o ] .  
table 1 summarized the dominated peak band in mpt mass spectra of rees in this study .
rees can form the anion in the mpt plasma when the number of the attached no3 
  exceeds , so that rees can be detected in negative mode .
in fact , the negative mode should be more suitable for the detection of rees since the negative mpt mass spectra are generally noisier than the positive mpt mass spectra . 

figure 4 is the typical mpt mass spectrum of cerium in negative mode collected in the range of m / z 320420 , where a 10  mg / l cerium standard solution was used . clearly , there are two pairs of peak , m / z 342 and 344 as well as m / z 388 and 390 , with intensity ratio of 7.65 and 8 .
cerium has four natural isotopes , ce(0.19% ) , ce(0.25% ) , ce(88.48% ) , and ce(11.07% ) .
ignoring the former two isotopes since their abundance is too small , the abundance ratio of the latter two isotopes is 7.99 , which nearly equals the intensity ratio of the two peaks in latter pair .
based on the fine matching , combined with the empirical molecular formula , the ion of m / z 342 can be assigned to [ ceo(no3)3 ] and the ion of m / z 388 is [ ce(no3)4 ] .
it is easy to see that the structure of [ ce(no3)4 ] is more symmetric so that the signals are more intense and the intensity ratio is much closer to the theoretical values .
analogously , the mpt tandem mass spectrometry in negative mode also provides partial evidences about the structure of the rees anions .
figures 5(a)5(c ) are the ms mass spectra of the precursor ions at m / z 388 , n = 2 , 3 , 4 , respectively .
as shown , the precursor ions of m / z 388 produce first the fragment of m / z 342 by losing a no2 group . in ms ,
subprecursor of m / z 342 can produce the minor fragment of m / z 312 by the loss of no group and the major fragment of m / z 296 by throwing away a second no2 group , and the fragment of m / z 296 yields the peak at m / z 250 by the loss of a third no2 group and yields the peak at m / z 234 by losing a no3 group .
the ions of m / z 234 would dissociate further under current experimental conditions possibly because the signal intensity in ms is low . however , sequential loss of 46-d group ( no2 ) hints that the precursor ion of m / z 388 is polynitrogen complex .
figures 5(d)5(f ) depict the tandem mass spectra of the ions of m / z 390 showing parallel isotopic pattern .
thus combined with the abundant ration , the fact that the ion of m / z 388 was assigned to [ ce(no3)4 ] is reasonable .
the characteristic mpt anionic peaks of seven rees in this study were also summarized in table 1 .
mpt - ltq ms in negative running mode is less noisier and more efficient than that in positive mode , and the quantitative performance of mpt - ltq negative ms is better than that of mpt - ltq positive ms in this study .
therefore , we adopted the negative mode to measure semiquantitatively the concentration of single ree in water to verify the capacity of mpt - ms in the detection of rees .
in addition , to reduce the possibility of a false signal originated from the isotopes of adjacent rees , the major fragments in the mpt - ms mass spectra were proposed as the signal for the qualification of rees in water samples .
the linear relationships between the measured fragment signal intensities and the concentrations of each spiked ree in water were measured with the concentration range of 0 to several hundred ppb ( ng / ml ) , at least two orders of magnitude , with the determination coefficient r 
  larger than 0.99 . to be noticed , for each data point , six measurements were repeated with the relative standard deviation ( rsd ) almost less than 15% .
the lod of this method were estimated at the level of 0.1  ng / ml according to triple noise level at the lowest concentration .
the lod for praseodymium is even as low as 0.04  ng / ml , almost close to the level of icp - ms [ 2 , 10 ]
. the largest values of lod appear in yttrium , the only nonlanthanide rees in this study , arriving at 0.5  ng / ml .
in addition , adding recovery experiments ( for almost all rees in this study , 5  ng / ml solutions were used , but for yttrium 7.5  ng / ml solution was used ) also give the recovery rates of these seven rees in a reasonable interval of 97.6%122.1% .
totally , although these lod values are higher about one magnitude than those obtained in icp - ms , this mpt - ltq - ms is still valuable in particular applications as an alternative supplement of icp - ms for the field analysis of rees in water . as a primary application , according to these calibration curves , the two practical aqueous samples were directly analyzed to obtain the content of these seven rees .
conclusively , a sensitive method based on mpt - ltq ms has been developed to determine individual content of some rees in water at trace levels , including yttrium , lanthanum , cerium , praseodymium , neodymium , samarium , and europium . by means of tandem mass spectrometry and isotopic abundant ratio ,
characteristic mpt mass spectra of these seven rees were identified both in positive and negative modes ; meanwhile the main fragment signals in ms mass spectra were also quantitative measurement .
the results showed that this mpt - ms method has enough sensitivity to determine trace levels of these seven rees in water with the lod values at ~0.1  ng / ml level with reasonable semiquantitative performance .
therefore , this method has potential applications in quality monitoring of single ree in water .
nevertheless , in real applications , this mpt - ms ought to combine with some extraction and separation methods to isolate single ree for accurate analysis .
however , as a low - cost and sensitive method , it is still promising and can be used as a supplement of icp applied in field analysis of single ree in water .
 a sensitive mass spectrometric analysis method based on the microwave plasma technique is developed for the fast detection of trace rare earth elements ( rees ) in aqueous solution . 
 the plasma was produced from a microwave plasma torch ( mpt ) under atmospheric pressure and was used as ambient ion source of a linear ion trap mass spectrometer ( ltq ) . 
 water samples were directly pneumatically nebulized to flow into the plasma through the central tube of mpt . for some rees , 
 the generated composite ions were detected in both positive and negative ion modes and further characterized in tandem mass spectrometry . under the optimized conditions , the limit of detection ( lod ) was at the level 0.1  ng / ml using ms2 procedure in negative mode . 
 a single ree analysis can be completed within 2~3 minutes with the relative standard deviation ranging between 2.4% and 21.2% ( six repeated measurements ) for the 5 experimental runs . moreover , the recovery rates of these rees are between the range of 97.6%122.1% . 
 two real samples have also been analyzed , including well and orange juice . 
 these experimental data demonstrated that this method is a useful tool for the field analysis of rees in water and can be used as an alternative supplement of icp - ms . 
1. Introduction
2. Materials and Methods
3. Results and Discussion
4. Conclusions