am I able to determine Ca in the protein sample with ICP-MS 7800 ( Include Helium collision and High Matrix)?

Question

my sample is expected to contain Calcium at the level of 20ppm. we are trying to determine it with 7800 ICP-MS (includes Helium collision and High Matrix). 
 my questions are following: 
 1. can we reduce FP from 1550 W to 900W, so we can use cold plasma to test 40Ca? 
 2. how do we define "total dissolved solid" to the digested sample? 
 for example, sample weight 0.5g, after digested , then diluted to 50mL. is TDS 1%? 
 we need to evaporate digested liquid sample , then get some solid left, dilute to 50mL. Then we calculate TDS by solid weight divided by 50. then TDS is much less than 1%.

Agilent · Accepted Answer

Hello Di Xu, 
 I think you are getting this low SN because 44Ca has only 2% abundance. For sure using HMI will reduce your sensitivity (less sample being introduced). 
 Let me ask - why you need to work with SN>10? Is this a quality norm? 
 You can change the sample depth to 8.0 mm, that will give some more signal to you. 
 Are you doing the autotune before starting the tests?

Agilent · Answer

Hello Di Xu, 
 You will increase the signal with 8.0 mm because the ion transfer will be more effective, in a general way. On the other hand, if the sample has too much TDS, the cones will get dirt faster. It is a compromise situation when you have to choose between sensibility vs productivity. 
 In general, the introduction of He as the collision gas will reduce sensitivity because there will be collisions of your analyte with the He, hence diminishing its kinetic energy and reducing the ion transfer efficiency. The kinetic energy discriminator (KED) (set as "Energy Discriminator") will make a barrier, in which species with higher kinetic energy (KE) will pass through this barrier and species with lower KE will be blocked. If you set the Energy Discriminator too high, nothing will pass, if it is too low, your analyte and interferences (such as ArHe and SiO) will likely pass through the barrier and go to the MS. Generally we use between 3-5 V in normal conditions. Although there are these drawbacks, KED with He has been proven to be the best choice when working with ICP-MS (H2 and other reactive gases can give you good results, but in a single quadrupole the reaction is not always selective and you might get inaccurate results). 
 Which USP are you following?

Agilent · Answer

Hello Kitty2022 , 
 1) I would not reduce the FP to try 40Ca, keep it to 1500W and use 44Ca, you will have enough signal since Ca ionizes easily in the plasma. 
 2) The total dissolved solid of a sample is normally defined as the residual mass from the evaporation of the solvent (in the case of waters). In your case, 0,5 g of a sample, when digested and diluted to 50 mL, will have a maximum TDS of 1%. I say maximum because you may have compounds that will be mineralized... organic matter, for instance, will be taken to C and H (or simpler derivatives, depending on how aggressive is your digestion). If you digest 0,5 g of NaCl and take to 50 mL, you will have 1% of NaCl in the sample as Na+ and Cl-, along with the counter anions from the acids used. 
 
 Lets say you have 20 mg/Kg of Ca in your sample, and with this sample preparation, your final Ca concentration will be around 0.20 mg/L. That is more than enough to run on the 7800. I would even suggest using 0.50 L/min as the neb gas and 0.50 L/min as dilution gas to further dilute the tertiary aerosol and reduce the Ca signal, plus reducing the interface of the TDS in your sample. Please note you have to apply aerosol dilution in your blank, standards and solution. Using He at 4.3 - 5.0 mL/min will also give good results. Please check if this works with other elements that might be determined along Ca. 
 
 If determining 20 mg/L of Ca is your actual need, you can also use AD at a higher dilution proportion and force the detector to work in analog mode. To do this, you have to go to "options", then "system settings" and tick "force analog mode". In the element table, you can mark Ca to be detected only in analog mode, so you don't overcharge your EM with too much signal. 
 
 Or maybe just dilute the sample to a lower concentration - bare in mind that successively dilutions will increase your method's uncertainty. HMI (AD) helps in this way since it is an online process and the dilution error is minimized. 
 Hope this helps! 
 Rodolfo

	44Ca[He]
	CPS	CPS RSD	Average	S/N
Blank	1654.57	5.2	1615	8.020433
Blank	1617.89	3.5
Blank	1646.78	5.9
Blank	1587.88	7.9
Blank	1570.11	1.9
0.2ppm	12750.42	1.1	12953
0.2ppm	12990.66	1.8
0.2ppm	13032.91	1.6
0.2ppm	13036.23	1.9

am I able to determine Ca in the protein sample with ICP-MS 7800 ( Include Helium collision and High Matrix)?

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